A vacuum drying intelligent control method for freeze-dried strawberry physical lock fresh bacteria inhibition

Abstract

The application relates to the technical field of vacuum freeze-drying and automatic control, and discloses a vacuum drying intelligent control method for physical lock-fresh and bacterium inhibition of strawberry freeze-drying. In the main drying stage, the pressure rising rate is obtained by closing an isolation valve, the mass transfer resistance and the sublimation interface temperature are inverted, the heating power is dynamically limited in combination with the collapse critical temperature, and the sublimation overheating leading to the micro-melting collapse of the skeleton is prevented. In the analysis stage, the final mass transfer resistance is extracted to calculate the tortuosity factor, the anti-condensation critical temperature is calculated according to the residual water vapor partial pressure, the refrigeration waste heat is utilized to carry out constant-temperature heat storage on the inert gas in a buffer tank, and then based on the preset pressure rising rate effective lower limit value and the pressure rising rate structure safety upper limit value, the vacuum pump and the adjusting valve are controlled to execute the alternating pressure cycle. The application realizes the physical bacterium inhibition without chemical addition by utilizing the alternating pressure difference to tear the cell membrane, the injected heat storage gas can compensate the expansion cooling, the secondary condensation of water vapor is prevented, and the drying and dehydration efficiency is improved.

Description

Technical Field

[0001] This invention relates to the field of vacuum freeze-drying and automated control technology, specifically to an intelligent control method for vacuum drying of strawberries using physical methods to lock in freshness and inhibit bacterial growth. Background Technology

[0002] Strawberries and other berries have high sugar and initial water content, making their cell structure prone to shrinkage and browning during traditional dehydration processes. To better preserve the natural color, flavor, and heat-sensitive nutrients of fruits and vegetables, vacuum freeze-drying technology, with its physical mechanism of low-temperature sublimation dehydration, is increasingly being widely used in the intensive processing of high-end agricultural products.

[0003] In existing vacuum freeze-drying engineering applications, equipment largely relies on static control programs set based on experience. Operators typically pre-set the heating plate temperature, chamber vacuum level, and operating time for the sublimation and desorption drying stages, ensuring continuous operation of the equipment under constant parameters to provide latent heat of sublimation and remove water vapor. Regarding antibacterial and preservation measures, conventional processing methods either add chemical preservatives during pretreatment or rely solely on low temperature and low pressure static vacuum environments during drying to inhibit microbial activity. When entering the desorption stage to remove bound water, the equipment typically maintains an extremely low static vacuum level, relying on prolonged heat preservation and evacuation to slowly diffuse residual moisture.

[0004] However, existing static drying processes are ill-suited to the dynamic changes in the physical state of materials during dehydration. As the external porous drying layer thickens, the mass transfer resistance of water vapor escaping increases. If a constant heating power is maintained, the heat absorbed at the ice crystal sublimation interface cannot dissipate quickly enough, causing a rapid temperature rise in the unsublimated ice core region. Once the micro-melting point of the high-sugar matrix is ​​exceeded, the porous framework undergoes plastic closure, leading to collapse. Conventional static vacuum low-oxygen conditions only induce dormancy for microorganisms attached deep within the capillaries; these microorganisms are easily revived and multiply upon rehydration or contact with air. Forcibly increasing the desorption temperature for sterilization severely damages heat-sensitive components such as anthocyanins. If pulsed pressure is introduced during the desorption stage to accelerate the removal of bound water, the gas entering the high-vacuum chamber undergoes adiabatic expansion, and the rapid cooling of the airflow easily causes the water vapor that has not yet escaped from the material to reach its dew point again, resulting in secondary condensation deep within the pores. Therefore, this invention provides a smart control method for vacuum drying of freeze-dried strawberries to physically lock in freshness and inhibit bacterial growth, addressing the shortcomings of existing technologies. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a smart control method for vacuum drying of strawberries using physical methods to lock in freshness and inhibit bacterial growth. This method solves the problems of existing vacuum freeze-drying processes for high-sugar fruits and vegetables, which rely on constant temperature and pressure control and are unable to adapt to dynamic changes in the physical state of the materials. These problems include overheating at the sublimation interface leading to micro-melting and structural collapse of the porous framework, and the fact that the purely physical low-pressure environment only induces dormancy in microorganisms, lacking effective antibacterial measures without chemical additives.

[0006] To achieve the above objectives, the present invention provides a vacuum drying intelligent control method for freeze-drying strawberries to physically lock in freshness and inhibit bacterial growth, comprising the following steps: S100, the refrigeration unit is started to control the temperature of the environment where the material is located to drop, so that the moisture inside the material undergoes a phase change to form a solid ice crystal structure and a frozen skeleton. After the solid ice crystal structure sublimates, it forms a porous matrix. S200, reduce the pressure in the drying chamber and output a power control signal to the heating plate, close the isolation valve to obtain the pressure rise rate, calculate the mass transfer resistance and ice crystal sublimation interface temperature based on the pressure rise rate, compare the ice crystal sublimation interface temperature with the collapse critical temperature and adjust the power control signal. S300, when the pressure rise rate decreases to the set sublimation end threshold, the tortuosity factor is calculated based on the mass transfer resistance, and the residual water vapor partial pressure in the drying chamber is obtained. The anti-condensation critical temperature is calculated based on the residual water vapor partial pressure, inert gas is introduced into the buffer tank, and the inert gas is heated to a temperature not lower than the heating target temperature determined based on the anti-condensation critical temperature using the high-temperature gaseous refrigerant in the exhaust bypass. S400: Calculate the upper limit value of the pressure rise rate structure based on the tortuosity factor, and obtain the preset lower limit value of the pressure rise rate effective value. Reduce the pressure of the drying chamber and maintain the pressure. Open the regulating valve to inject inert gas into the drying chamber. Constrain the pressure rise rate to be between the lower limit value of the pressure rise rate effective value and the upper limit value of the pressure rise rate structure to form an alternating pressure cycle. S500 acquires material quality data and calculates residual moisture content. When the residual moisture content reaches the preset target residual moisture content requirement, it stops the alternating pressure cycle and restores ambient normal pressure.

[0007] Furthermore, the step of starting the refrigeration unit to control the decrease in the ambient temperature of the material, causing the internal moisture of the material to undergo a phase change and form a solid ice crystal structure and a frozen skeleton, includes: acquiring the initial temperature data of the material and the set target temperature range; setting the cooling rate and adjusting the refrigeration unit to maintain a refrigeration load that meets the cooling rate so that the ambient temperature in the drying chamber decreases according to the cooling rate; the internal moisture of the material enters the maximum ice crystal formation zone, and by maintaining the cooling rate, the time the material spends in the maximum ice crystal formation zone is shortened, so that the main liquid phase moisture inside the material is transformed into dispersed solid ice crystals; after the liquid phase moisture crystallizes and undergoes phase change, the solid ice crystal structure forms and fills the interior of the material cells and intercellular spaces, combining with the solid dry matter contained in the material to form a frozen skeleton that maintains the macroscopic morphology of the material. By constraining the cooling rate through the above steps, the nucleation rate of moisture crystallization is increased and the growth cycle of individual ice crystals is limited, the volume expansion stress generated by moisture crystallization is dispersed, and the physical damage of large ice crystals to the cell walls of the material is reduced.

[0008] Furthermore, the steps of reducing the pressure in the drying chamber and outputting a power control signal to the heating plate, closing the isolation valve to obtain the pressure rise rate, and calculating the mass transfer resistance based on the pressure rise rate include: reducing the absolute pressure inside the drying chamber to the working vacuum range; sending an electromagnetic closing command to cut off the gas flow pipeline between the drying chamber and the cold trap according to a set time period; continuously acquiring transient pressure data points inside the drying chamber; calculating the pressure rise rate inside the drying chamber contributed by material sublimation during the isolation valve closure period through fitting and correction; and calculating the mass transfer resistance of the material drying layer to water vapor migration by combining the pressure rise rate with the effective sublimation surface area, the saturated vapor pressure of the ice crystal sublimation interface, and the ambient gas phase temperature. Through the above steps, based on fluid dynamics relationships, the macroscopic volumetric pressure rise data is transformed into the microscopic mass transfer resistance of the drying layer, realizing online acquisition of the internal mass transfer state of the material without contacting the material structure.

[0009] Furthermore, the step of calculating the mass transfer resistance and ice crystal sublimation interface temperature based on the pressure rise rate, comparing the ice crystal sublimation interface temperature with the collapse critical temperature, and adjusting the power control signal includes: iteratively inverting the transient temperature of the ice crystal sublimation interface based on the mass transfer resistance, ice surface saturated vapor pressure data table, and thermodynamic equations; calling the collapse temperature dynamic decay equation, combining the mass transfer resistance with the initial collapse temperature baseline value and the resistance temperature drift coefficient, and dynamically calculating the collapse critical temperature at the current moment; calculating the safety margin between the transient temperature and the collapse critical temperature, and when the safety margin is less than or equal to the set safety temperature difference threshold, reducing the duty cycle of the pulse width modulation signal sent to the heating plate, forcing the transient temperature to fall back and remain below the collapse critical temperature. This control method establishes a dynamic mechanical boundary where the material collapse threshold decreases as the drying resistance increases, and by limiting the rise in transient temperature, it prevents the heating rate from exceeding the sublimation endothermic rate, thus preventing micro-melting and collapse of the porous skeleton.

[0010] Furthermore, the calculation of the tortuosity factor based on the mass transfer resistance includes: continuously tracking the pressure rise rate trend of each measurement cycle; when the pressure rise rate is lower than the sublimation termination threshold for a set number of consecutive measurement cycles, extracting the mass transfer resistance value of the last valid measurement cycle as the final-state mass transfer resistance; normalizing the final-state mass transfer resistance with the system's reference flow resistance constant and then performing a logarithmic operation to generate a dimensionless tortuosity factor as a subsequent mechanical input parameter. This calculation process quantifies the connectivity complexity of the micro-channels inside the porous matrix into a specific value, providing a physical basis for evaluating the pressure bearing capacity limit of the material skeleton in the subsequent analysis stage.

[0011] Furthermore, the calculation of the anti-condensation critical temperature, which involves introducing inert gas into the buffer tank and heating the inert gas using high-temperature gaseous refrigerant in the exhaust bypass, includes: obtaining the residual water vapor partial pressure in the drying chamber; applying the gas-liquid phase change equilibrium theory and temperature-deformation formula; and calculating the anti-condensation critical temperature of the deep capillaries based on the residual water vapor partial pressure; setting the anti-condensation critical temperature superimposed with a thermodynamically compensated safety temperature difference as the heating target temperature; introducing inert gas into the buffer tank until the set initial working pressure is reached; and adjusting the refrigerant diversion valve on the exhaust bypass to introduce the high-temperature gaseous refrigerant into the heat exchange jacket, obtaining the actual temperature of the inert gas and comparing it with the heating target temperature to dynamically control the opening of the diversion valve. This control strategy transfers the exhaust waste heat carried by the high-temperature gaseous refrigerant in the refrigeration unit's exhaust bypass to the inert gas to form an enthalpy reserve, compensating for the adiabatic expansion and cooling effect generated when the inert gas is subsequently released into a low-pressure environment, and preventing secondary condensation of residual water vapor in the material pores.

[0012] Furthermore, the calculation of the structural safety upper limit value of the pressure rise rate based on the tortuosity factor includes: obtaining a preset effective lower limit value of the pressure rise rate as the control lower limit of the pressure rise rate when the regulating valve is opened, ensuring that the alternating pressure cycle has an inhibitory effect on the target microorganisms; obtaining the yield stress constant of the dry matter skeleton of the material under a preset low pressure state and a preset structural safety calibration coefficient, and determining the structural safety upper limit value of the pressure rise rate based on the tortuosity factor, yield stress constant, and structural safety calibration coefficient. This preset low pressure state corresponds to the pressure reduction and pressure holding sub-step in the analytical drying stage, preferably a state where the absolute pressure of the drying chamber is within the preset ultra-low pressure reference lower limit range. This calculation process integrates the microbial mechanical damage threshold and the physical strength of the dried material itself, establishing a safe control range that balances physical antibacterial action and prevention of material damage.

[0013] Furthermore, the process of reducing and maintaining the pressure in the drying chamber, injecting inert gas into the drying chamber through the regulating valve, and constraining the pressure rise rate to be between the effective lower limit and the structurally safe upper limit to form an alternating pressure cycle includes: disconnecting the airflow connection between the drying chamber and the buffer tank; reducing the absolute pressure in the drying chamber to a preset extremely low pressure reference lower limit and maintaining the pressure for a specified time, so that the cell structure of the target microorganism is under the combined action of dehydration stress and differential pressure stress; outputting an analog opening command to the regulating valve to allow the heated inert gas in the buffer tank to enter the drying chamber; monitoring the pressure rise rate in the drying chamber in real time and correcting the opening signal of the regulating valve; constraining the actual pressure rise rate to be between the effective lower limit and the structurally safe upper limit; closing the regulating valve and stabilizing it after a delay when the absolute pressure rises to the set upper limit; and restarting the pumping action. Under the continuous alternating pressure cycle, the external pressure of the microbial cells reverses rapidly, causing mechanical damage to the cell membrane and cell wall, thereby weakening the microbial reproductive capacity and achieving physical antibacterial action.

[0014] Furthermore, the step of acquiring material mass data to calculate residual moisture content, and stopping the alternating pressure cycle when the residual moisture content reaches the preset target residual moisture content requirement, includes: continuously acquiring the electrical signal of mechanical deformation caused by the material's gravity through an online weighing module, subtracting the sensor reference data under different vacuum levels as the tare weight, and dynamically correcting it using a temperature compensation algorithm to obtain the transient mass; retrieving the initial total mass and initial wet basis moisture content obtained by weighing the material before it enters the drying chamber, and calculating the residual moisture content by combining it with the transient mass; comparing the residual moisture content with the preset target residual moisture content threshold in real time, and determining that the material has reached the target drying state when the residual moisture content calculated multiple times within a continuously set time window is lower than the target residual moisture content threshold. This step, through dynamic tare weight correction and continuous time window determination mechanism, eliminates sensor signal drift interference caused by extremely low pressure and temperature changes, improving the accuracy of the drying endpoint determination.

[0015] Furthermore, the restoration of ambient atmospheric pressure includes: outputting a control signal to the pressure relief valve connected to the air inlet of the drying chamber; setting an intermediate pressure plateau as a dwell point within the vacuum to atmospheric pressure range; constraining the opening of the pressure relief valve to ensure that the pressure rise rate inside the drying chamber does not exceed a preset vacuum breaking pressure rise rate threshold; controlling the pressure relief valve to close when the absolute pressure inside the drying chamber rises to the intermediate pressure plateau; maintaining this position for a set time to allow the air pressure inside and outside the porous skeleton of the material to reach equilibrium; then reopening the pressure relief valve until the interior of the drying chamber is completely restored to ambient atmospheric pressure. By setting the dwell platform in a stepped manner to buffer the pressure rise rate, the transient strong airflow is prevented from impacting and damaging the fragile porous skeleton after dehydration.

[0016] This invention provides a smart control method for vacuum drying of strawberries using physical methods to lock in freshness and inhibit bacterial growth. It offers the following advantages: 1. This invention continuously collects the pressure rise rate during valve closure to calculate mass transfer resistance, and inversely calculates the ice crystal sublimation interface temperature based on thermodynamic equations, thereby constructing a dynamic decay mechanical boundary for the collapse threshold as the resistance changes. By calculating the safety margin of transient temperature and collapse critical temperature in real time and implementing closed-loop limiting control of heating power, it can prevent the heating rate from exceeding the sublimation endothermic rate without contacting the material, effectively avoiding skeletal micro-melting and structural collapse of high-sugar materials during dehydration, and maintaining the physical morphology and porous structure of the freeze-dried product; 2. This invention correlates the lower limit of microbial lethal stress with the yield strength of the porous framework of the material. During the desorption drying stage, an alternating pressure cycle is executed, constrained by both the effective lower limit of the pressure rise rate and the structural safety upper limit of the pressure rise rate. Under continuous depressurization, pressure holding, and repressurization operations, the external pressure of the microbial cells rapidly reverses, causing mechanical damage to the cell membrane and cell wall under the combined action of dehydration stress and fluid pressure differential stress, thereby weakening the survival and reproduction ability of aerobic bacteria and molds deep within the capillaries. 3. This invention utilizes the waste heat from the exhaust bypass of the refrigeration unit to store heat at a constant temperature in the inert gas within the buffer tank, and uses the anti-condensation critical temperature calculated by the gas-liquid phase change equilibrium theory as the lower limit of the heat storage target. During alternating pressure cycling, the enthalpy carried by the preheated gas injected into the vacuum chamber effectively compensates for the adiabatic expansion cooling effect, preventing secondary condensation of residual water vapor in deep pores, maintaining the thermodynamic driving force for water vapor diffusion outwards, and achieving resource utilization of equipment waste heat while shortening the drying cycle. Attached Figure Description

[0017] Figure 1 This is a system architecture process diagram of the present invention; Figure 2 This is a flowchart of the method of the present invention; Figure 3 This is a flow chart of the cooling process in the pre-freezing phase change step of the present invention. Figure 4 This is a closed-loop control logic diagram of the main drying control step of the present invention; Figure 5 This is a signal and energy flow diagram for the parameter extraction and heat storage steps of the present invention; Figure 6 This is a flowchart of the alternating pressure cycle procedure of the present invention; Figure 7 This is a flowchart illustrating the timing control of the endpoint determination and the stepped vacuum breaking procedure of the present invention. Figure 8 This is a multi-parameter closed-loop response curve of the main drying stage of the present invention. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] See attached document Figure 1 The freeze-drying equipment includes a drying chamber and a cold trap. The drying chamber and the cold trap are connected by pipelines. Isolation valves are installed on the pipelines. The isolation valves receive electrical signals and perform opening or closing actions to cut off or open the gas passage between the drying chamber and the cold trap.

[0020] The drying chamber is equipped with a vacuum gauge, an ambient temperature sensor, and a heating plate. The vacuum gauge measures the absolute pressure inside the drying chamber, and the ambient temperature sensor acquires the gas phase temperature data within the drying chamber. The heating plate has a pipeline for circulating heat-conducting fluid. This pipeline connects to the low-temperature heat exchange branch of the refrigeration unit and the heating circuit via a cooling / heating switching pipeline. This allows the heating plate to function as a cooling support during the pre-freezing stage and as a heating support during the drying stage, transferring heat to the material placed on it. A temperature sensor is installed on the support surface of the heating plate, and an online weighing module is located at the bottom of the heating plate's support structure. The temperature sensor acquires the temperature data of the support surface, and the online weighing module acquires real-time mass data of the material during the drying process.

[0021] The freeze-drying equipment is equipped with a refrigeration unit and a vacuum pump unit. The evaporator end of the refrigeration unit is connected to the heat exchange coil inside the cold trap, and is also connected to the heat-conducting fluid pipeline inside the heating plate through a low-temperature heat exchange branch to provide cooling to the heating plate during the pre-freezing stage. The refrigeration unit is equipped with an exhaust bypass. The vacuum pump unit is connected to the drying chamber and the cold trap through an exhaust pipeline to reduce the absolute pressure inside the drying chamber.

[0022] A buffer tank is installed outside the drying chamber. A heat exchange jacket is installed around the buffer tank. An exhaust bypass connects to the heat exchange jacket of the buffer tank. The buffer tank is connected to the drying chamber via a pipeline equipped with a regulating valve. The regulating valve receives an analog signal to adjust its opening degree to control the gas flow rate. The buffer tank is also connected to an inert gas source, and a gas source valve and a sterile filter are installed on the pipeline between the inert gas source and the buffer tank. A temperature sensor and a pressure sensor are installed inside the buffer tank. The drying chamber is also connected to a vacuum-breaking inlet pipeline, which is equipped with a pressure relief valve and a sterile filter for introducing sterile air or inert gas into the drying chamber in stages after drying is complete.

[0023] The freeze-drying equipment is equipped with a controller. The controller is electrically connected to the isolation valve, vacuum gauge, heating plate, refrigeration unit, and regulating valve, and is also signal- or electrically connected to the ambient temperature sensor, temperature sensor, online weighing module, vacuum pump unit, gas source valve, buffer tank temperature sensor, buffer tank pressure sensor, and pressure relief valve. The controller has a built-in fluid calculation module and thermodynamic module.

[0024] See attached document Figure 2 This invention provides a smart control method for vacuum drying of strawberries using physical methods to lock in freshness and inhibit bacterial growth, comprising the following steps: S100, the controller starts the refrigeration unit, controls the temperature of the material bearing surface to drop through the low temperature heat exchange branch of the heating plate, and controls the temperature of the environment where the material is located to drop in conjunction with the heat exchange effect of the cold trap. The water inside the material undergoes a phase change to form a solid ice crystal structure and a frozen skeleton. After the solid ice crystal structure sublimates, it forms water vapor migration channels, thus forming a porous matrix.

[0025] S200, the controller controls the vacuum pump group to reduce the pressure in the drying chamber to the set range and outputs a power control signal to the heating plate. The controller sends a closing command to the isolation valve according to the set cycle. During the isolation valve closure, the controller obtains the pressure data output by the vacuum gauge to calculate the pressure rise rate. Based on the pressure rise rate, the controller calculates the mass transfer resistance and back-calculates the ice crystal sublimation interface temperature. The controller compares the ice crystal sublimation interface temperature with the collapse critical temperature and adjusts the output power of the heating plate. S300: When the pressure rise rate decreases to the set sublimation end threshold, the controller calculates the tortuosity factor based on the final mass transfer resistance and obtains the residual water vapor partial pressure in the drying chamber. Based on the residual water vapor partial pressure, the controller calculates the anti-condensation critical temperature, which includes the critical dew point temperature or the critical frost point temperature. At the same time, the controller controls the buffer tank to be connected to inert gas and controls the exhaust bypass to be connected to the heat exchange jacket of the buffer tank to heat the inert gas to a temperature not lower than the heating target temperature determined based on the anti-condensation critical temperature. S400: The controller obtains the preset effective lower limit value of the pressure rise rate and calculates the structural safety upper limit value of the pressure rise rate of the regulating valve based on the tortuosity factor. It controls the vacuum pump group to reduce the pressure in the drying chamber to the reference lower limit and maintain it for a set time. After the pressure holding is completed, it outputs an opening command to the regulating valve to allow the inert gas in the buffer tank to enter the drying chamber. The controller constrains the opening of the regulating valve so that the pressure rise rate in the drying chamber is between the effective lower limit value of the pressure rise rate and the structural safety upper limit value of the pressure rise rate. When the absolute pressure in the drying chamber rises to the set pressure upper limit value, a repressurization process is formed, which together with the aforementioned pressure reduction and holding process forms an alternating pressure cycle. S500: The controller acquires the material's quality data and calculates the residual moisture content. When the residual moisture content reaches the preset target residual moisture content requirement, it stops the alternating pressure cycle, shuts off the heating plate, and controls the gas to enter the drying chamber until the ambient pressure is restored.

[0026] The technical solution of the present invention will be described in detail below in conjunction with the above-described operational steps.

[0027] See attached document Figure 3 In step S100, the controller starts the refrigeration unit to control the decrease in ambient temperature, causing a phase change in the moisture inside the material, thereby forming a solid ice crystal structure and a frozen skeleton. The frozen skeleton then transforms into a porous matrix during subsequent sublimation. Specifically, step S100 may include the following steps: S110, the controller acquires the temperature data of the bearing surface through a temperature sensor installed on the bearing surface of the heating plate, and estimates the initial temperature data of the material by combining the material slice thickness, the initial feeding temperature, and a preset heat conduction empirical compensation formula. It should be noted that in embodiments with a material temperature sensor, the controller can also directly acquire the temperature data of the material center.

[0028] After acquiring the material temperature data, the controller sends an operating command to the refrigeration unit. Based on the preset cooling sequence curve, the controller outputs control signals to adjust the operating frequency of the compressor and the opening of the electronic expansion valve in the refrigeration unit.

[0029] By adjusting the operating load of the refrigeration unit, the controller controls the evaporation temperature of the heat exchange coils in the cold trap, and at the same time controls the low-temperature heat exchange branch of the refrigeration unit to provide cooling capacity to the heat transfer fluid pipeline in the heating plate, so that the ambient temperature and the material bearing surface temperature in the drying chamber decrease at the set cooling rate.

[0030] S120, the controller monitors the decrease in ambient temperature and lowers the ambient temperature of the material to a target temperature range, preferably set to -30℃ to -35℃. During this process, heat is transferred from the material to the outside, and the temperature decreases accordingly.

[0031] The controller sets and constrains the cooling rate of the material. The calculation formula is: ; in, The initial phase transition temperature is the temperature at which the moisture inside the material undergoes a crystallization phase transition. This initial phase transition temperature is determined by looking up a table based on the sugar content of the material. For strawberry materials, the value is usually between -1℃ and -2℃. The set target temperature; This is the time it takes for the material temperature to drop from the initial phase transition temperature to the target temperature.

[0032] The controller maintains a cooling load that meets the cooling rate by adjusting the refrigeration unit, which will take time. The maximum value is limited to 15 to 30 minutes, which limits the cooling rate. To meet the process requirements for rapid crystallization of liquid phase water.

[0033] S130, as the ambient temperature inside the drying chamber continues to decrease, the liquid phase moisture inside the material crystallizes under the action of phase change thermodynamics.

[0034] When the material temperature drops below freezing, the liquid water phase enters the zone of maximum ice crystal formation, which is typically between -1°C and -5°C. Within this temperature range, a large amount of water crystallizes inside the material.

[0035] The controller maintains the cooling rate determined by the above calculation, shortens the time the material is in the maximum ice crystal formation zone, thereby increasing the nucleation rate of water crystals and limiting the growth cycle of individual ice crystals, so that the main liquid phase water inside the material is transformed into dispersed solid phase ice crystals.

[0036] In S140, after most of the liquid water undergoes a crystallization phase transition, a solid ice crystal structure is formed inside the material. A large number of fine solid ice crystals fill the interior of the material cells and the intercellular spaces to disperse the volume expansion stress generated by water crystallization and reduce the physical damage to the cell walls caused by excessively large ice crystals.

[0037] The solid dry matter of the material combines with the solid ice crystals that originally occupied the moisture to form a frozen framework that maintains the macroscopic shape of the material. In the subsequent sublimation process, the solid ice crystals are directly converted from solid to water vapor and migrate out of the material. The space originally occupied by the ice crystals is transformed into channels for the water vapor to migrate outward. The channels and the solid dry matter framework together form a porous matrix.

[0038] See attached document Figure 4 In step S200, the controller controls the vacuum pump assembly to reduce the pressure inside the drying chamber to a set range and outputs a power control signal to the heating plate. The controller sends a closing command to the isolation valve at a set cycle to collect the pressure rise rate, thereby calculating the temperature and adjusting the heating plate. Specifically, step S200 may include the following steps: S210, after the material enters the sublimation drying state, the controller controls the vacuum pump group to reduce the absolute pressure in the drying chamber to the set working vacuum range, for example, between 0.5Pa and 30Pa, and at the same time outputs a pulse width modulation signal to the heating plate to provide the latent heat of sublimation. In this embodiment, the power control signal is a pulse width modulation signal.

[0039] During this process, the controller triggers pressure acquisition according to a set time cycle, typically set to 10 to 30 minutes. The controller sends an electromagnetic closing command to the isolation valve to instantly cut off the gas flow between the drying chamber and the cold trap. Within the limited time window of the isolation valve closure, such as 3 to 5 seconds, the absolute pressure inside the drying chamber increases accordingly as water vapor is generated by the continuous sublimation of solid ice crystals inside the material and cannot be discharged to the outside.

[0040] During this period, the vacuum gauge continuously acquires transient pressure data points inside the drying chamber at a high-frequency sampling rate. The controller reads these data points and performs linear fitting using the least squares method. Then, based on the background leakage rate obtained from the equipment's no-load test, the fitting slope is subtracted and corrected. Finally, the pressure rise rate inside the drying chamber contributed by material sublimation during the isolation valve closure is calculated. .

[0041] S220, the controller determines the pressure rise rate based on the acquired pressure rise rate. Calculate the mass transfer resistance of the material drying layer. Since the migration of water vapor from the sublimation interface is physically hindered by the dried porous matrix, the controller invokes the mass transfer resistance calculation. The fluid dynamics calculation formula is as follows: ; in, The effective sublimation surface area of ​​the material is estimated by the controller based on the pre-input single batch feed mass and the geometric dimensions of the material slices. The saturated vapor pressure at the ice crystal sublimation interface is determined by the controller through exponential fitting of transient pressure data during the closure of the isolation valve, and by taking the asymptotic extreme value of the fitting curve in combination with the pressure change boundary before and after valve closure. The absolute pressure for stable operation within the drying chamber before the isolation valve closes; The effective volume constant of the drying chamber; It is the ideal gas constant; The ambient gas phase temperature inside the drying chamber is obtained through an ambient temperature sensor.

[0042] Obtaining mass transfer resistance Then, the controller calls the temperature solution deformation formula of the Clausius-Clapeyron equation to calculate the transient temperature of the actual ice crystal sublimation interface inside the material. Based on the relationship between the saturated vapor pressure and the physical temperature of the ice crystal sublimation interface, the controller can adjust the saturated vapor pressure accordingly. The transient temperature is obtained by iterating over a pre-set table of saturated vapor pressure data for the ice surface or a thermodynamic equation. .

[0043] S230, considering the risk of structural collapse of high-sugar materials during vacuum sublimation, and the fact that its critical collapse temperature decreases and drifts with changes in the thickness of the porous drying layer and the mass transfer resistance, a dynamic decay equation for the collapse temperature is established within the controller, based on the mass transfer resistance obtained from the aforementioned calculations. Dynamically calculate the critical collapse temperature at the current moment. .

[0044] The dynamic decay equation of the collapse temperature is defined as follows: .in, The initial collapse temperature baseline value for this type of material is pre-recorded by the system. It is determined by the sugar content of the strawberry material, the slice thickness, and the pre-established material collapse temperature calibration table. For example, the collapse temperature of strawberry material is usually recorded between -15℃ and -25℃. The resistance temperature drift coefficient, which is affected by the sugar content of the material, is called by the controller according to the preset material type parameters, or written into the memory through the pre-experimental calibration data of similar materials.

[0045] Subsequently, the controller calculates the transient temperature. With the critical temperature of collapse Safety margin between ,Right now When this safety margin When the temperature difference is less than or equal to the set safe temperature threshold, such as 2°C, it indicates that the internal heating rate exceeds the sublimation heat absorption rate, posing a risk of matrix collapse. In this case, the controller reduces the duty cycle of the pulse width modulation signal sent to the heating plate, thereby limiting heat output and forcing the transient temperature to... It fell back and remained below the critical temperature for collapse.

[0046] S240, the controller continuously tracks the pressure rise rate during each measurement cycle in the main drying stage. Trend of change. When the controller determines the rate of pressure rise within a set number of measurement cycles, such as three consecutive measurement cycles. When all values ​​are below the set sublimation end threshold, and the change between adjacent measurement cycles is less than the stability judgment threshold and close to the background leakage rate of the drying chamber, it is confirmed that the solid ice crystals inside the material have basically completed sublimation, and the main drying stage ends.

[0047] After the main drying stage is completed, the controller extracts the mass transfer resistance value of the last valid measurement cycle as the final state mass transfer resistance. It is then stored in an internal register as the initial input parameter for the subsequent parsing and drying process.

[0048] See attached document Figure 5In step S300, when the pressure rise rate of the main drying stage decreases to the sublimation termination threshold, the controller calculates the tortuosity factor based on the final mass transfer resistance and the anti-condensation critical temperature based on the residual water vapor partial pressure. Simultaneously, the controller controls the buffer tank to connect to inert gas and utilizes the residual heat carried by the high-temperature gaseous refrigerant in the exhaust bypass to heat the inert gas. Specifically, step S300 may include the following steps: S310, the controller retrieves the final-state mass transfer resistance extracted and stored at the end of the main drying stage. Since the connectivity complexity of the micro-channels within the porous matrix and its mass transfer resistance can be characterized by empirical correlations pre-calibrated in the equipment, the controller calculates the tortuosity factor of the porous matrix accordingly. The calculation formula is as follows: ; in, It is a dimensionless tortuosity factor, representing the degree of tortuosity of the migration path of gas molecules inside the porous matrix; This is the system reference flow resistance constant for the freeze-drying equipment under no-load conditions.

[0049] This constant is obtained by the equipment under no-material conditions through a standard dynamic pressure rise test, and is pre-written as a system constant into the controller's memory. During the calculation, the controller incorporates the final-state mass transfer resistance. With respect to the system reference flow resistance constant Normalization is performed to obtain dimensionless ratios for logarithmic operations, from which the tortuosity factor is calculated. This is used as a mechanical input parameter for subsequent evaluation of the material's pressure-bearing capacity.

[0050] S320, the controller calculates the critical dew point temperature or critical frost point temperature to prevent water vapor condensation, and records both as the anti-condensation critical temperature. Specifically, the controller first obtains the residual water vapor partial pressure in the drying chamber at the current stage. The partial pressure value is calculated by subtracting the equipment's inherent leakage limit pressure from the absolute pressure data inside the chamber collected by the vacuum gauge. This limit pressure is obtained by the controller through an unloaded limit vacuum test before the equipment is put into operation and is then fixed as a system constant.

[0051] To prevent water vapor in the deep capillaries of the material from undergoing phase change and condensation due to the injection of large amounts of gas during subsequent alternating pressure differential operations, the controller calculates the critical anti-condensation temperature of the deep capillaries based on the gas-liquid phase change equilibrium theory by using the temperature-derived formula of the Antoine equation. The specific relationships are as follows: ; in, Represented by constants The natural logarithm with base 0; , , These are all Antoine empirical constants for water vapor in a low-pressure vacuum environment. They are fixed values ​​within a specific working vacuum range and are directly called by the thermodynamic calculation module built into the controller.

[0052] The controller reduces the partial pressure of residual water vapor before invoking the Antoine equation. Unified conversion to empirical constants , , The corresponding pressure unit. For example, a constant corresponding to the physical properties of a water system. The value is set to 8.07131, a constant. The value is 1730.63, a constant. The value is 233.426. When the calculated phase change equilibrium temperature is below 0℃, the controller calls the empirical formula for the saturated vapor pressure of ice surface to correct the calculation result, and uses the corrected temperature as the critical frost point temperature, which serves as the critical anti-condensation temperature to prevent secondary condensation of moisture inside the material.

[0053] S330, the controller is based on the calculated anti-condensation critical temperature. Set the target heating temperature of the buffer tank. Its formula is .in, The thermodynamic compensation safety temperature difference set for the system is typically between 5°C and 15°C, taking into account the heat loss during the gas flow through valves and pipelines.

[0054] After the temperature setting operation is completed, the controller outputs an open signal to the gas source valve, allowing the high-purity nitrogen and other inert gases in the inert gas source to enter the buffer tank after being filtered through a sterile filter. When the pressure inside the buffer tank reaches the set initial working pressure, the controller confirms this pressure value through the buffer tank pressure sensor, and then closes the gas source valve to maintain the tank's seal. At this time, this initial pressure value is set according to the upper limit of the subsequent alternating pressure difference requirement, generally limited to between 0.1MPa and 0.3MPa.

[0055] After the buffer tank is fully charged (S340), the controller performs waste heat recovery and constant temperature heat storage operations. It sends a command to the refrigeration unit to adjust the refrigerant diversion valve on the exhaust bypass, introducing the high-temperature gaseous refrigerant discharged from the compressor into the heat exchange jacket surrounding the buffer tank. The high-temperature gaseous refrigerant transfers its heat to the inert gas inside the buffer tank through the heat exchange jacket, increasing the enthalpy and gradually raising the temperature of the inert gas.

[0056] During this process, the controller obtains the actual temperature of the inert gas through the buffer tank temperature sensor and compares it with the set heating target temperature. The system performs a comparison and then dynamically controls the opening of the refrigerant diversion valve in the exhaust bypass based on the temperature difference feedback data, so that the inert gas in the buffer tank is heated and stably maintained at the target heating temperature. This allows for the storage of thermodynamic energy for subsequent alternating pressure differential operations.

[0057] See attached document Figure 6 In step S400, the controller determines the pressure rise rate control range of the regulating valve based on the preset effective lower limit value of the pressure rise rate and the structural safety upper limit value of the pressure rise rate determined by the tortuosity factor. It then controls the vacuum pump unit and the regulating valve to operate alternately, thereby forming an alternating pressure cycle to achieve physical antibacterial action and promote the removal of bound water. Specifically, step S400 may include the following steps: S410, the material enters the desorption and drying stage to remove bound water. Before executing the alternating pressure injection, the controller obtains the preset effective lower limit value of the pressure rise rate. And based on the obtained tortuosity factor Yield stress constant of the dry matter skeleton of the material under a preset low-pressure state and preset structural safety calibration coefficients Determine the upper limit of the structural safety value for the pressure rise rate when the regulating valve opens. The effective lower limit of the pressure rise rate can be obtained through a target microorganism pressure response calibration experiment. Specifically, under the same material moisture content, temperature, and vacuum conditions, different pressure rise rates are applied to the target microorganism in repeated pressure cycles, and the total number of colonies, survival rate, or cell membrane integrity after treatment are detected. When the decrease in the total number of colonies, the decrease in the survival rate, or the proportion of membrane damage corresponding to a certain pressure rise rate reaches the preset antibacterial judgment standard, the pressure rise rate is determined as the effective lower limit of the pressure rise rate for the corresponding target microorganism and stored in the empirical threshold table of the controller.

[0058] The effective lower limit of the pressure rise rate is a control threshold pre-stored in the controller. This control threshold can be determined based on data such as the decrease in total colony count, changes in survival rate, or membrane structure damage of the target microorganism under different pressure rise rates. This effective lower limit ensures that alternating pressure cycling can inhibit the target microorganism, while the structural safety upper limit of the pressure rise rate prevents the hydrodynamic impulse from exceeding the structural limit of the porous matrix. The constraint relationship between the two is as follows: ; Specifically, The actual pressure rise rate within the drying chamber during alternating pressure cycles; the effective lower limit value of the pressure rise rate. The threshold can be predetermined based on relevant experimental data of the target microorganism and written into the controller in the form of an empirical threshold table; for common aerobic bacteria, yeasts and molds in strawberry processing, the controller can directly call the corresponding threshold in the empirical threshold table.

[0059] Pressure rise rate structural safety limit value This is used to limit the transient fluid impact intensity on the porous matrix during inert gas repressurization. The controller acquires the yield stress constant of the material's dry matter skeleton under a preset low-pressure state. And call the preset structural safety calibration coefficient. According to the tortuosity factor Yield stress constant and structural safety calibration coefficient Determine the structural safety upper limit value for the rate of pressure rise. In one embodiment, the preset low-pressure calibration state is a state where the absolute pressure of the drying chamber is between 1 Pa and 5 Pa, or any calibration pressure point within this pressure range.

[0060] In one implementation, the structural safety upper limit value for the pressure rise rate is... The calculation relationship is as follows: ; in, The structural safety calibration coefficient can be obtained by calibrating the structural retention rate, pore breakage rate or volume collapse rate of similar freeze-dried strawberry materials under different pressure rise rates and recompression conditions. The yield stress constant of the dry matter skeleton of the material under a preset low pressure state is obtained by static compression test on porous skeleton samples of similar sugar-containing materials after the main drying is completed during the equipment commissioning phase, under the preset low pressure calibration state and the temperature conditions of the analytical drying stage. The tortuosity factor characterizes the degree of tortuosity of the gas migration path within the porous matrix. The controller uses this relationship to determine the structural safety upper limit for the pressure rise rate, ensuring that the pressure rise rate during the repressurization process does not exceed the structural safety range that the porous framework of the material can withstand.

[0061] When the controller determines that the effective lower limit value is greater than the structural safety upper limit value of the pressure rise rate, it prioritizes limiting the opening of the regulating valve to protect the material structure and outputs a process parameter adjustment prompt to re-satisfy the above constraint relationship by increasing the number of cycles, reducing the loading thickness, or adjusting the upper limit of pressure.

[0062] S420, the controller starts the pressure reduction and pressure holding sub-step of the alternating pressure cycle program, which reduces the absolute pressure in the drying chamber to an extremely low pressure reference lower limit, for example, between 1 Pa and 5 Pa, by cutting off the airflow connection between the drying chamber and the buffer tank and controlling the operation of the vacuum pump group.

[0063] When the pressure reaches the lower limit of the reference, the controller maintains the vacuum state for a set pressure holding time, such as 30 to 60 seconds, so that the microbial cells attached to the porous skeleton of the material and deep in the micropores are in an extremely low external pressure environment, which causes the pressure balance of the cell membrane, cell wall and intracellular residual water to change, thereby putting the cell structure of the target microorganism in a sensitive state under the combined action of dehydration stress and pressure difference stress.

[0064] S430, after the pressure holding time ends, the controller starts the pressure recovery sub-step, outputs an analog opening command to the regulating valve, so that the preheated inert gas in the buffer tank enters the low-pressure drying chamber under the pressure difference drive.

[0065] During this process, the controller monitors the pressure rise rate in the drying chamber in real time through a vacuum gauge, and combines a closed-loop proportional-integral-derivative algorithm to correct the opening signal of the regulating valve in real time, so as to constrain the actual pressure rise rate in the drying chamber to be between the effective lower limit of the pressure rise rate and the structural safety upper limit of the pressure rise rate, and preferably make the actual pressure rise rate approach but not exceed the structural safety upper limit of the pressure rise rate.

[0066] As inert gas is injected, the absolute pressure inside the drying chamber rapidly increases to a set upper pressure limit, for example, between 100 Pa and 500 Pa. The controller then closes the regulating valve and waits for a set time to allow the airflow inside the chamber to stabilize before restarting the vacuum pump unit for the next pumping operation. This pressure reduction, holding, and repressurization constitute a complete alternating pressure cycle. In one specific embodiment, the controller sets the number of alternating pressure cycles to 20 to 60, or determines the cycle endpoint based on the decreasing trend of residual moisture content and a preset minimum inhibitory cycle count.

[0067] In S440, during the execution of the complex pressure step, the alternating pressure field induces two main physical responses within the porous matrix.

[0068] First, the rapid increase in pressure from extremely low to the upper limit within a short period causes a rapid change in the pressure state outside the microbial cells. Through repeated cycles of depressurization, pressure maintenance, and repressurization, the microbial cell membrane, cell wall, and surrounding water environment are repeatedly affected by dehydration stress, differential pressure stress, and airflow shearing. This alternating effect increases cell membrane permeability and creates a tendency for membrane structure damage, thereby reducing the microbial reproductive capacity in subsequent rehydration environments and achieving a physical antibacterial effect without chemical additives.

[0069] Since the initial temperature of the inert gas injected into the buffer tank is already higher than the critical temperature for preventing condensation, when the relatively high-pressure inert gas is throttled through the regulating valve into the extremely low-pressure drying chamber, the expansion and cooling effect during pressure release and volume expansion is effectively compensated by the enthalpy carried by the gas beforehand. The gas carrying the enthalpy can more easily maintain its temperature above the safe temperature for preventing condensation after expansion. This compensation mechanism prevents residual water vapor that has not yet escaped from the deep pores of the material from undergoing a phase change and condensing back into liquid water due to cooling, thereby maintaining the thermodynamic driving force for water vapor diffusion and improving the efficiency of bound water removal.

[0070] See attached document Figure 7 In step S500, the controller acquires the material's mass data and calculates the residual moisture content. When the residual moisture content reaches the preset target residual moisture content requirement, the alternating pressure cycle stops, the heating plate is turned off, and gas is controlled to enter the drying chamber until ambient pressure is restored. Specifically, step S500 may include the following steps: During the desorption and drying stage of the S510, the controller continuously collects the real-time weight signal of the material through the online weighing module set at the bottom of the heating plate support structure, and then converts the mechanical deformation caused by the gravity of the material into an electrical signal and transmits it to the internal data processing.

[0071] In this process, in order to eliminate the measurement drift caused by the equipment under extremely low pressure and temperature change conditions, the controller pre-records the sensor reference data under no-load conditions, no-load conditions of the heating plate, and different vacuum degree ranges as tare weight for deduction, and combines the built-in temperature compensation algorithm to dynamically correct the acquired transient electrical signals.

[0072] The S520 controller converts the acquired transient mass data into residual moisture content. The residual moisture content is calculated based on the wet basis moisture content, and the calculation formula is as follows: ; in, The transient mass obtained by the online weighing module at the current moment and after compensation and correction; The initial total mass obtained by weighing the material before it enters the drying chamber; The initial wet basis moisture content of the material is obtained by the operator through sampling with an external moisture meter before the start of the process and is entered as a fixed system parameter.

[0073] Using the above formula, the controller maps macroscopic quality changes to the removal state of microscopic moisture in the material in real time. In the formula... The item represents the absolute dry matter mass of the material that does not participate in evaporation.

[0074] S530, the controller will calculate the residual moisture content. The residual moisture content is compared in real time with the system's preset target threshold. Taking the freeze-drying process of strawberry materials with this threshold set to 3.5% as an example, the controller calculates the residual moisture content multiple times within a continuously set time window of 5 to 10 minutes. When the concentration of each component is below 3.5% and the number of alternating pressure cycles reaches the preset minimum number of antibacterial cycles, the controller determines that the material has reached the target dryness state.

[0075] At this point, the controller outputs a stop command to terminate the alternating pressure cycle program, while simultaneously cutting off the heating circuit of the heat-conducting fluid inside the heating plate and sending a shutdown command to shut down the vacuum pump unit.

[0076] After the alternating pressure cycle program terminates, the controller performs a step-by-step vacuum breaking operation to restore ambient normal pressure. It outputs an analog control signal to the pressure relief valve connected to the air inlet of the drying chamber, controlling the sterile air or inert gas to enter the drying chamber in stages after being filtered by a sterile filter.

[0077] Specifically, the controller sets intermediate pressure platforms such as 30000Pa and 70000Pa as dwell points within the range from vacuum to atmospheric pressure. During the depressurization process, the opening of the pressure relief valve is constrained based on the pressure data fed back by the vacuum gauge, so that the pressure rise rate in the drying chamber does not exceed the preset vacuum breaking pressure rise rate threshold.

[0078] When the absolute pressure inside the drying chamber rises to the intermediate pressure plateau, the controller controls the pressure relief valve to close and maintains it for a set time of 1 to 3 minutes to allow the air pressure inside and outside the porous skeleton of the material to reach equilibrium.

[0079] After the residence time of the intermediate pressure platform ends, the controller opens the pressure relief valve again until the inside of the drying chamber is completely restored to the ambient normal pressure. This avoids the physical impact of transient strong airflow on the dehydrated porous skeleton by limiting the rate of increase of airflow pressure, thus maintaining the structural integrity of the finished product and completing the entire drying process.

[0080] In a specific application embodiment, taking the freeze-drying process of a single batch of 10kg high-sugar strawberry slices as an example, the above control process is calculated and explained in conjunction with specific engineering values.

[0081] The pre-freezing phase change step in equipment startup procedure S100. Initial wet basis moisture content of strawberry material. The initial phase transition temperature was determined to be 90% based on a built-in lookup table. The target pre-freezing temperature is -1.5℃. The temperature is set at -35°C, with a limited duration of cooling. The maximum duration is 20 minutes. Based on the cooling rate constraint formula, the controller must maintain the output load of the refrigeration unit to ensure the cooling rate... At a rate of 1.675℃ / min, the water inside the strawberry rapidly crosses the maximum ice crystal formation zone to form dispersed solid ice crystals.

[0082] In step S200, the main drying control step is initiated, and the controller sets the working vacuum level to 20 Pa. During a certain measurement cycle, the isolation valve closes for 4 seconds, and the vacuum gauge measures a linear increase in the absolute pressure inside the chamber from 20 Pa to 26 Pa. The pressure rise rate is then calculated. The rate is 1.5 Pa / s. The controller substitutes this rate into the mass transfer resistance calculation formula, and after subtracting the system's background leakage rate, calculates the current mass transfer resistance of the drying layer. 2.5×10 4 .

[0083] Furthermore, the controller calculates and confirms the transient temperature of the current ice crystal sublimation interface. The initial temperature is -29℃. The dynamic decay equation for strawberry collapse temperature is applied, setting the initial collapse temperature as the baseline value. The resistance temperature drift coefficient is -20℃. Take 1.2 × 10 -4 The current critical temperature for collapse was calculated. At this point, the safety margin is... If the temperature difference exceeds the set safety threshold of 2°C, the controller determines that there is no risk of collapse and maintains the current heating power of the heating plate.

[0084] At the end of the main drying stage, step S300 is executed, and the controller extracts the final state mass transfer resistance. 8.5×10 4 Combined with no-load reference constant The tortuosity factor was calculated. At this time, the partial pressure of residual water vapor inside the chamber... Extremely low, the critical temperature for preventing condensation was calculated using a modified formula derived from the Antoine equation. The temperature is -32.5℃. The controller adds a 10℃ thermodynamic compensation safety temperature difference to set the target heating temperature of the buffer tank. The temperature is -22.5℃, and the waste heat from the exhaust bypass is used to store the nitrogen in the tank.

[0085] Proceed to step S400, the alternating pressure cycle step. The critical stress constant for common aerobic bacteria in strawberries is known. Set to 1.5×10 5 Pa / s, yield stress constant of strawberry dry matter skeleton 4.5×10 4 Pa. The controller calculates the structural safety limit value for the pressure rise rate. The controller constrains the opening of the regulating valve accordingly, performing isenthalpic pressure cycling and alternating pressure cycles at a pressure rise rate not exceeding 1.55 Pa / s between 1 Pa and 200 Pa, ensuring that the strawberry micropores do not rupture while tearing the cell membrane.

[0086] In the endpoint determination stage of step S500, the online weighing module obtains the current transient mass after temperature and vacuum buoyancy compensation. The mass is 1.036 kg. The initial total mass is known. For 10 kg, initial wet basis moisture content The residual moisture content is 90%, and the absolute dry matter mass of the material is 1 kg. The controller calculates the current residual moisture content. If the value is below the target residual moisture content threshold of 3.5% and remains stable, the controller determines that the target dry state has been reached, terminates the alternating program, and executes a step-by-step vacuum breaking process.

[0087] This application provides a set of comparative verification experimental data.

[0088] The experiment used strawberry materials from the same batch, with similar sugar content and uniform slice thickness, and divided them into a control group and an experimental group, with each group having a loading of 10 kg.

[0089] The control group used a conventional static program-controlled drying method, with the pre-freezing temperature fixed at -30°C, the heating plate temperature fixed at 40°C during the sublimation drying stage, and the vacuum level maintained at 20 Pa during the desorption drying stage. The experimental group used the intelligent vacuum drying control method of this invention, performing adaptive closed-loop regulation and alternating pressure cycling according to the dynamic correlation parameters of the above application examples.

[0090] After processing, the core physicochemical properties of the two finished products were tested, and the results are compared below: See attached document Figure 8 In the control group, due to the lack of temperature limiting control based on mass transfer resistance during the main drying stage, some high-sugar slices showed micro-melting inside, with a volume collapse rate of 14.5%. The experimental group effectively avoided overheating of the sublimation interface through real-time suppression using the dynamic decay equation, keeping the volume collapse rate below 1.2%, maintaining the integrity of the porous matrix structure, and exhibiting better rehydration properties than the control group.

[0091] The total bacterial count (CFU / g) in the finished product was tested. In the control group, without chemical preservatives, the total bacterial count decreased by only about 1 log reduction, and vacuum-resistant bacteria hidden deep in the capillaries still survived. In the experimental group, after undergoing physical damage from alternating pressure differences during the desorption and drying stage, the cell membranes were mechanically torn over a large area, resulting in a decrease of more than 3 log reductions in the total bacterial count, and the antibacterial effect met the hygiene and safety standards.

[0092] Because the experimental group injected isenthalpic preheated gas into the alternating cycle, the anti-condensation mechanism maintained the vaporization driving force within the micropores, significantly increasing the removal rate of bound water. The total time required for the experimental group to reach the target moisture content of 3.5% was shortened by 18% compared to the control group. Simultaneously, the experimental group utilized the waste heat from the refrigeration unit's exhaust for gas preheating, reducing the overall energy consumption of the equipment by approximately 12%.

[0093] While specific embodiments of the present invention have been described above, those skilled in the art should understand that these specific embodiments are merely illustrative. Those skilled in the art can omit, substitute, and modify the details of the above methods and systems in various ways without departing from the principles and essence of the present invention. For example, combining the above method steps to perform substantially the same function and achieve substantially the same result using substantially the same method falls within the scope of the present invention.

Claims

1. A vacuum drying intelligent control method for physically locking freshness and inhibiting bacteria in freeze-dried strawberries, characterized in that, Includes the following steps: S100, the refrigeration unit is started to control the temperature of the environment where the material is located to drop, so that the moisture inside the material undergoes a phase change to form a solid ice crystal structure and a frozen skeleton. After the solid ice crystal structure sublimates, it forms a porous matrix. S200, reduce the pressure in the drying chamber and output a power control signal to the heating plate, close the isolation valve to obtain the pressure rise rate, calculate the mass transfer resistance and ice crystal sublimation interface temperature based on the pressure rise rate, compare the ice crystal sublimation interface temperature with the collapse critical temperature and adjust the power control signal. S300, when the pressure rise rate decreases to the set sublimation end threshold, the tortuosity factor is calculated based on the mass transfer resistance, and the residual water vapor partial pressure in the drying chamber is obtained. The anti-condensation critical temperature is calculated based on the residual water vapor partial pressure, inert gas is introduced into the buffer tank, and the inert gas is heated to a temperature not lower than the heating target temperature determined based on the anti-condensation critical temperature using the high-temperature gaseous refrigerant in the exhaust bypass. S400: Calculate the upper limit value of the pressure rise rate structure based on the tortuosity factor, reduce and maintain the pressure in the drying chamber, open the regulating valve to inject inert gas into the drying chamber, and constrain the pressure rise rate in the drying chamber to be between the lower limit value of the pressure rise rate structure and the upper limit value of the pressure rise rate structure according to the preset lower limit value of the pressure rise rate and the upper limit value of the pressure rise rate structure, so as to form an alternating pressure cycle. S500 acquires material quality data and calculates residual moisture content. When the residual moisture content reaches the preset target residual moisture content requirement, it stops the alternating pressure cycle and restores ambient pressure.

2. The intelligent control method for vacuum drying of strawberries using physical freshness locking and antibacterial properties as described in claim 1, characterized in that, The S100 step further includes: Acquire the initial temperature data of the material and the set target temperature range, set the cooling rate and adjust the refrigeration unit to maintain the refrigeration load that meets the cooling rate so that the ambient temperature in the drying chamber decreases according to the cooling rate. Moisture inside the material enters the maximum ice crystal formation zone. By maintaining the cooling rate, the time the material spends in the maximum ice crystal formation zone is shortened, causing the main liquid phase moisture inside the material to be converted into dispersed solid phase ice crystals. After the liquid phase water crystallizes and undergoes a phase change, it forms a solid ice crystal structure that fills the interior of the material cells and the intercellular spaces. It combines with the solid dry matter contained in the material to form a frozen skeleton that maintains the macroscopic shape of the material.

3. The intelligent control method for vacuum drying of strawberries using physical freshness locking and antibacterial properties as described in claim 1, characterized in that, The S200 step further includes: Reduce the absolute pressure inside the drying chamber to the working vacuum range, and send an electromagnetic closing command according to the set time cycle to cut off the gas flow pipeline between the drying chamber and the cold trap. Transient pressure data points inside the drying chamber are continuously acquired, and the rate of pressure rise inside the drying chamber contributed by material sublimation during the closure of the isolation valve is calculated through fitting correction. By combining the pressure rise rate with the effective sublimation surface area, the saturated vapor pressure at the ice crystal sublimation interface, and the ambient gas phase temperature, the mass transfer resistance of the material drying layer to water vapor migration is calculated.

4. The intelligent control method for vacuum drying of strawberries using physical freshness locking and antibacterial properties as described in claim 1, characterized in that, In step S200, the step of calculating the mass transfer resistance and the ice crystal sublimation interface temperature based on the pressure rise rate, comparing the ice crystal sublimation interface temperature with the collapse critical temperature, and adjusting the power control signal includes: Based on mass transfer resistance, ice surface saturated vapor pressure data, and thermodynamic equations, the transient temperature of the ice crystal sublimation interface is obtained through iterative inversion. The collapse temperature dynamic decay equation is invoked to combine the mass transfer resistance with the initial collapse temperature baseline value and the resistance temperature drift coefficient to dynamically calculate the collapse critical temperature at the current moment. Calculate the safety margin between the transient temperature and the collapse critical temperature. When the safety margin is less than or equal to the set safety temperature difference threshold, reduce the duty cycle of the pulse width modulation signal sent to the heating plate to force the transient temperature to drop and remain below the collapse critical temperature.

5. The intelligent control method for vacuum drying of strawberries using physical freshness locking and antibacterial properties as described in claim 1, characterized in that, In step S300, the calculation of the tortuosity factor specifically includes the following steps: The pressure rise rate is continuously tracked in each measurement cycle. When the pressure rise rate is lower than the sublimation end threshold for a set number of consecutive measurement cycles, the mass transfer resistance value of the last effective measurement cycle is extracted as the final state mass transfer resistance. After normalizing the final-state mass transfer resistance and the system's reference flow resistance constant, a logarithmic operation is performed to generate a dimensionless tortuosity factor as a mechanical input parameter.

6. The intelligent control method for vacuum drying of strawberries using physical freshness locking and antibacterial properties as described in claim 1, characterized in that, The S300 step further includes: The residual water vapor partial pressure in the drying chamber is obtained, and the gas-liquid phase change equilibrium theory and temperature solution deformation formula are called. The critical temperature for preventing condensation in deep capillaries is obtained by combining the residual water vapor partial pressure calculation. The anti-condensation critical temperature is superimposed with the thermodynamic compensation safety temperature difference and set as the heating target temperature. Inert gas is introduced into the buffer tank until the set initial working pressure is reached. Adjusting the refrigerant diversion valve on the exhaust bypass introduces high-temperature gaseous refrigerant into the heat exchange jacket, obtains the actual temperature of the inert gas and compares it with the heating target temperature to dynamically control the opening of the diversion valve.

7. The intelligent control method for vacuum drying of strawberries using physical freshness locking and antibacterial properties as described in claim 1, characterized in that, The S400 step further includes: The preset effective lower limit value of the pressure rise rate is obtained and used as the lower limit value of the pressure rise rate control when the regulating valve is opened, so as to ensure that the alternating pressure cycle has an inhibitory effect on the target microorganisms. Obtain the yield stress constant and the preset structural safety calibration coefficient of the dry matter skeleton of the material under a preset low pressure state, and determine the upper limit value of the structural safety of the pressure rise rate based on the tortuosity factor, yield stress constant and structural safety calibration coefficient.

8. The intelligent control method for vacuum drying of strawberries using physical freshness locking and antibacterial properties as described in claim 7, characterized in that, The formation of the alternating pressure cycle includes the following steps: Cut off the airflow connection between the drying chamber and the buffer tank, reduce the absolute pressure in the drying chamber to the preset ultra-low pressure reference lower limit and maintain the pressure for a period of time, so that the cell structure of the target microorganism is in a state of combined dehydration stress and pressure difference stress. An analog opening command is output to the regulating valve to allow the heated inert gas in the buffer tank to enter the drying chamber. The pressure rise rate in the drying chamber is monitored in real time and the regulating valve opening signal is corrected accordingly. The actual pressure rise rate is constrained to be between the effective lower limit of the pressure rise rate and the structural safety upper limit of the pressure rise rate. When the absolute pressure rises to the set upper limit of the pressure, the regulating valve is closed and stabilized after a delay, and the pumping action is restarted.

9. The intelligent control method for vacuum drying of strawberries using physical freshness locking and antibacterial properties according to claim 1, characterized in that, In step S500, the calculation of residual moisture content by obtaining material mass data specifically includes the following steps: The system continuously collects electrical signals of mechanical deformation caused by the gravity of materials through an online weighing module. The sensor reference data under different vacuum ranges is subtracted to obtain the tare weight, and the transient mass is obtained by dynamic correction in combination with a temperature compensation algorithm. The initial total mass and initial wet basis moisture content of the material are obtained by weighing it before it enters the drying chamber, and the residual moisture content is obtained by combining the transient mass calculation. The residual moisture content is compared with the preset target residual moisture content threshold in real time. When the residual moisture content calculated multiple times within a continuously set time window is lower than the target residual moisture content threshold, the material is determined to have reached the target dry state.

10. The intelligent control method for vacuum drying of strawberries using physical freshness locking and antibacterial properties according to claim 1, characterized in that, In step S500, the restoration of ambient pressure specifically includes the following steps: A control signal is output to the pressure relief valve connected to the air inlet of the drying chamber. An intermediate pressure platform is set as a dwell point in the range from vacuum to atmospheric pressure to constrain the opening of the pressure relief valve so that the pressure rise rate in the drying chamber does not exceed the preset vacuum breaking pressure rise rate threshold. When the absolute pressure inside the drying chamber rises to the intermediate pressure plateau, the pressure relief valve is closed. After maintaining this position for a set time to allow the air pressure inside and outside the porous skeleton of the material to reach equilibrium, the pressure relief valve is reopened until the interior of the drying chamber is completely restored to ambient atmospheric pressure.

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