Method for unfreezing litchis by coupling electrostatic field with water medium

Abstract

The invention relates to the technical field of thawing of frozen litchis, in particular to a method for thawing litchis by coupling an electrostatic field with a water medium. The method for unfreezing the litchis by coupling the electrostatic field with the water medium comprises the following steps: S100, bagging and freezing fresh litchis, and then freezing and storing the litchis in an ice house; s200, putting the frozen litchis into an electric field freezing and refrigerating box; s300, water is injected into the electric field freezing and refrigerating box to immerse the frozen litchis; and S400, an electrostatic field with the voltage of 5-15 kV is provided for the electric field freezing and refrigerating box, and the frozen litchis are unfrozen under the action of the electrostatic field. According to the method for unfreezing the litchis by coupling the electrostatic field with the water medium, the unfreezing time of the frozen sleep litchis can be shortened, and the unfreezing efficiency is improved; moreover, the juice loss and the nutrition loss are less, and the appearance, the taste and the flavor of the thawed litchis are closer to those of fresh litchis.

Description

Technical Field

[0001] This application relates to the technical field of thawing frozen lychees, and particularly to a method for thawing lychees by coupling an electrostatic field with an aqueous medium. Background Art

[0002] Fresh lychees have a unique taste and are rich in nutrients. They are representative fruits of tropical and subtropical regions. However, the harvesting time of fresh lychees is concentrated, and they are extremely perishable after harvesting. During storage and logistics transportation after harvesting, quality deterioration is likely to occur due to various factors, which poses a great challenge to the lychee industry. Therefore, for the lychee industry, preservation technologies that can quickly play a role are particularly crucial. Freezing preservation is one of the most effective technical means at present. It can not only significantly extend the shelf life of lychees, allowing consumers to taste fresh lychees throughout the year, but also greatly alleviate the sales pressure brought by concentrated listing and meet the demand for high-quality raw materials for annual production and processing.

[0003] However, the problem that lychees face after freezing is thawing. During the thawing process, problems such as juice and nutrient loss, softening of the taste, and decline in flavor may occur. These problems not only affect the sensory quality of lychees but also their commercial value. Currently, traditional thawing methods include air thawing, water immersion thawing, ultrasonic thawing, microwave thawing, etc. To a certain extent, these thawing methods can retain the original quality of frozen products, but these thawing means are often accompanied by problems such as juice loss, flavor deterioration, appearance browning, and long thawing time. Summary of the Invention

[0004] Based on this, this application provides a method for thawing lychees by coupling an electrostatic field with an aqueous medium, which can shorten the thawing time of frozen lychees, improve the thawing efficiency; and has less juice loss and less nutrient loss, and the appearance, taste, and flavor of the thawed lychees are closer to those of fresh lychees.

[0005] This application provides a method for thawing lychees by coupling an electrostatic field with an aqueous medium, including the following steps:

[0006] S100. Pack fresh lychees and freeze them, then store them in an ice storage;

[0007] S200. Place the frozen lychees in an electric field freezing and refrigerating box;

[0008] S300. Inject water into the electric field freezing and refrigerating box to submerge the frozen lychees;

[0009] S400. Provide an electrostatic field with a voltage of 5 - 15 kV to the electric field freezing and refrigerating box, and let the frozen lychees thaw under the action of the electrostatic field.

[0010] In one embodiment, in step S400, provide an electrostatic field with a voltage of 5 kV to the electric field freezing and refrigerating box, and let the frozen lychees thaw under the action of the electrostatic field.

[0011] In one embodiment, in step S400, an electrostatic field with a voltage of 10 kV is provided to the electric-field freezing and refrigerating box, and the frozen litchis are thawed under the action of the electrostatic field.

[0012] In one embodiment, in step S400, an electrostatic field with a voltage of 15 kV is provided to the electric-field freezing and refrigerating box, and the frozen litchis are thawed under the action of the electrostatic field.

[0013] In one embodiment, in step S400, when the frozen litchis are thawed under the action of the electrostatic field, the external ambient temperature is 22 - 28°C.

[0014] In one embodiment, after step S400, the following steps are further included:

[0015] During the thawing process of the frozen litchis, a thermocouple is inserted into the internal part of the sample litchi to detect the core temperature of the litchi sample in real time and record the thawing time. The thawing time is the time required for the core temperature of the litchi sample to rise from the initial temperature to the final thawing temperature. Herein, the initial temperature is -8 to -12°C, and the final thawing temperature is 2 to 6°C.

[0016] In one embodiment, the electrostatic field provided to the electric-field freezing and refrigerating box is a non-pulsed DC electrostatic field. The DC electrostatic field is uniformly distributed above and below the electrode plates, and the needle-shaped electrodes on the electrode plates have tips.

[0017] In one embodiment, the electrode plates are made of bare stainless steel material, and the distance from the electrode plates to the grounding end is 18 - 22 cm.

[0018] In one embodiment, in step S200, the bagged frozen litchis are placed in the electric-field freezing and refrigerating box and laid flat so that the distance between the frozen litchis and the electrode plates is 2 - 4 cm.

[0019] Compared with the prior art, in the electrostatic field thawing technology of this embodiment, during the thawing process of the hibernating litchis, the hibernating litchi samples are immersed in water, and an electrostatic field of 5 - 15 kV is applied to polarize and vibrate the water molecules in the hibernating litchi samples, accelerating the movement of water molecules and thus accelerating the melting of ice crystals. The temperature distribution of each part of the frozen objects in the thawing chamber is more uniform, thereby shortening the thawing time and reducing the common quality loss problem in traditional thawing; thawing with water as the medium can evenly cover the temperature around the frozen products, with higher heat transfer efficiency, and can avoid the phenomenon that the outer layer of the frozen products melts while the inside is still in a frozen state; in addition, electrostatic field thawing does not generate excessive heat, and can effectively avoid the degradation of heat-sensitive nutrients and the deterioration of flavor. Description of the Drawings

[0020] Figure 1In an embodiment of the present application, it is a flowchart of a method for thawing litchi by electrostatic field coupling with water medium.

[0021] Figure 2 In an embodiment of the present application, it is a test result graph of the litchi thawing curve.

[0022] Figure 3 In an embodiment of the present application, it is a test result graph of the appearance and BI of litchi.

[0023] Figure 4 In an embodiment of the present application, it is a test result graph of the weight loss rate of thawed litchi.

[0024] Figure 5 In an embodiment of the present application, it is a test result graph of the electronic nose radar map and PCA analysis of thawed litchi. Detailed implementation manners

[0025] Here, the exemplary embodiments will be described in detail, and the examples are shown in the drawings. When the following description refers to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. Moreover, the implementation manners described in the following exemplary embodiments do not limit the present invention, and any structural, method, or functional transformation made by those of ordinary skill in the art based on these implementation manners is included in the protection scope of the present invention.

[0026] The terms used in the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The singular forms "a", "the", and "said" used in the present invention and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term "and / or" used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0027] It should be understood that although the terms first, second, third, etc. may be used in the present invention to describe various information, such information should not be limited to these terms. These terms are only used to distinguish the same type of information from each other. For example, without departing from the scope of the present invention, the first information may also be referred to as the second information, and similarly, the second information may also be referred to as the first information. Depending on the context, the word "if" as used herein may be interpreted as "when" or "while" or "in response to a determination".

[0028] In view of the technical problems mentioned in the background art, the embodiment of the present application provides a method for thawing litchi by electrostatic field coupling with water medium. This method submerges the frozen litchi with water as the heat transfer medium and combines electrostatic field assistance for thawing, which can shorten the thawing time of the frozen litchi, improve the thawing efficiency; and has less juice loss and less nutrient loss, and the appearance and taste of the thawed litchi are closer to those of fresh litchi.

[0029] The following is a more specific description of a method for thawing litchi with an electrostatic field-coupled water medium according to an embodiment of the present invention, but it should not be limited thereto.

[0030] As Figure 1 shown, Figure 1 is a flowchart of a method for thawing litchi with an electrostatic field-coupled water medium according to an exemplary embodiment of the present application. A method for thawing litchi with an electrostatic field-coupled water medium provided by an embodiment of the present application includes the following steps:

[0031] S100: Pack fresh litchi in bags, freeze them, and store them in an ice storage.

[0032] S200: Place the frozen and stored litchi in an electric field freezer-refrigerator.

[0033] S300: Inject water into the electric field freezer-refrigerator to submerge the frozen and stored litchi.

[0034] S400: Provide an electrostatic field with a voltage of 5 - 15 kV to the electric field freezer-refrigerator, and let the frozen and stored litchi thaw under the action of the electrostatic field.

[0035] In the electrostatic field thawing technology of this embodiment, during the thawing process of hibernating litchi, the hibernating litchi samples are submerged in water, and an electrostatic field of 5 - 15 kV is applied for assisted thawing. Thawing with water as the medium can evenly cover the frozen product with temperature, and the heat transfer efficiency is higher, which can avoid the phenomenon that the outer layer of the frozen product melts while the inside is still in a frozen state; providing an electrostatic field for assisted thawing can polarize and vibrate the water molecules in the hibernating litchi samples, accelerate the movement of water molecules, thereby accelerating the melting of ice crystals, making the temperature distribution of each part of the frozen object in the thawing warehouse more uniform, thus shortening the thawing time and reducing the common quality loss problems in traditional thawing. Moreover, electrostatic field thawing does not generate too much heat, and can effectively avoid the degradation of thermosensitive nutrients and the deterioration of flavor.

[0036] Using the method for thawing litchi with an electrostatic field-coupled water medium according to the embodiment of the present application has the following advantages: (1) Compared with air thawing without using an electrostatic field, the quality of the thawed samples is improved by more than 430%; (2) The thawing time of the samples is greatly shortened, reducing by more than 270% compared with air thawing without using an electrostatic field; (3) The appearance and taste flavor of the samples after thawing are closer to fresh samples; (4) The juice loss rate of the samples after thawing is reduced, and the nutrient retention rate is increased by more than 10% compared with the thawing group using air as the medium without using an electrostatic field.

[0037] In this embodiment, the electrostatic field provided to the electric-field freezing and refrigerating box is a non-pulsed DC electrostatic field. The DC electrostatic field is uniformly distributed above and below the electrode plates, and the needle-shaped electrodes on the electrode plates have tips. The electrode plates are made of bare stainless steel, and the distance from the electrode plates to the grounding end is any value between 18 and 22 cm, preferably 20 cm.

[0038] In step S200, the bagged frozen litchis are placed in the electric-field freezing and refrigerating box and laid flat so that the distance between the frozen litchis and the electrode plates is any distance between 2 and 4 cm, preferably 3 cm.

[0039] In step 300, a container filled with an appropriate amount of water can also be used. The bagged frozen litchis are placed in the container and laid flat, and the water submerges the bagged frozen litchis. Then the container is placed in the thawing cavity of the electric-field freezing and refrigerating box, and the distance between the electrodes and the sample is 3 cm.

[0040] In step S400, an electrostatic field with a voltage of 5 kV, or 10 kV, or 15 kV, and any intensity between 5 and 15 kV is provided to the electric-field freezing and refrigerating box, and the frozen litchis are thawed under the action of the electrostatic field. Optionally, when the frozen litchis are thawed under the action of the electrostatic field, the ambient external temperature is any temperature between 22 and 28 °C, preferably the ambient external temperature is 25 °C. In this embodiment, providing an electrostatic field with a voltage of 5 to 15 kV to the electric-field freezing and refrigerating box means providing an electric field strength of 1.5 to 5 kV / cm to the electric-field freezing and refrigerating box.

[0041] Optionally, after step S400, the following steps are further included: During the thawing process of the frozen litchis, a thermocouple is inserted into the internal part of the sample litchis to detect the core temperature of the litchi sample in real time and record the thawing time. The thawing time is the time required for the core temperature of the litchi sample to rise from the initial temperature to the final thawing temperature, where the initial temperature is any temperature between -8 and -12 °C, and the final thawing temperature is any temperature between 2 and 6 °C. Preferably, the initial temperature can be -10 °C, and the final thawing temperature is 4 °C.

[0042] Since the thermocouple needs to be connected to the outside by wires, preferably, the litchi sample used to measure the internal temperature is packaged in a vacuum packaging different from other litchis, which can enable the wires of the thermocouple to extend out of the packaging bag and be connected to the outside.

[0043] The following describes in detail a method for thawing litchis by coupling water medium with an electrostatic field according to the present application through several specific embodiments.

[0044] In the following examples, the litchi variety used was Jingganghongnuo litchi, picked from a commercial orchard, with a maturity of 8 to 9 degrees. Litchis of uniform size without pests or diseases were selected, and fresh litchis were bagged, about 240 grams per bag, and then frozen to obtain frozen products, which were stored in an ice storage. At an external environmental temperature of 25°C, the bagged frozen litchis were placed flat in an electric field freezing and refrigerating box, with the frozen litchis 3 centimeters away from the electrode plate, and the frozen litchis were thawed under the action of an electrostatic field. After thawing, the samples were taken out of the electric field freezing and refrigerating box for standby.

[0045] Example 1

[0046] The heat transfer medium was air, and an electrostatic field with a voltage of 5 kV was provided to the electric field freezing and refrigerating box for thawing. During the thawing process, a T-type thermocouple was inserted into the geometric center of the sample and connected to a data recorder to monitor the core temperature of the litchi during thawing. The data acquisition instrument recorded the time data required for the core temperature of the sample to change from the initial temperature (-10°C) to the final thawing temperature (4°C) every 1 second.

[0047] Example 2

[0048] The heat transfer medium was air, and an electrostatic field with a voltage of 10 kV was provided to the electric field freezing and refrigerating box for thawing. During the thawing process, a T-type thermocouple was inserted into the geometric center of the sample and connected to a data recorder to monitor the core temperature of the litchi during thawing. The data acquisition instrument recorded the time data required for the core temperature of the sample to change from the initial temperature (-10°C) to the final thawing temperature (4°C) every 1 second.

[0049] Example 3

[0050] The heat transfer medium was air, and an electrostatic field with a voltage of 15 kV was provided to the electric field freezing and refrigerating box for thawing. During the thawing process, a T-type thermocouple was inserted into the geometric center of the sample and connected to a data recorder to monitor the core temperature of the litchi during thawing. The data acquisition instrument recorded the time data required for the core temperature of the sample to change from the initial temperature (-10°C) to the final thawing temperature (4°C) every 1 second.

[0051] Example 4

[0052] The heat transfer medium was water. Water was injected into the electric field freezing and refrigerating box to submerge the frozen litchis, and an electrostatic field with a voltage of 5 kV was provided to the electric field freezing and refrigerating box for thawing. During the thawing process, a T-type thermocouple was inserted into the geometric center of the sample and connected to a data recorder to monitor the core temperature of the litchi during thawing. The data acquisition instrument recorded the time data required for the core temperature of the sample to change from the initial temperature (-10°C) to the final thawing temperature (4°C) every 1 second.

[0053] Example 5

[0054] The heat transfer medium is water. Water is injected into the electric field freezer-refrigerator to submerge the frozen litchis, and an electrostatic field with a voltage of 10 kV is applied to the electric field freezer-refrigerator for thawing. During the thawing process, a T-type thermocouple is inserted into the geometric center of the sample and connected to a data logger to monitor the core temperature of the litchis during thawing. The data acquisition instrument records the time data required for the core temperature of the sample to reach the final thawing temperature (4 °C) from the initial temperature (-10 °C) every 1 second.

[0055] Example 6

[0056] The heat transfer medium is water. Water is injected into the electric field freezer-refrigerator to submerge the frozen litchis, and an electrostatic field with a voltage of 10 kV is applied to the electric field freezer-refrigerator for thawing. During the thawing process, a T-type thermocouple is inserted into the geometric center of the sample and connected to a data logger to monitor the core temperature of the litchis during thawing. The data acquisition instrument records the time data required for the core temperature of the sample to reach the final thawing temperature (4 °C) from the initial temperature (-10 °C) every 1 second.

[0057] Comparative Example 1: Freshly picked litchis.

[0058] Comparative Example 2: A thawing method for fruits and vegetables, used for thawing litchis. The thawing method is the same as that in Examples 1, 2, and 3, except that the electric field strength in Comparative Example 1 is 0 kV.

[0059] Comparative Example 3: A thawing method for fruits and vegetables, used for thawing litchis. The thawing method is the same as that in Examples 4, 5, and 6, except that the electric field strength in Comparative Example 1 is 0 kV.

[0060] Test Example 1: Observation of thawing curve and thawing efficiency.

[0061] Test method: The time required from the initial temperature (-10 °C) to the final thawing temperature (4 °C) is measured and recorded by a T-type thermocouple and a temperature recorder. The formula for the freezing rate (Freezing Rate, k) is Formula 1: where k is the thawing rate (°C / min), Q is the thawing time (s), Q0 is the initial temperature of the litchis (°C), and Qt is the final temperature of the litchis at the end of thawing (°C).

[0062] Test results: See Figure 2 and Table 1 below.

[0063] In Table 1, different lowercase letters in the same row indicate significant differences (P < 0.05).

[0064]

[0065] Table 1

[0066] Result analysis: Combining Figure 2From Table 1, it can be seen that the thawing rate in the examples is significantly increased by 1.5 to 3.2 times. The thawing rate of the 1-3 groups of the examples is 3.2 times that of the 2 groups of the comparative examples, and the thawing time is shortened by 68.75%. However, the thawing efficiency of the 2 group and the 3 group of the examples is only about 2.5 times that of the 2 group of the comparative examples. When the electric field strength reaches 10 kV in the 4-6 groups of the examples, the thawing rate is significantly increased to 2.5 times that of the 3 group of the comparative examples. The time for raising the core temperature of litchi from -10 °C to 0 °C is shortened from 139 min to 56 min, and the total thawing time is significantly shortened by 59.83%. However, the thawing rates of the 4 group and the 6 group of the examples decrease to 1.77 and 1.47 times that of the 3 group of the comparative examples. It shows that when air is used as the heat transfer medium, the thawing rate is the highest when the electric field strength is 5 kV; when water is used as the heat transfer medium, the electric field strength of 10 kV has the most obvious effect. When the voltage reaches 15 kV, the thawing rate is significantly reduced. Further analysis reveals that the thawing efficiency of the 2 group of the comparative examples is about 2.13 times that of the 3 group of the comparative examples, and the average thawing rate of the 4-6 groups of the examples is about 44.14% higher than that of the 1-3 groups of the examples, indicating that there are obvious advantages in thawing litchi with water as the medium.

[0067] Test Example 2: Appearance and Color Difference

[0068] Test method: Use an UltraScan VIS automatic color difference meter (reflection mode) to measure the color of litchi, and the color difference results are expressed in L*, a*, b* and Browning Index (BI).

[0069] The calculation of BI is shown in Formula 2 and Formula 3. Formula 2 is: Formula 3 is:

[0070] Test results: See Figure 3 and Table 2 below. In Table 2, different lowercase letters in the same column indicate significant differences (P < 0.05).

[0071]

[0072] Table 2

[0073] Result analysis:

[0074] Combined with Table 2 and Figure 3Looking at the sensory color of lychees, it was found that the color change trends of the thawed lychees in Example Groups 4-6 and Example Groups 1-3 were similar, and the differences were not obvious. However, severe browning occurred in Comparative Example 2 and Comparative Example 3 groups, and obvious brown spots appeared on the surface of the lychees. Example 4, Example 5 and Example 1 showed the lowest degree of browning and presented an attractive bright red color. After thawing, the L* and a* values of the lychees increased significantly with the increase of voltage, and Example Groups 4-6 were significantly higher than Example Groups 1-3. The b* value showed a trend of first decreasing and then increasing with the increase of voltage. Among them, the yellowness of Example 1, Example 4 groups and Example 2, Example 5 groups was about 54% lower than that of their Comparative Example 2 groups and Comparative Example 3 groups. The BI of Example 1, Example 4 groups and Example 2, Example 5 groups decreased significantly, and decreased by about 62% compared with Comparative Example 2 and the Comparative Example groups. At the same voltage, the ΔE of Comparative Example 3 group was significantly higher than that of Comparative Example 2 group, and there was no significant difference in the ΔE values between Example Groups 1-3 and Example 4-6; at different voltages, the ΔE values of each group were: Example 1, 4 groups < Example 2, 5 groups < Example 3, 6 groups < Comparative Example 2, 3 groups. It was obvious that the ΔE values of Example 1, 4 groups were 14% - 50% higher than those of other groups, indicating that electrostatic field-assisted thawing had obvious advantages in maintaining the color of lychees. It should be noted that the ΔE values of Example Groups 1-3 decreased by 15% - 49% compared with Comparative Example 2 group, and Example Groups 4-6 decreased by 19% - 66% compared with Comparative Example 3 group, indicating that with water as the medium, the color protection effect was more significant under the assistance of an electric field. The ΔE value of Example 4 group was the lowest, making the color of its lychees closest to that of fresh lychees. Generally speaking, in terms of the color protection effect of lychee thawing, water as the heat transfer medium and a voltage of 5 kV were relatively the most suitable.

[0075] Test Example 3: Texture and weight loss rate

[0076] Testing method:

[0077] The texture of lychee samples after different thawing treatments was evaluated by a texture analyzer (TA.XT Plus). The probe model was P / 50, the compression rate was 30%, the pre-test speed was 1.60 mm / s, the test speed was 0.80 mm / s, the post-test speed was 2.00 mm / s, the trigger force was 5 g, and the time was 10 s. All samples were measured at least 6 times each. The masses of the samples before and after thawing were accurately weighed and recorded as g1 and g2 respectively.

[0078] The calculation formula for the thawing loss rate (Thawing loss rat, r) is as follows:

[0079] Test results: See Table 3 below and Figure 4 .

[0080] In Table 3, different lowercase letters in the same column indicate significant differences (P < 0.05).

[0081]

[0082] Table 3

[0083] Result analysis:

[0084] Combined with Table 3 and Figure 4 it can be seen that compared with fresh litchis, the hardness, adhesiveness, and chewiness of litchis thawed naturally (in Comparative Example 2 group and Comparative Example 3 group) decreased by about 50%, 30%, and 33% respectively, and there were no significant changes in elasticity, cohesiveness, and resilience, indicating that the sensory taste became worse. The hardness, adhesiveness, and chewiness of the groups of Examples 1 - 3 increased by 13% - 68%, 2% - 57%, and -2% - 60% respectively compared with those of Comparative Example 2 group, and the hardness, adhesiveness, and chewiness of the groups of Examples 4 - 6 increased by 8% - 35%, 5% - 31%, and 8% - 39% respectively compared with those of Comparative Example 3 group, indicating that the effect of electric field-assisted thawing was significant. At the same voltage, the hardness, adhesiveness, and chewiness of Comparative Example 3 group were significantly higher than those of Comparative Example 2 group, and similar trends were shown in the groups of Examples 1 - 3 and the groups of Examples 4 - 6 at 5kV, 10kV, and 15kV; the texture of Example 6 group was closest to that of fresh litchis and there was no significant difference from that of Comparative Example 1 group. It shows that litchis thawed by electric field-assisted (15kV) using water as the heat transfer medium can better retain the texture characteristics of thawed litchis.

[0085] In addition, the weight loss rate of Comparative Example 3 group decreased by about 21% compared with that of Comparative Example 2 group, and the weight loss rate of Example 4 group decreased by about 30% compared with that of Example 1 group. With the increase of voltage, the hardness, adhesiveness, and chewiness of thawed litchis gradually increased, while the weight loss rate gradually decreased. When the voltage rose to 10kV and 15kV, there was no significant difference between the groups of Examples 1 - 3 and the groups of Examples 4 - 6, indicating that when the electric field was higher than 10kV, the effect of combining the electric field with the water medium to protect thawed litchis would be weakened. The weight loss rates of the 10kV and 15kV groups were the lowest, only about 30% of that of the 0kV group. Generally speaking, using electric field (10kV, 15kV) to assist thawing and water as the heat transfer medium can significantly improve the texture characteristics of litchis and help them maintain a delicate and crispy taste closer to the fresh state.

[0086] Test Example 4: Flavor (electronic nose)

[0087] Test method: Weigh 5g of ground litchi puree and put it into a 50mL special bottle and seal it. Immerse the bottle in a 45°C water bath for 30min to fully volatilize the aroma of litchis. The electronic nose adopts the dynamic headspace sampling method, and the injection flow rate of the sample gas is 0.7L / min. The detection time for each sample is 120 seconds, wait for 10 seconds before measuring the next sample, and each sample is tested three times repeatedly.

[0088] Test results: See Figure 5

[0089] Result analysis: An electronic nose is an electronic system that is selective for different molecules by mimicking the structure of the human nose. It is usually connected to 5 to 100 sensitive sensors and can distinguish subtle changes in volatile compounds. From Figure 5 it can be seen that the relatively odor-responsive sensors are W5S (short-chain alkane aromatic components), W2W (aromatic components, sensitive to organic sulfides), and W1W (sensitive to sulfides). The response values of these three sensors gradually increase with the increase of voltage. Under the same voltage, the response values of Examples 4-6 are higher, and the response value of the Example 4 group is closest to that of Comparative Example 1.

[0090] Principal Components Analysis (PCA) was performed on the volatile odors of litchi pulp after thawing based on the response values of the electronic nose sensors to more intuitively compare the flavor differences between fresh litchi and the litchi in Example Groups 1-3 and Example Groups 4-6. As Figure 5 shown, PC1 and PC2 contributed 94.7% and 3% of the total variance respectively, indicating that PCA can reflect all the characteristics of fresh litchi and thawed litchi (Example Groups 4-6 and Example Groups 1-3). The differences among the three groups are mainly reflected in PC1. The response values of Example Groups 4-6 are closer to the fresh group in the PC1 direction than those of Example Groups 1-3, further illustrating the advantage of thawing with water as the medium in reducing flavor deterioration. Among Example Groups 4-6, Example 4 is closest to the fresh group, indicating that the Example 4 group is closest to Comparative Example 1 in the electronic nose system.

[0091] In summary, the present invention compared litchi thawed with air as the medium (Examples 1-3 and Comparative Example 2) and water as the medium (Examples 3-6 and Comparative Example 3) in an electrostatic field, measured the thawing efficiency, thawing weight loss rate, color difference, texture, and flavor of the comparative examples and thawed litchi, explored the litchi thawing law, and concluded that it is more suitable to thaw litchi with water as the heat transfer medium, which can shorten the thawing time by more than 50% compared with thawing litchi with air as the medium, and the sensory quality is improved by 10%-50% (appearance, color difference, texture). It was also found by electronic nose analysis that the flavor of Example Groups 3-6 is closer to that of Comparative Example 1.

[0092] The more suitable voltage for litchi thawing is 5-10 kV. For litchi thawed with water as the medium (Examples 4 and 5) at these two voltages, the thawing efficiency is significantly higher than that of other groups, and their appearance is closest to that of fresh litchi and the browning degree is the lowest (the △E values are 7.99 and 12.04 respectively, and the BI values are 87.47 and 129.42), and the texture and flavor are well maintained.

[0093] Taking all factors into consideration, it is relatively appropriate to thaw fruits and vegetables such as litchi with water medium supplemented with a static voltage of 5 - 10 kV.

[0094] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0095] The above-described embodiments only represent several implementation manners of the present application. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several modifications and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the appended claims.

Claims

1. A method for thawing litchi in an electrostatic field-coupled water medium, characterized in that, It includes the following steps: S100: Pack fresh lychees in bags, freeze them, and store them in a freezer; S200: Place the frozen lychees in an electric field freezer-refrigerator; S300: Inject water into the electric field freezer-refrigerator to submerge the frozen lychees; S400: Provide an electrostatic field with a voltage of 5 - 15 kV to the electric field freezer-refrigerator, and let the frozen lychees thaw under the action of the electrostatic field.

2. The method for thawing lychees by electrostatic field coupling with water medium according to claim 1, wherein in step S400, provide an electrostatic field with a voltage of 5 kV to the electric field freezer-refrigerator, and let the frozen lychees thaw under the action of the electrostatic field.

3. The method for thawing lychees by electrostatic field coupling with water medium according to claim 1, wherein in step S400, provide an electrostatic field with a voltage of 10 kV to the electric field freezer-refrigerator, and let the frozen lychees thaw under the action of the electrostatic field.

4. The method for thawing lychees by electrostatic field coupling with water medium according to claim 1, wherein in step S400, provide an electrostatic field with a voltage of 15 kV to the electric field freezer-refrigerator, and let the frozen lychees thaw under the action of the electrostatic field.

5. The method for thawing litchi with water medium by electrostatic field coupling according to any one of claims 1 to 4, characterized in that, In step S400, when the frozen lychees thaw under the action of the electrostatic field, the external ambient temperature is 22 - 28 °C.

6. The method for thawing lychees by electrostatic field coupling with water medium according to any one of claims 1 - 4, wherein after step S400, the following steps are further included: During the thawing process of the frozen lychees, insert a thermocouple into the internal part of the sample lychees, detect the core temperature of the lychee samples in real time and record the thawing time. The thawing time is: the time required for the core temperature of the lychee samples to rise from the initial temperature to the final thawing temperature, wherein the initial temperature is -8 - -12 °C and the final thawing temperature is 2 - 6 °C.

7. The method for thawing lychees by electrostatic field coupling with water medium according to any one of claims 1 - 4, wherein the electrostatic field provided to the electric field freezer-refrigerator is a non-pulsed DC electrostatic field, and the DC electrostatic field is evenly distributed above and below the electrode plates. The needle-shaped electrodes on the electrode plates have tips.

8. The method for thawing lychees by electrostatic field coupling with water medium according to claim 7, wherein the electrode plates are made of bare stainless steel, and the distance from the electrode plates to the ground terminal is 18 - 22 cm.

9. The method for thawing lychees by electrostatic field coupling with water medium according to claim 7, wherein in step S200, place the bagged frozen lychees in the electric field freezer-refrigerator, and lay them flat so that the distance between the frozen lychees and the electrode plates is 2 - 4 cm.

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