Freeze-dried fresh-keeping vegetables and preparation method thereof

Abstract

The invention relates to the technical field of vegetable fresh-keeping, in particular to freeze-dried fresh-keeping vegetables and a preparation method thereof. The method comprises a raw material pretreatment stage, blanching enzyme deactivation, fluidized quick-freezing and vacuum freeze-drying, color protection treatment needs to be carried out after cutting in the raw material pretreatment stage, and in the color protection treatment process, a composite color protection solution is adopted to carry out soaking color protection on cut vegetables; the composite color protection liquid comprises ascorbic acid, tea polyphenol, citric acid and deionized water; the activity of polyphenol oxidase (PPO) is directly inhibited through ascorbic acid (reducibility) in the composite color-protecting solution, tea polyphenol is competitively combined with an enzyme activity center and doubly blocks browning reaction, so that the surfaces of freeze-dried vegetables are free of obvious brown spots, the color is close to a fresh-cut state, and the composite color-protecting solution is capable of protecting the surfaces of the freeze-dried vegetables through redox relay in cooperation with vacuum freeze-drying. The retention rate of vitamin C is increased, the nutritional value of vegetables is retained, the safety risk of chemical additives is avoided, and the food safety detection standard is met.

Description

Technical Field

[0001] The present invention relates to the technical field of vegetable fresh-keeping, and more specifically, to a freeze-dried fresh-keeping vegetable and a preparation method thereof. Background Art

[0002] As a non-staple food, vegetables are essential in people's daily lives. With the improvement of people's living standards, they are gradually developing towards being beneficial to bodybuilding, healthy, diverse, and multi-flavored. However, in some areas of our country, especially in the Northeast region, due to geographical environmental conditions, only some cold-resistant vegetables can be produced, and the variety is relatively monotonous. Therefore, a considerable part of the vegetables need to be transported from the central plains region.

[0003] Before transporting vegetables, it is necessary to carry out freeze-drying and fresh-keeping on the vegetables. During the pre-treatment process of the vegetables, traditional color protection methods use sulfites (such as sodium metabisulfite) or synthetic antioxidants (such as BHA) to effectively protect color and keep fresh. In addition, there is also a green color protection technology, which uses single ascorbic acid for color protection.

[0004] Currently, the traditional color protection technology has the following key defects: (1) Sulfite residues cause allergic reactions (the risk for asthma patients increases by 50%), and may generate carcinogens (such as nitrosamines), posing a safety risk to the human body; (2) Single ascorbic acid has unstable color protection effects (the browning index fluctuates by ±2.0), poor synergism, resulting in a loss of vitamin C (>40%), a chlorophyll degradation rate (>30%), causing a loss of the nutritional value of the vegetables themselves, and leading to the generation of pheophytin due to the demetallization reaction of the vegetables, resulting in color browning. Summary of the Invention

[0005] The purpose of the present invention is to provide a freeze-dried fresh-keeping vegetable and a preparation method thereof. By using a composite color protection solution of ascorbic acid, citric acid, and tea polyphenols, the activity of polyphenol oxidase is directly inhibited, the browning reaction is blocked, and the internal nutritional components of the vegetables are retained to solve the problems in the above background art, that is:

[0006] The traditional pre-treatment of vegetables using sulfites and single ascorbic acid for color protection results in nutritional loss of vegetables and poor browning inhibition effect.

[0007] To achieve the above purpose, the preparation method of the freeze-dried fresh-keeping vegetable includes a color protection treatment in the raw material pre-treatment stage. During the color protection treatment, the cut vegetables are soaked and protected from color using a composite color protection solution;

[0008] The composite color protection solution includes the following components:

[0009] Ascorbic acid, accounting for 0.1% - 0.2% of the total solution mass;

[0010] Tea polyphenols, accounting for 0.03% - 0.05% of the total solution mass;

[0011] Citric acid, accounting for 0.05% of the total solution mass;

[0012] Deionized water, accounting for 99.7% - 99.82% of the total solution mass.

[0013] In the above technical solution, the ratio of ascorbic acid, tea polyphenols and citric acid is 0.2% ascorbic acid, 0.03% tea polyphenols and 0.05% citric acid. Among them, ascorbic acid reduces quinone substances and inhibits the Maillard reaction, i.e., non-enzymatic browning. Tea polyphenols can scavenge free radicals, block the lipid oxidation chain reaction, and reduce the degradation of photosensitive pigments such as chlorophyll and carotenoids. Citric acid can chelate metal ions (Fe 3+ / Cu 2+ ), reducing the metal-catalyzed oxidation rate.

[0014] On this basis, tea polyphenols (lipid-soluble antioxidants) and ascorbic acid (water-soluble antioxidants) form a cross-phase synergy to form an antioxidant network, comprehensively protecting water-soluble and lipid-soluble nutrients, avoiding the loss of vitamin C and polyphenolic substances, preserving the nutritional components of vegetables themselves. Citric acid maintains the pH value and inhibits the autooxidation of vitamin C in an acidic environment, while delaying the non-enzymatic browning of vegetables.

[0015] In another technical solution, 0.2% ascorbic acid, without affecting the sensory quality of vegetables themselves and considering the diminishing marginal benefit, can maximize the retention rate of vitamin C in vegetables and the browning inhibition effect;

[0016] The use of 0.03% tea polyphenols can effectively inhibit polyphenol oxidase, and the production of quinones is reduced by 80% - 95%, effectively reducing the browning index of freeze-dried vegetables. The use of 0.05% tea polyphenols is also applicable to dark-colored fruits and vegetables and cold-chain prefabricated dishes with long-term storage requirements, making the color protection effect of vegetables last. Tea polyphenols and ascorbic acid act synergistically, penetrate into the interior of vegetables, and inhibit the browning of deep tissues of vegetables;

[0017] The use of 0.05% citric acid maintains the pH value stability of the composite color protection solution, prevents the autooxidation of ascorbic acid, and enables the synergistic effect of tea polyphenols and ascorbic acid in the composite color protection solution, allowing tea polyphenols and ascorbic acid to form a reduction cycle.

[0018] On the other hand, the present invention provides a preparation method for ascorbic acid, tea polyphenols, and citric acid described in any one of the above, including:

[0019] Preparation of ascorbic acid: The raw materials (taking acerola cherry as an example) are crushed with a blade rotation speed of 3000 - 4500 rpm, and ultrasonic-assisted extraction is used with a frequency of 20 kHz for 30 min. After extraction, macroporous resin is used for purification with a particle size of 0.3 - 1.25 mm and an adsorption capacity of > 80 mg / g. The purified ascorbic acid is vacuum-concentrated through a falling-film evaporator and finally vacuum-dried for crystallization to obtain.

[0020] Preparation of tea polyphenols: Fresh tea leaves are dried in hot air at 80 °C until the moisture content ≤ 5%, and then crushed to a particle size ≤ 0.5 mm. Ultrasonic extraction is used with a frequency of 28 kHz, a power density of 120 W / L, and a time of 15 min. Nanofiltration membrane is used for purification, and finally, a centrifugal spray drying tower is used to dry the raw materials with an inlet air temperature of 180 °C and an outlet air temperature of 80 °C.

[0021] Preparation of citric acid: After preparing the culture medium, molasses is used as the main fermentation substrate and fermented in a stirred ventilation fermentation tank at a set temperature of 30 - 32 °C. After fermentation, calcium citrate is obtained by the calcium salt precipitation method. At this time, the ion exchange purification process is used to remove Ca 2+ , and purification is carried out. Finally, concentrated crystallization is performed to obtain citric acid crystals with a purity ≥ 99.5%.

[0022] According to Figure 1 shown, the present invention provides a preparation method of a composite color protection solution, including:

[0023] S1. Heat deionized water to 40 °C, add citric acid and stir until completely dissolved. The mass of citric acid is 0.5 g, accounting for 0.05% of the total mass of the color protection solution;

[0024] S2. When the citric acid solution cools to below 30 °C, add ascorbic acid and continue to stir and dissolve. The mass of ascorbic acid is 2 g, accounting for 0.2% of the total mass of the color protection solution;

[0025] S3. Add tea polyphenols and stir for 10 - 15 minutes until the solution is clear and free of precipitation. The mass of tea polyphenols is 0.3 g, accounting for 0.03% of the total mass of the color protection solution.

[0026] According to Figure 1 shown, the present invention also provides a preparation method of freeze-dried fresh-keeping vegetables, including:

[0027] S4. Sorting, cleaning and cutting of vegetable raw materials. After cutting, soak the vegetables in the composite color protection solution for 10 min;

[0028] S5. Blanch and inactivate enzymes of the color-protected vegetables using a steam blanching machine at a temperature of 95 - 105 °C; steam pressure: 0.1 - 0.3 MPa;

[0029] S6. Pre-cool the blanched vegetables to 0 - 4°C to reduce the freezing load. Blow cold air at -30°C to -40°C into the bottom at a wind speed of 5 - 10 m / s to suspend the food particles and make them evenly cooled.

[0030] S7. Use an industrial freeze dryer to conduct vacuum freeze-drying on the vegetables. The pre-freezing temperature is -40°C to -50°C, and the vacuum degree is 10 - 50 Pa.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] 1. In this freeze-dried fresh-keeping vegetable and its preparation method, the ascorbic acid (reductive property) in the composite color protection liquid directly inhibits the activity of polyphenol oxidase (PPO). Tea polyphenols competitively bind to the enzyme active center, doubly blocking the browning reaction, making the surface of the freeze-dried vegetables have no obvious brown spots and the color approaching the fresh-cut state. The composite color protection liquid, through "oxidation-reduction relay", cooperates with vacuum freeze-drying to improve the retention rate of vitamin C and retain the nutritional value of the vegetables themselves.

[0033] 2. In this freeze-dried fresh-keeping vegetable and its preparation method, the ascorbic acid, tea polyphenols and citric acid components in the composite color protection liquid are all prepared by natural preparation methods. During the fresh-keeping process of the freeze-dried vegetables, the safety risks of chemical additives are avoided, there is no toxic residue of sulfite, and the generation of drug resistance by molds and yeasts is avoided, meeting the food safety detection standards. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is the overall process flow block diagram of the preparation method of the freeze-dried fresh-keeping vegetable of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present invention.

[0036] Example 1: Preparation of ascorbic acid solution and color protection effect and VC retention rate (taking spinach as an example)

[0037] (1) Preparation: Heat deionized water to 40 ± 2°C. Add deionized water to a light-shielding container. Take ascorbic acid powder (0.1% - 0.2% w / v) and dissolve it in deionized water. Stir in the dark for 20 min until completely dissolved (under low light conditions (<50 Lux)). Add citric acid (0.05% w / v) to the ascorbic acid solution. By stirring, adjust the pH value of the ascorbic acid solution to 3.8 - 4.2 (for inhibiting the activity of polyphenol oxidase).

[0038] (2) Experimental data: Fresh spinach (with intact leaves and a moisture content of 92%-95%) was immersed in a color-preserving solution (solution temperature at 40±2°C) for 10 minutes. The immersion results are shown in the following table.

[0039] Table 1 Inhibitory effect of ascorbic acid concentration on spinach browning (ΔE value):

[0040]

[0041]

[0042] It can be seen from Table 1 that when the ascorbic acid concentration is 0.3%, the browning index ΔE of fresh spinach is <2.7, and the chlorophyll retention rate is 89%, with the best effect.

[0043] Table 2 Reference table of ascorbic acid concentration for VC retention rate of spinach:

[0044] Ascorbic acid concentration (%) Initial VC content (mg / 100g) VC retention rate after freeze-drying (%) 0 (control group) 45.2±1.5 50±3 0.1 45.2±1.5 88±2 0.15 45.2±1.5 89±1 0.2 45.2±1.5 90±1 0.3 45.2±1.5 91±1

[0045] In Table 2, the retention rate of VC is above 88% after color preservation with ascorbic acid from the initial content of 45.2±1.5 mg / 100 g of fresh spinach. Ascorbic acid reduces the oxidative loss of VC by reducing quinone substances and competitively inhibiting polyphenol oxidase (PPO).

[0046] From the above two tables, it can be seen that when the ascorbic acid concentration is 0.3%, the browning inhibition rate is the best and the oxidative loss of VC is the least. However, when the ascorbic acid concentration is greater than 0.2%, the reaction gradually approaches the enzyme inhibition saturation point or substrate exhaustion, and the inhibition rate increases slowly, resulting in a decreasing marginal benefit. Also, considering the real - world scenario during consumption, when the ascorbic acid concentration >0.2%, the sour taste of ascorbic acid is significantly enhanced, affecting the sensory quality of vegetables and causing a sour and astringent feeling. Therefore, in this solution, an ascorbic acid concentration of 0.1%-0.2% is preferably used.

[0047] Example 2: Preparation of citric acid solution and its marginal benefit test

[0048] Preparation: Heat deionized water to 40±2°C, add citric acid (0.05% w / w), stir until completely dissolved, with a stirring speed of 300 rpm for 5 minutes, and then detect the pH value of the solution, with the target range of 3.8 - 4.2.

[0049] Synergistic effect and marginal benefit test of the retention rate of citric acid VC: When the concentration of citric acid is 0.05%, the pH is 4.2, and the PPO activity is inhibited by more than 90%. Citric acid inhibits the activity of polyphenol oxidase (PPO) by reducing the pH of the color protection solution (from 6.8 to 4.2), blocking the enzymatic browning chain reaction. However, when the concentration > 0.05%, the pH further decreases (e.g., from 0.1% concentration to pH 3.9), but the browning inhibition rate only increases by 6.3% (ΔE increases from 1.8 to 1.6), and the marginal benefit significantly decreases. Moreover, when the concentration of citric acid is 0.05%, it can prevent the auto-oxidation of ascorbic acid, enabling the synergistic effect of polyphenols and vitamins in the composite color protection solution, and allowing tea polyphenols and ascorbic acid to form a reduction cycle. Therefore, in this example, a citric acid concentration of 0.05% is preferably used.

[0050] Example 3: Preparation and performance test of tea polyphenols

[0051] Preparation: Crush the green tea or tea powder raw materials to a particle size ≤ 0.5 mm, and control the moisture content ≤ 5% to reduce oxidation. Then, perform ultrasonic extraction at a frequency of 28 kHz, a power density of 120 W / L, and a time of 15 min to fully dissolve the tea polyphenols. After the extract is vacuum filtered to remove the residue, it is concentrated to 1 / 10 of the original volume by rotary evaporation (temperature ≤ 50°C, vacuum degree -0.08 MPa), and then purified using a nanofiltration membrane with an operating pressure of 2.0 MPa. Finally, the raw materials are dried using a centrifugal spray drying tower with an inlet air temperature of 180°C and an outlet air temperature of 80°C.

[0052] Browning inhibition performance test: Cut fresh vegetables (such as spinach) into thin slices of the same specification (thickness 1 - 2 mm), and soak them in tea polyphenol solutions with concentrations of 0.03% and 0.05% in combination with ascorbic acid with a concentration of 0.2% for 10 minutes; after draining, place them in an environment of 25°C and a relative humidity of 85% for 24 hours, and use a color difference meter to measure the L (lightness), a (red - green value), and b (yellow - blue value) of the treated vegetables. The test results show that the addition of tea polyphenols in combination with ascorbic acid can form an oxidation - reduction relay, reducing the amount of quinone generated by 80% - 95%. At the same time, it makes up for the problem that the penetration ability of ascorbic acid cannot reach the deep tissues of vegetables. Considering that real - life edible vegetables will not have a bitter taste, in this example, a tea polyphenol concentration of 0.03% is preferably used.

[0053] Example 4: The present invention provides verification of the production of a composite color protection solution, vegetable browning inhibition, and VC retention rate

[0054] (1) Add 0.3 g of tea polyphenols to deionized water at 50°C, stir until completely dissolved (rotation speed 500 rpm, time 10 minutes). At the same time, place 2 g of ascorbic acid and 0.5 g of citric acid in a separate light - proof container, add the remaining normal - temperature deionized water, and stir to dissolve (rotation speed 300 rpm, time 5 minutes);

[0055] After the raw materials are dissolved and premixed, slowly add the tea polyphenol solution to the ascorbic acid-citric acid mixture, turn on the high-speed shear emulsifier, with a rotation speed of 2000 rpm and a time of 15 minutes, and control the temperature ≤ 40 °C to homogenize the solution;

[0056] After homogenization, pass through the ultrafiltration membrane system (pressure 0.2 MPa, flow rate 50 L / h) to remove macromolecular impurities, and finally perform UHT instant sterilization and cool to 25 °C to obtain a composite color protection solution (0.03% tea polyphenol + 0.2% ascorbic acid + 0.05% citric acid).

[0057] Vegetable immersion color protection and freshness preservation experiment test: The treatment time gradients are 5 minutes, 10 minutes, 20 minutes (the concentration of the color protection solution is fixed at 100%), and the concentration gradients of the composite color protection solution are 50%, 100%, 150% (the treatment time is fixed at 10 minutes). The experimental data are shown in the following table:

[0058] Table 3 Test table at different immersion times when the concentration of the color protection solution is fixed at 100%

[0059] Treatment conditions Browning index (ΔE) VC retention rate (%) Soaking for 5 minutes 3.2±0.3 75±2 Soaking for 10 minutes 2.1±0.2 82±1 Soaking for 20 minutes 1.9±0.3 80±2

[0060] Table 4 Test table at different concentrations of the color protection solution when the treatment time is fixed at 10 minutes

[0061] Treatment conditions Browning index (ΔE) VC retention rate (%) Color protection solution with 50% concentration 4.5±0.4 65±3 Color protection solution with 100% concentration 2.1±0.2 82±1 Color protection solution with 150% concentration 2.0±0.3 78±2

[0062] According to Table 3, when the concentration of the composite color protection solution is fixed at 100%, the 10-minute immersion is the optimal condition, with a significant decrease in the browning index and the best VC retention rate;

[0063] According to Table 4, when the treatment time is fixed at 10 minutes, the concentration of the composite color protection solution remaining at 100% is the optimal condition, also maintaining a significant optimization of the browning index and the highest VC retention rate. When the concentration is higher, the VC retention rate will show a downward trend, and in reality, there will be astringency in the taste of consumption.

[0064] Example 5: Compared with Example 4, in this example, the process is adjusted. The concentration of ascorbic acid in the composite color protection solution is reduced to 0.1%, and other parameters are the same as in Example 4. The tea polyphenol concentration is 0.03% and the citric acid concentration is 0.05%. The test results are shown in Table 5.

[0065] Table 5 Comparative test between Example 4 and Example 5

[0066]

[0067]

[0068] As shown in Table 5, with the decrease in ascorbic acid concentration, the browning index increases, the chlorophyll retention rate decreases, and the sour taste sensory score increases. It is suitable for fruits and vegetables sensitive to acid, such as tender leafy vegetables (such as lettuce, spinach seedlings). Reducing the acid concentration can avoid the decrease in leaf brittleness while maintaining the color protection effect.

[0069] Example 6: Compared with Example 4, in this example, the process was adjusted, and the tea polyphenol concentration in the compound color protection liquid was increased to 0.05%, and other parameters were the same as those in Example 4. The test results are shown in Table 6.

[0070] Table 6 Comparative Test between Example 4 and Example 6

[0071] Index Example 6 Example 4 Browning index (ΔE) 2.1±0.3 2.7±0.3 PPO inhibition rate (%) 82±4 65±3 Sensory score of sour taste ( / 10) 7.6±0.5 (obvious bitter taste) 8.5±0.3 (slight sour taste)

[0072] As shown in Table 6, with the increase in tea polyphenol concentration, the browning index decreases, the inhibition rate of PPO (polyphenol oxidase) increases, and the sour taste sensory score decreases. It is suitable for dark-colored fruits and vegetables (such as purple cabbage, blueberries) and cold-chain prefabricated dishes with long-term storage requirements. The color protection effect is lasting, the chlorophyll / anthocyanin retention rate increases, and in dark-colored fruits and vegetables such as purple cabbage or blueberries, the bitter taste is easily masked, and at the same time, the stability of antioxidant pigments (anthocyanins) increases.

[0073] Comparative Example 1: Compared with Example 4, in this comparative example, ascorbic acid was not used, and it only contained tea polyphenol and citric acid.

[0074] Test data:

[0075] Browning out of control: The inhibition of the activity of polyphenol oxidase (PPO) is insufficient, and quinone substances cannot be reduced in time, resulting in the rapid accumulation of melanin; in fresh-cut potato samples, the browning index (ΔE) deteriorated from 2.1 ± 0.2 (soaked in 100% concentration color protection liquid for 10 minutes) in Example 4 to 6.2 ± 0.7.

[0076] Color distortion: Although the acidic environment (citric acid) inhibits PPO, the lack of the repair effect of ascorbic acid on oxidized pigments makes the food materials prone to become dull;

[0077] Decrease in metal chelation efficiency: Ascorbic acid can assist citric acid in binding Fe 3+ / Cu 2+ , and after removal, the metal ion residue increases by 20%, catalyzing the oxidation reaction.

[0078] Comparative Example 2: Compared with Example 4, in this comparative example, tea polyphenol was not used, and it only contained ascorbic acid and citric acid.

[0079] Test data:

[0080] Decline in color protection uniformity: The browning index (ΔE) deteriorated from 2.1 ± 0.2 in Example 4 (soaked in 100% concentration color protection solution for 10 minutes) to 4.8 ± 0.6. Additionally, the permeability of ascorbic acid is weaker than that of tea polyphenols, and the inhibition of browning in deep tissues is insufficient (such as the center of thick-cut potatoes turning black);

[0081] Nutritional value: The loss of polyphenols and fat-soluble vitamins increases, the stability of ascorbic acid decreases, and there is a lack of antioxidant protection of tea polyphenols. The oxidative loss of endogenous polyphenols in the food material (such as rutin and quercetin) increases by 15%.

[0082] Comparative Example 3: Compared with Example 4, this comparative example does not use citric acid and only contains ascorbic acid and tea polyphenols.

[0083] Test results:

[0084] Out-of-control enzymatic browning: The increase in pH value leads to the recovery of the activity of polyphenol oxidase (PPO), and the browning reaction rate increases by 2 to 3 times. The pH value in Example 4 is stable at 2.8 - 3.2. After removing citric acid, the pH value rises to 4.0 - 4.5;

[0085] Rapid loss of vitamin C: The auto-oxidation rate of ascorbic acid increases when pH > 4.0, and the loss rate reaches 35% in 24 hours;

[0086] Synergistic failure of tea polyphenols and ascorbic acid: Tea polyphenols are oxidized to quinones due to the increase in PPO activity and cannot form a reduction cycle with ascorbic acid.

[0087] As shown in Table 5, Table 6, and Comparative Examples 1, 2, and 3, in the present invention, ascorbic acid, citric acid, and tea polyphenols act synergistically. (1) Citric acid reduces the pH to 3.0 (inhibiting PPO activity), and tea polyphenols directly bind to Cu 2+ (double-blocking enzyme activity), forming the first browning inhibition process for fresh vegetables to avoid enzymatic browning; (2) Ascorbic acid directly reduces quinone substances and cooperates with tea polyphenols to scavenge free radicals (synergistic antioxidant), forming the second browning inhibition process to avoid non-enzymatic oxidative browning. (3) Citric acid (strong chelation) + tea polyphenols (auxiliary chelation) synergistically chelate Fe 3+ / Cu 2+ forming the third browning inhibition process for fresh vegetables to avoid metal-catalyzed browning, and meeting the food safety standards: Tea polyphenols (plant extraction), ascorbic acid (vitamin C), and citric acid are all food additives permitted by GB 2760, and there is no need for components such as EDTA and phytic acid that may introduce metal chelation residues.

[0088] Example 7: Compared with Example 4, this comparative example only uses ascorbic acid (i.e., the single ascorbic acid in the traditional color protection technology).

[0089] Table 7 Comparative Test between Example 4 and Example 7 (24 hours)

[0090] Index Example 4 Example 7 Browning index (ΔE) 2.1±0.3 4.5±0.5 DPPH scavenging rate 92％±2％ 78％±3％ pH stability 3.4 (original 3.2) 3.8 (original 2.8)

[0091] As shown in Table 7, compared with the single ascorbic acid in Example 7, the composite color protection liquid in Example 4 has a browning index of 4.5±0.5 for the single ascorbic acid within 24 hours after color protection of vegetables, while the browning index of the composite color protection liquid is 2.1±0.3, and the browning inhibition rate is increased by 65%, showing better performance in delaying oxidation and discoloration;

[0092] In addition, in terms of the DPPH scavenging rate, the scavenging rate of the composite color protection liquid is higher, because tea polyphenols synergistically enhance the free radical scavenging ability with ascorbic acid;

[0093] In addition, in terms of the pH value stability of the color protection liquid, the pH value of the composite color protection liquid only increases from the original 3.2 to 3.4. On the contrary, the pH value of the single ascorbic acid increases from the original 2.8 to 3.8, with a larger fluctuation range. Relatively speaking, the composite color protection liquid has more stable pH and can effectively inhibit the activity of polyphenol oxidase.

[0094] Secondly, the composite color protection liquid in the present invention maintains the physical stability of freeze-dried vegetables (taking freeze-dried spinach as an example). The rehydration rate after 12 months of storage can still be increased from 68% of the rehydration rate of untreated freeze-dried spinach to 92%; and the integrity rate of the crispy slices is maintained above 90% (ascorbic acid in the composite color protection liquid penetrates into the cell gaps, reducing the mechanical damage of ice crystals to the cell wall during the freeze-drying process); Long-term storage performance: The acidic environment (pH 3.0-3.5) inhibits the growth of bacteria. Combined with the antibacterial effect of tea polyphenols, the total number of colonies is reduced by two orders of magnitude, and the total number of microorganisms < 10 3 .

[0095] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and the descriptions in the specification are only preferred examples of the present invention and are not used to limit the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing freeze-dried fresh-keeping vegetables, characterized in that: It includes a raw material pretreatment stage, blanching and enzyme inactivation, fluidized quick-freezing, and vacuum freeze-drying. Among them, after slicing in the raw material pretreatment stage, a color protection treatment is required. During the color protection treatment, the sliced vegetables are soaked in a composite color protection liquid for color protection. The composite color protection liquid includes the following components: Ascorbic acid, accounting for 0.1%-0.2% of the total solution mass; Tea polyphenols, accounting for 0.03%-0.05% of the total solution mass; Citric acid, accounting for 0.05% of the total solution mass; Deionized water, accounting for 99.7%-99.82% of the total solution mass.

2. The preparation method of freeze-dried fresh-keeping vegetables according to claim 1, wherein, The preparation of the ascorbic acid includes the following steps: S1.

1. Crush the raw materials and use ultrasonic-assisted extraction; S1.

2. Purify using macroporous resin with a particle size of 0.3-1.25 mm and an adsorption capacity > 80 mg / g; S1.

2. Vacuum-concentrate the purified ascorbic acid through a falling-film evaporator, and then vacuum-dry to obtain ascorbic acid crystals with a purity ≥ 99%.

3. The preparation method of freeze-dried fresh-keeping vegetables according to claim 1, wherein, The preparation of the tea polyphenols includes the following steps: S2.

1. Dry fresh tea leaves at 80°C by hot air until the moisture content ≤ 5%, and crush them to a particle size ≤ 0.5 mm; S2.

2. Use ultrasonic extraction with a frequency of 28 kHz, a power density of 120 W / L, and a time of 15 min; S2.

3. Purify using a nanofiltration membrane, and finally dry the raw materials using a centrifugal spray drying tower with an inlet air temperature of 180°C and an outlet air temperature of 80°C.

4. The preparation method of freeze-dried fresh-keeping vegetables according to claim 1, characterized in that, The preparation of the citric acid includes the following steps: S3.

1. Prepare a culture medium and ferment it in a fermenter; S3.

2. Use the calcium salt precipitation method to obtain calcium citrate; S3.

3. Remove Ca in it by using the ion exchange purification process 2+ , and conduct purification. Finally, concentrate and crystallize to obtain citric acid crystals with a purity of ≥99.5%.

5. The preparation method of freeze-dried fresh-keeping vegetables according to claim 1, characterized in that, The preparation method of the composite color protection liquid includes the following steps: S4.

1. Heat the deionized water to 40°C, add citric acid, and stir until completely dissolved; S4.

2. When the citric acid solution cools to below 30°C, add ascorbic acid and continue to stir and dissolve; S4.

3. Add tea polyphenols and stir for 10-15 minutes until the solution is clear and free of precipitation.

6. The preparation method of freeze-dried fresh-keeping vegetables according to claim 5, characterized in that: In S4.1, when stirring the citric acid solution, the rotation speed is 300 rpm, the time is 5 min, and the pH value is controlled at 3.8-4.

2.

7. The preparation method of freeze-dried fresh-keeping vegetables according to claim 1, characterized in that: When preparing the composite color protection liquid, a light-proof container needs to be used to avoid photocatalytic oxidation.

8. The preparation method of freeze-dried fresh-keeping vegetables according to claim 1, characterized in that, The raw material pretreatment stage and blanching and enzyme inactivation include the following steps: S5.

1. Sort, wash, and slice vegetable raw materials. After slicing, soak the vegetables in the composite color protection liquid for 10 min; S5.

2. Use a steam blanching machine to blanch and inactivate the enzymes of the color-protected vegetables at a temperature of 95-105°C and a steam pressure of 0.1-0.3 MPa.

9. The preparation method of freeze-dried fresh-keeping vegetables according to claim 1, characterized in that, The fluidized quick-freezing and vacuum freeze-drying include the following steps: S6.

1. Pre-cool the blanched vegetables to 0-4°C to reduce the freezing load. Blow cold air at -30°C to -40°C into the bottom at a wind speed of 5-10 m / s to make the food particles suspended and evenly cooled; S6.

2. Use an industrial freeze-dryer to vacuum freeze-dry the vegetables. The pre-freezing temperature is -40°C to -50°C, and the vacuum degree is 10-50 Pa.

10. A freeze-dried fresh-keeping vegetable prepared by the method for preparing a freeze-dried fresh-keeping vegetable according to any one of claims 1-9, characterized in that: Freeze-dried fresh-keeping vegetables color-protected by a compound color-protecting liquid, including lettuce, spinach, kale, and potatoes, with no obvious brown spots on the surface, color close to the freshly cut state, and no chemical residues.

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