Liquid nitrogen quick-freezing fresh-keeping method for soft-shelled turtle prefabricated product

Abstract

The invention provides a liquid nitrogen quick-freezing fresh-keeping method for a soft-shelled turtle prefabricated product, which comprises the following steps: (1) preparing a quick-freezing device: the quick-freezing device is a tunnel type liquid nitrogen quick-freezing machine, and the liquid nitrogen spraying speed, the liquid nitrogen conveying speed and the cooling air speed can be adjusted; (2) pre-cooling and placing on a tray: pre-cooling the cooked soft-shelled turtles, and uniformly placing the soft-shelled turtles on the tray after the surface of the soft-shelled turtles is cooled; (3) liquid nitrogen freezing: conveying the tray to a feeding cabin of a liquid nitrogen instant freezer, and starting a liquid nitrogen spraying system and a speed transmission device; (4) ice crystal inhibition and texture protection: regulating and controlling the liquid nitrogen spraying rate, the conveying rate and the air blowing speed, and performing three-stage quick freezing; (5) taking out of a cabin and post-processing: quickly taking out of the cabin after freezing, and quickly sealing and packaging in vacuum; and (6) storage and transportation: boxing, storing in a refrigeration house, and transporting by using a refrigeration vehicle. The temperature of the soft-shelled turtle prefabricated product can be reduced to-18 DEG C or below within 3 min, the freezing time is greatly shortened, the temperature in the freezing process is uniform, the original flavor, taste and nutritional value of the product are effectively kept, and the shelf life reaches 12 months or above.

Description

Technical Field

[0001] The invention belongs to the technical field of food quick-freezing and fresh-keeping, and in particular relates to a liquid nitrogen quick-freezing and fresh-keeping method for a turtle preform. Background Art

[0002] Turtle, also known as water fish, round fish, etc., has delicious meat and rich nutrition. It contains amino acids needed by the human body and is rich in methionine that is lacking in general food. It also contains trace elements such as calcium, phosphorus, iron, and vitamins such as thionine, riboflavin, and niacin. Turtle has the effects of clearing heat and nourishing yin, calming the liver and extinguishing wind, softening and dispersing lumps. It is not only a delicious dish on the table, but also a widely used nourishing raw material, which is very popular among people. At present, the national output of soft-shelled turtles has reached 360,000 tons, with the highest output in Hunan, Hubei, Jiangxi, Anhui, Jiangsu and other places. However, my country's soft-shelled turtle processing industry is weak, and for ordinary consumers, soft-shelled turtles are difficult to select, slaughter and cook. At present, 90% of soft-shelled turtles on the market are mainly sold fresh. With the improvement of people's living standards, healthy, nutritious, delicious, and long-shelf-life soft-shelled turtle convenience foods have broad market space and development potential.

[0003] Since soft-shell turtles are rich in nutrients and have a high water content, they are a good natural culture medium for microorganisms. If the soft-shell turtles are not kept fresh during processing, the taste of the product will easily be distorted or even spoiled, causing economic losses. Therefore, quick freezing and preservation of processed soft-shell turtle products is of great significance and value for the development of healthy and safe clean-dish soft-shell turtle pre-prepared products. Traditional freezing methods (such as mechanical freezing) may cause damage to the meat tissue structure, affecting the taste and nutritional content. Liquid nitrogen quick freezing, as an advanced freezing technology, can quickly reduce the product temperature, reduce ice crystal particles, and significantly improve the quality of meat products after freezing. In view of the characteristics of soft-shell turtle pre-prepared products, it is particularly important to design a liquid nitrogen quick freezing preservation method to extend the shelf life and maintain the original flavor and taste. Summary of the invention

[0004] In response to the above problems, the technical problem solved by the present invention is to propose a liquid nitrogen quick-freezing and preservation method for soft-shelled turtle preforms. Through ultra-low temperature rapid freezing technology, the complete protection of the internal meat cell structure of the soft-shelled turtle preform can be achieved, the damage to the tissue by ice crystal particles can be reduced, the problems of tissue structure destruction, flavor loss and bacterial growth in traditional freezing processes can be solved, and the original flavor, taste and nutritional value of the soft-shelled turtle preforms can be effectively maintained.

[0005] The present invention provides a method for quick-freezing and preserving a turtle preformed product with liquid nitrogen, the quick-freezing and preserving method comprising the following steps: (1) Preparation of quick freezing device: The quick freezing device is a tunnel-type liquid nitrogen quick freezing machine, which is equipped with a liquid nitrogen spray speed adjustment device, a freezing channel capable of adjusting the transmission speed, and a blowing device on both sides of the channel capable of adjusting the wind speed; (2) Precooling and plating: The cooked turtle meat is quickly placed in the cooling room for precooling. After the surface temperature drops, it is evenly placed on the quick-freezing tray at a specific distance to prevent them from sticking to each other during freezing; (3) Liquid nitrogen freezing: The tray of preformed turtles is transferred to the feed compartment of the liquid nitrogen quick-freezing machine, the liquid nitrogen spraying system and the speed transmission device are started, and the tray is sent into the freezing channel for cooling and quick freezing; (4) Ice crystal inhibition and texture protection: After the tray enters the freezing channel, the liquid nitrogen spray rate, conveying rate and blowing speed are adjusted according to the weight of the turtle preformed product to be frozen in the entire freezing channel, and three-stage quick freezing is carried out to complete ice crystal inhibition and texture protection of the turtle meat; (5) Unloading and post-processing: After freezing is completed, the tray is quickly unloaded from the discharge chamber of the liquid nitrogen quick freezer and transferred to the vacuum packaging workshop within a specific time to avoid frost on the food surface due to excessive temperature difference in the environment. The frozen turtle pre-made products are quickly vacuum-sealed and packaged using a vacuum packaging machine to reduce the risk of oxidation and bacterial contamination; (6) Storage and transportation: The packaged soft-shell turtle preforms are packed and then stored in a cold warehouse. Refrigerated trucks are used for product transportation.

[0006] Preferably, the operating temperature of the liquid nitrogen quick freezer in step (1) is -196°C, the temperature control accuracy is ±2°C, and the purity of the liquid nitrogen is not less than 99.9%.

[0007] Preferably, the temperature of the cooling room in step (2) is 0-4°C, the surface temperature is reduced to 20-25°C, and the specific spacing is 5-10 mm between each piece of meat.

[0008] Preferably, the liquid nitrogen spraying system in step (3) is designed with a multi-layer nozzle, and the time required to reduce the surface temperature of the frozen items to -18°C can be controlled within 3 minutes.

[0009] There are two types of water contained in soft-shelled turtle preforms. One is free water, whose ice crystal formation stage is at 0~5℃; the other is colloidally bound water, which is attached to the surface of meat molecules. When the frozen product is at -18℃, the crystallization rate is above 95%. Storing food in this low temperature environment can effectively inhibit the activity and reproduction of microorganisms.

[0010] Preferably, in step (4), the spray density of liquid nitrogen is 5-15 L / kg food, the liquid nitrogen spray rate is 10-50 L / h, the conveying rate is 0.5-5 m / min, the blowing speed is 2-10 m / s, and the three-stage quick freezing is divided into a starting section, a middle section and an ending section according to the length of the cooling channel. The cooling efficiencies of the three sections are set according to 1-2:1-1.5:0.5-1, respectively.

[0011] During the freezing process, the uniform distribution of liquid nitrogen can avoid local overfreezing or incomplete freezing to a limited extent. Rapid freezing converts the free water in the meat into small and uniform ice crystals in a very short time, minimizing the formation of large ice crystals, thereby reducing the damage of ice crystals to cell walls. After rapid freezing, the muscle fiber structure of the meat remains basically intact, and there is no obvious juice loss or taste change after thawing.

[0012] In the process of liquid nitrogen quick freezing, there is a close relationship between air supply and freezing effect. The design and adjustment of the air supply system directly affects the speed, uniformity and final quality of food freezing. The following analyzes the relationship between air supply and freezing effect from three aspects: physical mechanism, actual effect and design optimization: The essence of freezing is the heat exchange process between the food surface and the refrigerant. Air supply accelerates the flow of refrigerant, improves the efficiency of convection heat transfer, and can take away the heat from the food surface more quickly. Reasonable air supply design can avoid local temperatures being too high or too low and maintain the uniformity of the temperature gradient between the food surface and the core.

[0013] Preferably, the specific time of transfer in step (5) is no more than 15 minutes, and the temperature of the vacuum packaging workshop is 0-5°C.

[0014] Preferably, in step (6), the packing environment temperature does not exceed 12°C, the storage cold storage temperature is not higher than -18°C, and the transportation refrigeration vehicle temperature is not higher than -15°C.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The liquid nitrogen quick-freezing technology proposed in the present invention can rapidly reduce the core temperature of the turtle preformed product to below -18°C within 3 minutes, greatly shortening the freezing time; the temperature uniformity during the freezing process can be ensured by adjusting the liquid nitrogen spray rate, transmission rate, and wind speed, thereby avoiding incomplete freezing or partial overfreezing; the three-stage quick-freezing performance design and the ultra-low temperature of liquid nitrogen can rapidly freeze the water in the turtle preformed product into fine ice crystals, thereby effectively protecting the cell wall and meat structure; freezing is combined with vacuum packaging to inhibit microbial growth and oxidation reactions to the greatest extent, thereby retaining the original flavor, taste, and nutritional value of the turtle preformed product, with a shelf life of more than 12 months. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 It is a technical flow chart of the present invention; DETAILED DESCRIPTION

[0018] The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention. Without departing from the spirit and substance of the present invention, modifications or substitutions made to the method, steps or conditions of the present invention are within the scope of the present invention. Unless otherwise specified, the chemical reagents used in the examples are conventional commercial reagents, and the technical means used in the examples are conventional means well known to those skilled in the art.

[0019] Example 1 Preparation of quick freezing device: The quick freezing device is a tunnel-type liquid nitrogen quick freezing machine, which is equipped with a liquid nitrogen spray speed regulating device. The working temperature of the liquid nitrogen quick freezing machine is -196°C, the temperature control accuracy is ±2°C, the purity of liquid nitrogen is not less than 99.9%, the freezing channel can adjust the transmission speed, and both sides of the channel are equipped with blowing devices, and the wind speed can be adjusted; Pre-cooling and plating: quickly put the cooked soft-shelled turtle meat into a cooling room at 0°C for pre-cooling. After the surface temperature drops to 20°C, place each piece of meat on the quick-freezing tray with a 5mm spacing to prevent them from sticking to each other during freezing; Liquid nitrogen freezing: The tray of pre-made soft-shell turtles is transferred to the feed chamber of the liquid nitrogen quick-freezing machine, the liquid nitrogen spray system and the speed transmission device are started, and the tray is sent to the freezing channel for cooling and quick freezing. The liquid nitrogen spray system adopts a multi-layer nozzle design, and the time required to reduce the surface temperature of the frozen items to -18°C can be controlled within 3 minutes.

[0020] Ice crystal inhibition and texture protection: After the tray enters the freezing channel, the liquid nitrogen flow rate is set according to the spray density of 5L / kg of food in the entire freezing channel, the liquid nitrogen spray rate is adjusted to 12L / h, the transmission rate is 0.5 / min, and the blowing speed is 8m / s. Three-stage quick freezing is carried out in the cooling channel, and the cooling efficiency of the three stages is set according to 2:1.2:1.

[0021] Unloading and post-processing: After freezing is completed, the tray is quickly unloaded from the discharge chamber of the liquid nitrogen quick freezer and transferred to the vacuum packaging workshop within 15 minutes. The temperature of the vacuum packaging workshop is set at 2°C, and the frozen turtle preforms are quickly vacuum-sealed and packaged; Storage and transportation: The packaged soft-shell turtle preforms are packed in boxes at an ambient temperature of 12°C and then stored in a frozen warehouse at -18°C. When the products are transported, the temperature of the refrigerated truck is -15°C.

[0022] Example 2 Preparation of quick freezing device: The quick freezing device is a tunnel-type liquid nitrogen quick freezing machine, which is equipped with a liquid nitrogen spray speed regulating device. The working temperature of the liquid nitrogen quick freezing machine is -196°C, the temperature control accuracy is ±2°C, the purity of liquid nitrogen is not less than 99.9%, the freezing channel can adjust the transmission speed, and both sides of the channel are equipped with blowing devices, and the wind speed can be adjusted; Pre-cooling and plating: quickly put the cooked soft-shelled turtle meat into a cooling room at 2°C for pre-cooling. After the surface temperature drops to 23°C, place each piece of meat on the quick-freezing tray with a spacing of 8mm to prevent them from sticking to each other during freezing. Liquid nitrogen freezing: The tray of pre-made soft-shell turtles is transferred to the feed chamber of the liquid nitrogen quick-freezing machine, the liquid nitrogen spray system and the speed transmission device are started, and the tray is sent to the freezing channel for cooling and quick freezing. The liquid nitrogen spray system adopts a multi-layer nozzle design, and the time required to reduce the surface temperature of the frozen items to -18°C can be controlled within 3 minutes.

[0023] Ice crystal inhibition and texture protection: After the tray enters the freezing channel, the liquid nitrogen flow rate is set according to the spray density of 10L / kg of food in the entire freezing channel, the liquid nitrogen spray rate is adjusted to 15L / h, the transmission rate is 1m / min, and the blowing speed is 6m / s. Three-stage quick freezing is carried out in the cooling channel, and the cooling efficiency of the three stages is set according to 1.5:1:0.8.

[0024] Unloading and post-processing: After freezing is completed, the tray is quickly unloaded from the discharge chamber of the liquid nitrogen quick freezer and transferred to the vacuum packaging workshop within no more than 15 minutes. The temperature of the vacuum packaging workshop is set at 3°C, and the frozen turtle preforms are quickly vacuum-sealed and packaged; Storage and transportation: The packaged soft-shell turtle preforms are packed in boxes at an ambient temperature of 10°C, and then stored in a frozen warehouse at -20°C. When the products are transported, the temperature of the refrigerated truck is -16°C.

[0025] Example 3 Preparation of quick freezing device: The quick freezing device is a tunnel-type liquid nitrogen quick freezing machine, which is equipped with a liquid nitrogen spray speed regulating device. The working temperature of the liquid nitrogen quick freezing machine is -196°C, the temperature control accuracy is ±2°C, the purity of liquid nitrogen is not less than 99.9%, the freezing channel can adjust the transmission speed, and both sides of the channel are equipped with blowing devices, and the wind speed can be adjusted; Pre-cooling and plating: quickly put the cooked soft-shelled turtle meat into a cooling room at 4°C for pre-cooling. After the surface temperature drops to 25°C, place each piece of meat on the quick-freezing tray with a distance of 10mm to prevent them from sticking to each other during freezing. Liquid nitrogen freezing: The tray of pre-made soft-shell turtles is transferred to the feed chamber of the liquid nitrogen quick-freezing machine, the liquid nitrogen spray system and the speed transmission device are started, and the tray is sent to the freezing channel for cooling and quick freezing. The liquid nitrogen spray system adopts a multi-layer nozzle design, and the time required to reduce the surface temperature of the frozen items to -18°C can be controlled within 3 minutes.

[0026] Ice crystal inhibition and texture protection: After the tray enters the freezing channel, the liquid nitrogen flow rate is set according to the spray density of 15L / kg of food in the entire freezing channel, the liquid nitrogen spray rate is adjusted to 20L / h, the transmission rate is 2m / min, and the blowing speed is 4m / s. Three-stage quick freezing is carried out in the cooling channel, and the cooling efficiency of the three stages is set according to 1.2:1:0.6.

[0027] Unloading and post-processing: After freezing is completed, the tray is quickly unloaded from the discharge chamber of the liquid nitrogen quick freezer and transferred to the vacuum packaging workshop within no more than 15 minutes. The temperature of the vacuum packaging workshop is set at 5°C, and the frozen turtle preforms are quickly vacuum-sealed and packaged; Storage and transportation: The packaged soft-shell turtle preforms are packed in boxes at an ambient temperature of 10°C, and then stored in a frozen warehouse at -18°C. When the products are transported, the temperature of the refrigerated truck is -15°C.

[0028] The core temperature of the turtle preformed products in the above-mentioned embodiments 1 to 3 can be quickly reduced to below -18°C within 3 minutes, the temperature is uniform during the freezing process, and there is no incomplete freezing or local overfreezing. The water in the turtle preformed products after ultra-low temperature quick freezing with liquid nitrogen is quickly frozen into fine ice crystals, which effectively protects the cell wall and meat structure and reduces the damage of ice crystal particles to the tissue. After thawing, the turtle preformed products retain the original flavor, taste and nutritional value, and the shelf life of the turtle preformed products reaches more than 12 months.

[0029] Although the present invention has been described in detail above with general descriptions and specific embodiments, it is obvious to those skilled in the art that some modifications or improvements may be made thereto based on the present invention. Therefore, these modifications or improvements made without departing from the spirit of the present invention all fall within the scope of protection claimed by the present invention.

Claims

1. A method for quick freezing and preserving a turtle preform by liquid nitrogen, characterized in that: The fresh-keeping method comprises the following steps: (1) Preparation of quick freezing device: The quick freezing device is a tunnel-type liquid nitrogen quick freezing machine, which is equipped with a liquid nitrogen spray speed adjustment device, a freezing channel capable of adjusting the transmission speed, and a blowing device on both sides of the channel capable of adjusting the wind speed; (2) Precooling and plating: The cooked turtle meat is quickly placed in the cooling room for precooling. After the surface temperature drops, it is evenly placed on the quick-freezing tray at a specific distance to prevent them from sticking to each other during freezing; (3) Liquid nitrogen freezing: The tray of preformed turtles is transferred to the feed compartment of the liquid nitrogen quick-freezing machine, the liquid nitrogen spraying system and the speed transmission device are started, and the tray is sent into the freezing channel for cooling and quick freezing; (4) Ice crystal inhibition and texture protection: After the tray enters the freezing channel, the liquid nitrogen spray rate, conveying rate and blowing speed are adjusted according to the weight of the turtle preformed product to be frozen in the entire freezing channel, and three-stage quick freezing is carried out to complete ice crystal inhibition and texture protection of the turtle meat; (5) Unloading and post-processing: After freezing is completed, the tray is quickly unloaded from the discharge chamber of the liquid nitrogen quick freezer and transferred to the vacuum packaging workshop within a specific time to avoid frost on the food surface due to excessive temperature difference in the environment. The frozen turtle pre-made products are quickly vacuum-sealed and packaged using a vacuum packaging machine to reduce the risk of oxidation and bacterial contamination; (6) Storage and transportation: The packaged soft-shell turtle preforms are packed and then stored in a cold warehouse. Refrigerated trucks are used for product transportation.

2. The method for quick-freezing and preserving turtle preforms by liquid nitrogen according to claim 1, characterized in that: The operating temperature of the liquid nitrogen quick freezer in step (1) is -196°C, the temperature control accuracy is ±2°C, and the purity of the liquid nitrogen is not less than 99.9%.

3. The method for quick-freezing and preserving turtle preforms by liquid nitrogen according to claim 1, characterized in that: The temperature of the cooling room in step (2) is 0-4°C, the surface temperature is reduced to 20-25°C, and the specific spacing is 5-10 mm between each piece of meat.

4. The method for quick-freezing and preserving turtle preforms by liquid nitrogen according to claim 1, characterized in that: In step (3), the liquid nitrogen spraying system is designed with a multi-layer nozzle, and the time required to reduce the surface temperature of the frozen items to -18°C can be controlled within 3 minutes.

5. The method for quick-freezing and preserving turtle preforms by liquid nitrogen according to claim 1, characterized in that: In the step (4), the spray density of liquid nitrogen is 5-15 L / kg food, the liquid nitrogen spray rate is 10-50 L / h, the conveying rate is 0.5-5 m / min, and the blowing speed is 2-10 m / s.

6. The method for quick-freezing and preserving turtle preforms by liquid nitrogen according to claim 1, characterized in that: The three-stage quick freezing in step (4) is divided into a starting section, a middle section and an ending section evenly according to the length of the cooling channel, and the cooling efficiency of the three sections is set according to 1~2:1~1.5:0.5~1 respectively.

7. The method for quick-freezing and preserving turtle preforms by liquid nitrogen according to claim 1, characterized in that: The specific time of transfer in step (5) is no more than 15 minutes, and the temperature of the vacuum packaging workshop is 0-5°C.

8. The method for quick-freezing and preserving turtle preforms by liquid nitrogen according to claim 1, characterized in that: In step (6), the packing environment temperature does not exceed 12°C, the storage cold storage temperature does not exceed -18°C, and the transportation refrigeration vehicle temperature does not exceed -15°C.

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