A method of inhibiting browning and improving quality of frozen fruit pieces

Abstract

The application provides a method for inhibiting browning and improving quality of frozen and thawed fruit pieces, comprising the following steps: soaking fruit pieces into an epigallocatechin gallate aqueous solution, and performing pulse ultrasonic treatment during the soaking process; and sealing the fruit pieces and storing them in a frozen state. The application adopts a combined treatment mode of pulse ultrasonic and epigallocatechin gallate, the pulse ultrasonic can effectively promote the penetration of the epigallocatechin gallate solution into the fruit pieces, relieve the tissue damage of the fruit pieces caused by conventional continuous ultrasonic, and improve the preservation effect; and the subsequent combination of the traditional freezing technology can complete the freezing of the fruit and vegetable, without the need of making substantial modification to the existing freezing equipment, and the process has strong adaptability and is easy to implement.

Description

Technical Field

[0001] This invention belongs to the field of fruit and vegetable processing technology, and in particular relates to a method for inhibiting browning of frozen and thawed fruit pieces and improving their quality. Background Technology

[0002] Apples are rich in nutrients and have a pleasant flavor, making them one of the most consumed fruits globally and highly favored by consumers. Processed products made from apples, such as apple puree, juice, and tea, are also emerging in large numbers, with market demand continuing to grow. However, fresh apples are difficult to maintain in excellent quality during long-term storage, transportation, and processing, easily suffering from damage to their appearance and loss of nutrients. This significantly reduces their commercial value and has become a major factor restricting the large-scale development of the apple industry.

[0003] Low-temperature freezing is a widely used green and efficient storage method in the food industry. With its advantages of simple operation and good preservation effect, it can effectively alleviate the quality deterioration of fresh apples during storage and transportation, and is currently the mainstream technology for apple preservation. However, after freezing and thawing, apples are extremely prone to severe browning and quality deterioration, which not only damages the sensory quality and edible value of the product, but also creates many obstacles for subsequent deep processing, seriously limiting the further application of low-temperature freezing technology in the apple processing field.

[0004] Ultrasonic technology (such as pulsed ultrasound), as a green, non-thermal processing technology, shows excellent application prospects in the food industry due to its ability to effectively promote the uniform penetration of exogenous substances into food tissues. Existing research has shown that ultrasonic technology can significantly improve food freezing efficiency and enhance the storage and edibility of frozen foods. Its working principle is that when ultrasound acts on a medium, it generates periodic cavitation, mechanical, and thermal effects. These effects not only regulate the structure and function of biomolecules and cells but also reduce the formation of large ice crystals during apple freezing, accelerating the fruit freezing process and thus mitigating freeze-thaw damage to cells. Simultaneously, it can directly inhibit the activity of polyphenol oxidase (PPO), peroxidase (POD), and cell wall degrading enzymes, thereby inhibiting fruit browning to a certain extent. However, conventional ultrasound, due to its continuous operation, can lead to excessive accumulation of localized temperatures within the medium and food, resulting in ultrasonic thermal effects. This not only increases energy consumption but also affects food freezing efficiency; furthermore, its continuous mechanical action can easily damage the integrity of fruit cell tissues and cell wall structure. This accelerates browning and quality deterioration of the fruit. The degree of damage to the cell morphology and cell wall structure of the fruit is positively correlated with fruit browning, but negatively correlated with fruit quality. Pulsed ultrasound, through its intermittent working mode, serves as an effective solution to reduce the duration of mechanical action, thermal effects, and energy consumption. It can further improve the uniformity of ice crystal formation and enhance freezing efficiency, thereby maintaining cell morphology and cell wall structure.

[0005] Natural active substances have shown good application effects in inhibiting fruit browning, among which flavonoids, a type of phenolic compound, have a particularly significant browning-inhibiting effect. Epigallocatechin gallate, as the core active ingredient of natural flavonoid-3-ol polyphenols, has become a research hotspot in the field of food preservation due to its excellent antioxidant, free radical scavenging, anti-inflammatory, and antibacterial activities. Meanwhile, its direct effect in reducing the degree of browning in food is also gradually attracting attention. Related studies have confirmed that its browning-inhibiting mechanism mainly includes inhibiting polyphenol oxidase (PPO) activity, chelating browning-related metal ions, and scavenging free radicals generated by the browning reaction. Based on this, it can be inferred that epigallocatechin gallate has potential application value in inhibiting browning in freeze-thawed apples. However, epigallocatechin gallate suffers from poor tissue penetration, making it difficult to effectively penetrate the apple tissue barrier and fully contact browning-related enzymes and substrates within cells. This limits its browning-inhibiting efficacy and greatly restricts its practical application in the freeze-thaw preservation of apples.

[0006] Although both epigallocatechin gallate and pulsed ultrasound have potential advantages in inhibiting fruit browning, both have insurmountable limitations when used as a single treatment method: epigallocatechin gallate's low tissue penetration efficiency significantly reduces its browning-inhibiting effect; while pulsed ultrasound can only achieve short-term inhibition of browning-related enzymes through physical action, lacking a sustained enzyme-inhibiting effect and failing to fundamentally solve the browning problem of freeze-thawed apples. Therefore, there is an urgent need in this field to develop a composite treatment technology that combines epigallocatechin gallate, which has targeted enzyme-inhibiting effects, with pulsed ultrasound, which has physically enhanced penetration and auxiliary browning-inhibiting effects, to achieve efficient and sustained inhibition of browning in freeze-thawed apples. This would lead to the development of green, high-quality frozen apple products, solving the core problems of browning and quality deterioration in existing frozen apple processing methods. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method to inhibit browning of frozen and thawed fruit pieces and improve their quality, so as to solve the current problems of frozen / thawed apples not being resistant to storage, easily browning after being cut and easily losing nutrients.

[0008] To address the aforementioned technical problems, this invention provides a method for inhibiting browning in frozen-thawed fruit pieces and improving their quality, the method comprising: S1. Soak the fruit pieces in an aqueous solution of epigallocatechin gallate, and perform pulsed ultrasound treatment during the soaking process. S2. Seal the fruit pieces thoroughly and freeze for preservation.

[0009] Further, in the above method, the concentration of epigallocatechin gallate in the aqueous solution of epigallocatechin gallate is 1.0 g / 100 mL to 3.0 g / 100 mL.

[0010] Furthermore, in the above method, the power of the pulsed ultrasound is 100 watts to 300 watts, the ultrasound interval is 10 seconds to 60 seconds, and the total ultrasound time is 5 minutes to 10 minutes.

[0011] In the above method, further, in S1, the temperature of the epigallocatechin gallate aqueous solution is maintained at 10°C to 18°C.

[0012] Further, in the above method, the cryopreservation in S2 specifically involves: immediately placing the container in a freezer at -18 to -25°C for freezing, with the center temperature of the freezer below -18°C.

[0013] Furthermore, in the above method, the sealed fruit pieces in S2 are spaced 2 to 5 centimeters apart.

[0014] Compared with the prior art, the advantages of the present invention are as follows: (1) This invention provides a method for inhibiting browning of frozen and thawed fruit pieces and improving their quality. A combined treatment method of pulsed ultrasound and epigallocatechin gallate is adopted. Pulsed ultrasound can effectively promote the penetration of epigallocatechin gallate solution into the fruit pieces, thereby improving the preservation effect. The subsequent freezing of fruits and vegetables is completed by combining traditional freezing technology. There is no need to make major modifications to the existing freezing equipment. The process is highly adaptable and easy to implement.

[0015] (2) This invention provides a method for inhibiting browning of frozen and thawed fruit pieces and improving their quality. Compared with traditional hot-blanching preservation methods, this invention can maintain the color, nutrition and flavor quality of fruit pieces after freezing and thawing to the greatest extent. While ensuring the core edible quality of apples, it precisely solves the industry problem that apples are not resistant to storage, are prone to browning after fresh cutting, and are prone to spoilage. At the same time, this process is also applicable to apples and other fruits, and the preservation effect has universality.

[0016] (3) This invention provides a method for inhibiting browning of frozen and thawed fruit pieces and improving their quality. This invention has high promotional value for the fruit processing industry. It enables large-scale purchase and rapid freezing of fruits such as apples during peak seasons, effectively solving the problem of unsold fresh-cut fruits and reducing production and sales losses for fruit farmers and processing enterprises. Frozen fruit pieces can support continuous production by processing enterprises throughout the year, breaking the limitation of short production period of fruit raw materials, avoiding long-term idleness of processing equipment, greatly improving equipment utilization, and reducing the overall production cost of the industry.

[0017] (4) This invention provides a method for inhibiting browning of frozen and thawed fruit pieces and improving their quality. This invention not only provides a core method for treating frozen fruit pieces with ultrasonic-assisted epigallocatechin gallate to inhibit browning during thawing and improve their quality, but also extends this method to the production of fresh-cut apple pieces. The application scenario of the technology is clear, and it can be further promoted to the processing and production of various fresh-cut and frozen fruits. The technology has both practicality and scalability. Attached Figure Description

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0019] Figure 1 This is a flowchart of Embodiment 1 of the present invention.

[0020] Figure 2 These are product images of frozen-thawed apple chunks from Embodiment 1 and Comparative Examples 1-5 of the present invention.

[0021] Figure 3 The color and browning index of frozen-thawed apple pieces in Example 1 and Comparative Examples 1-5 of this invention are shown.

[0022] Figure 4 Images, colors, and browning indices of frozen-thawed apple chunks from Examples 1 and Comparative Examples 2 and 6 of this invention.

[0023] Figure 5 The total phenols, total flavonoids, EGCG content, and DPPH free radical scavenging rate of frozen-thawed apple pieces in Examples 1 and Comparative Examples 2 and 5 of this invention are given.

[0024] Figure 6 The total phenols and total flavonoids content of frozen-thawed apple pieces in Examples 1 and Comparative Examples 2 and 6 of this invention.

[0025] Figure 7 The PPO, POD, LOX, and PAL enzyme activities of frozen-thawed apple pieces in Examples 1 and Comparative Examples 2 and 5 of this invention are shown.

[0026] Figure 8 The vitamin C (VC) content of frozen-thawed apple pieces in Examples 1 and Comparative Examples 2 and 6 of this invention.

[0027] Figure 9 The content of aroma substances in frozen-thawed apple pieces in Examples 1 and Comparative Examples 2 and 6 of the present invention.

[0028] Figure 10 The cell structure and cell wall structure of the frozen-thawed apple pieces in Example 1 and Comparative Examples 1 and 7 of this invention are shown. Detailed Implementation

[0029] The present invention will be further described below with reference to specific preferred embodiments, but this does not limit the scope of protection of the present invention.

[0030] The materials, reagents, and instruments used in the following examples were all commercially available. Unless otherwise specified, the experimental methods used in the following examples are conventional methods in the art. Units mM and μM are mmol / L and μmol / L, respectively.

[0031] Example 1 A method for inhibiting browning and improving the quality of frozen-thawed fruit pieces by ultrasonic-assisted treatment with epigallocatechin gallate esters; the process is described in [link to procedure]. Figure 1 This includes the following steps: (1) Pretreatment: Select fresh, uncontaminated, rot-free, and pest-free apple raw materials and place them in a cool place to cool down. The raw materials should be handled gently during loading and unloading. Wash the surface of the raw materials with clean water in a clean water pool and drain the wastewater. Quickly remove the peel and core from the apples; cut the flesh into square pieces.

[0032] (2) Low-temperature soaking for color protection: Epigallocatechin gallate was dissolved in pure water at a ratio of 2.0 g: 100 ml to obtain an epigallocatechin gallate aqueous solution. The fruit pieces cut in step (1) were placed into the prepared low-temperature epigallocatechin gallate aqueous solution for uniform soaking, while pulsed ultrasound was performed at a power of 200 watts, with a time interval of 30 seconds and a time of 8 minutes, maintaining the temperature at 15℃.

[0033] (3) After the fruit pieces have been soaked in cold water for color protection, drain off excess moisture from the surface, remove any defective, discolored, or broken pieces, and pack them into a freezer box according to the standard weight. The fruit pieces should be neatly arranged and in appropriate quantities. Pack them into a freezer bag and seal it tightly. Immediately place them in a freezer for freezing storage at a temperature of -18 to -25°C, with the core temperature below -18°C. Store the fruit pulp in a freezer according to its specifications and batches, keeping the temperature below -20°C.

[0034] Comparative Example 1 Select fresh, uncontaminated, rot-free, and pest-free apples and place them in a cool, shaded area to cool. Handle the apples gently during loading and unloading. Rinse the apples thoroughly with clean water in a clean water tank and drain the wastewater. Quickly remove the peel and core from the apples; cut the flesh into square pieces. Store directly at room temperature.

[0035] Comparative Example 2 The difference from Example 1 is that in step (2), the sample was not soaked in an aqueous solution of epigallocatechin gallate, but soaked in pure water.

[0036] Comparative Example 3 The difference from Example 1 is that in step (2), the sample was not soaked in an aqueous solution of epigallocatechin gallate, but in a vitamin C solution.

[0037] Comparative Example 4 The difference from Example 1 is that in step (2), the sample was not soaked in an aqueous solution of epigallocatechin gallate, but in a citric acid solution.

[0038] Comparative Example 5 The difference from Example 1 is that pulsed ultrasound processing was not performed in step (2).

[0039] Comparative Example 6 (1) Pretreatment: Select fresh, uncontaminated, rot-free, and pest-free apple raw materials and place them in a cool place to cool down. The raw materials should be handled gently during loading and unloading. Wash the surface of the raw materials with clean water in a clean water pool and drain the wastewater. Quickly remove the peel and core from the apples; cut the flesh into square pieces.

[0040] (2) Place the fruit pieces in a 90℃~95℃ hot water bath for 5~10 minutes.

[0041] (3) Drain thoroughly, remove defective, discolored and broken fruit, and pack into fresh-keeping boxes according to standard weight; the fruit pieces in the boxes should be neatly arranged and in appropriate quantities. Pack into fresh-keeping bags and seal thoroughly. Immediately place in the freezer for freezing storage at a temperature of -18 to -25℃, with the core temperature below -18℃. Store the fresh-keeping boxes in the freezer according to their specifications and batches, keeping the temperature below -20℃.

[0042] Comparative Example 7 The difference from Example 1 is that in step (2), pulsed ultrasound treatment was not performed, but conventional continuous ultrasound treatment was used with a power of 200 watts, and ultrasound was applied continuously for 8 minutes, maintaining the temperature at 15℃~25℃.

[0043] Experiment 1: To examine the color and browning index of fruit pieces from Example 1 and Comparative Examples 1-5.

[0044] The fruit pieces from Examples 1 to 5 were thawed, and the methods for detecting the color and quality of the fruit pieces are as follows: (1) Color: The color difference of the fruit pieces was measured by an automatic colorimeter.

[0045] (2) Browning Index (BI): In terms of L a and b The BI value is calculated using the following formula: (1)

[0046] Figure 2 These are product images of frozen-thawed apple chunks from Embodiment 1 and Comparative Examples 1-5 of the present invention. Figure 3 The figures show the color and browning index of frozen-thawed apple pieces in Example 1 and Comparative Examples 1-5 of this invention. A in the figures represents L. Value, B in the diagram is a Value, C in the diagram is b The value, D in the graph, represents the browning index. From Figure 2 External observation and Figure 3 As can be seen from the color parameters (L*, a*, b*) and browning index (BI), the degree of browning in all freeze-thaw treatments (Comparative Examples 2-5, Example 1) was significantly higher than that in the fresh control group (Comparative Example 1), with a decrease in L and b values ​​and an increase in a value and total color difference BI value. This indicates that freeze-thaw treatment damages cell structure and accelerates the enzymatic browning process. Among them, the browning in Comparative Example 2 was the most severe, while the treatment in Example 1 was the most effective in inhibiting browning. Its L and b values ​​decreased the least, and its a and BI values ​​increased the least. It showed the best color protection effect at all time points and was significantly better than other freeze-thaw treatment groups.

[0047] Figure 4 Images of frozen-thawed apple pieces from Examples 1, 2, and 6 of this invention, along with their browning, are shown. In the figures, A represents the product image, and B represents the product image (L). Value, C in the figure is a Value, D in the diagram is b The values ​​are shown in the figure, where E represents the browning index. As can be seen from the figure, the fruit pieces treated with pulsed ultrasound-assisted epigallocatechin gallate in Example 1 showed significantly higher L values ​​after thawing compared to Comparative Example 2, and the a and b values ​​were also significantly higher. * The color retention is better, and the degree of browning is significantly reduced; however, the color retention and browning inhibition of Example 1 are not as good as those of Comparative Example 6, which is treated with conventional heat treatment. However, Example 1 is significantly better than Comparative Example 6, which is treated with conventional heat treatment, in terms of texture.

[0048] comprehensive Figure 2 , Figure 3 and Figure 4 The results show that freezing Thawing destroys the cell structure of apple pieces and accelerates enzymatic browning, resulting in a significantly higher degree of browning than fresh samples, manifested as a decrease in L* and b values, and an increase in a value and browning index (BI).

[0049] In various frozen During the thawing process, pulsed ultrasound-assisted treatment with epigallocatechin gallate (Example 1) showed the best effect in inhibiting browning and protecting the color of apple pieces, significantly superior to other freezing methods. Thawing group. The color retention and browning inhibition effects of Example 1 were not as good as those of the traditional hot-stamping treatment (Comparative Example 6), but the texture quality was significantly better than that of the traditional hot-stamping treatment group.

[0050] Experiment 2: The total phenolic content, total flavonoid content, DPPH free radical scavenging rate, and relative content of epigallocatechin gallate (EGCG) were investigated in Examples 1 and Comparative Examples 1-6.

[0051] The method for detecting total phenol content was as follows: The Folin-phenol colorimetric method was used. Based on the fact that polyphenols can reduce phosphomolybdic acid-phosphotungstic acid to form a blue complex under alkaline conditions, the absorbance was measured at 760 nm, and gallic acid was used as a standard for quantification.

[0052] The method for detecting total flavonoid content is the sodium nitrite-aluminum nitrate-sodium hydroxide colorimetric method (NaNO2-Al(NO2)2-NaOH method). This method is based on the fact that the phenolic hydroxyl groups of flavonoids form stable colored complexes with aluminum ions, which have a maximum absorption peak at a wavelength of 510 nm. Rutin is used as a standard for quantification.

[0053] DPPH radical scavenging rate: The DPPH method was used. Approximately 1.0 g of sample was mixed with 5 mL of 75% ethanol, then sonicated for 40 min, followed by centrifugation at 10000 g for 20 min at 4 °C. Subsequently, 0.1 mL of the supernatant was added to 3.9 mL of 0.1 mmol / L... - ¹ DPPH solution. After reacting in the dark for 30 minutes, the absorbance of the resulting mixture was measured at 517 nm.

[0054] Relative EGCG content: determined and analyzed using a liquid chromatography-UPLC-MS / MS system.

[0055] Figure 5 Figures show the total phenol, total flavonoid, EGCG content, and DPPH free radical scavenging rate of frozen-thawed apple pieces in Examples 1 and Comparative Examples 2 and 5 of this invention. In the figures, A represents the total phenol content; B represents the total flavonoid content; C represents the DPPH free radical scavenging rate; and D represents the relative EGCG content.

[0056] from Figure 5As shown in A and B, after freezing and thawing, the total phenol and total flavonoid content of each group of samples showed a decreasing trend over time. Among them, the total phenol and total flavonoid content of Example 1 was always significantly higher than that of Comparative Example 2 and Comparative Example 5, and the decrease was the smallest. This indicates that the treatment can effectively delay the degradation of phenols and flavonoids in fruits and vegetables, thereby better preserving their antioxidant activity and nutritional quality.

[0057] from Figure 5 As shown in C, after freezing and thawing, the DPPH free radical scavenging rate of each group showed a decreasing trend over time. Among them, Example 1, which was treated with pulsed ultrasound-assisted epigallocatechin gallate, had a DPPH free radical scavenging rate that was always significantly higher than that of Comparative Example 2 and Comparative Example 5, and the decrease was the smallest. This indicates that the treatment can effectively maintain the antioxidant activity of frozen fruit pieces after thawing, reduce oxidative damage, thereby inhibiting browning and improving their quality.

[0058] from Figure 5 As can be seen from D, in Example 1, which was treated with pulsed ultrasound-assisted epigallocatechin gallate ester 4 hours after freezing-thawing, the relative content of EGCG was significantly higher than that of Comparative Example 2 and Comparative Example 5. This indicates that the treatment can effectively retain EGCG in the fruit pieces, thereby enhancing their antioxidant capacity and achieving the effect of inhibiting thawing browning and improving product quality.

[0059] Figure 6 The figures show the total phenolic content (A) and total flavonoid content (B) of frozen-thawed apple pieces in Examples 1 and Comparative Examples 2 and 6 of this invention. As can be seen from the figures, in Example 1, the fruit pieces treated with pulsed ultrasound-assisted epigallocatechin gallate showed significantly higher total phenolic and total flavonoid content after thawing compared to the blank control (frozen-thawed) Comparative Example 2 and the blanched Comparative Example 6. This indicates that the treatment can effectively delay the degradation of phenolic and flavonoid antioxidants, thereby enhancing the antioxidant capacity of frozen fruit pieces, inhibiting thawing browning, and improving product quality. The effect is significantly better than the blank control and the blanching treatment.

[0060] In summary Figure 5 and Figure 6The results showed that freezing-thawing treatment led to a decrease in the total phenolic and total flavonoid content and DPPH free radical scavenging rate of apple pieces over time, resulting in the degradation of antioxidant active substances and a reduction in antioxidant capacity. However, pulsed ultrasound-assisted epigallocatechin gallate (EGCG) treatment (Example 1) effectively delayed the degradation of antioxidant substances such as total phenolic and total flavonoids, significantly improved EGCG retention and DPPH free radical scavenging rate, and better maintained the antioxidant activity of frozen-thawed apple pieces. This treatment was significantly better than the ordinary freezing-thawing control group and the traditional blanching treatment group in terms of antioxidant retention and antioxidant capacity enhancement, and could effectively improve the overall quality of frozen-thawed apple pieces from the perspective of inhibiting oxidation and reducing browning.

[0061] Experiment 3: To examine the PPO, POD, LOX and PAL enzyme activities of Example 1 and Comparative Examples 2 and 5.

[0062] The detection method for polyphenol oxidase (PPO) activity is as follows: the catechol colorimetric method is used. The principle is that catechol is oxidized under the catalysis of PPO to generate colored quinones, which have a characteristic absorption peak at a wavelength of 420 nm. The enzyme activity is calculated by measuring the rate of change of absorbance over time.

[0063] The method for detecting peroxidase (POD) activity is as follows: The guaiacol colorimetric method is used. POD catalyzes the oxidation reaction of guaiacol with hydrogen peroxide (H2O2) to produce colored tetraguaiacol, which has a characteristic absorption peak at a wavelength of 470 nm. The detection method for lipoxygenase (LOX) activity: Ultraviolet spectrophotometry is used. LOX catalyzes the oxidation of substrates such as linoleic acid to generate hydroperoxides with conjugated double bonds. This product has a characteristic absorption peak at a wavelength of 234 nm. Methods for detecting phenylalanine ammonia-lyase (PAL) activity: Ultraviolet spectrophotometry is used. The principle is that PAL can catalyze the deamination of L-phenylalanine to produce trans-cinnamic acid, which has a characteristic absorption peak at a wavelength of 290 nm.

[0064] Figure 7The figures show the enzyme activities of PPO (A), POD (B), LOX (C), and PAL (D) in frozen-thawed apple pieces from Examples 1 and Comparative Examples 2 and 5 of this invention. PPO is a major driver of browning and quality deterioration in apples because it catalyzes the oxidation of catechol to neoquinones; this unstable intermediate undergoes complex polymerization reactions to form melanin. POD can catalyze hydrogen peroxide, reducing reactive oxygen species (ROS) damage to cells, and can also oxidize and polymerize with phenolic compounds, leading to tissue browning. Inhibiting the activity of PPO and POD can delay fruit browning. LOX catalyzes the production of malondialdehyde, generating a large amount of reactive oxygen species, which induces membrane lipid peroxidation. PAL, as a key enzyme involved in the production of flavonoids and other phenolic substances, also plays an important role in the enzymatic browning of apples. As can be seen from the figure, in Example 1, which was treated with pulsed ultrasound-assisted epigallocatechin gallate, the activity levels of PPO, POD, and LOX enzymes were significantly lower than those in Comparative Examples 2 and 5 throughout the entire storage process. Only the PAL enzyme activity showed a moderate increasing trend. This indicates that the treatment can effectively inhibit the PPO, POD, and LOX enzyme activities associated with browning and quality deterioration. At the same time, by moderately increasing the PAL enzyme activity, the synthesis of phenolic antioxidants is promoted, thereby achieving the technical effect of inhibiting the browning of frozen fruit pieces during thawing and delaying quality deterioration.

[0065] Experiment 4: Examine the vitamin C content of Example 1 and Comparative Examples 2 and 6.

[0066] Vitamin C content: The 2,6-dichlorophenolindophenol titration method was used. Reduced ascorbic acid has strong reducing properties and can reduce the blue sodium 2,6-dichlorophenolindophenol (DCPIP) to a colorless phenolic imine compound. DCPIP is pink under acidic conditions. The titration endpoint is reached when the solution becomes a stable pink color (not fading for 15 seconds). The content of reduced vitamin C in the sample was calculated based on the amount of titrant consumed.

[0067] Figure 8 The figures show the vitamin C (VC) content of frozen-thawed apple pieces in Examples 1 and Comparative Examples 2 and 6 of this invention. As can be seen from the figures, the vitamin C content of the fruit pieces in Example 1, treated with pulsed ultrasound-assisted epigallocatechin gallate, was significantly higher after thawing than that in the blank control (comparative Example 2, freeze-thawed) and the blanched control (comparative Example 6). This indicates that the treatment can effectively reduce the oxidative degradation of vitamin C, thereby better preserving the nutritional quality of the frozen fruit pieces, and its effect is significantly better than the blank control and the blanching treatment.

[0068] Experiment 5: Examine the aroma content of Example 1 and Comparative Examples 2 and 6.

[0069] The aroma compound content was determined using headspace solid-phase microextraction (HSPME) and GC-MS. 50 / 30 μm SPME fibers were extracted, equilibrated at 50°C for 15 minutes, and then adsorbed for 30 minutes. The interface temperature of the GC-MS9 (Shimadzu GCMS-2010) was 250°C, the ion source temperature was set to 200°C, and the quadrupole temperature was 150°C. The ionization mode was EI, the electron energy was 70 electron volts, and the mass scan range was 30–550 mz. After extraction, the SPME fibers were inserted into the GC inlet set to 250°C and subjected to a non-fragmentation mode for 5 minutes. The helium flow rate on the HP-5MS capillary column was 1.2 mL / min. The temperature range was as follows: initial column temperature of 40°C for 5 minutes, followed by a ramp to 120°C at a rate of 4°C / min, then a ramp to 230°C at a rate of 10°C / min, and finally held at the final temperature for 6 minutes. The volatile compounds detected by GC-MS analysis were matched by computer with reference mass spectra from NIST and retention time (RT) MS libraries.

[0070] Figure 9 The figures show the aroma substance content of frozen-thawed apple pieces in Examples 1 and Comparative Examples 2 and 6 of this invention. In the figures, A represents the content of aroma substances such as aldehydes, esters, and terpenes; B represents the content of aldehydes such as hexanal and (E)-2-hexenal; C represents the content of esters (such as butyl acetate and hexyl acetate); and D represents the content of characteristic aroma substances such as terpenes (such as D-limonene and α-farnesene).

[0071] from Figure 9 As can be seen in A, in Example 1, which was treated with pulsed ultrasound-assisted epigallocatechin gallate, the formation of undesirable flavor compounds such as aldehydes was significantly inhibited after thawing, while the content of aroma compounds such as esters and terpenes was significantly increased. Compared with the blank control (freeze-thaw comparison example 2) and the blanching treatment (comparative example 6), this treatment can effectively improve the flavor quality of frozen fruit pieces. Figure 9As shown in Figure B, the content of aldehydes such as hexanal and (E)-2-hexenal in Example 1 is significantly lower than that in Comparative Example 2 (freeze-thaw), while in Comparative Example 6, aldehydes are almost completely absent. This indicates that the treatment can effectively inhibit undesirable flavors such as rancidity and grassiness caused by lipid oxidation and enzymatic browning. As shown in Figures C and D, the content of characteristic aroma compounds such as esters (e.g., butyl acetate, hexyl acetate) and terpenes (e.g., D-limonene, α-farnesene) in Example 1 is significantly higher than that in Comparative Examples 2 and 6. Esters contribute fruity aroma and sweetness, while terpenes impart a fresh aroma, indicating that the treatment can better preserve the original pleasant aroma of the fruit. Compared with the blanching treatment (Comparative Example 6), which only inhibits undesirable flavors but also loses a large amount of aroma compounds, the treatment of the present invention can more effectively preserve and enhance the aroma compounds of esters and terpenes while inhibiting aldehyde off-flavors, achieving precise flavor control, which is significantly better than the blank control and traditional blanching treatment.

[0072] Experiment 6: Examine the changes in cell structure and cell wall in Example 1 and Comparative Examples 1 and 7.

[0073] Observation of fruit pulp cell structure: The microstructure of the fruit pieces was observed using scanning electron microscopy (SEM). Each group of samples was cut into 1.0 cm × 1.0 cm pieces, then fixed and dried. The samples were fixed in 2.5% glutaraldehyde solution, and then subjected to gradient dehydration with 30%, 50%, 70%, 90%, and 100% ethanol, 10 min each time. The samples were then placed in a critical point dehydrator (Tousimis Autosamdri). The samples were critically dried for approximately 1 hour using the 815, Series. The samples were then fixed to the sample stage with conductive adhesive, sputtered with gold using an ion sputtering system (HIYACHI MC1000), and the microstructure of the cells and tissues was imaged using a Hitachi SU8020 scanning electron microscope at an accelerating voltage of 5 kV.

[0074] Observation of fruit pulp cell walls: The pre-fixation and dehydration procedures for samples used in transmission electron microscopy (TEM) were the same as for SEM samples. After dehydration with a series of ethanol and acetone solutions of increasing concentration, the samples were immersed in a mixture of embedding agent and propylene oxide, and finally sectioned on an ultramicrotome for observation using a FEI Tecnai G2 12 transmission electron microscope (FEI, USA).

[0075] Figure 10 The figures show the cell structure and cell wall structure of frozen-thawed apple pieces in Example 1 and Comparative Examples 1 and 7 of this invention. In the figures, A represents the cell microstructure; B represents the cell wall microstructure.

[0076] from Figure 10In Example A, cell morphology and structural integrity were observed. Comparative Example 1 cells exhibited a regular, tightly packed honeycomb structure with clear cell wall outlines, intact structure, and small intercellular spaces, maintaining the typical dense cellular structure of fresh apple segments. In Example 1, cells treated with pulsed ultrasound-assisted epigallocatechin gallate esters still maintained a relatively intact honeycomb framework. Although the cell walls showed some expansion, no large-scale rupture occurred, and intercellular connections remained relatively stable, demonstrating significantly better structural integrity. In contrast, Comparative Example 7 (the group treated with conventional ultrasound combined with epigallocatechin gallate esters) showed more severe cell structural damage, with large-scale wrinkling, partial collapse, and even rupture of the cell walls. The honeycomb structure partially disappeared, intercellular spaces significantly increased, and numerous cell fragments and loose, flocculent structures appeared. This indicates that pulsed ultrasound-assisted epigallocatechin gallate ester treatment is significantly superior to conventional ultrasound treatment in maintaining the morphology and structural integrity of apple segment cells.

[0077] from Figure 10 In Example B, the microstructure of the cell wall can be observed. In Comparative Example 1, the cell wall layers are clear, the thickness is uniform, the structure is dense, and there is no obvious material deposition or structural disorder. In Example 1, treated with pulsed ultrasound-assisted epigallocatechin gallate, the cell wall structure remained largely continuous. A small amount of granular material (presumably from epigallocatechin gallate treatment) was uniformly attached to the cell wall surface, and the cell wall layers were still discernible, with no obvious breakage or dissolution. In contrast, in Comparative Example 7 (the group treated with conventional ultrasound combined with epigallocatechin gallate), the cell wall structure was more disordered, the layers were less defined, and local breakage and damage occurred, resulting in more severe damage to cell wall integrity. This indicates that pulsed ultrasound-assisted epigallocatechin gallate treatment is significantly more effective than conventional ultrasound treatment in maintaining the cell wall structure of the fruit. The degree of damage to the cell morphology and cell wall structure of the fruit is positively correlated with fruit browning but negatively correlated with fruit quality; therefore, maintaining a good cell structure is crucial for alleviating the quality of the fruit after freezing and thawing and inhibiting browning. This invention reveals that, compared with conventional ultrasound combined with gallocatechin gallate treatment, pulsed ultrasound-assisted treatment of gallocatechin gallate is more advantageous in maintaining the cell morphology and cell wall structure integrity of apple segments.

[0078] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention using the methods and techniques disclosed above, or modify them into equivalent embodiments with equivalent changes, without departing from the spirit and technical essence of the present invention. Therefore, any simple modifications, equivalent substitutions, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall still fall within the protection scope of the technical solutions of the present invention.

Claims

1. A method for inhibiting browning of frozen-thawed fruit pieces and improving their quality, characterized in that, The method includes: S1. Soak the fruit pieces in an aqueous solution of epigallocatechin gallate, and perform pulsed ultrasound treatment during the soaking process. S2. Seal the fruit pieces thoroughly and freeze for storage.

2. The method according to claim 1, characterized in that, The concentration of epigallocatechin gallate in the aqueous solution is 1.0 g / 100 mL to 3.0 g / 100 mL.

3. The method according to claim 1, characterized in that, The power of the pulsed ultrasound is 100 watts to 300 watts, the ultrasound interval is 10 seconds to 60 seconds, and the total ultrasound time is 5 minutes to 10 minutes.

4. The method according to claim 1, characterized in that, In step S1, the temperature of the epigallocatechin gallate aqueous solution is maintained at 10°C to 18°C.

5. The method according to claim 1, characterized in that, The cryopreservation described in S2 specifically involves immediately placing the container in a freezer at -18 to -25°C for freezing, with the center temperature of the freezer below -18°C.

6. The method according to claim 1, characterized in that, The interval between the sealed fruit pieces in S2 is 2 cm to 5 cm.

Images