Ultrasonic-assisted freeze-thaw fruit preservation processing method suitable for litchi

Abstract

The application relates to the technical field of fruit preservation processing, and discloses an ultrasonic-assisted freeze-thaw fruit preservation processing method suitable for litchi. The method comprises the following steps: packaging litchi samples and precooling treatment; placing the precooled litchi samples into a quick-freezing bin, immersing in a freezing liquid, starting ultrasonic-assisted immersion quick-freezing treatment, stopping the ultrasonic treatment when the maximum ice crystal generation zone is passed, and continuing the immersion quick-freezing until the sample temperature is reduced to the freezing endpoint temperature; transferring the frozen litchi samples to a freezer for storage; taking out the frozen litchi samples and placing them in a thawing bin, immersing in a thawing liquid, starting ultrasonic-assisted immersion thawing treatment, and stopping the ultrasonic treatment when the frozen litchi sample temperature is increased to the thawing endpoint temperature; and taking out the thawed litchi samples and placing them in a refrigeration chamber. Through the synergistic treatment of ultrasonic-assisted immersion quick-freezing and thawing, the cell damage in the freeze-thaw process of the litchi is effectively reduced, and the preservation quality of the litchi is improved.

Description

Technical Field

[0001] This application relates to the field of fruit preservation and processing technology, and in particular to an ultrasonic-assisted freeze-thaw fruit preservation and processing method adapted for lychee. Background Technology

[0002] Lychee is a tropical and subtropical fruit with white, translucent flesh and a sweet, fragrant flavor. However, it experiences vigorous respiration after harvest, its peel is prone to browning, and the fruit is extremely susceptible to dehydration and rotting, resulting in a very short shelf life at room temperature. Freezing is one of the effective methods to extend the supply period of lychee, inhibiting microbial growth and enzymatic reactions, and maintaining the basic quality of the flesh. However, the size and distribution of ice crystals are difficult to control during conventional freezing. Large ice crystals can pierce cell walls and cell membranes, leading to significant juice loss, soft and mushy flesh, and deteriorated flavor after thawing, resulting in a significant decline in quality. To address these issues, researchers have attempted to improve ice crystal morphology using new technologies such as immersion freezing and ultrasound-assisted freezing. Immersion freezing involves direct contact between the food and a low-temperature liquid with a high thermal conductivity, resulting in high heat exchange efficiency and rapid freezing. The cavitation effect and microjets generated by ultrasound can induce the formation of numerous crystal nuclei within the supercooled liquid, promoting the formation of small and uniform ice crystals.

[0003] However, the existing technologies mentioned above only explore the freezing process and do not conduct in-depth research on the entire freeze-thaw process. They fail to achieve systematic control over the quality of the entire freeze-thaw process. In particular, how to avoid secondary crystallization, reduce juice loss, and ensure the integrity of the fruit pulp during the thawing stage has become a key bottleneck restricting the commercialization of frozen storage of lychees.

[0004] Therefore, there is an urgent need to develop a freeze-thaw preservation method that can effectively inhibit ice crystal growth, reduce thawing losses, and preserve the original flavor and nutrients of lychees. Summary of the Invention

[0005] To address the aforementioned technical issues, this application provides an ultrasonic-assisted freeze-thaw fruit preservation processing method adapted for lychees. This method systematically optimizes the entire freeze-thaw process for lychees, simultaneously improving both freezing and thawing efficiency while maximizing the preservation of the original quality of the lychees.

[0006] This application provides an ultrasonic-assisted freeze-thaw fruit preservation processing method adapted for lychee, comprising the following steps: S1. Pack the freshly picked lychees and pre-cool them; S2. Place the pre-cooled lychee sample into the quick-freezing chamber, immerse it in the freezing liquid, turn on the ultrasound to perform ultrasonic-assisted quick-freezing treatment on the lychee sample until the temperature of the lychee sample passes through the maximum ice crystal formation zone, then turn off the ultrasound treatment and continue quick-freezing until the sample temperature drops to the freezing endpoint temperature. S3. After freezing, transfer the litchi samples to a cold storage room for storage. The temperature difference between the cold storage temperature and the freezing endpoint temperature shall be less than or equal to ±1℃. S4. After taking out the frozen lychee sample, place it in the thawing chamber and immerse it in thawing solution. Turn on the ultrasound to perform ultrasound-assisted thawing treatment on the frozen lychee sample until the temperature of the frozen lychee sample rises to the thawing endpoint temperature and then turn off the ultrasound treatment. S5. After thawing, remove the lychee samples from the thawing chamber and place them in the refrigerator.

[0007] Optionally, the coupled thawing rate of the ultrasound-assisted impregnation thawing process is matched with the freezing rate of the ultrasound-assisted impregnation quick-freezing process.

[0008] Optionally, the freezing rate of the ultrasonic-assisted impregnation quick-freezing treatment is 0.65–1.0 °C / min, and the coupled thawing rate of the ultrasonic-assisted impregnation thawing treatment is 0.65–1.0 °C / min, and the absolute value of the difference between the coupled thawing rate and the freezing rate is less than or equal to 0.35 °C / min, or, The freezing rate of the ultrasonic-assisted impregnation quick-freezing treatment is 0.4–0.6 °C / min, the coupled thawing rate of the ultrasonic-assisted impregnation thawing treatment is 0.4–0.6 °C / min, and the absolute value of the difference between the coupled thawing rate and the freezing rate is less than or equal to 0.2 °C / min.

[0009] Optionally, in step S2, after the ultrasound is turned on, it is intermittently turned on and off according to a first preset time interval, the first preset time interval being 10–50 seconds; and / or, In step S4, after the ultrasound is turned on, it is turned on and off intermittently according to a second preset time interval, which is 10 to 50 seconds.

[0010] Optionally, in step S2, the temperature of the cryogenic fluid is -25 to -40°C, the ultrasonic treatment frequency is 20 to 80 kHz, and the ultrasonic treatment power is 200 to 1200 W.

[0011] Optionally, the temperature of the cryogenic fluid is -30°C, the ultrasonic treatment frequency is 28kHz, and the ultrasonic treatment power is 632W.

[0012] Optionally, in step S4, the temperature of the thawing solution is 18–25°C, the ultrasonic treatment frequency is 20–80 kHz, and the ultrasonic treatment power is 300–1200 W.

[0013] Optionally, the temperature of the thawing fluid is 20°C, the ultrasonic treatment frequency is 28kHz, and the ultrasonic treatment power is 632W.

[0014] Optionally, the temperature difference between the cold storage temperature and the freezing endpoint temperature is less than or equal to ±1℃; the thawing endpoint temperature of the frozen lychees is 2 to 4℃.

[0015] Optionally, the thawing solution is used to flush the surface of the lychee in a circulating manner.

[0016] The lychee freeze-thaw preservation method provided in this application combines ultrasound-assisted technology with immersion freezing and thawing. During the quick-freezing stage, the ultrasonic cavitation effect induces the formation of numerous small crystal nuclei within the critical temperature range of the "maximum ice crystal formation zone," replacing the growth path of a few large ice crystals in the traditional freezing process. Simultaneously, the use of intermittent ultrasound avoids excessive localized cavitation damage caused by continuous ultrasound. During the thawing stage, intermittent ultrasound accelerates the thawing process, shortens the thawing time, and reduces juice loss due to prolonged immersion. At the same time, the mechanical effect of ultrasonic vibration promotes the rapid separation of ice crystals from the fruit pulp, ensuring good juice retention in the thawed fruit. Therefore, the method of this application achieves quality control throughout the entire process from freezing to thawing, ensuring that the lychees retain plump pulp, a low juice loss rate, and a flavor close to that of fresh lychees after thawing. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of an ultrasonic-assisted freeze-thaw fruit preservation processing method according to one embodiment of this application.

[0018] Figure 2 This is a schematic diagram of the time-temperature curves for different freezing methods used in this application to process lychees.

[0019] Figure 3 A schematic diagram of temperature-time curves for treating lychees using different thawing methods according to this application.

[0020] Figure 4 This is a schematic diagram illustrating the effects of different freeze-thaw treatments on the appearance of lychees in this application.

[0021] Figure 5 For different freeze-thaw treatments of lychee L*, a*, b*, etc. in this application Schematic diagram illustrating the effect of E value.

[0022] Figure 6 This is a schematic diagram illustrating the effect of different freeze-thaw treatments on the hardness of litchi in this application.

[0023] Figure 7 The T2 curves show the relaxation times of litchi under different freeze-thaw treatments in this application.

[0024] Figure 8 The percentage of the T2 peak area represents the relaxation time of litchi under different freeze-thaw treatments in this application. Detailed Implementation

[0025] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description refers to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. Furthermore, the embodiments described in the following exemplary embodiments do not limit the invention, and any structural, methodological, or functional modifications made by those skilled in the art based on these embodiments are included within the scope of protection of this invention.

[0026] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The singular forms “a,” “the,” and “the” used in this 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” as 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 this invention to describe various information, this information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, first information may also be referred to as second information without departing from the scope of this invention, and similarly, second information may also be referred to as first information. Depending on the context, the word "if" as used herein may be interpreted as "when," "when," or "in response to a determination."

[0028] In response to the technical problems mentioned in the background, this application provides an ultrasonic-assisted freeze-thaw fruit preservation processing method adapted for lychee, which systematically optimizes the entire process of freeze-thaw lychee, and can simultaneously improve freezing efficiency and thawing efficiency, while maintaining the original quality of lychee to the greatest extent.

[0029] The following is an explanation of each name.

[0030] The "maximum ice crystal formation zone" mentioned in this application refers to the temperature range in which the latent heat of crystallization is released most concentratedly during the freezing process of food, and this range varies for different foods. For lychee fruit, the temperature range of the maximum ice crystal formation zone is typically -1℃ to -5℃. Within this temperature range, a large number of water molecules transform from a liquid state into solid ice crystals. If the freezing time is too long, the ice crystals will grow rapidly into large crystals, causing irreversible damage to the cell structure. In this application, during the quick-freezing process, crystal nuclei are induced by ultrasound, supplemented by intermittent ultrasound to regulate the ice crystal morphology, ensuring that a large number of small and uniform ice crystals are formed when passing through this temperature range, thereby maximizing the protection of cell integrity. The maximum ice crystal formation zone is determined by differential scanning calorimetry (DSC).

[0031] The "ultrasonic-assisted immersion quick-freezing" described in this application refers to directly immersing the food to be frozen in a low-temperature freezing liquid while simultaneously applying ultrasonic treatment. The ultrasonic cavitation effect generates a large number of microbubbles within the freezing liquid. When these microbubbles collapse, they release localized high pressure and high temperature, significantly reducing the supercooling required for ice crystal nucleation, thereby generating a large number of crystal nuclei in a short time. Simultaneously, the oscillation effect of ultrasound can break up larger ice crystals that have already formed, further refining the crystal grains. The intermittent ultrasonic mode used in this application (alternating between ultrasonic on / off cycles, typically with an interval of 30 seconds) can achieve a dynamic balance between nucleation and ripening while avoiding localized overheating and mechanical damage to the material caused by continuous cavitation. This ensures a sufficient number of crystal nuclei while preventing ice crystals from agglomerating due to excessive ultrasound.

[0032] The "ultrasonic-assisted immersion thawing" described in this application refers to immersing frozen lychees in a flowing thawing solution and applying ultrasonic waves. The "thermal effect" of ultrasound originates from the attenuation of sound energy in the medium, which is converted into heat energy, enabling uniform heating from the inside of the material and avoiding the "external heat and internal cold" phenomenon that occurs during traditional air thawing or static water thawing. The "mechanical effect" of ultrasound enhances the convective heat transfer between the thawing solution and the food surface through acoustic flow and microjets, significantly shortening the thawing time. The intermittent ultrasonic thawing mode also helps prevent localized overheating during the thawing process, allowing the water from the melted ice crystals to fully permeate back into the intercellular spaces, thereby reducing juice loss.

[0033] The "vacuum packaging" described in this application refers to placing lychees in a packaging bag, removing the internal air, and sealing the bag to maintain a negative pressure state inside. During frozen storage, vacuum packaging prevents oxygen from directly contacting the fruit peel, effectively inhibiting enzymatic browning reactions such as polyphenol oxidase (PPO), and reducing wilting of the peel due to water loss. Simultaneously, vacuum packaging also prevents the loss of lychee flavor compounds and contamination by external odors during frozen storage. The synergistic effect of vacuum packaging and ultrasound-assisted freeze-thaw technology can comprehensively improve the sensory quality of lychees after thawing.

[0034] The "thawing fluid flow" mentioned in this application refers to the circulation of thawing fluid during the thawing process using a pump or agitator. The flowing thawing fluid continuously washes over the surface of the lychee, breaking down the localized low-temperature boundary layer caused by thawing, maintaining a high convective heat transfer coefficient, and further increasing the thawing rate. At the same time, the mechanical action of the flowing liquid helps to quickly remove the melted juice from the fruit surface, reducing the chance of microbial growth on the surface.

[0035] The following describes in more detail the ultrasonic-assisted freeze-thaw fruit preservation processing method adapted for lychee based on specific implementation methods, but should not be limited thereto.

[0036] like Figure 1 As shown, Figure 1This is a flowchart illustrating an ultrasonic-assisted freeze-thaw fruit preservation processing method adapted for lychee, according to an exemplary embodiment of this application.

[0037] This application provides an ultrasonic-assisted freeze-thaw fruit preservation processing method adapted for lychee, comprising the following steps: S1. Pack the freshly picked lychees and pre-cool them; S2. Place the pre-cooled lychee sample into the quick-freezing chamber, immerse it in the freezing liquid, turn on the ultrasound to perform ultrasonic-assisted quick-freezing treatment on the lychee sample until the temperature of the lychee sample passes through the maximum ice crystal formation zone, then turn off the ultrasound treatment and continue quick-freezing until the sample temperature drops to the freezing endpoint temperature. S3. After freezing, transfer the litchi samples to a cold storage room for storage. The temperature difference between the cold storage temperature and the freezing endpoint temperature shall be less than or equal to ±1℃. S4. After taking out the frozen lychee sample, place it in the thawing chamber and immerse it in thawing solution. Turn on the ultrasound to perform ultrasound-assisted thawing treatment on the frozen lychee sample until the temperature of the frozen lychee sample rises to the thawing endpoint temperature and then turn off the ultrasound treatment. S5. After thawing, remove the lychee samples from the thawing chamber and place them in the refrigerator.

[0038] The lychee freeze-thaw preservation method provided in this application combines ultrasound-assisted technology with immersion freezing and thawing. During the quick-freezing stage, the ultrasonic cavitation effect induces the formation of numerous small crystal nuclei within the critical temperature range of the "maximum ice crystal formation zone," replacing the growth path of a few large ice crystals in the traditional freezing process. Simultaneously, the use of intermittent ultrasound avoids excessive localized cavitation damage caused by continuous ultrasound. During the thawing stage, intermittent ultrasound accelerates the thawing process, shortens the thawing time, and reduces juice loss due to prolonged immersion. At the same time, the mechanical effect of ultrasonic vibration promotes the rapid separation of ice crystals from the fruit pulp, ensuring good juice retention in the thawed fruit. Therefore, the method of this application achieves quality control throughout the entire process from freezing to thawing, ensuring that the lychees retain plump pulp, a low juice loss rate, and a flavor close to that of fresh lychees after thawing.

[0039] Freshly picked lychees can be of any variety, including Guiwei, Feizixiao, Nuomici, and Heiye. After harvesting, they should be refrigerated and transported to the laboratory within 6 hours. Select fruits of moderate size, free from mechanical damage, pests, and disease, and without browning of the peel. Remove the stems and vacuum pack 9 fruits per nylon vacuum bag, pre-cooling them in a 4℃ cold storage for 10 hours or more. Alternatively, vacuum packaging can be used with polyethylene (PE) plastic bags or polyvinylidene chloride (PVDC) composite bags, with the vacuum level controlled to -0.08 to -0.09 MPa.

[0040] In the ultrasonic-assisted impregnation and quick-freezing process, the freezing liquid can be a food-grade liquid medium, such as any one selected from a calcium chloride aqueous solution with a mass fraction of 30%–40% or an ethylene glycol aqueous solution with a mass fraction of 20%–30%. The temperature range of the freezing liquid is -25 to -40°C, that is, it can be arbitrarily selected between -25°C, -28°C, -30°C, -32°C, -35°C, -38°C, and -40°C. Preferably, the temperature of the freezing liquid is -30°C. The ultrasonic treatment frequency is 20–80 kHz, that is, it can be arbitrarily selected from values ​​such as 20 kHz, 28 kHz, 40 kHz, 60 kHz, and 80 kHz, with 28 kHz being the preferred ultrasonic treatment frequency. The ultrasonic treatment power is 200–1200 W, that is, it can be arbitrarily selected from values ​​such as 200 W, 300 W, 360 W, 420 W, 480 W, 600 W, 720 W, and 1200 W, with 632 W being the preferred ultrasonic treatment power. The above parameter selection can achieve ideal cooling efficiency and nucleation efficiency. Under optimal conditions, the ice crystal refinement effect and energy consumption are optimally balanced.

[0041] During the ultrasonic-assisted impregnation and quick-freezing process, ultrasonic treatment can be started immediately after the lychee sample is placed in the impregnation tank, or it can be started when the temperature of the lychee sample drops to the beginning of the maximum ice crystal formation zone, and then turned off when the temperature of the lychee sample passes through the maximum ice crystal formation zone.

[0042] Optionally, the ultrasonic generating device can be a probe-type ultrasonic transducer or a jacket-type ultrasonic generator, which is immersed and installed on the side wall of the quick-freezing chamber.

[0043] In a preferred embodiment, during ultrasonic quick-freezing, the ultrasound is intermittently turned on and off according to a first preset time interval. The first preset time interval (quick-freezing interval) can be arbitrarily selected from 10s, 15s, 20s, 25s, 30s, 35s, 40s, 45s, and 50s, with 30s being the preferred interval. During the immersion quick-freezing stage, when the time interval is set to 30s, ultrasonic treatment is first turned on for 30s, then off for 30s (other conditions remain unchanged during the ultrasonic off period), then on for 30s again, and then off for 30s again, repeating this cycle of ultrasonic action. The same procedure applies to other time intervals. At this interval, each ultrasonic activation efficiently excites crystal nuclei, while the off-time allows for sufficient temperature conduction, forming a uniform and dense ice crystal network.

[0044] Optionally, the freezing endpoint temperature can be any value among -18℃, -20℃, -25℃, and -28℃, with -20℃ being the preferred temperature. The temperature difference between the cold storage temperature and the freezing endpoint temperature is less than or equal to ±1℃.

[0045] During ultrasound-assisted thawing, the thawing solution can be purified water or food-grade physiological saline. The temperature range of the thawing solution is 18–25℃, meaning the temperature can be arbitrarily selected from values ​​such as 18℃, 19℃, 20℃, 21℃, 22℃, and 24℃, with 20℃ being the preferred temperature. The ultrasonic treatment frequency is any frequency between 20 and 80 kHz, meaning it can be arbitrarily selected from values ​​such as 20 kHz, 28 kHz, 40 kHz, 60 kHz, and 80 kHz, with 28 kHz being the preferred ultrasonic frequency. The ultrasonic treatment power range is 200–1200 W, meaning the ultrasonic treatment power can be arbitrarily selected from values ​​such as 200 W, 300 W, 360 W, 420 W, 480 W, 600 W, 720 W, and 1200 W, with 632 W being the preferred ultrasonic treatment power. Under the above thawing conditions, the thawing speed is moderate, allowing for rapid passage through the temperature range where microorganisms are prone to growth while avoiding localized overheating that could cause fruit pulp denaturation.

[0046] In a preferred embodiment, the second preset time interval (thawing interval) during the thawing process can be arbitrarily selected from 10s, 15s, 20s, 25s, 30s, 35s, 40s, 45s, and 50s, with a preferred time interval of 30s. During the thawing stage, when the set time interval is 30s, the ultrasonic treatment is first turned on for 30s, then turned off for 30s (while other conditions remain unchanged), then the ultrasonic treatment is turned on again for 30s, and then turned off for 30s, and so on, repeating the ultrasonic treatment cycle. The same applies to other time intervals.

[0047] The final thawing temperature for frozen lychees is 2–4℃, specifically any temperature among 2℃, 2.5℃, 3℃, 3.5℃, and 4℃.

[0048] In a preferred embodiment, the thawing fluid in the thawing chamber is circulated to wash the surface of the lychees. Optionally, a stirrer or circulation pump may be installed in the thawing chamber to keep the thawing fluid flowing, for example, at a flow rate of 0.1 to 0.5 m / s.

[0049] In a preferred embodiment, the coupled thawing rate of the ultrasound-assisted immersion thawing treatment is matched with the freezing rate of the ultrasound-assisted immersion rapid freezing treatment in step S2. The freezing rate is the average freezing rate of the litchi sample from 10°C to -18°C; the coupled thawing rate is the average thawing rate of the frozen litchi sample from -18°C to 4°C.

[0050] Optionally, the freezing rate of the ultrasonic-assisted impregnation quick-freezing treatment is any value within the range of 0.65 to 1.0℃ / min, and the coupled thawing rate of the ultrasonic-assisted impregnation thawing treatment is any value within the range of 0.65 to 1.0℃ / min, and the absolute value of the difference between the coupled thawing rate and the freezing rate is less than or equal to 0.35℃ / min.

[0051] In another embodiment, the freezing rate of the ultrasonic-assisted impregnation quick-freezing treatment is any value within the range of 0.4 to 0.6 °C / min, and the coupled thawing rate of the ultrasonic-assisted impregnation thawing treatment is any value within the range of 0.4 to 0.6 °C / min, and the absolute value of the difference between the coupled thawing rate and the freezing rate is less than or equal to 0.2 °C / min.

[0052] The thawing rate and freezing rate should be matched. The freezing rate and the coupled thawing rate should be divided into two independent matching ranges of high and low, and they should not be matched across ranges. The two should have a positive and synergistic relationship, with the thawing rate increasing as the freezing rate increases. If a relatively low freezing rate is matched with a high thawing rate, or if there is a mismatch between the ranges, the preservation effect will be reduced. The difference between the freeze-thaw rate and the freezing rate within the same range should be controlled within the corresponding limit. The low-speed range has a higher requirement for the rate matching degree.

[0053] Matching the thawing and freezing rates together yields the following technical benefits: The freezing rate directly determines the morphology, size, and distribution of ice crystals within the litchi pulp cells. When the freezing rate is fast (corresponding to smaller, more uniform ice crystals), increasing the coupled thawing rate ensures a high degree of synchronization between the heat input rate and the ice crystal melting rate. This allows tiny ice crystals to melt instantly in a very short time, effectively preventing the recrystallization and merging of small ice crystals into larger ones during slow thawing due to the Ostwald ripening effect. This avoids mechanical damage caused by large ice crystals piercing the cell walls. Conversely, when the freezing rate is slow (corresponding to larger, more irregular ice crystals), appropriately decreasing the coupled thawing rate avoids the risk of a rapid increase in the internal temperature gradient of the pulp due to excessive thawing, which could lead to cell wall rupture due to differences in thermal expansion coefficients. This ensures that the water migration path during thawing is highly reversible compared to the ice crystal formation path during freezing, maximizing the protection of the semi-permeable cell membranes and the supporting structure of the spongy tissue in the litchi pulp.

[0054] This method employs ultrasound-assisted immersion freezing during the quick-freezing stage, inducing numerous tiny ice crystal nuclei within the maximum ice crystal formation zone to form a uniform and dense ice crystal structure. This avoids the mechanical damage to cells caused by large ice crystals in traditional freezing, significantly improving the integrity of the pulp and juice retention after thawing. Intermittent ultrasound-assisted immersion thawing during the thawing stage accelerates the thawing process, reducing juice loss due to prolonged immersion. Simultaneously, the mechanical effect of ultrasound cavitation promotes the separation of ice crystals from the pulp, resulting in plump and juicy pulp after thawing. Constant temperature storage, consistent with the freezing endpoint temperature, prevents ice crystal recrystallization caused by temperature fluctuations, further ensuring frozen storage quality. The synergistic effect of vacuum packaging and the ultrasound-assisted freeze-thaw process ensures that the color, flavor, and texture of the thawed lychees are close to that of fresh lychees, providing a reliable technical solution for the long-term commercial frozen storage of lychees.

[0055] To study the effects of different freeze-thaw rate combinations on litchi quality, a total of 5 groups (6 examples and 7 comparative examples) of different freeze-thaw treatments were designed, as detailed in Table 1.

[0056] Examples 1 to 6 all involve ultrasound-assisted freeze-thaw (UIF-UIT).

[0057] Example 1 The lychee variety was Guiwei, and it was harvested from a commercial orchard in Zengcheng District, Guangzhou City, Guangdong Province. The ripeness was 80-90%. The average diameter of a single Guiwei lychee was 2.5-3cm, and the average weight was 15-18g. Lychees of uniform size and free from pests and diseases were selected as samples.

[0058] The ultrasonic-assisted freeze-thaw fruit preservation processing method adapted for lychees includes the following steps: (1) Pre-cooling treatment and vacuum packaging: Select freshly picked Guiwei lychees and transport them to the laboratory within 4 hours after picking via cold chain (4℃). After removing inferior fruits, put them into PE vacuum bags and seal them after vacuuming to a vacuum degree of -0.085MPa on a vacuum packaging machine. Each bag contains nine lychees and the vacuum-packed lychees are placed in a 4℃ low-temperature cold storage for 10 hours to cool (pre-cool).

[0059] (2) Ultrasonic-assisted immersion freezing: The pre-cooled litchi samples were placed in a quick-freezing chamber containing a 35% calcium chloride aqueous solution as the freezing liquid at a temperature of -30℃. The ultrasonic transducer (28kHz, 632W) was immersed in the freezing liquid. The ultrasound was started, and the timing was started simultaneously. The ultrasound working mode was intermittent: it was on for 30 seconds and then off for 30 seconds, continuously cycling intermittently. The temperature of the litchi center was monitored. When the temperature passed the maximum ice crystal formation zone (-1 to -5℃), the ultrasound was turned off, and immersion freezing continued until the center temperature dropped to -18℃ (i.e., the freezing endpoint temperature). The freezing rate was 0.75℃ / min.

[0060] (3) Constant temperature storage: After the frozen lychees are taken out, they should be immediately transferred to a -18℃ freezer for storage to maintain a constant temperature.

[0061] (4) Ultrasonic-assisted thawing: Frozen lychees with an initial temperature of -18℃ were placed in a thawing chamber, and the thawing solution was purified water (temperature 20℃), immersing the lychees. Ultrasonic waves (28kHz, 632W) were turned on for assisted thawing, with the ultrasonic mode being intermittent: on for 30s and off for 30s, continuously cycling intermittently. The thawing solution was kept flowing during the thawing process (flow rate 0.3m / s). The temperature at the center of the lychee was monitored, and the ultrasonic waves were turned off when the temperature reached 2℃ (the final thawing temperature). The coupled thawing rate was 0.81℃ / min, and the difference between the coupled thawing rate and the freezing-thawing rate was 0.06℃ / min.

[0062] (5) Post-refrigeration treatment: After thawing, take out the lychees, drain the surface water, and store them temporarily in a 4℃ refrigerator.

[0063] Example 2 The difference from Example 1 is that in step (2), the temperature of the cryosol is -35℃, the ultrasonic frequency is 40kHz, and the ultrasonic power is 480W. Preparation details are as follows: (1) Vacuum packaging and pre-cooling treatment: Same as in Example 1.

[0064] (2) Ultrasonic-assisted impregnation and quick-freezing: The freezing solution was a 35% (w / w) calcium chloride aqueous solution, and the temperature was -35℃; the ultrasonic frequency was 40kHz, the power was 480W, and the ultrasonic mode was intermittent: it was turned on for 30s and then off for 30s, and the cycle was repeated continuously. After the temperature of the litchi center passed the maximum ice crystal formation zone, the ultrasonic was turned off, and impregnation continued until -18℃, with a freezing rate of 0.65℃ / min. The remaining conditions were the same as in Example 1.

[0065] (3) Constant temperature storage: Same as in Example 1.

[0066] (4) Ultrasonic-assisted immersion thawing: The thawing solution was pure water, the temperature was 22℃, the ultrasonic frequency was 40kHz, the power was 480W, and the ultrasonic mode was intermittent: it was turned on for 30s and then turned off for 30s, continuously cycling intermittently, with the thawing solution flowing. The thawing endpoint temperature was 4℃. The coupled thawing rate was 0.65℃ / min.

[0067] (5) Refrigeration: Same as in Example 1.

[0068] Example 3 The difference from Example 1 is that in step (2), the temperature of the cryosol is -25°C, the ultrasonic frequency is 28kHz, and the ultrasonic power is 400W. Preparation details are as follows: (1) Vacuum packaging and pre-cooling treatment: Same as in Example 1.

[0069] (2) Ultrasonic-assisted impregnation and quick-freezing: The freezing liquid was a 35% calcium chloride aqueous solution at a temperature of -25℃; the ultrasonic frequency was 28kHz, the power was 600W, and the ultrasonic mode was intermittent: it was turned on for 30s and then off for 30s, and the cycle was repeated continuously. After the temperature of the litchi center passed the maximum ice crystal formation zone, the ultrasonic was turned off, and impregnation continued until -18℃, with a freezing rate of 1.0℃ / min.

[0070] (3) Constant temperature storage: Same as in Example 1.

[0071] (4) Ultrasonic-assisted immersion thawing: The thawing solution is pure water, the temperature is 18℃, the ultrasonic frequency is 28kHz, the power is 600W, the ultrasonic mode is intermittent on: turn on for 30s and then turn off for 30s, continuously intermittently cyclical, the thawing solution flows, the thawing endpoint temperature is 2℃, and the coupled thawing rate is 1.0℃ / min.

[0072] (5) Refrigeration: Same as in Example 1.

[0073] Example 4 The difference from Example 1 is that the quick-freezing interval is changed to 20 seconds (on for 20 seconds, then off for 20 seconds, continuously cycling); the thawing interval is also changed to 20 seconds (on for 20 seconds, then off for 20 seconds, continuously cycling). The specific steps are as follows: (1) Vacuum packaging and pre-cooling treatment: Same as in Example 1.

[0074] (2) Ultrasonic-assisted immersion quick-freezing: freezing liquid temperature -30℃, ultrasound 40kHz, 440W, intermittent mode: turn on for 20s and then turn off for 20s, continuously intermittently cycled, freezing rate is 0.4℃ / min, the rest is the same as in Example 1.

[0075] (3) Constant temperature storage: Same as in Example 1.

[0076] (4) Ultrasonic-assisted immersion thawing: thawing solution temperature 20℃, ultrasound 40kHz, 440W, intermittent mode: turn on for 20s and turn off for 20s, continuously intermittently cycled, thawing endpoint temperature 2℃, coupled thawing rate is 0.4℃ / min.

[0077] (5) Refrigeration: Same as in Example 1.

[0078] Example 5 (Continuous Ultrasonic-Assisted Rapid Freezing and Continuous Ultrasonic-Assisted Thawing UIF-UIT): The key difference between this comparative example and Example 1 is that ultrasound was applied continuously (without interruption) during the quick-freezing and thawing processes.

[0079] (1) Vacuum packaging and freezing pretreatment: Same as in Example 1.

[0080] (2) Ultrasonic-assisted immersion and quick-freezing (continuous ultrasound): the temperature of the freezing liquid is -30℃, the ultrasound is 60kHz, 440W, and is continuously turned on (without interruption) until the temperature of the center of the litchi passes through the maximum ice crystal formation zone and then it is turned off. Then the immersion continues to -18℃, and the freezing rate is 0.6℃ / min.

[0081] (3) Constant temperature storage: Same as in Example 1.

[0082] (4) Ultrasonic-assisted immersion thawing (continuous ultrasound): thawing fluid temperature 20℃, ultrasound 60kHz, 440W, continuously on (without interruption), thawing fluid flow, until the center temperature is 2℃, coupled thawing rate is 0.6℃ / min.

[0083] (5) Refrigeration: Same as in Example 1.

[0084] Example 6 The litchi variety used was Fei Zi Xiao, and the other conditions were the same as in Example 1.

[0085] Comparative Example 1 (Conventional Impregnation Freeze-Thaw Intervention without Ultrasonic Assistance): This comparative example provides a method for freezing and thawing lychees, the key difference from Example 1 being that no ultrasound is applied during the quick-freezing and thawing process.

[0086] (1) Vacuum packaging and freezing pretreatment: Same as in Example 1.

[0087] (2) Immersion and quick-freezing (IF without ultrasound): The pretreated lychees are directly immersed in a freezing liquid (35% calcium chloride aqueous solution) at -30℃ without turning on the ultrasound until the temperature of the center of the lychee drops to -18℃ (about 10 minutes to pass through the maximum ice crystal formation zone).

[0088] (3) Constant temperature storage: Same as in Example 1.

[0089] (4) Immersion thawing (IT without ultrasound): Immerse the frozen lychees in pure water at 20°C without turning on the ultrasound, and keep the thawing solution flowing (0.3m / s) until the core temperature rises to 2°C (total thawing time is about 12min).

[0090] (5) Refrigeration: Same as in Example 1.

[0091] Comparative Example 2 (Air freezing and air thawing without ultrasound assistance AF-AT): The key difference between this comparative example and Example 1 is that the freezing and thawing processes are both natural, without any assistance.

[0092] (1) Vacuum packaging and freezing pretreatment: Same as in Example 1.

[0093] (2) Place the vacuum-packed and pre-cooled lychees in a freezer at -20℃ and freeze naturally.

[0094] (3) Constant temperature storage: Same as in Example 1.

[0095] (4) Place the frozen lychees in the refrigerator at 4°C to thaw until the time-temperature curve is stable, and then thaw naturally.

[0096] (5) Refrigeration: Same as in Example 1.

[0097] Comparative Example 3 (Ultrasonic-assisted rapid freezing and UIF-IT without ultrasonic-assisted thawing): The key difference between this comparative example and Example 1 is that the quick-freezing process is intermittently assisted by ultrasound, while the thawing process is not assisted by ultrasound.

[0098] (1) Vacuum packaging and freezing pretreatment: Same as in Example 1.

[0099] (2) Ultrasonic-assisted immersion and quick-freezing: freezing liquid temperature -30℃, ultrasonic 28kHz, 632W, ultrasonic mode is intermittent: turn on for 30s and then turn off for 30s, continuously cycle intermittently until the temperature of the center of the litchi passes through the maximum ice crystal formation zone and then turn off, and continue immersion to -18℃.

[0100] (3) Constant temperature storage: Same as in Example 1.

[0101] (4) Thawing without ultrasonic assistance: The thawing solution temperature is 20°C, and the thawing solution flows until the center temperature is 2°C.

[0102] (5) Refrigeration: Same as in Example 1.

[0103] Comparative Example 4 (rapid freezing without ultrasound assistance and intermittent ultrasound-assisted thawing IF-UIT): The key difference between this comparative example and Example 1 is that there is no ultrasonic assistance during the quick-freezing process, while the thawing process is intermittently assisted by ultrasound.

[0104] (1) Vacuum packaging and freezing pretreatment: Same as in Example 1.

[0105] (2) Quick-freezing without ultrasonic assistance: the temperature of the freezing liquid is -30℃, and the freezing liquid is immersed to -18℃.

[0106] (3) Constant temperature storage: Same as in Example 1.

[0107] (4) Ultrasonic-assisted immersion thawing: Thawing fluid temperature 20℃, ultrasound 28kHz, 632W, ultrasound mode is intermittent on: turn on for 30s and then turn off for 30s, continuously intermittently cycle, thawing fluid flows, until the center temperature is 2℃.

[0108] (5) Refrigeration: Same as in Example 1.

[0109] Comparative Example 5 (IF-AT: Fast Freezing without Ultrasonic Impregnation and Air Thawing): The key difference between this comparative example and Example 1 is that there is no ultrasonic assistance during the quick-freezing process, and the thawing process is air thawing.

[0110] (1) Vacuum packaging and freezing pretreatment: Same as in Example 1.

[0111] (2) Immersion and quick-freezing (IF without ultrasound): The pretreated lychees are directly immersed in a freezing liquid (35% calcium chloride aqueous solution) at -30℃ without turning on the ultrasound until the temperature of the center of the lychee drops to -18℃ (about 10 minutes to pass through the maximum ice crystal formation zone).

[0112] (3) Constant temperature storage: Same as in Example 1.

[0113] (4) Place the frozen lychees in the refrigerator at 4°C to thaw until the time-temperature curve is stable, and then thaw naturally.

[0114] (5) Refrigeration: Same as in Example 1.

[0115] Comparative Example 6 (Ultrasonic-Assisted Immersion Freezing and Air Thawing UIF-AT): The key difference between this comparative example and Example 1 is that the thawing process is air thawing.

[0116] (1) Vacuum packaging and freezing pretreatment: Same as in Example 1.

[0117] (2) Same as in Example 1.

[0118] (3) Constant temperature storage: Same as in Example 1.

[0119] (4) Place the frozen lychees in the refrigerator at 4°C to thaw until the time-temperature curve is stable, and then thaw naturally.

[0120] (5) Refrigeration: Same as in Example 1.

[0121] Comparative Example 7 (Air Freezing vs. AF-IT without Ultrasonic Impregnation): The key difference between this comparative example and Example 1 is that the freezing process is air freezing, and the thawing process is not ultrasonically assisted.

[0122] (1) Vacuum packaging and freezing pretreatment: Same as in Example 1.

[0123] (2) Place the vacuum-packed and pre-cooled lychees in a freezer at -20℃ and freeze naturally.

[0124] (3) Constant temperature storage: Same as in Example 1.

[0125] (4) After taking out the frozen lychees, immerse them in the flow tank of the immersion freezing equipment, and set the flow water temperature to 20°C.

[0126] (5) Refrigeration: Same as in Example 1.

[0127] The following analysis is based on the experimental test results of the above embodiments and comparative examples.

[0128] I. Analysis of improvements in freezing and thawing rates: Freezing stage: Time-temperature curves for lychees treated with different freezing methods are shown below. Figure 2 As shown, ultrasound-assisted immersion freezing (UIF) has the fastest cooling rate, reducing the sample center temperature from 10℃ to below -18℃ in just 41.5 minutes, with a freezing rate of 0.67℃ / min. Immersion freezing (IF) is the second fastest, taking 68 minutes with a freezing rate of 0.41℃ / min, while air freezing (AF) is the slowest, requiring 212.5 minutes to reach the same temperature, with a freezing rate of 0.13℃ / min. UIF improves the freezing rate by nearly 60% compared to IF, and is approximately 5 times faster than AF.

[0129] according to Figure 2 Ultrasonic-assisted impregnation freezing enhances the freezing rate primarily by rapidly traversing the zone of maximum ice crystal formation. This accelerates the freezing process by strengthening ice nucleation and increasing mass and heat transfer rates. Furthermore, ultrasound can reduce ice crystal size during freezing, resulting in a more uniform ice crystal distribution. Ultrasonic waves generate cavitation in the medium, creating cavitation bubbles that act as nuclei, promoting ice crystal formation and inhibiting ice crystal growth. This, in turn, increases the freezing rate and reduces the damage caused by ice crystals to food. Therefore, ultrasonic-assisted impregnation freezing demonstrates a highly efficient freezing-promoting capability, significantly shortening freezing time and effectively reducing energy consumption, making it a promising application in industrial freezing.

[0130] Thawing stage: The specific thawing temperature-time curves for different thawing methods are as follows: Figure 3 As shown, the total thawing time is the time it takes for the sample center temperature to rise from -15℃ to 4℃. Among the three different thawing methods, air thawing (AT) had the longest thawing time, exceeding 15 hours. This is mainly due to the inherent properties of air as a heat transfer medium, resulting in extremely low heat transfer efficiency. Compared to immersion thawing (43.3 min), ultrasonic-assisted immersion thawing (UIT) only took 28.5 min, shortening the thawing time by approximately 30%. This is primarily due to the ultrasonic thermal effect; during ultrasonic thawing, the acoustic energy of ultrasound is converted into heat energy, raising the internal temperature of the material and achieving the thawing effect. Secondly, ultrasound induces intense turbulence in the medium, thereby enhancing the heat transfer coefficient and increasing the thawing rate. Comparing the thawing curves of IT and UIT, the improvement in thawing rate by ultrasound is mainly within the temperature range of -5 to 0℃; ultrasonic treatment within the temperature range of -15 to -5℃ has no significant effect on improving the thawing rate.

[0131] II. Quality Analysis of Frozen-Thawed Lychees: Lychee browning (including enzymatic browning and non-enzymatic browning) causes the color of the peel or flesh to gradually change from bright red, white, etc. to brown. Therefore, the degree of lychee browning can be reflected by changes in color difference.

[0132] like Figure 4As shown, there were significant differences in the appearance and color of the litchi from different treatment groups. The litchi in the slow thawing (AT) treatment group showed more severe browning, especially the litchi treated with slow freeze-slow thawing (AF-AT), which exhibited obvious collapse in appearance. In contrast, the litchi in the rapid thawing treatment groups (IF, UIT) maintained their color better, showed less browning, and had a more intact overall appearance.

[0133] Chromaticity values ​​of different processing groups, such as Figure 5 As shown, the ΔE values ​​(14.7-19.5) of the slow thawing treatment (QF-ST, SF-ST) were significantly higher than those of the rapid thawing treatment group (7.8-9.8). Compared with fresh lychees (CK), the brightness value (L* value) of the slow thawing treatment group was significantly lower, the red-green value (a* value) was slightly lower, and the yellow-blue value (b* value) was significantly higher (P<0.05), indicating that its browning phenomenon was more severe. This phenomenon is mainly attributed to the excessively long thawing time, which led to increased contact between the sample and oxygen, thereby promoting the occurrence of enzymatic browning and non-enzymatic browning. In the slow thawing treatment group, the total color difference of the UIF group ( The E value was significantly lower than that of the AF group and the IF group by 25.8% and 20.9%, respectively. This is because the ultrasonic treatment inhibited the activity of PPO enzyme, thereby delaying the enzymatic browning of litchi.

[0134] Rapid defrosting treatments (QF-QT, SF-QT) have a short defrosting time. The E values ​​were all at low levels, closer to the color of fresh lychees. For the QF-QT treatment group, the UIF-IT treatment... The E value was significantly lower than that of UIF-UIT and IF-UIT, indicating that UIF-IT can more effectively maintain the original color of lychees and reduce browning. This result further confirms the positive effect of moderate ultrasound-assisted treatment in inhibiting enzymatic browning of lychees, especially when combined with rapid freezing, where the effect is more significant. In contrast, the rapid freeze-thaw treatment group without ultrasound assistance (IF-IT) also showed good color retention, but its browning degree was still slightly higher compared to UIF-IT. In addition, the brightness value (L* value) of the rapid thawing treatment group was closer to that of fresh lychees, and the changes in red-green value (a* value) and yellow-blue value (b* value) were relatively small, indicating that rapid thawing helps maintain the bright color of lychees and reduces color deterioration. In summary, ultrasound-assisted immersion freezing combined with rapid thawing is an ideal freeze-thaw treatment method for maintaining the appearance and color of lychees.

[0135] The hardness of a thawed sample is a core indicator reflecting its textural properties. The better the hardness is maintained, the higher the integrity of the cell structure and the better the quality.

[0136] The hardness of samples treated differently, such as Figure 6As shown, the hardness of litchi decreased by more than 57% after different freeze-thaw treatments. The hardness of litchi in the rapid freezing group (QF-QT, QF-ST) was significantly higher than that in the slow freezing group (SF-QT, SF-ST) (P<0.05), indicating that the freezing method has a significant impact on the texture of litchi. Combined with the weight loss rate analysis, it was found that the change in litchi hardness and the weight loss rate were negatively correlated. Rapid freezing can quickly pass through the maximum ice crystal formation zone, forming small and uniform ice crystals with relatively intact cell structure, which not only significantly reduces the loss of juice during thawing but also better maintains the firmness of the pulp. In contrast, the coarsening of ice crystals during slow freezing causes serious damage to the cell structure.

[0137] For litchi treated with rapid freezing, there was no significant difference in fruit firmness, indicating that the actual impact of different thawing methods on sample firmness after rapid freezing is relatively small. Within the QF-QT treatment group, litchi treated with ultrasound-assisted thawing (UIF-UIT, IF-UIT) had significantly higher firmness (P<0.05) than litchi treated with only immersion thawing (IF-IT, UIF-IT), indicating that ultrasound thawing is more beneficial for improving firmness after rapid freezing. Based on rapid freezing, the effect of ultrasound-assisted thawing on firmness improvement is mainly reflected in the comparison with immersion thawing, and this improvement is related to the specific ultrasound treatment method. Specifically, the UIF-UIT treatment group showed the most significant firmness improvement. In contrast, IF-IT, lacking the assistance of ultrasound, may have resulted in more concentrated melting of ice crystals during thawing, leading to greater pressure on local cell structures and consequently a decrease in firmness. Within the QF-ST group, the lychees treated with UIF-AT showed significantly higher hardness than those treated with IF-AT, indicating that ultrasound also demonstrated a positive effect on improving hardness during the freezing process when rapid freezing was combined with slow thawing.

[0138] In the slow-freezing groups (SF-QT, SF-ST), regardless of the thawing method, the firmness of the litchi was significantly lower than that of the rapid-freezing group, further confirming the crucial influence of freezing rate on cell structural integrity. It is noteworthy that within the slow-freezing SF-QT group, while AF-UIT improved firmness to some extent compared to AF-IT, the improvement was limited. This may be related to the severe damage to cell structure caused by slow freezing itself, making subsequent sonication less effective in repairing cells. The litchi treated with AF-AT had the lowest firmness, making this treatment method the least suitable.

[0139] In summary, the freezing method is the main factor affecting the firmness of lychees after thawing, with rapid freezing significantly improving firmness. Combining rapid freezing with ultrasonic-assisted thawing (especially UIF-UIT) can further enhance firmness, with the UIF-UIT treatment group exhibiting the best firmness retention. Therefore, if high firmness of lychees after thawing is desired, rapid freezing combined with ultrasonic-assisted thawing should be the preferred method.

[0140] The high proportion of free water in lychee pulp indicates strong water flow and juicy, plump pulp.

[0141] Figure 7 The T2 spectra of litchi relaxation times under different freeze-thaw treatments show that the T20 peak (relaxation time 0–10 ms) represents bound water, which is mainly tightly bound to intracellular biomolecules, including proteins, polysaccharides (such as cellulose and pectin), and nucleic acids. The T21 peak (relaxation time 10–100 ms) represents semi-bound water, which mainly exists in intercellular spaces and small intracellular spaces, interacting with biomolecules to some extent, but with relatively weak binding forces. The T22 peak (relaxation time 100–1000 ms) represents free water, which mainly exists in larger spaces such as intercellular spaces, is highly mobile, and has very weak binding forces with biomolecules, being almost unbound by them. All litchi samples showed only two peaks in their spectra, corresponding to immobile water and free water, respectively, while the content of bound water was relatively low. The T22 peak had the largest area and the strongest signal amplitude, indicating that the water in litchi mainly exists in the form of free water. From the perspective of relaxation time, the relaxation time T20 of the bound water region in ultrasound-assisted cryotherapy (UIF-UIT, UIF-IT, UIF-AT) shifted to the right compared to other groups, indicating that some strongly bound water had been converted into hemibound water, suggesting that the interaction forces between water and tissues and various macromolecules were weakened. This is because the ultrasound-assisted cryotherapy process weakens the strong binding effect of the matrix on water, reducing the degree of binding of water that was originally tightly bound.

[0142] Depend on Figure 8 The results showed that the proportion of free water in litchi varied from 68.9% to 75.6% after different freeze-thaw treatments. The two rapid-freezing groups (QF-ST and QF-QT) showed a significant increase of 0.5% to 6.8% compared to the two slow-freezing groups (SF-ST). This is because slow freezing causes severe cell damage, leading to significant juice loss and a reduction in free water, resulting in a relatively higher proportion of non-flowing water. In contrast, rapid freezing, due to the rapid and small formation of ice crystals, causes less damage to cell structure and thus retains more free water. This result indicates that the proportion of free water is mainly affected by the freezing method, with rapid freezing maintaining a higher proportion of free water while reducing juice loss, making it a more ideal freezing method.

[0143] For the rapid freezing group, the coupled thawing method had little impact on the free water ratio; a similar situation existed in the slow freezing group, indicating that the thawing method had a relatively limited effect on the free water ratio. Within the QF-QT group, the free water ratio was UIF-IT≈IF-UIT≥UIF-UIT≥IF-IT, indicating that when both freezing and thawing rates are relatively fast, adding ultrasonic assistance has a certain effect on increasing the free water ratio. The same pattern in the QF-QT group further suggests that when both freezing and thawing rates are at a high level, adding ultrasonic assistance has a certain effect on increasing the free water ratio.

[0144] The proportion of free water in the QF-ST treatment was also at a high level, which has a positive impact on reducing juice loss and maintaining sample texture; however, the difference between it and the QF-QT treatment was not significant. Given that slow thawing takes a long time and results in severe browning, rapid freezing combined with ultrasonic treatment is a better way to maintain the water distribution of litchi pulp and preserve its juicy and plump characteristics.

[0145] III. Comparison of intermittent and continuous ultrasound: Balancing Cavitation and Thermal Effects: Intermittent Ultrasound (Examples 1-4): An "on-off" cycle (e.g., 30s / 30s) is employed. During the on-off phase, the microjets and shock waves generated by cavitation effectively break up ice crystals, promote heat and mass transfer, and accelerate the freezing / thawing process. During the off-off phase, relaxation time is provided, allowing the local Joule heat generated by continuous ultrasound operation to dissipate rapidly, effectively preventing abnormal temperature rises in the cryo- and thawing fluids. This mode balances the physical enhancement effect of ultrasound with the thermal stability of the system. Continuous Ultrasound (Example 5): Although continuous cavitation theoretically provides a stronger driving force for heat and mass transfer, the prolonged operation of the ultrasound transducer leads to continuous heat accumulation, causing the cryo-fluid temperature to rise, weakening the cooling capacity of the freezing medium, and instead prolonging the time to pass through the maximum ice crystal formation zone. During thawing, continuous heat generation easily causes local overheating, exacerbating the temperature difference between the sample surface and center.

[0146] Ice crystal formation zone passage efficiency: The preferred scheme (UIF, i.e., ultrasound-assisted immersion freezing) has a freezing time of only 41.5 min. Intermittent ultrasound (Example 1) is shut off when passing through the maximum ice crystal formation zone (-1 to -5°C). This design cleverly avoids the problem of decreased freezing rate due to thermal effects in continuous ultrasound, ensuring that the ice crystals are small and uniformly distributed. In contrast, due to the heat accumulation effect, the actual effective freezing rate of continuous ultrasound (Example 5) is lower than that of the intermittent group, and the ice crystals are more prone to secondary crystallization or coarsening.

[0147] Comparing Example 1 (30s / 30s) and Example 4 (20s / 20s), the 30s interval period is the preferred option. This is because a 30s off time is sufficient to restore a uniform temperature field in the system, resulting in more thorough heat dissipation; while a 20s off time is relatively short, leading to insufficient relaxation and a slight heat accumulation effect. Therefore, the 30s on / 30s off mode used in Example 1 achieves the best balance between efficiency and quality.

[0148] In summary, intermittent ultrasound (Examples 1-4), through "pulsed" energy input, effectively avoids the positive benefits of cavitation enhancement and the negative effects of heat. While shortening the total freezing / thawing time, it significantly preserves the color, texture, cell structure, and nutritional components of the lychee. The "30s on / 30s off" intermittent ultrasound-assisted immersion freeze-thaw process, represented by Example 1, demonstrates the best overall performance in terms of freezing efficiency, thawing rate, and final product quality, providing the best practice solution for lychee freeze-thaw preservation processing.

[0149] The technical features of the above embodiments can be combined in any way. For the sake of brevity, 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, they should be considered to be within the scope of this specification.

[0150] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A method for ultrasonic-assisted freeze-thaw fruit preservation processing adapted for lychee, characterized in that, The steps include the following: S1. Pack the freshly picked lychees and pre-cool them; S2. Place the pre-cooled lychee sample into the quick-freezing chamber, immerse it in the freezing liquid, turn on the ultrasound to perform ultrasonic-assisted quick-freezing treatment on the lychee sample until the temperature of the lychee sample passes through the maximum ice crystal formation zone, then turn off the ultrasound treatment and continue quick-freezing until the sample temperature drops to the freezing endpoint temperature. S3. After freezing, transfer the litchi samples to a cold storage room for storage. The temperature difference between the cold storage temperature and the freezing endpoint temperature shall be less than or equal to ±1℃. S4. After taking out the frozen lychee sample, place it in the thawing chamber and immerse it in thawing solution. Turn on the ultrasound to perform ultrasound-assisted thawing treatment on the frozen lychee sample until the temperature of the frozen lychee sample rises to the thawing endpoint temperature and then turn off the ultrasound treatment. S5. After thawing, remove the lychee samples from the thawing chamber and place them in the refrigerator.

2. The ultrasonic-assisted freeze-thaw fruit preservation processing method adapted for lychee according to claim 1, characterized in that, The coupled thawing rate of the ultrasound-assisted impregnation thawing process is matched with the freezing rate of the ultrasound-assisted impregnation quick-freezing process.

3. The ultrasonic-assisted freeze-thaw fruit preservation processing method adapted for lychee according to claim 2, characterized in that, The freezing rate of the ultrasonic-assisted impregnation quick-freezing treatment is 0.65–1.0 °C / min, and the coupled thawing rate of the ultrasonic-assisted impregnation thawing treatment is 0.65–1.0 °C / min, and the absolute value of the difference between the coupled thawing rate and the freezing rate is less than or equal to 0.35 °C / min; or, The freezing rate of the ultrasonic-assisted impregnation quick-freezing treatment is 0.4–0.6 °C / min, the coupled thawing rate of the ultrasonic-assisted impregnation thawing treatment is 0.4–0.6 °C / min, and the absolute value of the difference between the coupled thawing rate and the freezing rate is less than or equal to 0.2 °C / min.

4. The ultrasonic-assisted freeze-thaw fruit preservation processing method adapted for lychee according to claim 1, characterized in that, In step S2, after the ultrasound is turned on, it is intermittently turned on and off according to a first preset time interval, the first preset time interval being 10–50 seconds; and / or, In step S4, after the ultrasound is turned on, it is turned on and off intermittently according to a second preset time interval, which is 10 to 50 seconds.

5. The ultrasonic-assisted freeze-thaw fruit preservation processing method adapted for lychee according to claim 1, characterized in that, In step S2, the temperature of the cryogenic fluid is -25 to -40°C, the ultrasonic treatment frequency is 20 to 80 kHz, and the ultrasonic treatment power is 200 to 1200 W.

6. The ultrasonic-assisted freeze-thaw fruit preservation processing method adapted for lychee according to claim 5, characterized in that, In step S2, the temperature of the cryogenic fluid is -30°C, the ultrasonic treatment frequency is 28kHz, and the ultrasonic treatment power is 632W.

7. The ultrasonic-assisted freeze-thaw fruit preservation processing method adapted for lychee according to claim 1, characterized in that, In step S4, the temperature of the thawing solution is 18–25°C, the ultrasonic treatment frequency is 20–80 kHz, and the ultrasonic treatment power is 200–1200 W.

8. The ultrasonic-assisted freeze-thaw fruit preservation processing method adapted for lychee according to claim 7, characterized in that, In step S4, the temperature of the thawing solution is 20°C, the ultrasonic treatment frequency is 28kHz, and the ultrasonic treatment power is 632W.

9. The ultrasonic-assisted freeze-thaw fruit preservation processing method for lychee according to any one of claims 1 to 8, characterized in that, The thawing solution is used to flush the surface of the lychees in a circulating manner.

10. The ultrasonic-assisted freeze-thaw fruit preservation processing method for lychee according to any one of claims 1 to 8, characterized in that, The freezing endpoint temperature is -18 to -28°C.

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