A liquid nitrogen preservation control system and method for fruit and vegetable warehouses

Abstract

This invention relates to the field of industrial control technology, specifically disclosing a liquid nitrogen preservation control system and method for fruit and vegetable warehouses. The method includes the following steps: S1: Obtain the current temperature of the fruit and vegetable warehouse and calculate the ideal amount of liquid nitrogen to be used based on the optimal storage temperature of the fruits and vegetables; S2: Calculate the single usage amount, add liquid nitrogen at a preset addition point, update the position of the addition point, and continue adding, repeating the operation; S3: Determine whether the fruits and vegetables have suffered frost damage based on hardness testing. If frost damage is found, adjust the liquid nitrogen addition method; S4: Calculate the average surface temperature of the fruits and vegetables, and use a carbon dioxide-compressor cooling method to lower the surface temperature of the fruits and vegetables to the optimal storage temperature. This invention designs a liquid nitrogen preservation control system and method for fruit and vegetable warehouses, avoiding the formation of large ice crystals inside the fruits and vegetables during liquid nitrogen preservation, thereby maximizing the preservation of the freshness, taste, and nutritional components of the fruits and vegetables.

Description

Technical Field

[0001] This invention relates to the field of industrial control technology, specifically to a liquid nitrogen preservation control system and method for fruit and vegetable warehouses. Background Technology

[0002] Liquid nitrogen is a colorless, odorless, low-viscosity transparent liquid with a boiling point of -196℃ at normal pressure. Utilizing the low-temperature properties of liquid nitrogen, fruits and vegetables can be rapidly cooled to a suitable preservation temperature range (generally -18℃ to -25℃), thereby inhibiting respiration, metabolism, and the growth and reproduction of microorganisms, achieving long-term preservation.

[0003] When liquid nitrogen comes into contact with fruits and vegetables, it rapidly absorbs a large amount of heat, freezing them in a short time. This rapid freezing process effectively reduces the formation of ice crystals. In conventional freezing processes, larger ice crystals damage the cellular structure of fruits and vegetables, leading to nutrient loss and a deterioration in taste. However, the tiny ice crystals formed by liquid nitrogen flash freezing cause less damage to the cellular structure, thus preserving the original taste and nutritional value of the fruits and vegetables.

[0004] While liquid nitrogen preservation can rapidly freeze food, excessively rapid or uneven freezing can cause large ice crystals to form inside fruits and vegetables, damaging their cellular structure and affecting their taste and texture. Therefore, a liquid nitrogen preservation control system and method for fruit and vegetable warehouses are needed to prevent the formation of large ice crystals during the liquid nitrogen preservation process, thus preventing nutrient loss and deterioration in taste. Summary of the Invention

[0005] The purpose of this invention is to provide a liquid nitrogen preservation control system and method for fruit and vegetable warehouses, thereby solving the above-mentioned technical problems.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A method for controlling the preservation of fruits and vegetables in a liquid nitrogen warehouse includes the following steps:

[0008] Step S1: Obtain the current temperature W of the fruit and vegetable warehouse, based on the optimal storage temperature W for fruits and vegetables. best Calculate the ideal usage amount M of liquid nitrogen;

[0009] Step S2: Calculate the single usage amount m = M / N, where N represents the preset number of uses. Arrange the fruits and vegetables in a preset formation and add liquid nitrogen of mass m at the preset addition point. Update the position of the addition point and continue to add liquid nitrogen of mass m. Repeat the above steps until the total amount of liquid nitrogen used equals the ideal usage amount.

[0010] Step S3: Determine whether the fruits and vegetables have suffered frost damage based on the hardness test. If frost damage is found, adjust the liquid nitrogen addition method. The liquid nitrogen addition method includes:

[0011] Calculate the proportion (B) of frost-damaged fruits and vegetables to the total amount of fruits and vegetables, and update the single-use amount of liquid nitrogen. Calculate the proportion of frost-damaged fruits and vegetables to the total amount of fruits and vegetables after a single usage update, Bs. Stop adjusting when the proportion Bs ≤ 5%.

[0012] Step S4: Calculate the average temperature W of the fruit and vegetable skin. ave If the mean W ave >W best Then, the carbon dioxide-compressor cooling method is used to lower the surface temperature of fruits and vegetables to the optimal storage temperature W. best .

[0013] Preferably, in step S1, the optimal storage temperature W for fruits and vegetables is used. best The methods for calculating the ideal usage amount M of liquid nitrogen include:

[0014] Calculate the temperature difference ΔW = WW best The heat consumption Q = c1ΔW is calculated based on the temperature difference ΔW, where c1 represents the specific heat capacity of air. The ideal usage of liquid nitrogen is then calculated. Here, c2 represents the heat of vaporization of liquid nitrogen.

[0015] Preferably, in step S2, the preset number of uses N ≥ 4, so that the time consumed to add liquid nitrogen under different number of uses N is equal to the preset specified time.

[0016] Preferably, in step S2, the method for updating the position of the added point specifically includes:

[0017] Divide the circular area with the distance between the center and the addition point as the radius. The center of the circle is the center point of the fruit and vegetable arrangement. Rotate the addition point clockwise by θ degrees along the boundary of the circular area to obtain a new addition point, where θ = 2π / N.

[0018] Preferably, in step S3, the method for determining whether fruits and vegetables have suffered frost damage based on a hardness test specifically includes:

[0019] K detection points are pre-set on the surface of fruits and vegetables. A preset value of pressure is applied to the detection points, and the indentation distance L of the fruit and vegetable surface is obtained. If there is a detection point with an indentation distance L greater than or equal to the preset judgment distance, it is determined that the fruit and vegetable has frost damage.

[0020] Preferably, in step S3, another method for determining whether fruits and vegetables have suffered frost damage specifically includes:

[0021] The ethylene content Y in the air before liquid nitrogen is added is obtained. When liquid nitrogen of mass m is added, the ethylene content Ys in the air is updated. If the updated ethylene content Ys > Y + η, it is determined that the fruits and vegetables have suffered frost damage. Here, η represents the preset ethylene content fluctuation value.

[0022] Preferably, in step S4, the carbon dioxide-compressor cooling method includes:

[0023] Add a preset volume of carbon dioxide to the fruits and vegetables, measure the surface temperature of the fruits and vegetables at this time, and if the surface temperature of the fruits and vegetables is higher than the optimal storage temperature, then use a compressor to lower the surface temperature of the fruits and vegetables to the optimal storage temperature.

[0024] A liquid nitrogen preservation control system for fruit and vegetable warehouses includes:

[0025] Calculation module: Obtains the current temperature W of the fruit and vegetable warehouse, and calculates the optimal storage temperature W for the fruits and vegetables. best Calculate the ideal usage amount M of liquid nitrogen;

[0026] Cooling module: Calculate the single usage amount m = M / N, where N represents the preset number of uses. Arrange the fruits and vegetables in a preset formation and add liquid nitrogen of mass m at the preset addition point. Update the position of the addition point and continue to add liquid nitrogen of mass m. Repeat the above steps until the total amount of liquid nitrogen used equals the ideal usage amount.

[0027] Adjustment module: Based on hardness testing, it determines whether fruits and vegetables have suffered frost damage. If frost damage is found, the liquid nitrogen addition method is adjusted. The liquid nitrogen addition method includes:

[0028] Calculate the proportion (B) of frost-damaged fruits and vegetables to the total amount of fruits and vegetables, and update the single-use amount of liquid nitrogen. Calculate the proportion of frost-damaged fruits and vegetables to the total amount of fruits and vegetables after a single usage update, Bs. Stop adjusting when the proportion Bs ≤ 5%.

[0029] Compensation module: Calculates the average temperature (W) of fruit and vegetable skins. ave If the mean W ave >W best Then, the carbon dioxide-compressor cooling method is used to lower the surface temperature of fruits and vegetables to the optimal storage temperature W. best .

[0030] The beneficial effects of this invention are:

[0031] 1. Adding liquid nitrogen in batches. By rationally planning the amount and time interval of each injection, it is possible to avoid freezing damage to fruits and vegetables caused by excessively rapid temperature drops, while ensuring that the temperature in the warehouse decreases steadily and gradually reaches and is maintained within the optimal preservation temperature range required by fruits and vegetables. This improves the preservation effect of fruits and vegetables, extends their shelf life, reduces losses caused by improper storage, and protects the economic benefits for merchants and the user experience for consumers.

[0032] 2. To accurately determine which liquid nitrogen application method can effectively reduce frost damage to fruits and vegetables, an evaluation system will be constructed. This system will dynamically adjust and update the single-use liquid nitrogen dosage based on the key indicator of the proportion of frost-damaged fruits and vegetables in the total produce. Continuous adjustment and optimization of the single-use liquid nitrogen dosage will maximize the preservation of the fruits and vegetables' integrity.

[0033] 3. Introducing the carbon dioxide-compressor cooling method: A predetermined volume of carbon dioxide gas is added to the storage space for fruits and vegetables, and the compressor is activated to extract heat from the surrounding air. This gradually lowers the surface temperature of the fruits and vegetables until it reaches the preset optimal storage temperature. The presence of carbon dioxide and the suitable low-temperature environment work together to inhibit the respiration intensity of fruit and vegetable cells, maintaining their freshness, taste, and color. Attached Figure Description

[0034] The invention will now be further described with reference to the accompanying drawings.

[0035] Figure 1 This is a flowchart illustrating a liquid nitrogen preservation control method for fruit and vegetable warehouses according to the present invention. Detailed Implementation

[0036] 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.

[0037] Please see Figure 1 As shown, this invention is a method for controlling the preservation of fruits and vegetables in a liquid nitrogen warehouse, comprising the following steps:

[0038] Step S1: Obtain the current temperature W of the fruit and vegetable warehouse, based on the optimal storage temperature W for fruits and vegetables. best Calculate the ideal usage amount M of liquid nitrogen;

[0039] Step S2: Calculate the single usage amount m = M / N, where N represents the preset number of uses. Arrange the fruits and vegetables in a preset formation and add liquid nitrogen of mass m at the preset addition point. Update the position of the addition point and continue to add liquid nitrogen of mass m. Repeat the above steps until the total amount of liquid nitrogen used equals the ideal usage amount.

[0040] Step S3: Determine whether the fruits and vegetables have suffered frost damage based on the hardness test. If frost damage is found, adjust the liquid nitrogen addition method. The liquid nitrogen addition method includes:

[0041] Calculate the proportion (B) of frost-damaged fruits and vegetables to the total amount of fruits and vegetables, and update the single-use amount of liquid nitrogen. Calculate the proportion of frost-damaged fruits and vegetables to the total amount of fruits and vegetables after a single usage update, Bs. Stop adjusting when the proportion Bs ≤ 5%.

[0042] Step S4: Calculate the average temperature W of the fruit and vegetable skin. ave If the mean W ave >W best Then, the carbon dioxide-compressor cooling method is used to lower the surface temperature of fruits and vegetables to the optimal storage temperature W. best .

[0043] It should be noted that in practice, using liquid nitrogen for fruit and vegetable preservation often presents two problems. First, when the entire dose of liquid nitrogen is poured into the fruit and vegetable warehouse at once, the extremely intense and low temperature of liquid nitrogen causes a rapid and drastic drop in the surrounding temperature. This rapid temperature change exceeds the tolerance range of the fruits and vegetables, resulting in severe damage to their internal structure and leading to frost damage. Once fruits and vegetables suffer frost damage, their cell tissues suffer irreversible damage, affecting not only their taste, color, and nutritional components but also significantly shortening their shelf life, resulting in substantial economic losses.

[0044] By controlling the amount and addition point of liquid nitrogen, it is possible to ensure that the surface temperature of fruits and vegetables is reduced to a specified temperature while avoiding the formation of excessive ice crystals. This reduces the probability of severe damage to the internal structure of fruits and vegetables, preventing freezing damage caused by excessively rapid temperature drops. It also ensures that the temperature in the warehouse continues to drop steadily, gradually reaching and maintaining the optimal preservation temperature range required by fruits and vegetables. This improves the preservation effect of fruits and vegetables, extends their shelf life, and reduces losses caused by improper storage.

[0045] In addition, the actual physical operation process needs to be considered in terms of time. From starting the liquid nitrogen addition to achieving the expected cooling effect and completing a series of adjustments, a certain amount of time will inevitably be consumed. During this period, due to the influence of the external environment and the heat exchange between the fruits and vegetables and their surroundings, the temperature of the fruit and vegetable skin will fluctuate to some extent, causing the actual temperature of the fruits and vegetables to deviate from their ideal storage temperature range.

[0046] To address this situation and ensure the quality and freshness of fruits and vegetables during storage, a temperature control method is needed: carbon dioxide-compressor cooling. Specifically, a predetermined volume of carbon dioxide gas is precisely added to the storage space. As an inert gas, the appropriate amount of carbon dioxide in the fruit and vegetable storage environment not only regulates the atmosphere but also inhibits, to some extent, the growth of microorganisms and the rate of oxidation on the surface of the fruits and vegetables.

[0047] Using a carbon dioxide-compressor cooling method for subsequent adjustments has two advantages. First, this temperature control method ensures that the surface temperature of fruits and vegetables remains stable at an optimal level throughout the entire storage period, avoiding problems such as quality deterioration and spoilage caused by excessive temperature fluctuations.

[0048] Secondly, the presence of carbon dioxide and a suitable low-temperature environment work together to significantly inhibit the respiration intensity of fruit and vegetable cells. After adding carbon dioxide, the surface temperature of the fruits and vegetables is measured in real time. If the monitoring data shows that the surface temperature of the fruits and vegetables is still higher than their optimal storage temperature, then the compressor equipment needs to be started to cool them down until the preset optimal storage temperature value is reached. This helps to extend the shelf life of fruits and vegetables and maintain their freshness, taste, color, and nutritional components and other key quality indicators at a high level.

[0049] In another preferred embodiment of the invention, based on the optimal storage temperature W for fruits and vegetables best The methods for calculating the ideal usage amount M of liquid nitrogen include:

[0050] Calculate the temperature difference ΔW = WW best The heat consumption Q = c1ΔW is calculated based on the temperature difference ΔW, where c1 represents the specific heat capacity of air. The ideal usage of liquid nitrogen is then calculated. Here, c2 represents the heat of vaporization of liquid nitrogen.

[0051] It is worth noting that when using the laws of thermodynamics to calculate the specific amount of liquid nitrogen needed, in a closed system, heat neither arises out of thin air nor disappears without a reason; it only flows from high-temperature regions to low-temperature regions, or is transferred and redistributed between different parts of the system.

[0052] In a specific environment, the fruit and vegetable warehouse can be approximated as a closed system, which reduces the amount of computation. Based on this principle, the ideal amount of liquid nitrogen can be accurately calculated according to the required temperature reduction. Specifically, the temperature difference between the current ambient temperature of the fruit and vegetables and their optimal storage temperature is first determined. This temperature difference is the key starting point for the entire calculation, because it is this temperature difference that causes heat to be transferred between the system (i.e., the warehouse and the fruit and vegetables stored within) and the liquid nitrogen.

[0053] When liquid nitrogen is introduced into the system, it begins to absorb heat from its surroundings, vaporizing from a liquid state to a gaseous state. The heat of vaporization of liquid nitrogen represents the amount of heat required to vaporize a unit mass of liquid nitrogen. After obtaining the heat of vaporization value of liquid nitrogen, a mathematical model is established and calculations are performed based on the heat change corresponding to the required temperature reduction, combined with the heat of vaporization of liquid nitrogen. The ideal amount of liquid nitrogen to use is determined through thermodynamic calculation formulas, ensuring that while meeting the cooling requirements, the efficiency of liquid nitrogen use is maximized, avoiding resource waste and potential adverse effects such as overcooling of fruits and vegetables due to excessive use of liquid nitrogen.

[0054] In another preferred embodiment of the present invention, the preset number of uses N≥4, so that the time consumed by adding liquid nitrogen under different number of uses N is equal to the preset specified time.

[0055] Understandably, the purpose of the preset number of uses is to reduce the risk of freezing damage to fruits and vegetables caused by pouring in excessive liquid nitrogen at once, while the time limit for adding liquid nitrogen is to reduce the impact of time on the cooling effect of liquid nitrogen.

[0056] In another preferred embodiment of the present invention, the method for updating the position of the added point specifically includes:

[0057] Divide the circular area with the distance between the center and the addition point as the radius. The center of the circle is the center point of the fruit and vegetable arrangement. Rotate the addition point clockwise by θ degrees along the boundary of the circular area to obtain a new addition point, where θ = 2π / N.

[0058] It is important to note that by continuously and stably rotating the containers or storage spaces holding fruits and vegetables, while simultaneously adding liquid nitrogen at a precisely controlled rate and amount, a highly effective temperature control mechanism is established, thereby minimizing the risk of freezing damage to the fruits and vegetables caused by liquid nitrogen. This rotational action promotes a continuous dynamic circulation and mixing process between the air within the warehouse and the microenvironment surrounding the fruits and vegetables. During this process, the low temperature carried by the liquid nitrogen is not concentrated at a single point or localized area, but is evenly dispersed with the airflow, gradually penetrating the entire storage space. Furthermore, this combination of rotation and liquid nitrogen addition also achieves the goal of uniform cooling of the fruits and vegetables.

[0059] In another preferred embodiment of the present invention, the method for determining whether fruits and vegetables have suffered frost damage based on a hardness test specifically includes:

[0060] K detection points are pre-set on the surface of fruits and vegetables. A preset value of pressure is applied to the detection points, and the indentation distance L of the fruit and vegetable surface is obtained. If there is a detection point with an indentation distance L greater than or equal to the preset judgment distance, it is determined that the fruit and vegetable has frost damage.

[0061] It should be noted that k representative and evenly distributed detection points are carefully pre-set on the surface of fruits and vegetables. These detection points can comprehensively and accurately reflect the state information of different parts of the fruit and vegetable surface. When it is necessary to detect whether fruits and vegetables have suffered frost damage, a preset pressure value is applied to each detection point using a professional pressure application device. After applying pressure, a high-precision displacement sensor is used to obtain the indentation distance L of the fruit and vegetable surface at this time. This indentation distance L directly reflects the degree of deformation of the fruit and vegetable surface under pressure. In order to accurately determine whether fruits and vegetables have suffered frost damage, a judgment distance threshold is preset. If the actual measured indentation distance L is greater than or equal to this preset judgment distance threshold, it can be clearly determined that the fruit and vegetables have suffered frost damage. This is because when fruits and vegetables suffer frost damage, their internal cell structure is damaged, causing changes in the toughness, elasticity and other mechanical properties of the surface, which in turn makes the degree of indentation of the surface greater than normal under the same pressure, exceeding the preset judgment distance threshold.

[0062] In another preferred embodiment of the present invention, another method for determining whether fruits and vegetables have suffered frost damage specifically includes:

[0063] The ethylene content Y in the air before liquid nitrogen is added is obtained. When liquid nitrogen of mass m is added, the ethylene content Ys in the air is updated. If the updated ethylene content Ys > Y + η, it is determined that the fruits and vegetables have suffered frost damage. Here, η represents the preset ethylene content fluctuation value.

[0064] Understandably, when fruits and vegetables suffer frost damage, a series of chemical reactions occur within them, among which the ethylene content increases. This increases as a signal indicator of whether the fruits and vegetables have suffered frost damage. Specifically, when frost damage occurs, the biochemical balance within the fruit and vegetable cells is disrupted. The previously stable order of life activities becomes disordered, and some enzymatic reactions deviate from their normal course. Simultaneously, intracellular energy metabolism is also affected, providing additional energy support for ethylene synthesis or reducing inhibitory factors on its synthesis.

[0065] Ethylene, a plant hormone, plays a crucial role in the ripening, senescence, and stress response of fruits and vegetables. Under normal circumstances, ethylene levels in fruits and vegetables are maintained at a relatively stable level, regulating the orderly progress of physiological processes. However, when frost damage causes an abnormal increase in its content, it further accelerates the physiological imbalance and quality deterioration of fruits and vegetables.

[0066] In another preferred embodiment of the present invention, the carbon dioxide-compressor cooling method includes:

[0067] Add a preset volume of carbon dioxide to the fruits and vegetables, measure the surface temperature of the fruits and vegetables at this time, and if the surface temperature of the fruits and vegetables is higher than the optimal storage temperature, then use a compressor to lower the surface temperature of the fruits and vegetables to the optimal storage temperature.

[0068] It's worth noting that this is a compensatory mechanism for changes in the surface temperature of fruits and vegetables. This regulation ensures that the surface temperature remains stable and optimal throughout storage, preventing quality deterioration and spoilage caused by excessive temperature fluctuations. On the other hand, the presence of carbon dioxide and a suitable low-temperature environment significantly inhibit the respiration of fruit and vegetable cells. As one of the main physiological processes by which fruits and vegetables consume their own nutrients, excessive inhibition of cellular respiration greatly reduces the rate at which they consume their own nutrients. This helps extend the shelf life of fruits and vegetables, maintaining their freshness, taste, color, and nutritional components at high levels, providing a strong guarantee for subsequent sales and consumption.

[0069] A liquid nitrogen preservation control system for fruit and vegetable warehouses includes:

[0070] Calculation module: Obtains the current temperature W of the fruit and vegetable warehouse, and calculates the optimal storage temperature W for the fruits and vegetables. best Calculate the ideal usage amount M of liquid nitrogen;

[0071] Cooling module: Calculate the single usage amount m = M / N, where N represents the preset number of uses. Arrange the fruits and vegetables in a preset formation and add liquid nitrogen of mass m at the preset addition point. Update the position of the addition point and continue to add liquid nitrogen of mass m. Repeat the above steps until the total amount of liquid nitrogen used equals the ideal usage amount.

[0072] Adjustment module: Based on hardness testing, it determines whether fruits and vegetables have suffered frost damage. If frost damage is found, the liquid nitrogen addition method is adjusted. The liquid nitrogen addition method includes:

[0073] Calculate the proportion (B) of frost-damaged fruits and vegetables to the total amount of fruits and vegetables, and update the single-use amount of liquid nitrogen. Calculate the proportion of frost-damaged fruits and vegetables to the total amount of fruits and vegetables after a single usage update, Bs. Stop adjusting when the proportion Bs ≤ 5%.

[0074] Compensation module: Calculates the average temperature (W) of fruit and vegetable skins. ave If the mean W ave >W best Then, the carbon dioxide-compressor cooling method is used to lower the surface temperature of fruits and vegetables to the optimal storage temperature W. best .

[0075] The foregoing has provided a detailed description of one embodiment of the present invention, but this description is merely a preferred embodiment and should not be construed as limiting the scope of the invention. All equivalent variations and modifications made within the scope of the claims of this invention should still fall within the patent coverage of this invention.

Claims

1. A method for controlling the preservation of fruits and vegetables in a liquid nitrogen warehouse, characterized in that, Includes the following steps: Step S1: Obtain the current temperature W of the fruit and vegetable warehouse, based on the optimal storage temperature W for fruits and vegetables. best Calculate the ideal usage amount M of liquid nitrogen; Step S2: Calculate the single usage amount m = M / N, where N represents the preset number of uses. Arrange the fruits and vegetables in a preset formation and add liquid nitrogen of mass m at the preset addition point. Update the position of the addition point and continue to add liquid nitrogen of mass m. Repeat the above steps until the total amount of liquid nitrogen used equals the ideal usage amount. Step S3: Determine whether the fruits and vegetables have suffered frost damage based on the hardness test. If frost damage is found, adjust the liquid nitrogen addition method. The liquid nitrogen addition method includes: Calculate the proportion (B) of frost-damaged fruits and vegetables to the total amount of fruits and vegetables, and update the single-use amount of liquid nitrogen. Calculate the proportion of frost-damaged fruits and vegetables to the total amount of fruits and vegetables after a single usage update, Bs. Stop adjusting when the proportion Bs ≤ 5%. Step S4: Calculate the average temperature W of the fruit and vegetable skin. ave If the mean W ave >W best Then, the carbon dioxide-compressor cooling method is used to lower the surface temperature of fruits and vegetables to the optimal storage temperature W. best .

2. The method for controlling liquid nitrogen preservation in a fruit and vegetable warehouse according to claim 1, characterized in that, In step S1, based on the optimal storage temperature W for fruits and vegetables best The specific methods for calculating the ideal usage amount M of liquid nitrogen include: Calculate the temperature difference ΔW = WW best The heat consumption Q = c1ΔW is calculated based on the temperature difference ΔW, where c1 represents the specific heat capacity of air. The ideal usage of liquid nitrogen is then calculated. Here, c2 represents the heat of vaporization of liquid nitrogen.

3. The method for controlling liquid nitrogen preservation in a fruit and vegetable warehouse according to claim 1, characterized in that, In step S2, the preset number of uses N≥4, so that the time consumed to add liquid nitrogen under different number of uses N is equal to the preset specified time.

4. The method for controlling liquid nitrogen preservation in a fruit and vegetable warehouse according to claim 1, characterized in that, In step S2, the method for updating the position of the added point specifically includes: Divide the circular area with the distance between the center and the addition point as the radius. The center of the circle is the center point of the fruit and vegetable arrangement. Rotate the addition point clockwise by θ degrees along the boundary of the circular area to obtain a new addition point, where θ = 2π / N.

5. The method for controlling liquid nitrogen preservation in a fruit and vegetable warehouse according to claim 1, characterized in that, In step S3, the method for determining whether fruits and vegetables have suffered frost damage based on a hardness test specifically includes: K detection points are pre-set on the surface of fruits and vegetables. A preset value of pressure is applied to the detection points, and the indentation distance L of the fruit and vegetable surface is obtained. If there is a detection point with an indentation distance L greater than or equal to the preset judgment distance, it is determined that the fruit and vegetable has frost damage.

6. The method for controlling liquid nitrogen preservation in a fruit and vegetable warehouse according to claim 1, characterized in that, In step S3, another method for determining whether fruits and vegetables have suffered frost damage specifically includes: The ethylene content Y in the air before liquid nitrogen is added is obtained. When liquid nitrogen of mass m is added, the ethylene content Ys in the air is updated. If the updated ethylene content Ys > Y + η, it is determined that the fruits and vegetables have suffered frost damage. Here, η represents the preset ethylene content fluctuation value.

7. The method for controlling liquid nitrogen preservation in a fruit and vegetable warehouse according to claim 1, characterized in that, In step S4, the carbon dioxide-compressor cooling method includes: Add a preset volume of carbon dioxide to the fruits and vegetables, measure the surface temperature of the fruits and vegetables at this time, and if the surface temperature of the fruits and vegetables is higher than the optimal storage temperature, then use a compressor to lower the surface temperature of the fruits and vegetables to the optimal storage temperature.

8. A liquid nitrogen preservation control system for fruit and vegetable warehouses, characterized in that, include: Calculation module: Obtains the current temperature W of the fruit and vegetable warehouse, and calculates the optimal storage temperature W for the fruits and vegetables. best Calculate the ideal usage amount M of liquid nitrogen; Cooling module: Calculate the single usage amount m = M / N, where N represents the preset number of uses. Arrange the fruits and vegetables in a preset formation and add liquid nitrogen of mass m at the preset addition point. Update the position of the addition point and continue to add liquid nitrogen of mass m. Repeat the above steps until the total amount of liquid nitrogen used equals the ideal usage amount. Adjustment module: Based on hardness testing, it determines whether fruits and vegetables have suffered frost damage. If frost damage is found, the liquid nitrogen addition method is adjusted. The liquid nitrogen addition method includes: Calculate the proportion (B) of frost-damaged fruits and vegetables to the total amount of fruits and vegetables, and update the single-use amount of liquid nitrogen. Calculate the proportion of frost-damaged fruits and vegetables to the total amount of fruits and vegetables after a single usage update, Bs. Stop adjusting when the proportion Bs ≤ 5%. Compensation module: Calculates the average temperature W of fruit and vegetable skins. ave If the mean W ave >W best Then, the carbon dioxide-compressor cooling method is used to lower the surface temperature of fruits and vegetables to the optimal storage temperature W. best .

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