Ultralow-temperature quick-freezing process for litchis

Abstract

The invention relates to the technical field of frozen storage of fruits and vegetables, in particular to an ultralow-temperature quick-freezing process for litchis. When liquid nitrogen is used for quick freezing, a three-stage graded cooling process is adopted, in the first stage, liquid nitrogen at the temperature ranging from-40 DEG C to-50 DEG C is adopted for reducing the center temperature of litchis to 4 DEG C to-1 DEG C; in the second stage, the central temperature of the litchis is reduced to-13 DEG C to-17 DEG C by adopting liquid nitrogen with the temperature of-60 DEG C to-100 DEG C; and in the third stage, reducing the central temperature of the litchis to-19 to-22 DEG C by adopting liquid nitrogen with the temperature of-50 to-60 DEG C. According to the process, the lychees subjected to liquid nitrogen quick-freezing treatment can form small ice crystals, the drying loss is low, the lychees have good low-temperature frost crack resistance, the integrity of product cells is guaranteed, the original color, freshness and flavor of the lychees can be kept, the shelf life of the lychees is greatly prolonged, and the fresh-keeping degree of the lychees is good. In addition, the lychee shells quick-frozen by the process are not easy to crack.

Description

Technical Field

[0001] The invention relates to the technical field of frozen storage of fruits and vegetables, and in particular to an ultra-low temperature quick freezing process for litchi. Background Art

[0002] Lychee is a tropical fruit. It is popular among people because of its crisp flesh, sweet and juicy taste, rich and delicious fruity aroma. However, the harvest period of lychees is concentrated in the hot summer season, and it is difficult to store and preserve them after picking. In the traditional lychee industry chain, lychees are produced and sold immediately, which can only meet the needs of consumers who consume them immediately in the season and are close to the production area. When the growing season of this fruit is exceeded, or because the transportation and preservation technology cannot meet the needs of long-distance consumers to consume such products, lychee liquid nitrogen quick freezing technology has also been developed. Liquid nitrogen ultra-low temperature quick freezing technology is a technology that freezes food by absorbing a large amount of heat through direct contact between liquid nitrogen and lychees. It is widely used as a green refrigerant because of its fast freezing speed, short time, high efficiency, safety and pollution-free. When using liquid nitrogen freezing, the freezing speed is one of the main reasons for the low-temperature fracture of lychees.

[0003] Based on this, the present invention is proposed. Summary of the invention

[0004] The purpose of the present invention is to solve the problem that the freezing speed of litchi affects the litchi shell, causing the litchi shell to crack, which seriously affects the appearance of the finished quick-frozen litchi product. In particular, it relates to a litchi ultra-low temperature quick-freezing process.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] The present invention provides a litchi ultra-low temperature quick-freezing process, comprising the following steps:

[0007] (1) Screening and pruning: removing bad fruits, rotten fruits and impurities, and cutting off the branches and leaves of freshly picked lychees to make them into single lychees;

[0008] (2) Cleaning: Put a single litchi into a clean water tank and introduce compressed air into the clean water tank to clean the litchi;

[0009] (3) Color and crispness protection: Put the washed lychees into a color and crispness protection agent, soak for 3 to 5 minutes, and introduce compressed air;

[0010] (4) Air drying: air drying the surface moisture of the lychees after color and crispness preservation;

[0011] (5) Classification: The air-dried lychees are classified according to their diameter;

[0012] (6) Inner packaging: inner packaging of graded lychees;

[0013] (7) Liquid nitrogen quick freezing: The inner-packaged lychees are subjected to graded liquid nitrogen quick freezing;

[0014] (8) Outer packaging: packing the liquid nitrogen graded quick-frozen lychees into packaging boxes to obtain packaged lychees;

[0015] (9) Frozen storage: The packaged lychees are frozen and stored.

[0016] Preferably, in step (2), the air flow rate when compressed air is introduced into the clean water tank is 100 to 300 L / min.

[0017] Preferably, the color-protecting and brittle-preserving agent in step (3) uses water as solvent and comprises the following components in mass concentrations: 0.5-1% calcium chloride, 0.2-0.5% vitamin C, and 3-5% citric acid.

[0018] Preferably, the air flow rate when the compressed air is introduced in step (3) is 100 to 300 L / min.

[0019] Preferably, the liquid nitrogen graded quick freezing step in step (7) comprises: firstly using liquid nitrogen at -40 to -50°C to reduce the central temperature of the litchi to 4 to -1°C to obtain pre-cooled litchi; then using liquid nitrogen at -60 to -100°C to reduce the central temperature of the pre-cooled litchi to -13 to -17°C to obtain frozen litchi; and finally using liquid nitrogen at -50 to -60°C to reduce the central temperature of the frozen litchi to -19 to -22°C to obtain liquid nitrogen graded quick frozen litchi.

[0020] Preferably, the packaging box described in step (8) is a foam packaging box.

[0021] Preferably, the frozen storage temperature in step (9) is -18 to -25°C.

[0022] The invention also provides application of the litchi ultra-low temperature quick freezing process in litchi preservation.

[0023] The present invention also provides application of the litchi ultra-low temperature quick freezing process in prolonging the shelf life of litchi.

[0024] The present invention provides a litchi ultra-low temperature quick-freezing process, and the ultra-low temperature quick-freezing process of the present invention has the following advantages:

[0025] When the present invention uses liquid nitrogen to quickly freeze litchi, a three-stage graded cooling process is adopted. In the first stage, liquid nitrogen at -40 to -50°C is used to reduce the central temperature of litchi to 4 to -1°C to obtain pre-cooled litchi; in this stage, the central temperature of litchi is reduced to the freezing point to dissipate sensible heat. In the second stage, liquid nitrogen at -60 to -100°C is used to reduce the central temperature of the pre-cooled litchi to -13 to -17°C to obtain frozen litchi; in the third stage, liquid nitrogen at -50 to -60°C is used to reduce the central temperature of the frozen litchi to -19 to -22°C to obtain liquid nitrogen graded quick-frozen litchi. The liquid nitrogen quick-freezing temperature is changed step by step in the three stages, the temperature difference between the equipment and the litchi is reduced in the pre-cooling period (first stage) and the freezing period (third stage), the cooling rate in the quick-freezing period (second stage) is increased, and the passing time of the maximum ice crystal formation zone is shortened. Under this process, the ice crystals formed by the liquid nitrogen quick-frozen lychees are small, the dry loss is low, and they have good resistance to low-temperature freezing and cracking. This not only ensures the integrity of the product cells, but also maintains its original color, freshness, and flavor, greatly extending its shelf life and keeping it fresh. In addition, the lychee shells quick-frozen by this process are not prone to cracking. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is the process flow chart of ultra-low temperature quick freezing of litchi. DETAILED DESCRIPTION

[0027] The present invention provides a litchi ultra-low temperature quick-freezing process, comprising the following steps:

[0028] (1) Screening and pruning: remove bad fruits, rotten fruits and impurities, and cut off the branches and leaves of freshly picked lychees to make them into single lychees; (2) Cleaning: put the single lychees into a clean water pool, and at the same time, pass compressed air into the clean water pool to clean the lychees; (3) Color protection and crispness preservation: put the washed lychees into a color protection and crispness preservation agent, soak for 3 to 5 minutes, and pass compressed air; (4) Air drying: air-dry the moisture on the surface of the lychees after color protection and crispness preservation; (5) Grading: grade the air-dried lychees according to their diameter; (6) Inner packaging: inner packaging the graded lychees; (7) Liquid nitrogen quick freezing: liquid nitrogen graded quick freezing the lychees that have been inner packaged; (8) Outer packaging: put the liquid nitrogen graded quick frozen lychees into a packaging box to obtain packaged lychees; (9) Frozen storage: freeze the packaged lychees.

[0029] In the present invention, the air flow rate when the compressed air is introduced into the clean water tank in step (2) is 100 to 300 L / min, preferably 200 L / min.

[0030] In the present invention, the color and brittleness-preserving agent in step (3) uses water as solvent and includes the following components in mass concentrations: calcium chloride 0.5-1%, preferably 0.8%; vitamin C 0.2-0.5%, preferably 0.3%; citric acid 3-5%, preferably 4%. The soaking time in step (3) is preferably 4 minutes.

[0031] In the present invention, the air flow rate when the compressed air is introduced in step (3) is 100 to 300 L / min, preferably 200 L / min.

[0032] In the present invention, the specific steps of the inner packaging in step (6) are: vacuum packaging the litchi using a stretch film vacuum packaging machine;

[0033] In the present invention, the step of liquid nitrogen graded quick freezing in step (7) is: firstly, using liquid nitrogen at -40 to -50°C to reduce the central temperature of the litchi to 4 to -1°C, so as to obtain pre-cooled litchi; then, using liquid nitrogen at -60 to -100°C to reduce the central temperature of the pre-cooled litchi to -13 to -17°C, so as to obtain frozen litchi; finally, using liquid nitrogen at -50 to -60°C to reduce the central temperature of the frozen litchi to -19 to -22°C, so as to obtain liquid nitrogen graded quick freezing litchi. The step of liquid nitrogen graded quick freezing is preferably: firstly, using liquid nitrogen at -45°C to reduce the central temperature of the litchi to 0°C, so as to obtain pre-cooled litchi; then, using liquid nitrogen at -80°C to reduce the central temperature of the pre-cooled litchi to -15°C, so as to obtain frozen litchi; finally, using liquid nitrogen at -55°C to reduce the central temperature of the frozen litchi to -20°C, so as to obtain liquid nitrogen graded quick freezing litchi.

[0034] In the present invention, the packaging box described in step (8) is a foam packaging box.

[0035] In the present invention, the frozen storage temperature in step (9) is -18 to -25°C, preferably -22°C.

[0036] The invention also provides application of the litchi ultra-low temperature quick freezing process in litchi preservation.

[0037] The present invention also provides application of the litchi ultra-low temperature quick freezing process in prolonging the shelf life of litchi.

[0038] The technical solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0039] Example 1

[0040] Preparation of graded ultra-low temperature quick-frozen litchi

[0041] (1) Screening and pruning: removing bad fruits, rotten fruits and impurities, and cutting off the branches and leaves of freshly picked lychees to make them into single lychees for subsequent packaging and quick freezing;

[0042] (2) Cleaning: Put a single litchi into a clean water tank, and at the same time, introduce compressed air into the clean water tank at an air flow rate of 200 L / min to clean the litchi;

[0043] (3) Color and crispness protection: The washed lychees were placed in a color and crispness protection agent, soaked for 4 min, and compressed air was introduced at an air flow rate of 200 L / min (the color and crispness protection agent uses water as a solvent and includes the following components in mass concentrations: 0.6% calcium chloride, 0.3% vitamin C, and 4% citric acid);

[0044] (4) air drying: the lychees that have been color-protected and crispy are passed through an air drying machine track to dry the moisture on the surface of the lychees, thereby obtaining air-dried lychees;

[0045] (5) Classification: The air-dried lychees are classified according to their diameter;

[0046] (6) Inner packaging: vacuum packing the litchi using a stretch film vacuum packaging machine and sealing at high temperature;

[0047] (7) Liquid nitrogen quick freezing: The inner packaged lychees are placed in a liquid nitrogen quick freezing tunnel. In the first stage (temperature zone), liquid nitrogen at -45°C is used to reduce the central temperature of the lychees to 0°C, thereby obtaining pre-cooled lychees. In the second stage (temperature zone), liquid nitrogen at -70°C is used to reduce the central temperature of the pre-cooled lychees to -15°C, thereby obtaining frozen lychees. In the third stage (temperature zone), liquid nitrogen at -55°C is used to reduce the central temperature of the frozen lychees to -20°C, thereby obtaining lychees that are graded and quick-frozen with liquid nitrogen.

[0048] (8) Outer packaging: putting the liquid nitrogen graded quick-frozen lychees into a foam packaging box and sealing it to obtain packaged lychees;

[0049] (9) Frozen storage: The packaged lychees are placed in a cold storage at -20°C for frozen storage.

[0050] Example 1 Flow chart of the ultra-low temperature quick freezing process of litchi Figure 1 shown.

[0051] Comparative Example 1

[0052] Preparation of ultra-low temperature quick-frozen litchi

[0053] (1) Screening and pruning: removing bad fruits, rotten fruits and impurities, and cutting off the branches and leaves of freshly picked lychees to make them into single lychees for subsequent packaging and quick freezing;

[0054] (2) Cleaning: Put a single litchi into a clean water tank, and at the same time, introduce compressed air into the clean water tank at an air flow rate of 200 L / min to clean the litchi;

[0055] (3) Color and crispness protection: The washed lychees were placed in a color and crispness protection agent, soaked for 4 min, and compressed air was introduced at an air flow rate of 200 L / min (the color and crispness protection agent uses water as a solvent and includes the following components in mass concentrations: 0.6% calcium chloride, 0.3% vitamin C, and 4% citric acid);

[0056] (4) air drying: the lychees that have been color-protected and crispy are passed through an air drying machine track to dry the moisture on the surface of the lychees, thereby obtaining air-dried lychees;

[0057] (5) Classification: The air-dried lychees are classified according to their diameter;

[0058] (6) Inner packaging: vacuum packing the litchi using a stretch film vacuum packaging machine and sealing at high temperature;

[0059] (7) Liquid nitrogen quick freezing: The inner-packaged lychees are directly quick-frozen using liquid nitrogen until the core temperature of the lychees is -20°C, thereby obtaining quick-frozen lychees;

[0060] (8) Outer packaging: Put the quick-frozen lychees into a foam packaging box and seal it;

[0061] (9) Frozen storage: The packaged lychees are placed in a cold storage at -20°C for frozen storage.

[0062] Comparative Example 2

[0063] Preparation of low temperature frozen litchi

[0064] (1) Screening and pruning: removing bad fruits, rotten fruits and impurities, and cutting off the branches and leaves of freshly picked lychees to make them into single lychees for subsequent packaging and quick freezing;

[0065] (2) Cleaning: Put a single litchi into a clean water tank, and at the same time, introduce compressed air into the clean water tank at an air flow rate of 200 L / min to clean the litchi;

[0066] (3) Color and crispness protection: The washed lychees were placed in a color and crispness protection agent, soaked for 4 min, and compressed air was introduced at an air flow rate of 200 L / min (the color and crispness protection agent uses water as a solvent and includes the following components in mass concentrations: 0.6% calcium chloride, 0.3% vitamin C, and 4% citric acid);

[0067] (4) air drying: the lychees that have been color-protected and crispy are passed through an air drying machine track to dry the moisture on the surface of the lychees, thereby obtaining air-dried lychees;

[0068] (5) Classification: The air-dried lychees are classified according to their diameter;

[0069] (6) Inner packaging: vacuum packing the litchi using a stretch film vacuum packaging machine and sealing at high temperature;

[0070] (7) Freezing: placing the inner-packaged lychees into the freezing chamber (-22° C.) of a common refrigerator, lowering the core temperature of the lychees to -20° C., thereby obtaining frozen lychees;

[0071] (8) Outer packaging: Put the quick-frozen lychees into a foam packaging box and seal it;

[0072] (9) Frozen storage: The packaged lychees are placed in a cold storage at -20°C for frozen storage.

[0073] Experimental Example 1

[0074] The quick-frozen or frozen lychees prepared in Example 1 and Comparative Examples 1-2 were frozen in a cold storage at -20°C for 6 months, and the fruit cracking rates of the lychees quick-frozen or frozen stored by different methods were compared. The results are shown in Table 1. Fruit cracking rate = number of cracked fruits / total number of quick-frozen lychees × 100%.

[0075] Table 1 Fruit cracking rate of quick-frozen or frozen litchi prepared by different methods

[0076] Group Example 1 Comparative Example 1 Comparative Example 2 Fruit cracking rate% 8％ 55％ 0％

[0077] After thawing the lychees at room temperature (25±1℃) to 4℃, wipe off the surface moisture and measure the color of the lychee peel at the equator with a colorimeter. The higher the L value, the brighter the color, that is, the lower the browning degree of the peel. The color changes of quick-frozen or frozen lychees prepared by different preparation processes after 6 months of frozen storage are shown in Table 2.

[0078] Table 2 Color changes of quick-frozen or frozen lychees obtained by different preparation processes after storage

[0079] Group L* Comparative Example 1 30.78±3.68 Comparative Example 2 25.20±2.25 Example 1 36.59±2.66

[0080] Table 2 shows that the shells of quick-frozen lychees prepared by the preparation process of Example 1 are brighter and less browned than those prepared by the other two methods, indicating that the method of the present invention can better ensure the freshness of lychees.

[0081] The lychees were thawed at room temperature (25 ± 1 °C) to 4 °C and weighed (M0). The thawed lychees were peeled and placed on a clean table to allow the juice to drip naturally for 5 minutes. The remaining juice on the surface of the pulp was wiped with a napkin. Finally, the pulp and peel were weighed (M1), and the juice loss was calculated. The results are shown in Table 3.

[0082] Juice loss (%) = (M0-M1) / M1×100.

[0083] Table 3 Juice loss of quick-frozen or frozen litchi prepared by different processes after frozen storage

[0084] Group Juice loss / % Comparative Example 1 1.29±0.36 Comparative Example 2 2.83±0.92 Example 1 1.14±0.12

[0085] Table 3 shows that the loss of juice of the quick-frozen lychees prepared by the preparation process of Example 1 after frozen storage is less than that of Comparative Examples 1-2, indicating that the preparation process of the present invention can ensure the taste and freshness of lychees.

[0086] After thawing to 4°C at room temperature (25±1°C), the lychees were peeled and placed on the platform of the texture analyzer. A cylindrical probe with a diameter of 36 mm was pressed on the center of the lychee. The following test parameters were used: 2.0 mm / s pre-test speed, 1.00 mm / s test speed, 5.0 mm / s post-test speed, 5.0 g trigger force, 30% deformation rate, and 5 seconds between two presses. The hardness, elasticity and chewiness of the lychees were measured. The results are shown in Table 4.

[0087] Table 4 Texture characteristics of quick-frozen or frozen litchi prepared by different processes after frozen storage

[0088] Group Hardness / gf elasticity Chewability / gf Comparative Example 1 948.74±196.23 0.71±0.05 405.23±132.21 Comparative Example 2 520.37±98.28 0.61±0.04 187.52±34.61 Example 1 1235.24±258.32 0.84±0.07 567.78±128.82

[0089] Table 4 shows that the hardness, elasticity and chewiness of the lychees quick-frozen by the quick-freezing method of the present invention are better than those of the lychees of Comparative Examples 1 to 2 after being frozen for 6 months, indicating that the method of the present invention can better ensure the texture characteristics of lychees.

[0090] In summary, the cracking rate of lychees quick-frozen by the method of comparative example 1 was relatively high due to the rapid cooling of the lychees after re-frozen storage. The frozen lychees of comparative example 2 did not crack due to the slow cooling after re-frozen storage, but the flavor of the lychees after freeze-thawing was quite different from that of fresh lychees, which was mainly reflected in the large loss of juice, severe brown peel, and obvious softening of the flesh. The cracking rate of lychees quick-frozen by the method of embodiment 1 was low, and the taste and flavor of the lychees were not much different from those of fresh lychees. It shows that the method of the present invention is more suitable for the quick freezing of lychees.

[0091] The quick freezing method of Example 1 of the present invention makes the ice crystals formed on the litchi very small and has good resistance to low-temperature freezing and cracking. It not only ensures the integrity of the product cells, but also the color and crispness retaining agent can maintain its original color, taste and juice during the soaking process in the process, greatly extending its shelf life. The freshness can also be closer to that of fresh litchi to a greater extent. The litchi shell after quick freezing by this process is not easy to crack.

[0092] It can be seen from the above embodiments that the present invention provides a process for ultra-low temperature quick freezing of lychees. When the present invention uses liquid nitrogen for quick freezing, a three-stage graded cooling process is adopted. In the first stage, liquid nitrogen at -40 to -50°C is used to reduce the central temperature of the lychees to 4 to -1°C to obtain pre-cooled lychees; in this stage, the central temperature of the lychees is reduced to the freezing point to dissipate sensible heat. In the second stage, liquid nitrogen at -60 to -100°C is used to reduce the central temperature of the pre-cooled lychees to -13 to -17°C to obtain frozen lychees; in the third stage, liquid nitrogen at -50 to -60°C is used to reduce the central temperature of the frozen lychees to -19 to -22°C to obtain lychees graded and quick frozen with liquid nitrogen. The liquid nitrogen quick freezing temperature is changed step by step in three stages, and the temperature difference between the equipment and the lychees is reduced in the pre-cooling period (first stage) and the freezing period (third stage), while the cooling rate in the quick freezing period (second stage) is increased, and the passage time of the maximum ice crystal formation zone is shortened. Under this process, the ice crystals formed by the liquid nitrogen quick-frozen lychees are small, the dry loss is low, and they have good resistance to low-temperature freezing and cracking. This not only ensures the integrity of the product cells, but also maintains its original color, freshness, and flavor, greatly extending its shelf life and keeping it fresh. In addition, the lychee shells quick-frozen by this process are not prone to cracking.

[0093] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A litchi ultra-low temperature quick freezing process, characterized in that: The steps include: (1) Screening and pruning: removing bad fruits, rotten fruits and impurities, and cutting off the branches and leaves of freshly picked lychees to make them into single lychees; (2) Cleaning: Put a single litchi into a clean water tank and introduce compressed air into the clean water tank to clean the litchi; (3) Color and crispness protection: Put the washed lychees into a color and crispness protection agent, soak for 3 to 5 minutes, and introduce compressed air; (4) Air drying: air drying the surface moisture of the lychees after color and crispness preservation; (5) Classification: The air-dried lychees are classified according to their diameter; (6) Inner packaging: inner packaging of graded lychees; (7) Liquid nitrogen quick freezing: The inner-packaged lychees are subjected to graded liquid nitrogen quick freezing; (8) Outer packaging: packing the liquid nitrogen graded quick-frozen lychees into packaging boxes to obtain packaged lychees; (9) Frozen storage: The packaged lychees are frozen and stored.

2. The ultra-low temperature quick freezing process for litchi according to claim 1, characterized in that: In step (2), the air flow rate when compressed air is introduced into the clean water tank is 100 to 300 L / min.

3. The ultra-low temperature quick freezing process for litchi according to claim 2, characterized in that: The color-protecting and brittle-preserving agent described in step (3) uses water as solvent and comprises the following components in mass concentrations: 0.5-1% calcium chloride, 0.2-0.5% vitamin C, and 3-5% citric acid.

4. The ultra-low temperature quick freezing process for litchi according to claim 3, characterized in that: The air flow rate when the compressed air is introduced in step (3) is 100 to 300 L / min.

5. The ultra-low temperature quick freezing process for litchi according to claim 4, characterized in that: The steps of liquid nitrogen graded quick freezing in step (7) are as follows: first, using liquid nitrogen at -40 to -50°C to reduce the central temperature of the litchi to 4 to -1°C to obtain pre-cooled litchi; then using liquid nitrogen at -60 to -100°C to reduce the central temperature of the pre-cooled litchi to -13 to -17°C to obtain frozen litchi; and finally using liquid nitrogen at -50 to -60°C to reduce the central temperature of the frozen litchi to -19 to -22°C to obtain liquid nitrogen graded quick frozen litchi.

6. The ultra-low temperature quick freezing process for litchi according to claim 5, characterized in that: The packaging box described in step (8) is a foam packaging box.

7. The ultra-low temperature quick freezing process for litchi according to claim 6, characterized in that: The frozen storage temperature in step (9) is -18 to -25°C.

8. Use of the litchi ultra-low temperature quick freezing process according to any one of claims 1 to 7 in the preservation of litchi.

9. Use of the litchi ultra-low temperature quick freezing process according to any one of claims 1 to 7 in extending the shelf life of litchi.

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