Online pasteurization method and system for desorption drying stage of freeze-dried food

Abstract

The invention discloses an online pasteurization method for a desorption drying stage of freeze-dried food, which comprises the following steps: step S1, in a freeze-drying desorption drying stage, introducing sterilization steam at 60-75 DEG C into a drying box, and maintaining for 30-90 minutes; s2, maintaining continuous operation of the water catcher and the refrigerating system, and adsorbing sterilization steam by using a frost layer on the surface of the water catcher; s3, dynamically controlling the steam temperature to be 5-25 DEG C higher than the material temperature; s4, the pressure in the drying box is controlled to be 1.5-3 times of the basic pressure in the desorption drying stage; and S5, after sterilization is completed, steam introduction is stopped, and desorption drying is automatically continued in the freeze-drying process until a freeze-dried finished product with the water content meeting the requirement is obtained. The invention further discloses an online pasteurization system for the desorption drying stage of the freeze-dried food. According to the method, steam is adsorbed by utilizing the golden time when the water activity in the desorption drying stage is still beneficial to sterilization and frost accumulated in the sublimation stage, so that the product quality is ensured, and the operation energy consumption and the investment cost of independent sterilization equipment are reduced.

Description

Technical Field

[0001] This invention belongs to the field of food processing technology, specifically a method and system for online pasteurization during the desorption and drying stage of freeze-dried food. Background Technology

[0002] In recent years, microbial contamination of low-moisture foods has become an increasingly serious problem in the field of food safety, with related foodborne illnesses and product recalls occurring frequently. These foods mainly include dried fruits, spices, and dry pet food. The most common pathogenic bacteria is Salmonella, followed by Listeria monocytogenes and Escherichia coli O157:H7. It is noteworthy that these pathogenic bacteria exhibit stronger heat resistance in low-moisture environments.

[0003] Water activity refers to the ratio of the partial pressure of water vapor on the surface of food in a closed environment to the saturated vapor pressure of pure water at the same temperature, reflecting the free water content in the food. Foods with a water activity below 0.7 are generally referred to as low water activity foods. Foodborne pathogens such as Salmonella cannot grow and reproduce in low water activity foods, but can survive for a long time in this environment. Sterilization of these foods often requires higher temperatures or longer heating times to thoroughly kill pathogens. However, high-temperature processing or extended heating times can affect product quality and yield, leading to nutrient loss, tissue deformation, color changes, and poor taste. Furthermore, this process is not only energy-intensive but also relatively inefficient.

[0004] Steam pasteurization effectively solves the above problems for general low-moisture active foods. Firstly, the penetrating power of high-temperature steam allows it to efficiently enter the porous structure of food. The presence of water molecules helps to break the hydrogen bonds and other weak interactions that maintain protein structure, thus easily achieving protein denaturation. Simultaneously, pasteurization temperatures are typically controlled between 60 and 90°C, which kills pathogenic bacteria while largely preserving the food's nutrients and flavor.

[0005] However, for freeze-dried foods, the water activity is about 0.1-0.3, which is an extremely low level. At this level, the heat resistance of pathogens is thousands of times that at high water activity. Therefore, traditional steam pasteurization methods cannot effectively inactivate pathogens without damaging their quality.

[0006] In existing technologies, such as patent US20200253244A1, a steam pasteurization method for freeze-dried foods is provided. This method requires deep pre-cooling of the freeze-dried food, followed by the introduction of steam to condense on the food to increase its water activity, then heating for sterilization, and finally vacuum drying to remove any added moisture. This method suffers from drawbacks such as complex processes, high energy consumption (requiring pre-cooling and re-drying), independent equipment, and additional production time. Furthermore, its method of increasing water activity through condensation is equivalent to partial rehydration of the freeze-dried product, followed by secondary drying, which inevitably affects the quality of the freeze-dried product. Most other surface sterilization technologies require pre-treatment or post-treatment of the food, making them difficult to integrate into continuous freeze-drying production lines, and they also pose a risk of altering the final moisture content and quality of the food.

[0007] Therefore, there is an urgent need in this field for an online sterilization solution that can be directly integrated into the freeze-drying process, without altering the final moisture state of the food, and is highly efficient and energy-saving. Summary of the Invention

[0008] To address the aforementioned problems in the prior art, this invention provides a highly efficient pasteurization method that can be completed directly within the freeze-drying process without pre-cooling and subsequent drying, and provides a system for implementing this method. This system can effectively utilize existing freeze-drying equipment, reducing energy consumption and equipment costs.

[0009] The technical solution to achieve the above objectives is: One of the present inventions provides an online pasteurization method for the desorption-drying stage of freeze-dried food, comprising: Step S1: During the freeze-drying and desorption drying stage, sterile steam at 60-75°C is introduced into the drying chamber and maintained for 30-90 minutes. Step S2: Maintain the continuous operation of the water trap and refrigeration system, and use the frost layer on the surface of the water trap to adsorb sterilization vapor; Step S3: Dynamically control the steam temperature to keep it 5-25°C higher than the material temperature; Step S4: Control the pressure inside the drying chamber to be 1.5-3 times the base pressure of the desorption stage; Step S5: After sterilization is completed, the steam supply is stopped, and the freeze-drying process continues automatically for desorption and drying until a freeze-dried product with the required moisture content is obtained.

[0010] Preferably, in step S1, when the material temperature is below 20°C, a staged heating program is adopted. In the first stage, the steam temperature = material temperature + (15-25)°C, and it is maintained for 20-30 minutes; in the second stage, the temperature is raised to 60-75°C and maintained for 30-60 minutes.

[0011] Preferably, in step S2, the refrigeration system dynamically adjusts its power according to the temperature of the frost layer, with the adjustment range being 5%-100% of the rated power.

[0012] A second invention provides an online pasteurization system for the desorption and drying stage of freeze-dried food, comprising: The drying chamber is used to hold the materials to be processed; The vacuum system is connected to the drying oven and is used to extract and maintain the vacuum inside the oven. The water trap is connected between the drying chamber and the vacuum system to capture sublimated water vapor; The refrigeration system is coupled to the water catcher and is used to provide cooling to the water catcher. A steam generation and supply module is connected to the drying chamber via pipeline and is used to supply sterilization steam with controllable temperature into the drying chamber; An adjustable power refrigeration control module, which is electrically connected to the refrigeration system, is used to dynamically adjust the output power of the refrigeration system; The control unit has its signal input terminals connected to the temperature sensor on the water catcher and the system's sensors, and its signal output terminals connected to the steam generation and supply module, the power adjustable refrigeration control module, and other moving parts of the system, respectively.

[0013] Preferably, the control unit is a main controller, which is configured as follows: Control the execution of the complete freeze-drying process, and when the process reaches the desorption drying stage: Receive the temperature signal from the water trap; Based on a preset program, the steam temperature and feed rate of the steam generation and supply module are controlled. Based on the temperature signal from the water trap, the adjustable power cooling control module is controlled to adjust the cooling power.

[0014] Compared with the prior art, the beneficial effects of the present invention are: the present invention directly embeds the sterilization process into the desorption drying stage, making full use of the "golden window period" when the material water activity is relatively high and the heat resistance of microorganisms is relatively weak. It eliminates the independent pre-cooling, humidification and re-drying steps that must be added for fully freeze-dried products due to their low water content, thus greatly reducing energy consumption and significantly improving production efficiency. By controlling the process with "continuous cooling + adjusting cooling power", the material is strictly prevented from becoming humid during sterilization, thus perfectly maintaining the porous and loose structure, excellent rehydration properties and original nutritional components of freeze-dried food, with a vitamin retention rate of over 90%. By utilizing the high penetrability of low-pressure steam to perform uniform and gentle heat treatment on materials, a pathogen inactivation rate of more than 5 log units (>5-log) can be achieved, which fully meets the requirements of commercial sterilization. There is no need to purchase separate sterilization equipment; it can be achieved by making minor modifications to existing freeze dryers and upgrading the control program, which significantly reduces equipment costs. Attached Figure Description

[0015] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a flowchart of an online pasteurization method for the desorption and drying stage of freeze-dried food according to the present invention; Figure 2 This is a block diagram of an online pasteurization system for the desorption and drying stage of freeze-dried food according to the present invention. Detailed Implementation

[0016] The technical solutions of 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.

[0017] like Figure 1 As shown, an online pasteurization method for the desorption-drying stage of freeze-dried food includes: Step S1: In the freeze-drying and desorption drying stage, sterile steam at 60-75°C is introduced into the drying chamber 1 and maintained for 30-90 minutes.

[0018] The 'desorption drying stage' of this invention refers to the process stage where most of the free water in the material has been removed, and the removal of bound water begins. This stage typically corresponds to a material moisture content range of 5% to 20%. During this stage, the material's water activity (Aw) is in the range of 0.4 to 0.6, and the heat resistance of microorganisms is significantly lower than that of the freeze-dried final product, making it the golden window for achieving online pasteurization. Completely freeze-dried products typically have a moisture content of less than 3%, at which point the water activity is very low. Pathogens such as Salmonella can survive in low Aw environments and exhibit extremely high heat resistance. Traditional heat sterilization methods struggle to achieve effective inactivation without compromising quality. Therefore, to improve sterilization efficiency, it is sometimes necessary to increase the humidity of the freeze-dried product.

[0019] In another embodiment, before introducing steam, pure water atomized particles with a particle size of 1-3 μm and a temperature of 15-25°C can be injected into the drying chamber 1 for pretreatment. This pretreatment step is not necessary, but it is particularly suitable for treating materials with strong surface hydrophobicity or extremely low initial moisture content, as it helps to wet the material surface and promotes the uniformity of subsequent steam heat transfer.

[0020] In the embodiments, the initial temperature of the material during the desorption stage may vary depending on the process. When processing certain heat-sensitive materials (such as probiotics, enzyme preparations, and certain vitamin-fortified foods) or when the material temperature is low (e.g., below 20°C) after the pre-freezing stage, directly introducing high-temperature steam at 60-75°C can cause a thermal shock due to the excessive temperature difference between the inside and outside of the material. This thermal shock may cause the heat-sensitive components to become instantly inactive, or cause the material surface to heat up too quickly while the inside remains at a low temperature, forming huge internal stress, damaging the porous structure of the material, and even causing surface hardening, melting, or collapse, ultimately affecting the product's quality, rehydration properties, and yield. To address the aforementioned technical challenges, this invention further provides a staged heating program. The core of this program is that when the material temperature is detected to be below 20°C, the control system automatically activates a two-stage heating mode. The first stage aims for gentle preheating: the steam temperature is set slightly higher than the current material temperature (15-25°C) and maintained for a period of time (20-30 minutes), allowing heat to be slowly and evenly conducted from the surface inwards, avoiding thermal shock and ensuring the overall material temperature rises steadily to a transitional plateau. The second stage aims for efficient sterilization: after the material temperature has risen and its overall condition has stabilized, the steam temperature is then increased to the target sterilization temperature (60-75°C) and maintained for a sufficient time (30-60 minutes) to ensure that the specified log reduction in pathogen kill is achieved. This program perfectly balances the conflict between protecting heat-sensitive materials and sterilization efficiency.

[0021] Step S2: Maintain the continuous operation of the water trap 3 and the refrigeration system 4, and use the frost layer on the surface of the water trap 3 to adsorb sterilization vapor.

[0022] In this embodiment, the refrigeration system 4 dynamically adjusts its power according to the temperature of the frost layer, with the adjustment range being 5%-100% of the rated power.

[0023] In this embodiment, during the sublimation drying stage of freeze-drying, the water trap 3 has already captured most of the vapor emitted by the sublimation of the product, forming a relatively thick frost layer on the water trap 3. This frost layer acts as a natural weapon for adsorbing sterilizing vapors, and often the existing frost layer alone can completely adsorb the sterilizing vapors. Keeping the refrigeration system 4 running continuously consumes very little energy to maintain the temperature of the frost layer. To ensure that the frost layer does not heat up above the freezing point and melt due to continuous vapor adsorption throughout the entire sterilization cycle (30-90 minutes), the refrigeration system needs to run continuously to counteract external heat intrusion (such as radiation and conduction) and the latent heat released by vapor condensation.

[0024] Step S3: Dynamically control the steam temperature to be 5-25°C higher than the material temperature to ensure that the steam heats the material in the form of sensible heat and avoids the steam condensing on the material surface, which would cause it to become humid.

[0025] Step S4: Control the pressure inside the drying oven 1 to 1.5-3 times the base pressure of the desorption stage. The purpose is to upgrade the system from the "high vacuum molecular flow" state to the "transition flow" state, and ensure the establishment of an effective convective heat transfer environment in a vacuum environment.

[0026] In step S5, after sterilization is completed, the steam supply is stopped, and the freeze-drying process automatically continues desorption drying until a freeze-dried product with the required moisture content is obtained. Thus, the steam pasteurization scheme proposed in this invention is perfectly integrated into the entire freeze-drying process.

[0027] like Figure 2 As shown, an online pasteurization system for the desorption and drying stage of freeze-dried food includes: Drying oven 1 is used to hold the materials to be processed; Vacuum system 2, connected to drying oven 1, is used to extract and maintain the vacuum inside the oven; Water trap 3, connected between drying chamber 1 and vacuum system 2, is used to capture sublimated water vapor; The refrigeration system 4, coupled to the water catcher 3, is used to provide cooling to the water catcher; The steam generation and supply module 5 is connected to the drying chamber 1 via a pipeline and is used to supply sterilization steam with controllable temperature to the drying chamber 1. The adjustable power cooling control module 6 is electrically connected to the cooling system 4 and is used to dynamically adjust the output power of the cooling system 4. The control unit 7 has its signal input terminal connected to the temperature sensor on the water catcher 3 and the system's sensors, and its signal output terminal connected to the steam generation and supply module 5, the power adjustable refrigeration control module 6, and other moving parts of the system, respectively.

[0028] In this embodiment, control unit 7 is the main controller of the freeze dryer, and it is configured as follows: Control the execution of the complete freeze-drying process, and when the process reaches the desorption drying stage: Receive the temperature signal from water trap 3; Based on a preset program, the steam temperature and feed rate of the steam generation and supply module 5 are controlled. Based on the temperature signal of the water trap 3, the adjustable cooling control module 6 is used to adjust the cooling power so that the temperature of the water trap 3 is maintained within the range where its frost layer can effectively adsorb the introduced steam.

[0029] Example 1: Sterilization of freeze-dried chicken parameter: Materials: Freeze-dried chicken (mid-desorption drying stage, moisture content 15%) Steam: Introduce at 65℃ and maintain for 50 minutes, with the pressure inside the chamber maintained at 120Pa (1.5 times the base pressure of 80Pa). Cooling: Continuously operates and dynamically adjusts cooling power to maintain frost temperature; ΔT: Dynamically controlled to be approximately 10°C above the material temperature; result: Sterilization efficiency: 6.1 log (Salmonella); Vitamin C retention rate: 92%; The final product moisture content was 4.8%, which was not significantly different from that before sterilization.

[0030] Example 2: Sterilization of freeze-dried probiotics parameter: Materials: Freeze-dried probiotics (mid-desorption drying stage, moisture content 12%) Special treatment: Staged temperature ramp-up program (40℃ / 20min → 55℃ / 20min → 65℃ / 50min); Cooling power: Continuously operates and dynamically adjusts cooling power to maintain frost temperature; result: Pathogen inactivation: 5.3 log; Probiotic survival rate: 93%; The final product has a water activity (Aw) of 0.18, which meets the shelf-life requirements.

[0031] Finally, it should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for online pasteurization during the desorption-drying stage of freeze-dried food, characterized in that, include: Step S1: During the freeze-drying and desorption drying stage, sterile steam at 60-75°C is introduced into the drying chamber (1) and maintained for 30-90 minutes. Step S2: Maintain the continuous operation of the water trap (3) and the refrigeration system (4) and use the frost layer on the surface of the water trap (3) to adsorb sterilization vapor; Step S3: Dynamically control the steam temperature to keep it 5-25°C higher than the material temperature; Step S4: Control the pressure inside the drying oven (1) to be 1.5-3 times the basic pressure of the desorption drying stage; Step S5: After sterilization is completed, the steam supply is stopped, and the freeze-drying process continues automatically for desorption and drying until a freeze-dried product with the required moisture content is obtained.

2. The method for online pasteurization during the desorption and drying stage of freeze-dried food according to claim 1, characterized in that, In step S1, when the material temperature is below 20°C, a staged heating program is adopted. In the first stage, the steam temperature = material temperature + (15-25)°C, and it is maintained for 20-30 minutes. In the second stage, the temperature is raised to 60-75°C and maintained for 30-60 minutes.

3. The method for online pasteurization during the desorption and drying stage of freeze-dried food according to claim 1, characterized in that, In step S2, the refrigeration system (4) dynamically adjusts its power according to the temperature of the frost layer, with the adjustment range being 5%-100% of the rated power.

4. An online pasteurization system based on the method of claim 1, characterized in that, include: The drying box (1) is used to hold the material to be processed; The vacuum system (2) is connected to the drying oven (1) and is used to extract and maintain the vacuum inside the oven; The water trap (3) is connected between the drying chamber (1) and the vacuum system (2) for capturing sublimated water vapor; The refrigeration system (4) is coupled to the water catcher (3) and is used to provide cooling capacity to the water catcher; The steam generation and supply module (5) is connected to the drying chamber (1) via a pipeline and is used to supply temperature-controlled sterilization steam to the drying chamber (1); A power-adjustable refrigeration control module (6) is electrically connected to the refrigeration system (4) and is used to dynamically adjust the output power of the refrigeration system (4); The control unit (7) has its signal input terminal connected to the temperature sensor on the water catcher (3) and other sensors of the system, and its signal output terminal connected to the steam generation and supply module (5), the power adjustable refrigeration control module (6), and other moving parts of the system, respectively.

5. The online pasteurization system according to claim 4, characterized in that, The control unit (7) is the main controller, and it is configured as follows: Control the execution of the complete freeze-drying process, and when the process reaches the desorption drying stage: Receive the temperature signal from the water trap (3); Based on a preset program, the steam temperature and feed rate of the steam generation and supply module (5) are controlled. Based on the temperature signal of the water trap (3), the power adjustable cooling control module (6) is controlled to adjust the cooling power.

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