Color protection method of frozen sleep litchis

Abstract

The invention relates to a color protection method of frozen sleep litchis, which comprises the following steps: soaking fresh litchis in an L-cysteine color protection solution, pouring the litchis and the color protection solution into an ultrasonic cleaning instrument, carrying out ultrasonic treatment, and reacting L-cysteine with an oxidative metabolite-quinones substance in the pericarp of the litchis to form a colorless compound so as to interrupt the browning reaction of the litchis. Meanwhile, the surface of the cell wall of the litchi peel is slightly damaged in cooperation with ultrasound, the intercellular space is increased, permeation of L-cysteine in the litchi peel is accelerated, the litchi is subjected to uniform color protection, and a simple, convenient and efficient mode is provided for color fixation of the freeze-sleep litchi.

Description

Technical Field

[0001] The invention relates to the technical field of food preservation, and in particular to a method for protecting the color of frozen lychees. Background Art

[0002] Litchi preservation technology is the key to the development of the lychee industry. Due to the particularity of lychees, the storage and transportation of lychees are more difficult than other fruits. On the one hand, lychee fruits have vigorous respiration and metabolism. The anaerobic respiration of the fruits after harvesting produces a large amount of alcohol, which causes the flavor to deteriorate and also causes rot, thus losing the edible value. On the other hand, the lychee peel and pulp contain a lot of oxidase. If the lychees are not protected from color and exposed to the air for a long time, they will easily turn brown within an hour, affecting the appearance of the product. In addition, lychees are prone to chilling damage under conventional low-temperature storage conditions, which shortens the shelf life. Therefore, color protection and preservation technology is of great significance to the preservation of lychees.

[0003] The emergence of frozen dormant lychees has greatly extended the shelf life of lychees and is a type of lychee preservation technology. Frozen dormant lychees refer to lychees that are quickly frozen in a sub-zero environment using a liquid with high thermal conductivity, forming uniform and dense ice crystals in the cells, which put the cells in a dormant state and prevent the lychees from turning brown and rotting during storage. Compared with traditional freezing technology, the shelf life of frozen dormant lychees is greatly extended, and the good lychee flavor is still preserved when thawed and eaten. However, the defect of frozen dormant lychees is that ice crystals are formed in the cells of lychees during freezing, causing irreversible damage to the lychee fruit. Uncolored frozen dormant lychees will be severely oxidized and browned within one hour after thawing. Therefore, frozen dormant lychees are more severely oxidized after thawing than lychees stored in ordinary refrigeration, and the browning reaction is more intense. The existing cold storage and color protection methods cannot solve the problem of severe browning of frozen dormant lychees during refrigeration and after thawing.

[0004] Existing lychee color protection technologies mainly include acid solution treatment, reducing gas treatment, low temperature preservation and other methods, among which acid solution treatment is relatively efficient and simple. CN102715469A discloses a processing method for dried lychees, which uses hot acid solution to passivate the oxidase of lychees, and protects the color of lychees while playing an antioxidant role. CN102018026A discloses a lychee color protection and preservation method and a lychee preservation treatment liquid, which uses organic acid and inorganic acid to reduce the pH of the peel to play a color protection role. Comprehensively reporting lychee color protection and preservation technologies, most of them focus on the application of organic acids and edible hydrochloric acid. Adding high-concentration acid solution to protect the color will cause the pulp to taste sour, which will damage the taste and flavor of lychees. In addition, there are also patents that combine micro-freezing technology to enhance the preservation effect, but the preservation objects of these methods are all freshly harvested lychees, and the preservation time is limited to about 40-60 days. In summary, the color preservation effect of the existing acid treatment is far inferior to that of frozen lychees, and it cannot have the same color preservation effect on frozen lychees, and it does not provide an effective solution to the browning problem of frozen lychees. At the same time, there are currently no reports on actual solutions for enhancing the color preservation effect of fruits through ultrasound. Summary of the invention

[0005] In view of the problem that frozen lychees turn brown quickly after thawing and affect the shelf life, the present invention provides a method for protecting the color of frozen lychees: fresh lychees are soaked in L-cysteine ​​color protection solution and ultrasonically treated. Cysteine ​​can react with quinone substances, the oxidative metabolites of lychees, thereby interrupting the browning reaction of lychees. At the same time, ultrasound is used to slightly damage the surface of the pericarp cell wall, increase the intercellular space, and accelerate the penetration of cysteine ​​in the lychees. Ultrasonic treatment can also enhance the mass transfer of L-cysteine, improve the color protection effect, and make the lychees uniformly color protected. Combined with cysteine ​​color protection solution treatment and ultrasonic treatment, the frozen lychees basically do not change color 3-4 hours after thawing, with excellent color protection effect and high safety. It can be used to protect the color of frozen lychees, improve their commercial value, and extend the shelf life.

[0006] A method for protecting the color of frozen litchi, characterized by comprising the following steps:

[0007] Choose lychees that are free of pests, diseases, and damage;

[0008] The litchi was immersed in a color-protecting solution containing L-cysteine ​​and subjected to ultrasonic treatment;

[0009] Place the lychees in a color-protecting solution containing L-cysteine;

[0010] The solution on the surface of the litchi is removed, the litchi is vacuum packed and frozen.

[0011] Preferably, the lychees are of similar size and shape, uniform color, free from insect pests, disease spots, and external mechanical damage. Litchi is preferred to ensure the quality of the raw materials and avoid affecting the preservation effect of the lychees.

[0012] Preferably, the weight of the litchi processed in a single time is less than or equal to 5 kg.

[0013] Furthermore, the optimal weight range of litchi for a single treatment is 3-3.5 kg, which can achieve sufficient color protection effect on litchi and avoid the problem of uneven penetration of color protection liquid.

[0014] Preferably, the L-cysteine ​​concentration of the color-protecting liquid is 1%-10%. Cysteine ​​is used as the color-protecting liquid, and its thiol group can interrupt the browning reaction of litchi and stably maintain the color of litchi fruit.

[0015] Furthermore, the optimal concentration range of the color protection liquid is 3%-5%, which can ensure the color protection effect of litchi while avoiding the waste of color protection liquid and maximizing the color protection efficiency.

[0016] Preferably, the mass ratio of the color protecting liquid to litchi is greater than 1.5:1, and the mass of the color protecting liquid is at least 1.5 times that of the litchi to ensure full contact between the litchi and the color protecting liquid, thereby avoiding uneven color protection.

[0017] Furthermore, the optimal mass ratio of the color protection liquid and litchi is 3:1, which can ensure the color protection effect of litchi while avoiding the waste of color protection liquid and maximizing the color protection efficiency.

[0018] Preferably, the ultrasonic treatment is to pour the litchi and the color protection solution into an ultrasonic cleaning instrument, set the power of the ultrasonic cleaning instrument to be greater than 100W, run the ultrasonic cleaning instrument for more than 1 minute, and start the ultrasonic cleaning instrument. Ultrasonic treatment of the litchi causes slight damage to the peel under the radiation and penetration of ultrasonic waves, increases the intercellular space of the peel, and is conducive to the penetration of the color protection solution.

[0019] Furthermore, the power of the ultrasonic treatment is 300-400W, and the ultrasonic treatment time is 4-5min. The parameters of the ultrasonic treatment can achieve a good penetration effect, which is more in line with the needs of reducing costs and increasing efficiency in production.

[0020] Preferably, the standing time is 1-15 minutes, and the litchi is placed in the color protection solution so that the color protection solution fully enters the litchi through the gaps between the pericarp cells and covers the entire litchi fruit, so as to obtain a good color protection effect.

[0021] Preferably, the total time of the ultrasonic treatment and standing is 1-25 min, and the ultrasonic treatment and standing are a continuous process, that is, the litchi should not be taken out after the ultrasonic treatment is completed, and the litchi should continue to be immersed in the color protection liquid to achieve a full color protection effect.

[0022] Preferably, the freezing treatment is to freeze the packaged lychees in an environment below -20°C.

[0023] Compared with the prior art, the beneficial effects of the present invention are specifically manifested as follows:

[0024] Anthocyanins in the cell sap are the main reason for the coloration of litchi peel. L-cysteine, as the main component of the color protection solution, contains more thiol functional groups, can maintain the stability of anthocyanins in cells, and has a strong anti-browning ability. L-cysteine ​​can not only react with quinone substances in litchi to form colorless compounds, interrupt the browning reaction, and thus maintain the color of litchi peel; it can also bind to copper ions on the active sites of enzymes to passivate the activity of oxidases such as peroxidase and polyphenol oxidase, accelerate the aging of free radicals, increase the content of ascorbic acid, 2,2-diphenyl-1-picryl-hydrazyl (DPPH) free radical scavenging activity, and the activity of antioxidants such as catalase and superoxide dismutase, reducing the oxidation rate of litchi when exposed to air. At the same time, L-cysteine ​​is one of the essential amino acids for the human body, and adding it to litchi will not affect its food safety; in addition, compared with the prior art of color protection through acid solution, the L-cysteine ​​added in this scheme will not change the flavor of litchi, which is more in line with market demand.

[0025] Soaking litchi in L-cysteine ​​color-protecting solution for a long time can also achieve a good color-protecting effect, but the soaking time is as long as several hours, the color-protecting efficiency is too low, and it is not suitable for large-scale production. Therefore, this scheme innovatively uses the cavitation effect of ultrasonic treatment to enhance the degree of energy transfer in the mass transfer process, causing slight damage to the tissue. Ultrasonic treatment of litchi fruit in color-protecting solution can enhance the transfer capacity of color-protecting solution and enhance the penetration and penetration ability of color-protecting solution; at the same time, it causes slight damage to the cell wall of litchi peel, enlarges the gap of the peel cell structure, and promotes the penetration of color-protecting solution, thereby enhancing the color-protecting effect, shortening the color-protecting treatment time, and improving the color-protecting efficiency.

[0026] Vacuum packaging effectively reduces the contact between litchi fruit and oxygen, slowing down the browning and oxidation process of litchi during frozen storage. It can also isolate external bacteria and ensure the food safety of litchi.

[0027] The present invention proposes for the first time a method for improving the color protection effect of litchi by combining color protection liquid treatment with ultrasonic treatment, and proposes an effective solution to the problem that frozen litchi quickly turns brown after thawing. The litchi fruit protected in color by the method can remain brown for 3-4 hours, while the litchi fruit without color protection turns brown severely within 1 hour. The method significantly extends the shelf life of litchi, is simple and convenient, and has good application prospects in litchi preservation. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0029] 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, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0030] In the attached figure:

[0031] Figure 1 : Test results of the color protection effect of color protection liquid with different L-cysteine ​​concentrations on frozen litchi.

[0032] Figure 2 : Test results of the color protection effect of frozen litchi under different standing times.

[0033] Figure 3 : Test results of color protection effect of frozen litchi under different ultrasonic power treatments.

[0034] Figure 4 : Test results of color protection effect of frozen litchi under different ultrasonic treatment time.

[0035] Figure 5 : Test results of color protection effect of different color protection treatments on frozen litchi.

[0036] Figure 6 : Flow chart of the color protection method for frozen lychees. DETAILED DESCRIPTION

[0037] 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 essence of the present invention, modifications or substitutions made to the methods, steps or conditions of the present invention all belong to the scope of protection of the present invention. Unless otherwise specified, the experimental materials, reagents, instruments, etc. used in the examples of the present invention are all commercially available; unless otherwise specified, all technical means in the examples of the present invention are conventional means well known to those skilled in the art.

[0038] Example 1

[0039] (1) Raw material selection: fresh lychees with uniform color, uniform size, free from pests and diseases, and free from mechanical damage were selected and washed with water. The lychees were divided into 5 groups, each group weighing 3.5 kg.

[0040] (2) Recording the initial color value: Place the colorimeter on the surface of the litchi peel, ensure that the measurement area is clean, flat, and non-reflective, and record the initial color value of each group of litchi peels.

[0041] (3) Preparation of color-protecting solution: The mass ratio of color-protecting solution to litchi is 3:1. 10.5 kg of color-protecting solution is required for each group of litchi. Color-protecting solutions with different L-cysteine ​​concentrations are prepared according to Table 1:

[0042] Table 1 Preparation method of color protection solution of different concentrations

[0043] Color protection solution concentration (%) L-Cysteine(g) Water (kg) 3.0 315.0 10.2 3.5 367.5 10.1 4.0 420.0 10.1 4.5 472.5 10.0 5.0 525.0 10.0

[0044] (4) Ultrasound: Set the ultrasonic power of the ultrasonic cleaner to 350 W and the ultrasonic time to 3 min. Pour the color protection solutions of different concentrations prepared in step (3) into different ultrasonic cleaners respectively. Put a group of lychees in each ultrasonic cleaner and immerse them completely in the color protection solution. Turn on the ultrasonic cleaner to perform ultrasound.

[0045] (5) Standing: After the ultrasonic treatment is completed, turn off the ultrasonic cleaning instrument and continue to place the litchi in the color protection solution for 12 minutes. The total time for ultrasonic treatment and standing is 15 minutes.

[0046] (6) Vacuum treatment: After standing, remove the litchi, drain the water, put the litchi into vacuum packaging and immediately evacuate the package.

[0047] (7) Frozen storage of lychees: The packaged lychees are frozen in liquid nitrogen and then stored at -20°C to obtain frozen lychees.

[0048] (8) Calculating the color difference ΔE: Take out the frozen lychees obtained in step (7), thaw the lychees at room temperature, and place a colorimeter on the surface of the lychee peel 3 hours after complete thawing, ensuring that the measurement area is clean, flat, and non-reflective, and record the color value of each group of lychee peels after treatment. Calculate the color difference ΔE using the formula:

[0049] Color difference ΔE = |initial color value of litchi peel - color value of litchi peel after treatment|

[0050] Depend on Figure 1 It can be seen that different letters indicate significant differences between different treatments (p < 0.05). When the L-cysteine ​​concentration in the color protection solution is 4.5%, the color difference of litchi before and after treatment is the smallest (ΔE < 10), indicating that this concentration of L-cysteine ​​has the best color protection effect. The effects of other concentrations of L-cysteine ​​color protection solutions are not significantly different.

[0051] Example 2

[0052] (1) Raw material selection: fresh lychees with uniform color, uniform size, free from pests and diseases, and free from mechanical damage were selected and washed with water. The lychees were divided into 5 groups, each group weighing 3.5 kg.

[0053] (2) Recording the initial color value: Place the colorimeter on the surface of the litchi peel, ensure that the measurement area is clean, flat, and non-reflective, and record the initial color value of each group of litchi peels.

[0054] (3) Preparation of color protection solution: The mass ratio of color protection solution to litchi is 3:1. For each group of litchi, 10.5 kg of color protection solution is required. 472.5 g of L-cysteine ​​crystals are completely dissolved in 10.0275 kg of water to prepare a 4.5% L-cysteine ​​color protection solution.

[0055] (4) Ultrasound: Set the ultrasonic power of the ultrasonic cleaner to 350 W and the ultrasonic time to 3 min. Pour all the color protection solution prepared in step (3) into the ultrasonic cleaner, completely immerse all the litchi in the color protection solution, and turn on the ultrasonic cleaner to perform ultrasound.

[0056] (5) Standing: After the ultrasonic treatment is completed, the ultrasonic cleaning instrument is turned off and the litchis are continued to be placed in the color protection solution for standing. A group of litchis are taken out at the 3rd, 6th, 9th, 12th and 15th minutes of standing. The total time of ultrasonic treatment and standing is 6, 9, 12, 15 and 18 minutes respectively.

[0057] (6) Vacuum treatment: After standing, remove the litchi, drain the water, put the litchi into vacuum packaging and immediately evacuate the package.

[0058] (7) Frozen storage of lychees: The packaged lychees are frozen in liquid nitrogen and then stored at -20°C to obtain frozen lychees.

[0059] (8) Calculating the color difference ΔE: Take out the frozen lychees obtained in step (7), thaw the lychees at room temperature, and place a colorimeter on the surface of the lychee peel 3 hours after complete thawing, ensuring that the measurement area is clean, flat, and non-reflective, and record the color value of each group of lychee peels after treatment. Calculate the color difference ΔE using the formula:

[0060] Color difference ΔE = |initial color value of litchi peel - color value of litchi peel after treatment|

[0061] Depend on Figure 2 It can be seen that different letters indicate significant differences between different treatments (p < 0.05). When the standing time is 12 minutes, the color difference of litchi before and after treatment is the smallest (ΔE < 12), indicating that the color protection effect of this standing time is the best compared with the treatment of standing for 3-9 minutes, and the difference is not significant compared with the treatment of standing for 15 minutes.

[0062] Example 3

[0063] (1) Raw material selection: fresh lychees with uniform color, uniform size, free from pests and diseases, and free from mechanical damage were selected and washed with water. The lychees were divided into 5 groups, each group weighing 3.5 kg.

[0064] (2) Recording the initial color value: Place the colorimeter on the surface of the litchi peel, ensure that the measurement area is clean, flat, and non-reflective, and record the initial color value of each group of litchi peels.

[0065] (3) Preparation of color protection solution: The mass ratio of color protection solution to litchi is 3:1. For each group of litchi, 10.5 kg of color protection solution is required. 472.5 g of L-cysteine ​​crystals are completely dissolved in 10.0275 kg of water to prepare a 4.5% L-cysteine ​​color protection solution.

[0066] (4) Ultrasound: Set the ultrasonic time of different ultrasonic cleaners to 3 min, and set the ultrasonic power to 200, 250, 300, 350, and 400 W, respectively. Pour all the color protection liquid prepared in step (3) into the ultrasonic cleaners. Place each group of litchi in different ultrasonic cleaners, completely immerse them in the color protection liquid, and turn on the ultrasonic cleaners for ultrasonic operation.

[0067] (5) Standing: After the ultrasonic treatment is completed, turn off the ultrasonic cleaning instrument and continue to place the litchi in the color protection solution for 12 minutes. The total time for ultrasonic treatment and standing is 15 minutes.

[0068] (6) Vacuum treatment: After standing, remove the litchi, drain the water, put the litchi into vacuum packaging and immediately evacuate the package.

[0069] (7) Frozen storage of lychees: The packaged lychees are frozen in liquid nitrogen and then stored at -20°C to obtain frozen lychees.

[0070] (8) Calculating the color difference ΔE: Take out the frozen lychees obtained in step (7), thaw the lychees at room temperature, and place a colorimeter on the surface of the lychee peel 3 hours after complete thawing, ensuring that the measurement area is clean, flat, and non-reflective, and record the color value of each group of lychee peels after treatment. Calculate the color difference ΔE using the formula:

[0071] Color difference ΔE = |initial color value of litchi peel - color value of litchi peel after treatment|

[0072] Depend on Figure 3 It can be seen that different letters indicate significant differences between different treatments (p < 0.05). When the ultrasonic power is 350W, the color difference of litchi before and after treatment is the smallest (ΔE < 9), indicating that this power has the best color protection effect compared with 250 and 300W ultrasonic treatments, and the effect is not significantly different compared with 200 and 400W ultrasonic treatments.

[0073] Example 4

[0074] (1) Raw material selection: fresh lychees with uniform color, uniform size, free from pests and diseases, and free from mechanical damage were selected and washed with water. The lychees were divided into 5 groups, each group weighing 3.5 kg.

[0075] (2) Recording the initial color value: Place the colorimeter on the surface of the litchi peel, ensure that the measurement area is clean, flat, and non-reflective, and record the initial color value of each group of litchi peels.

[0076] (3) Preparation of color protection solution: The mass ratio of color protection solution to litchi is 3:1. For each group of litchi, 10.5 kg of color protection solution is required. 472.5 g of L-cysteine ​​crystals are completely dissolved in 10.0275 kg of water to prepare a 4.5% L-cysteine ​​color protection solution.

[0077] (4) Ultrasound: Set the ultrasonic power of different ultrasonic cleaners to 350 W, and set the ultrasonic time to 1, 3, 5, 7, and 9 min, respectively. Pour all the color protection liquid prepared in step (3) into the ultrasonic cleaners. Place each group of litchi in different ultrasonic cleaners, completely immerse them in the color protection liquid, and turn on the ultrasonic cleaners to perform ultrasound.

[0078] (5) Standing: After the ultrasonic treatment, the ultrasonic cleaning instrument was turned off and the litchi was left standing in the color protection solution for 12 min. The total time for ultrasonic treatment and standing was 13, 15, 17, 19, and 21 min, respectively.

[0079] (6) Vacuum treatment: After standing, remove the litchi, drain the water, put the litchi into vacuum packaging and immediately evacuate the package.

[0080] (7) Frozen storage of lychees: The packaged lychees are frozen in liquid nitrogen and then stored at -20°C to obtain frozen lychees.

[0081] (8) Calculating the color difference ΔE: Take out the frozen lychees obtained in step (7), thaw the lychees at room temperature, and place a colorimeter on the surface of the lychee peel 3 hours after complete thawing, ensuring that the measurement area is clean, flat, and non-reflective, and record the color value of each group of lychee peels after treatment. Calculate the color difference ΔE using the formula:

[0082] Color difference ΔE = |initial color value of litchi peel - color value of litchi peel after treatment|

[0083] Depend on Figure 4 It can be seen that different letters indicate significant differences between different treatments (p < 0.05). When the ultrasonic time is 3 minutes, the color difference of litchi before and after treatment is the smallest, indicating that the color protection effect of this treatment time is the best (ΔE < 8), and the color protection effect of the ultrasonic time of 7-9 minutes is better than that of the treatment groups with ultrasonic time of 1 and 5 minutes.

[0084] Example 5

[0085] (1) Raw material selection: fresh lychees with uniform color, uniform size, free of pests and diseases, and free of mechanical damage were selected and washed with water. The lychees were divided into 4 groups, each group weighing 3.5 kg.

[0086] (2) Recording the initial color value: Place the colorimeter on the surface of the litchi peel, ensure that the measurement area is clean, flat, and non-reflective, and record the initial color value of each group of litchi peels.

[0087] (3) The litchis in each group were prepared with color protection solution, ultrasonicated, and left to stand according to Table 2. The specific steps were the same as those in Example 1.

[0088] Table 2 Related parameters of different litchi treatments

[0089]

[0090] (6) Vacuum treatment: After standing, remove the litchi, drain the water, put the litchi into vacuum packaging and immediately evacuate the package.

[0091] (7) Frozen storage of lychees: The packaged lychees are frozen in liquid nitrogen and then stored at -20°C to obtain frozen lychees.

[0092] (8) Calculating the color difference ΔE: Take out the frozen lychees obtained in step (7), thaw the lychees at room temperature, and place a colorimeter on the surface of the lychee peel 3 hours after complete thawing, ensuring that the measurement area is clean, flat, and non-reflective, and record the color value of each group of lychee peels after treatment. Calculate the color difference ΔE using the formula:

[0093] Color difference ΔE = |initial color value of litchi peel - color value of litchi peel after treatment|

[0094] It can be seen from Table 3 that the color retention rates of litchi treated differently are different. Among them, when the concentration of the color protection solution is 4.5%, the ultrasonic power is 350W, the ultrasonic time is 3min, and the standing time is 9min, the color difference of the litchi after color protection is the smallest, and the color retention rate is the best.

[0095] Table 3 Color retention rate of litchi with different treatments

[0096] Treatment Group 1 2 3 4 Color retention rate / % 91.37 85.10 82.80 76.28

[0097] Comparative Example 1

[0098] (1) Raw material selection: 3.5 kg of fresh lychees with uniform color, uniform size, free of pests and diseases, and without mechanical damage were selected and washed with water.

[0099] (2) Recording the initial color value: Place the colorimeter on the surface of the litchi peel, ensure that the measurement area is clean, flat, and non-reflective, and record the initial color value of the litchi peel.

[0100] (3) Preparation of color protection solution: The mass ratio of color protection solution to litchi is 3:1, and 10.5 kg of color protection solution is prepared. 472.5 g of L-cysteine ​​crystals are taken and completely dissolved in 10.0275 kg of water to prepare 4.5% L-cysteine ​​color protection solution.

[0101] (4) Standing: Soak the litchi completely in the color protection solution obtained in step (3) and let it stand for 15 minutes.

[0102] (5) Vacuum treatment: After standing, remove the litchi, drain the water, put the litchi into vacuum packaging and immediately evacuate the package.

[0103] (6) Frozen storage of litchi: The packaged litchi is frozen in liquid nitrogen and then stored in a -20°C environment to obtain frozen litchi.

[0104] (7) Calculating the color difference ΔE: Take out the frozen lychees obtained in step (7), thaw the lychees at room temperature, and place a colorimeter on the surface of the lychee peel 3 hours after complete thawing, ensuring that the measurement area is clean, flat, and non-reflective, and record the color value of the lychee peel after the color protection solution is treated alone. Calculate the color difference ΔE using the formula:

[0105] Color difference ΔE = |initial color value of litchi peel - color value of litchi peel after treatment|

[0106] Comparative Example 2

[0107] (1) Raw material selection: 3.5 kg of fresh lychees with uniform color, uniform size, free of pests and diseases, and without mechanical damage were selected and washed with water.

[0108] (2) Recording the initial color value: Place the colorimeter on the surface of the litchi peel, ensure that the measurement area is clean, flat, and non-reflective, and record the initial color value of the litchi peel.

[0109] (3) Ultrasound: Set the ultrasonic power of the ultrasonic cleaner to 350 W and the ultrasonic time to 3 min. Pour clean water into the ultrasonic cleaner, immerse the litchi completely in the clean water, and turn on the ultrasonic cleaner.

[0110] (4) Standing: After the ultrasonic treatment is completed, turn off the ultrasonic cleaning instrument and continue to place the litchi in the color protection solution for 12 minutes.

[0111] (5) Vacuum treatment: After standing, remove the litchi, drain the water, put the litchi into vacuum packaging and immediately evacuate the package.

[0112] (6) Frozen storage of lychees: The packaged lychees are frozen in liquid nitrogen and then stored in a -20°C environment to obtain frozen lychees.

[0113] (7) Calculation of color difference ΔE: Take out the frozen lychees obtained in step (7), thaw the lychees at room temperature, and place a colorimeter on the surface of the lychee peel 3 hours after complete thawing, ensuring that the measurement area is clean, flat, and non-reflective, and record the color value of the lychee peel after ultrasonic treatment alone. Calculate the color difference ΔE using the formula:

[0114] Color difference ΔE = |initial color value of litchi peel - color value of litchi peel after treatment|

[0115] Comparative Example 3

[0116] (1) Raw material selection: 3.5 kg of fresh lychees with uniform color, uniform size, free of pests and diseases, and without mechanical damage were selected and washed with water.

[0117] (2) Recording the initial color value: Place the colorimeter on the surface of the litchi peel, ensure that the measurement area is clean, flat, and non-reflective, and record the initial color value of the litchi peel.

[0118] (3) Vacuum treatment: Put the litchi into vacuum packaging and evacuate the package immediately.

[0119] (4) Frozen storage of litchi: The packaged litchi is frozen in liquid nitrogen and then stored in a -20°C environment to obtain frozen litchi.

[0120] (5) Calculation of color difference ΔE: Take out the frozen lychees obtained in step (7), thaw the lychees at room temperature, and place a colorimeter on the surface of the lychee peel 3 hours after complete thawing, ensuring that the measurement area is clean, flat, and non-reflective, and record the color value of the lychee peel after treatment. Calculate the color difference ΔE using the formula:

[0121] Color difference ΔE = |initial color value of litchi peel - color value of litchi peel after treatment|

[0122] Depend on Figure 5 It can be seen that different letters indicate significant differences between different treatments (p < 0.05). When ultrasonic treatment is combined with L-cysteine ​​color protection solution (Example 1, the concentration of color protection solution is 4.5%), the color difference value of litchi before and after treatment is the smallest (ΔE < 5), indicating that this method has the best color protection effect. Although the color difference values ​​of litchi treated with ultrasonic treatment alone (Comparative Example 2) and L-cysteine ​​color protection solution alone (Comparative Example 1) are significantly lower than those of the blank control (Comparative Example 3), the effect is poorer than that of ultrasonic treatment combined with L-cysteine ​​color protection solution (Example 1, the concentration of color protection solution is 4.5%).

[0123] In summary, ultrasonic treatment combined with L-cysteine ​​color protection liquid can significantly alleviate the color change problem of frozen lychees after thawing. Based on the results of all embodiments, 3.5 kg of lychees are treated each time, the concentration of L-cysteine ​​color protection liquid is set to 4.5%, the ultrasonic time is 3 min, the power is 350 W, and the standing time is 12 min, which is more in line with the needs of reducing costs and increasing efficiency in production, and can achieve better color protection effect.

[0124] It can be understood that the above embodiments only express the preferred implementation modes of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the patent scope of the present invention. It should be pointed out that, for ordinary technicians in this field, the above technical features can be freely combined without departing from the concept of the present invention, and several deformations and improvements can be made, which all belong to the protection scope of the present invention. Therefore, all equivalent changes and modifications made to the scope of the claims of the present invention should belong to the coverage of the claims of the present invention.

Claims

1. A method for protecting the color of frozen litchi, characterized in that: The following steps are involved: Choose lychees that are free of pests and damage; The litchi was immersed in a color-protecting solution containing L-cysteine ​​and subjected to ultrasonic treatment; Place the lychees in a color-protecting solution containing L-cysteine; The solution on the surface of the litchi is removed, the litchi is vacuum packed and frozen.

2. The color protection method according to claim 1, characterized in that: The weight of the litchi processed in a single time is less than or equal to 5 kg.

3. The color protection method according to claim 1, characterized in that: The L-cysteine ​​concentration of the color-protecting solution is 1%-10%.

4. The color protection method according to claim 3, characterized in that: The L-cysteine ​​concentration of the color-protecting solution is 3%-5%.

5. The color protection method according to claim 1, characterized in that: The mass ratio of the color protecting liquid to litchi is greater than 1.5:

1.

6. The color protection method according to claim 1, characterized in that: The ultrasonic treatment is to pour the lychee and the color-protecting liquid into an ultrasonic cleaning instrument, set the power of the ultrasonic cleaning instrument to be greater than 100W, run the ultrasonic cleaning instrument for more than 1 minute, and start the ultrasonic cleaning instrument.

7. The color protection method according to claim 6, characterized in that: The operation time of the ultrasonic cleaning instrument is 1-10 minutes.

8. The color protection method according to claim 1, characterized in that: The standing time is 1-15 min.

9. The color protection method according to claim 1, characterized in that: The total time of the ultrasonic treatment and standing is 1-25 min.

10. The color protection method according to claim 1, characterized in that: The frozen storage process is to place the packaged lychees in an environment below -20°C for freezing and storage.

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