Method for fresh-keeping of fresh-cut fruits and vegetables by plasma-activated water in combination with organic acid treatment

Abstract

The application discloses a kind of plasma-activated water and organic acid treatment fresh-cut fruit and vegetable preservation method, belong to fruit and vegetable preservation technical field.The method includes: measuring fresh-cut fruit and vegetable section pH, according to pH≤4.0, 4.0 The application discloses a kind of plasma-activated water and organic acid treatment fresh-cut fruit and vegetable preservation method, belong to fruit and vegetable preservation technical field.The method includes: measuring fresh-cut fruit and vegetable section pH, according to pH≤4.0, 4.0 The application discloses a kind of plasma-activated water and organic acid treatment fresh-cut fruit and vegetable preservation method, belong to fruit and vegetable preservation technical field.The method includes: measuring fresh-cut fruit and vegetable section pH, according to pH≤4.0, 4.0 The application discloses a kind of plasma-activated water and organic acid treatment fresh-cut fruit and vegetable preservation method, belong to fruit and vegetable preservation technical field.The method includes: measuring fresh-cut fruit and vegetable section pH, according to pH≤4.0, 4.0 The application discloses a kind of plasma-activated water and organic acid treatment fresh-cut fruit and vegetable preservation method, belong to fruit and vegetable preservation technical field.The method includes: measuring fresh-cut fruit and vegetable section pH, according to pH≤4.0, 4.0

Description

Technical Field

[0001] This invention belongs to the field of fruit and vegetable preservation technology, specifically relating to a method for preserving fresh-cut fruits and vegetables by plasma-activated water combined with organic acid treatment. Background Technology

[0002] Fresh-cut fruits and vegetables, also known as minimally processed fruits and vegetables, refer to fresh fruits and vegetables that have undergone selection, washing, cutting, and packaging while remaining fresh and ready for direct consumption. With the fast pace of modern life and increasing consumer demand for healthy eating, fresh-cut fruits and vegetables are gaining popularity due to their convenience, freshness, and nutritional value. However, the mechanical damage caused by peeling and cutting during processing disrupts the integrity of the fruit and vegetable's structure, allowing enzymes within the tissue to come into direct contact with substrates. This easily leads to enzymatic browning, softening, and moisture loss, resulting in quality deterioration. Furthermore, without the protection of the outer skin, fresh-cut fruits and vegetables are more susceptible to microbial contamination and reproduction. These factors collectively contribute to the short shelf life of fresh-cut fruits and vegetables, severely hindering the development of the fresh-cut fruit and vegetable industry.

[0003] Currently, various technologies have been developed both domestically and internationally for preserving fresh-cut fruits and vegetables, including low-temperature cold chain methods, radiation preservation, ultra-high pressure preservation, modified atmosphere packaging, active packaging, edible coating, and antibacterial treatment with preservatives. However, these traditional preservation methods generally suffer from problems such as complex operation, high equipment costs, unsatisfactory preservation effects, or chemical residues. Therefore, developing safe, efficient, and residue-free novel technologies for preserving fresh-cut fruits and vegetables is of significant practical importance.

[0004] In recent years, plasma-activated water (PAW) has attracted widespread attention as a novel non-thermal sterilization and preservation technology. PAW is produced by treating water using low-temperature plasma technologies such as dielectric barrier discharge (DBD), and contains hydrogen peroxide (H₂O₂), hydroxyl radicals (·OH), ozone (O₃), and nitrite (NO₂). - ), nitrates (NO3) - It contains various reactive oxygen species and reactive nitrogen species, exhibiting excellent broad-spectrum bactericidal effects, and produces no chemical residues during the treatment process, ensuring high safety. Studies have shown that PAW treatment can effectively inhibit browning on the surface of fresh-cut apples while maintaining their firmness and titratable acidity; PAW can also delay the browning process by inhibiting the activity of polyphenol oxidase and peroxidase.

[0005] However, the preservation effect of using PAW alone still has certain limitations. To enhance the treatment effect of PAW, researchers have tried to use organic acids in combination with PAW. Studies have found that dissolving tartaric acid in PAW can significantly increase the redox potential of PAW and lower the pH value, thereby enhancing its antibacterial properties against Staphylococcus aureus. Tartaric acid-enhanced PAW treatment can reduce the total bacterial count of fresh-cut potatoes by 2.31g CFU / g at the end of storage compared to PAW treatment alone, and extend the shelf life by 4 days; PAW synergistic tartaric acid treatment can also inhibit browning by destroying the hydroxyl and α-helical structures of polyphenol oxidase. A method for combining tartaric acid and plasma-activated water for fresh-cut fruits and vegetables has also been disclosed in the prior art.

[0006] However, the above-mentioned existing technologies all involve directly dissolving organic acids in plasma-activated water to form an "organic acid-plasma-activated water mixture" before soaking fresh-cut fruits and vegetables. Although this method can enhance the preservation effect of PAW to a certain extent, it still has the following shortcomings: (1) After organic acids and PAW are pre-mixed in the solution, some active species may react with organic acids in the liquid phase and be consumed, reducing the effective concentration of active species reaching the fruit and vegetable cut surfaces; (2) A large number of active species in the mixture are evenly distributed in the liquid phase, lacking a directional effect on the fruit and vegetable cut surfaces, and the processing efficiency needs to be improved; (3) The pH values ​​of the cut surfaces of different types of fresh-cut fruits and vegetables vary greatly, and it is difficult to achieve the best adaptation for fruits and vegetables with different pH characteristics by using a single organic acid or a fixed ratio of organic acids.

[0007] To address the shortcomings of the existing technology, this invention provides a method for preserving fresh-cut fruits and vegetables by plasma-activated water combined with organic acid treatment. Summary of the Invention

[0008] (a) Purpose of the invention

[0009] This invention addresses the technical problems of existing technologies that involve directly dissolving organic acids in plasma-activated water, such as high consumption of active species, lack of directional action, and poor adaptability to different fruits and vegetables. It provides a method for preserving fresh-cut fruits and vegetables by using plasma-activated water in conjunction with organic acid treatment.

[0010] (II) Technical Solution

[0011] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0012] A method for preserving fresh-cut fruits and vegetables using plasma-activated water and synergistic organic acid treatment includes the following steps:

[0013] (1) pH classification of cut surface: The pH of fresh cut surface of fresh fruits and vegetables was measured and classified as follows: pH≤4.0 as category A, 4.0<pH≤5.5 as category B, and pH>5.5 as category C.

[0014] (2) Preparation of organic acid pretreatment solution: Dissolve food-grade organic acid in water to prepare an aqueous solution with a total mass fraction of 0.8%–1.6% and a pH of 2.5–4.5; wherein:

[0015] The organic acid pretreatment solution corresponding to Class A consists of ascorbic acid, citric acid and tartaric acid in a mass ratio of (0.5-1.5):(0.3-0.8):(0.2-0.5);

[0016] The organic acid pretreatment solution corresponding to type B consists of tartaric acid, citric acid and lactic acid in a mass ratio of (0.3-0.8):(0.2-0.5):(0.1-0.3);

[0017] The organic acid pretreatment solution corresponding to type C consists of citric acid, tartaric acid, and malic acid in a mass ratio of (0.5–1.0):(0.2–0.5):(0.1–0.3).

[0018] (3) Formation of pre-acid film: Freshly cut fruits and vegetables are immersed in the pre-treatment solution obtained in step (2) at 4-15℃ for 1-5 minutes. After being taken out, they are not rinsed, so that the mass of the surface-attached liquid accounts for 0.5%-3.0% of the mass of the fruits and vegetables, and an organic acid pre-treatment film with a thickness of 5-40μm is formed on the cut surface. The organic acid pre-treatment film is not pre-mixed with plasma-activated water in subsequent steps, but is used as an independent attachment layer for fruits and vegetables before entering the activated water.

[0019] (4) Preparation of plasma-activated water: Pure water is treated with a dielectric barrier discharge device at a power of 300-500W for 30-60 minutes to obtain plasma-activated water with a pH of 2.0-3.5, an oxidation-reduction potential of 550-750mV, and a conductivity of 1500-3000μS / cm.

[0020] (5) Synergistic penetration soaking: The fresh-cut fruits and vegetables with the organic acid pre-film attached to the surface in step (3) are transferred into the plasma-activated water in step (4) and soaked at 4-15℃ for 3-10 minutes. During the soaking, intermittent ultrasound with a frequency of 28-35kHz and a power of 150-250W is applied. The ultrasound mode is 10 seconds of ultrasound every 30 seconds of soaking, so that the active oxygen / active nitrogen species in the plasma-activated water can penetrate the organic acid pre-film to the cut surface of the fruits and vegetables in a directional gradient.

[0021] (6) Post-processing: The fresh-cut fruits and vegetables treated in step (5) are rinsed with sterile water for 10-30 seconds, and then dried by airflow at ≤15℃ and wind speed of 1-3m / s until the surface water activity aw≤0.99. Then, modified atmosphere packaging is carried out, and the gas composition inside the packaging is O2 5%-10%, N2 80%-90%, CO2 5%-10%. Finally, it is refrigerated at 0-4℃.

[0022] Preferably, the fruits and vegetables of type A in step (1) are selected from strawberries, pineapples or citrus slices; the fruits and vegetables of type B are selected from cucumber slices or bell pepper strips; and the fruits and vegetables of type C are selected from apple slices, potato slices or pear slices.

[0023] Preferably, the local pH of the organic acid pre-membrane formed on the cut surface in step (3) is 2.5 to 4.5. During the settling period after the pre-membrane is transferred into plasma-activated water and before the ultrasonic treatment is started, there is only molecular diffusion-level material exchange between the liquid film on the cut surface and the main body of activated water. The pre-membrane maintains the phase structure integrity of the independent liquid film layer and establishes a gradient transition layer of active species concentration between the membrane phase and the main liquid phase of activated water.

[0024] Preferably, the intermittent ultrasound in step (5) is started within 1 to 2 minutes after the start of immersion, and the cumulative ultrasound time accounts for 25% to 35% of the total immersion time.

[0025] Preferably, the dielectric barrier discharge in step (4) adopts an underwater coaxial cylindrical electrode structure, the discharge gas is air, and the water temperature is controlled at 4 to 15°C during the process.

[0026] Preferably, after the synergistic penetration soaking in step (5), the mass of the organic acid pre-film remaining on the fruit and vegetable cut surface accounts for 40% to 70% of the initial amount of adhesion in step (3), and this residual film is partially removed in the sterile water rinsing step in step (6).

[0027] Preferably, in step (2), the ascorbic acid in the type A pretreatment solution is L-ascorbic acid, and the tartaric acid is D-tartaric acid; the malic acid in the type C pretreatment solution is L-malic acid.

[0028] Preferably, after the airflow drying is completed in step (6) and before modified atmosphere packaging, the surface temperature of the fruits and vegetables should not rise by more than 2°C compared to before drying.

[0029] Beneficial effects

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. This invention innovatively employs a two-step process: first, an organic acid pre-film is formed, and then PAW active species are allowed to penetrate the cut surface through the film in a directional gradient. This avoids the consumption of active species caused by pre-mixing organic acids and PAW in the liquid phase. Compared to a control method that directly dissolves organic acids of the same composition in plasma-activated water for the same dosage treatment, this method can increase the logarithmic reduction of total bacterial count in fresh-cut fruits and vegetables at the end of storage at 4°C by an additional ≥0.8lgCFU / g, and reduce the browning index ΔE of the cut surface by ≥15%.

[0032] 2. This invention categorizes fruits and vegetables into three types based on the pH of their cut surfaces, and designs specific compound organic acid formulations for each type. Type A (pH ≤ 4.0) uses a combination of ascorbic acid, citric acid, and tartaric acid, balancing antioxidant and antibacterial properties; Type B (4.0 < pH ≤ 5.5) uses a combination of tartaric acid, citric acid, and lactic acid, focusing on antibacterial activity and pH buffering; Type C (pH > 5.5) uses a combination of citric acid, tartaric acid, and malic acid, emphasizing browning inhibition and flavor preservation. This differentiated and precise design enables this invention to achieve excellent preservation effects for different types of fresh-cut fruits and vegetables.

[0033] 3. In this invention, intermittent ultrasound (28–35 kHz, 150–250 W, 10 s ultrasound every 30 s of immersion) at a specific frequency and power is applied during the synergistic penetration immersion stage. The cavitation effect and microjets of the ultrasound promote the directional migration of active species in the PAW (polyaluminum sorbent membrane) to the cut surface of fruits and vegetables. Simultaneously, the intermittent mode avoids the mechanical damage that continuous ultrasound may cause to the fruit and vegetable tissues. Before ultrasound initiation, the organic acid pre-membrane maintains its intact structure, ensuring the orderly progress of gradient penetration.

[0034] 4. This invention involves a series of processes after synergistic penetration soaking, including sterile water rinsing (to remove residual film and some surface microorganisms), low-temperature airflow drying (controlling water activity aw≤0.99), modified atmosphere packaging (O2 5%~10%, N2 80%~90%, CO2 5%~10%), and finally refrigeration at 0~4℃, forming a complete preservation technology chain to ensure the quality stability of fresh-cut fruits and vegetables during storage. Attached Figure Description

[0035] Figure 1 This is a schematic flowchart of the fresh-cut fruit and vegetable preservation method using plasma-activated water and organic acid treatment according to the present invention.

[0036] Figure 2 This is a bar chart comparing the browning index of fresh-cut apple slices stored at 4°C for 0–12 days, according to an embodiment of the present invention. Detailed Implementation

[0037] The present invention will be further described in detail below with reference to embodiments and comparative examples, but the present invention is not limited to the following embodiments. All raw materials used in the embodiments are food-grade, and all equipment used is conventional equipment in the art.

[0038] I. Preparation of Plasma-Activated Water

[0039] In the following examples and comparative examples, the preparation method of plasma activated water (PAW) is as follows: using a dielectric barrier discharge device with an underwater coaxial cylindrical electrode structure, using air as the discharge gas, pure water is treated at a power of 400W for 40 minutes, and the water temperature is controlled at 10℃ during the treatment process, to obtain plasma activated water with a pH of 2.8, an oxidation-reduction potential of 625mV, and a conductivity of 2200μS / cm.

[0040] II. Implementation Examples

[0041] Example 1

[0042] This embodiment uses fresh-cut apple slices (Category C, cut surface pH≈5.8) as the treatment object.

[0043] (1) pH classification of cut surface: The pH of fresh cut apple slices was determined to be 5.8, which is classified as C.

[0044] (2) Preparation of organic acid pretreatment solution: Dissolve food-grade citric acid, tartaric acid and L-malic acid in water at a mass ratio of 0.8:0.3:0.2 to prepare an aqueous solution with a total mass fraction of 1.2% and a pH of 3.5.

[0045] (3) Formation of pre-acid film: Freshly cut apple slices are immersed in the above pre-treatment solution at 10°C for 3 minutes. After being taken out, they are not rinsed, so that the mass of the surface-attached liquid accounts for 1.5% of the mass of the fruit and vegetables, forming an organic acid pre-treatment film with a thickness of about 20 μm on the cut surface.

[0046] (4) Preparation of plasma-activated water: PAW is prepared according to the above method.

[0047] (5) Synergistic penetration immersion: Freshly cut apple slices with organic acid pre-film on the surface were transferred into PAW and immersed at 10°C for 6 min; intermittent ultrasound was started 1.5 min after the start of immersion (frequency 30kHz, power 200W, ultrasound for 10s every 30s of immersion, and the cumulative ultrasound time accounted for about 33% of the total immersion time).

[0048] (6) Post-processing: The processed fresh-cut apple slices are rinsed with sterile water for 20 seconds, then dried by airflow at 10℃ and 2m / s until the surface water activity aw≤0.99, then packaged with modified atmosphere packaging (O28%, N285%, CO27%), and finally refrigerated at 4℃.

[0049] Example 2

[0050] This embodiment uses fresh-cut pineapple slices (Category A, cut surface pH≈3.5) as the treatment object.

[0051] (1) pH classification of cut surface: The pH of fresh cut surface of fresh pineapple slices was determined to be 3.5, which was classified as category A.

[0052] (2) Preparation of organic acid pretreatment solution: Dissolve food-grade L-ascorbic acid, citric acid and D-tartaric acid in water at a mass ratio of 1.0:0.5:0.3 to prepare an aqueous solution with a total mass fraction of 1.0% and pH 3.0.

[0053] (3) Formation of pre-acid film: Immerse freshly cut pineapple slices in the above pre-treatment solution at 8°C for 2 minutes. After taking them out, do not rinse them. The mass of the surface-attached liquid accounts for 1.0% of the mass of the fruit and vegetables, forming an organic acid pre-treatment film with a thickness of about 15 μm on the cut surface.

[0054] (4) Preparation of plasma-activated water: Same as in Example 1.

[0055] (5) Synergistic penetration soaking: Freshly cut pineapple slices with organic acid pre-film on the surface are transferred into PAW and soaked at 8°C for 5 min; intermittent ultrasound is started 1 min after soaking begins (frequency 28kHz, power 180W, ultrasound for 10s every 30s of soaking).

[0056] (6) Post-processing: Same as in Example 1.

[0057] Example 3

[0058] This embodiment uses freshly cut potato slices (Category C, cut surface pH≈6.2) as the treatment object.

[0059] (1) pH classification of cut surface: The pH of fresh cut surface of fresh potato slices was measured to be 6.2, which was classified as C.

[0060] (2) Preparation of organic acid pretreatment solution: Dissolve food-grade citric acid, tartaric acid and L-malic acid in water at a mass ratio of 0.6:0.4:0.2 to prepare an aqueous solution with a total mass fraction of 1.4% and pH 3.8.

[0061] (3) Formation of pre-acid film: Freshly cut potato slices are immersed in the above pre-treatment solution at 12°C for 4 minutes. After being taken out, they are not rinsed, so that the mass of the surface-attached liquid accounts for 2.0% of the mass of the fruit and vegetables, forming an organic acid pre-treatment film with a thickness of about 30 μm on the cut surface.

[0062] (4) Preparation of plasma-activated water: Same as in Example 1.

[0063] (5) Synergistic penetration soaking: Freshly cut potato slices with organic acid pre-film on the surface are transferred into PAW and soaked at 12℃ for 8 min; intermittent ultrasound is started 2 min after the start of soaking (frequency 35kHz, power 220W, ultrasound for 10s every 30s of soaking).

[0064] (6) Post-processing: Same as in Example 1.

[0065] III. Comparative Example

[0066] Comparative Example 1 (without any processing)

[0067] Fresh-cut apple slices of the same variety and specifications as in Example 1 were taken and subjected to the same post-processing as in Example 1 (sterile water rinsing for 20 seconds, airflow drying, modified atmosphere packaging) without any preservation treatment, and then refrigerated at 4°C.

[0068] Comparative Example 2 (PAW Processing Only)

[0069] Fresh-cut apple slices of the same variety and specifications as in Example 1 were taken and directly immersed in PAW prepared with the same parameters as in Example 1 without undergoing organic acid pre-membrane treatment. They were soaked at 10°C for 6 minutes (without ultrasound). The remaining post-processing steps were the same as in Example 1, and the slices were refrigerated at 4°C.

[0070] Comparative Example 3 (Organic acid directly dissolved in PAW – control method)

[0071] Fresh-cut apple slices of the same variety and size as in Example 1 were used. Organic acids (citric acid, tartaric acid, and L-malic acid in a mass ratio of 0.8:0.3:0.2) with the same composition as the pretreatment solution in Example 1 (Category C) were directly dissolved in PAW (total organic acid mass fraction 1.2%) to prepare a mixed treatment solution. The fresh-cut apple slices were immersed in this mixed treatment solution at 10°C for 6 minutes (without ultrasound). The remaining post-treatment steps were the same as in Example 1, and the slices were refrigerated at 4°C. This comparative example simulates the prior art method of directly dissolving organic acids in PAW.

[0072] Comparative Example 4 (Organic acid pre-membrane treatment without ultrasound application)

[0073] Take fresh-cut apple slices of the same variety and specifications as in Example 1, form an organic acid pre-film according to steps (1) to (3) of Example 1, and then transfer them into PAW and soak them at 10°C for 6 minutes. However, no ultrasonic assistance is applied during the soaking period. The remaining steps are the same as in Example 1, and the slices are refrigerated at 4°C.

[0074] IV. Performance Testing and Result Analysis

[0075] 1. Testing Method

[0076] Fresh-cut fruit and vegetable samples treated in Examples 1-3 and Comparative Examples 1-4 were refrigerated at 4°C, and samples were taken on day 0, day 4, day 8, and day 12 of storage to determine the following indicators:

[0077] (1) Total bacterial count: The total bacterial count was determined in accordance with GB4789.2-2022 National Food Safety Standard for Microbiological Examination of Food, and the result was expressed as logarithm (lgCFU / g).

[0078] (2) Browning index (ΔE): The L, a, and b* values ​​of the cut surface of fruits and vegetables are measured using a colorimeter, and the color difference ΔE is calculated with an untreated fresh sample as a reference. The larger the ΔE value, the more severe the browning.

[0079] (3) Polyphenol oxidase (PPO) activity: determined by conventional methods and expressed as U / g.

[0080] 2. Test Results

[0081] Table 1. Changes in total bacterial count (lg CFU / g) of fresh-cut apple slices treated in each example and comparative example during storage at 4°C.

[0082] Example 1 2.1±0.2 3.4±0.3 4.8±0.4 5.9±0.3 Example 2 2.0±0.2 3.2±0.3 4.5±0.3 5.7±0.4 Example 3 2.3±0.3 3.6±0.3 5.0±0.4 6.1±0.4 Comparative Example 1 (Unprocessed) 3.5±0.3 6.2±0.4 8.5±0.5 9.8±0.5 Comparative Example 2 (PAW alone) 2.5±0.2 4.8±0.3 6.9±0.4 8.2±0.4 Comparative Example 3 (Organic acid directly dissolved in PAW) 2.3±0.2 4.1±0.3 5.9±0.4 7.2±0.4 Comparative Example 4 (Pre-coating membrane + no ultrasound) 2.2±0.2 3.9±0.3 5.5±0.4 6.8±0.4

[0083] As can be seen from the data in Table 1:

[0084] (1) The total number of colonies in the untreated Comparative Example 1 sample increased rapidly during storage, reaching 9.81g CFU / g on the 12th day, which was significantly higher than that of the treatment groups.

[0085] (2) Although Comparative Example 2 (PAW treatment alone) can inhibit microbial growth to a certain extent, the effect is limited. The total number of colonies on day 12 was 8.21g CFU / g.

[0086] (3) The antibacterial effect of Comparative Example 3 (treatment with organic acid directly dissolved in PAW) was better than that of PAW treatment alone. The total number of colonies on day 12 was 7.21g CFU / g, indicating that the synergistic effect of organic acid and PAW can indeed enhance the antibacterial effect.

[0087] (4) The antibacterial effect of Comparative Example 4 (pre-membrane treatment but without ultrasound) was better than that of Comparative Example 3. The total number of colonies on the 12th day was 6.81g CFU / g, indicating that the "pre-membrane" mode itself has certain advantages over the "organic acid directly dissolved in PAW" mode.

[0088] (5) Examples 1-3 (the complete scheme of the present invention) showed the best antibacterial effect, with total bacterial counts of 5.9, 5.7, and 6.11 g CFU / g on day 12, respectively. Compared with Comparative Example 3, the logarithmic value of the total bacterial count in Example 1 at the end of storage was 1.31 g CFU / g lower than that in Comparative Example 3. That is, compared with the control method of directly dissolving organic acids of the same composition in PAW, the method of the present invention reduced the total bacterial count by an additional 1.31 g CFU / g (superior to the technical effect of ≥0.81 g CFU / g in the claims).

[0089] Table 2. Changes in browning index (ΔE) of fresh-cut apple slices treated in each example and comparative example during storage at 4°C.

[0090] Example 1 2.5±0.3 5.8±0.5 9.2±0.6 12.5±0.7 Example 2 2.3±0.3 5.5±0.5 8.8±0.6 12.0±0.7 Example 3 2.8±0.3 6.1±0.5 9.6±0.6 13.0±0.7 Comparative Example 1 (Unprocessed) 4.2±0.4 10.5±0.7 16.8±0.8 22.3±0.9 Comparative Example 2 (PAW alone) 3.0±0.3 7.8±0.6 13.5±0.7 18.6±0.8 Comparative Example 3 (Organic acid directly dissolved in PAW) 2.8±0.3 7.0±0.5 11.8±0.7 16.2±0.8 Comparative Example 4 (Pre-coating membrane + no ultrasound) 2.6±0.3 6.5±0.5 10.5±0.6 14.8±0.7

[0091] As can be seen from the data in Table 2:

[0092] (1) The untreated Comparative Example 1 sample showed the most severe browning, with a ΔE value as high as 22.3 on day 12.

[0093] (2) Comparative Example 2 (PAW treatment alone) can inhibit browning to a certain extent, with a ΔE value of 18.6 on day 12. PAW treatment can reduce surface browning of fresh-cut apples, which is consistent with the literature.

[0094] (3) The browning inhibition effect of Comparative Example 3 (organic acid directly dissolved in PAW) was better than that of PAW alone, with a ΔE value of 16.2 on day 12. This is because the addition of organic acid further reduced the pH of the treatment solution, which enhanced the inhibitory effect of browning-related enzymes.

[0095] (4) The browning inhibition effect of Comparative Example 4 (pre-membrane treatment but no ultrasound) was even better than that of Comparative Example 3, with a ΔE value of 14.8 on day 12.

[0096] (5) Examples 1-3 (the complete scheme of the present invention) showed the best browning inhibition effect, with ΔE values ​​of 12.5, 12.0 and 13.0 on day 12, respectively. Compared with Comparative Example 3, the ΔE value of Example 1 at the end of storage was 3.7 lower than that of Comparative Example 3. That is, compared with the control method of directly dissolving organic acids of the same composition in PAW, the browning index ΔE of the method of the present invention was reduced by about 22.8% (ΔE decreased from 16.2 to 12.5, the reduction was (16.2-12.5) / 16.2×100%≈22.8%, which is better than the technical effect of ≥15% in the claims).

[0097] V. Conclusion and Analysis

[0098] Based on the test results of the above embodiments and comparative examples, the following conclusions can be drawn:

[0099] 1. The "pre-film-gradient penetration" synergistic treatment mode of this invention is significantly superior to existing technologies. Comparative Example 3 simulates the existing treatment method of directly dissolving organic acids in PAW, while Examples 1-3 adopt the treatment method of this invention, which involves "first forming an organic acid pre-film, then intermittently using ultrasound-assisted gradient penetration". Experimental data show that the method of this invention is significantly superior to the existing "organic acid directly dissolved in PAW" mode in terms of antibacterial effect (an additional reduction of ≥1.3lgCFU / g in total bacterial count at the end of storage) and browning inhibition effect (a relative reduction of ≥22.8% in ΔE). The fundamental reason for this advantage is that in this invention, the organic acid pre-film exists as an independent attachment layer on the cut surface of fruits and vegetables. The active species in PAW need to pass through this film to reach the cut surface, realizing "directional gradient penetration" and avoiding the consumption of active species caused by the pre-mixing of organic acids and PAW in the liquid phase.

[0100] 2. Intermittent ultrasound assistance is an indispensable key element of this invention. The only difference between Comparative Example 4 and Example 1 is whether or not intermittent ultrasound was applied. Experimental results show that Comparative Example 4, without ultrasound, was significantly lower than Example 1 in both antibacterial and browning inhibition (total colony count on day 12: 6.8 vs 5.9 lg CFU / g; ΔE: 14.8 vs 12.5). This indicates that the cavitation effect and microjets of intermittent ultrasound can effectively promote the directional migration of active species in PAW to the cut surface of fruits and vegetables, enhancing the efficiency of gradient penetration.

[0101] 3. The pH classification of cut surfaces and the differentiated organic acid compounding schemes have practical significance. Examples 1 (apple slices, type C), 2 (pineapple slices, type A), and 3 (potato slices, type C) all achieved good preservation effects, indicating that the three organic acid compounding schemes designed based on pH classification of cut surfaces in this invention have good adaptability to different types of fresh-cut fruits and vegetables.

[0102] 4. This invention has broad application prospects. The method of this invention is simple to operate, does not use chemical bactericides, and leaves no harmful residues. It can be widely used in the preservation of various fresh-cut fruits and vegetables such as apples, pears, potatoes, strawberries, pineapples, citrus fruits, cucumbers, and bell peppers, effectively extending shelf life and providing good economic and social benefits.

[0103] The above description is merely a preferred embodiment and comparative example of the present invention, and is not intended to limit the patent scope of the present invention. Any equivalent structural or procedural transformations made using the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preserving fresh-cut fruits and vegetables using plasma-activated water and synergistic organic acid treatment, characterized in that, Includes the following steps: (1) pH classification of cut surface: The pH of fresh cut surface of fresh fruits and vegetables was measured and classified as follows: pH≤4.0 into category A, 4.0<pH≤5.5 into category B, and pH>5.5 into category C; (2) Preparation of organic acid pretreatment solution: Dissolve food-grade organic acid in water to prepare an aqueous solution with a total mass fraction of 0.8%–1.6% and a pH of 2.5–4.5; wherein: The organic acid pretreatment solution corresponding to Class A consists of ascorbic acid, citric acid and tartaric acid in a mass ratio of (0.5-1.5):(0.3-0.8):(0.2-0.5); The organic acid pretreatment solution corresponding to type B consists of tartaric acid, citric acid and lactic acid in a mass ratio of (0.3-0.8):(0.2-0.5):(0.1-0.3); The organic acid pretreatment solution corresponding to type C consists of citric acid, tartaric acid, and malic acid in a mass ratio of (0.5–1.0):(0.2–0.5):(0.1–0.3). (3) Formation of pre-acid film: Freshly cut fruits and vegetables are immersed in the pre-treatment solution obtained in step (2) at 4-15℃ for 1-5 minutes. After being taken out, they are not rinsed, so that the mass of the surface-attached liquid accounts for 0.5%-3.0% of the mass of the fruits and vegetables, and an organic acid pre-treatment film with a thickness of 5-40 μm is formed on the cut surface. The organic acid pre-treatment film is not pre-mixed with plasma-activated water in subsequent steps, but is used as an independent attachment layer for fruits and vegetables before entering the activated water. (4) Preparation of plasma-activated water: Pure water is treated with a dielectric barrier discharge device at a power of 300-500W for 30-60 minutes to obtain plasma-activated water with a pH of 2.0-3.5, an oxidation-reduction potential of 550-750mV, and a conductivity of 1500-3000μS / cm; (5) Synergistic penetration soaking: The fresh-cut fruits and vegetables with the organic acid pre-film attached to the surface in step (3) are transferred into the plasma-activated water in step (4) and soaked at 4-15℃ for 3-10 minutes; during the soaking period, intermittent ultrasound with a frequency of 28-35kHz and a power of 150-250W is applied, and the ultrasound mode is 10 seconds of ultrasound every 30 seconds of soaking, so that the active oxygen / active nitrogen species in the plasma-activated water can penetrate the organic acid pre-film to the cut surface of the fruits and vegetables in a directional gradient. (6) Post-processing: The fresh-cut fruits and vegetables treated in step (5) are rinsed with sterile water for 10-30 seconds, and then dried by airflow at ≤15℃ and wind speed of 1-3m / s until the surface water activity aw≤0.

99. Then, modified atmosphere packaging is carried out, and the gas composition inside the packaging is O2 5%-10%, N2 80%-90%, CO2 5%-10%. Finally, it is refrigerated at 0-4℃.

2. The method according to claim 1, characterized in that, In step (1): Category A fruits and vegetables are selected from strawberries, pineapples or citrus slices; Category B fruits and vegetables are selected from cucumber slices or bell pepper strips; Category C fruits and vegetables are selected from apple slices, potato slices or pear slices.

3. The method according to claim 1, characterized in that, In step (3), the local pH of the organic acid pre-membrane formed on the cut surface is 2.5 to 4.

5. During the settling period after the pre-membrane is transferred into plasma-activated water and before the ultrasonic treatment is started, there is only molecular diffusion-level material exchange between the liquid film on the cut surface and the main body of activated water. The pre-membrane maintains the phase structure integrity of the independent liquid film layer and establishes a gradient transition layer of active species concentration between the membrane phase and the main liquid phase of activated water.

4. The method according to claim 1, characterized in that, In step (5), the intermittent ultrasound is started within 1 to 2 minutes after the start of immersion, and the cumulative ultrasound time accounts for 25% to 35% of the total immersion time.

5. The method according to claim 1, characterized in that, The dielectric barrier discharge described in step (4) uses an underwater coaxial cylindrical electrode structure, the discharge gas is air, and the water temperature is controlled at 4 to 15°C during the process.

6. The method according to claim 1, characterized in that, After the synergistic penetration soaking in step (5), the mass of the organic acid pre-film remaining on the fruit and vegetable cut surface accounts for 40% to 70% of the initial amount attached in step (3). This residual film is partially removed in the sterile water rinsing step in step (6).

7. The method according to claim 1, characterized in that, In step (2): the ascorbic acid in the type A pretreatment solution is L-ascorbic acid, and the tartaric acid is D-tartaric acid; the malic acid in the type C pretreatment solution is L-malic acid.

8. The method according to claim 1, characterized in that, In step (6), after the airflow drying is completed and before the modified atmosphere packaging is completed, the surface temperature of the fruits and vegetables should not rise by more than 2°C compared to before drying.

9. The method according to any one of claims 1 to 8, characterized in that, Compared to the control method that directly dissolves organic acids of the same composition in plasma-activated water and treats them with the same dosage, the method of the present invention increases the logarithmic reduction value of the total bacterial count of fresh-cut fruits and vegetables at the end of storage at 4°C by an additional ≥0.8lgCFU / g, and reduces the browning index ΔE of the cut surface by ≥15%.

10. The method according to any one of claims 1 to 8, characterized in that: The organic acid pretreatment solution is used only as a physical adhesion film on the cut surface of fresh-cut fruits and vegetables before they enter the plasma-activated water. It is not directly dissolved in the plasma-activated water, nor is it pre-dispersed in the plasma-activated water body. During the plasma-activated water soaking stage, the active species in the plasma-activated water are able to penetrate the cut surface of the fruits and vegetables through the organic acid pretreatment film via the intermittent ultrasound.

Images