A method for preventing browning and preserving green color of frozen vegetables

Abstract

The present application belongs to the technical field of frozen vegetable processing, and specifically provides a green-protecting and browning-preventing method for frozen vegetables, which comprises the following steps: washing fresh vegetables after cutting or without cutting; blanching the washed vegetables in weak alkaline electrolytic water, cooling the blanched vegetables, soaking the cooled vegetables in a color-protecting soaking solution, then draining, quick-freezing, sub-packaging, and finally freezing and storing. The green-protecting and browning-preventing method has the dual synergistic effect of weak alkaline electrolytic water blanching and endogenous peroxidase inhibitor soaking color protection, thereby improving the stability of chlorophyll, inhibiting the activity of residual peroxidase, and inhibiting the enzymatic browning of frozen vegetables, so that the color quality of green vegetables can be maintained during long-term freezing and storage, which meets the green and healthy concept and is easy to popularize and apply.

Description

Technical Field

[0001] This invention belongs to the field of frozen vegetable processing technology, specifically a method for protecting the green color and preventing browning of frozen vegetables. Background Technology

[0002] In terms of product structure, quick-frozen vegetables include representative products such as spinach, komatsuna, broccoli, edamame, snow peas, asparagus, and edible fungi, as well as local specialty vegetables like toon and bracken. These advantageous products not only meet domestic market demand but are also exported in large quantities to international markets such as Japan, the EU, and the US. Green vegetables occupy an important position and are a major category, with bright green color being a key evaluation indicator for quick-frozen green vegetables. However, quick-frozen green vegetables often lose their commercial value due to fading, yellowing, and browning during processing and storage, resulting in significant economic losses. The core technological bottleneck for quick-frozen green vegetables is the browning problem, which is mainly caused by two factors: enzymatic browning and chlorophyll degradation. The former involves polyphenol oxidase and peroxidase catalyzing the oxidation of phenolic substances to produce brown substances, while the latter involves chlorophyll demagnesinizing under acidic conditions to become yellow pheophytin. my country's entry-exit inspection and quarantine industry standard, "Inspection Procedures for Imported and Exported Quick-Frozen Vegetables" (SN / T 0626-2011), uses peroxidase residual activity as a physicochemical inspection indicator for imported and exported quick-frozen vegetables. Generally, passivation is based on at least a positive or negative result. Some literature also reports using 5-10% of the initial peroxidase activity as the standard for the degree of passivation.

[0003] Preventing browning and protecting the green color of vegetables has become an urgent problem for green vegetable processing plants and foreign trade enterprises, and has also been an important research topic for a long time. Currently, the main methods include adding metal ions (such as zinc or copper ions) to the blanching solution to replace magnesium ions in chloroplasts, adding sodium bicarbonate or disodium hydrogen phosphate to adjust the pH value, and adding antioxidants such as sodium sulfate or sodium metabisulfite, citric acid, ascorbic acid, and L-ascorbic acid. For example, Chinese invention patent application number CN201410018182.5, "A Method for Using a Green-Protecting and Preservative Compound for Water Celery," discloses a green-protecting and preservative compound consisting of an aqueous solution of 500–1500 mg / L calcium lactate, 50–150 mg / L zinc acetate, and 15–45 mg / L salicylic acid. The invention patent application CN201510359272.5, entitled "A Manufacturing Process of a Green-Protecting, Crisp-Preserving, and Fresh-Maintaining Agent for Suaeda salsa," describes a blanching and green-protecting solution containing 200ppm CuSO4, 200ppm Zn(CH3COO)2, and 200ppm Na2SO3, with the pH value adjusted to 6.0. The invention patent application CN202311356884.X, entitled "A Composite Color-Protecting Agent for Protecting the Color of Green Vegetables in Pre-Cooked Dishes," discloses a composite color-protecting agent composed of green tea extract, calcium ascorbate, sodium citrate, sodium D-isoascorbate, and sodium chloride.

[0004] Currently, preventing browning and maintaining green color often involves a combination of various chemical color-protecting agents. However, with increasing consumer awareness of food safety and the growing popularity of the "clean label" concept, developing naturally derived color-protecting agents has become an inevitable trend in the industry. Reducing the use of chemical color-protecting agents not only meets the market's demand for healthy and natural products but is also an important direction for the food industry's transformation towards green and sustainable practices. Summary of the Invention

[0005] In view of this, the purpose of this application is to provide a method for protecting the green color and preventing browning of frozen vegetables, so as to reduce the use of chemical color-protecting agents and improve the effect of protecting the green color and preventing browning.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: A method for protecting the green color and preventing browning of frozen vegetables includes the following steps: Wash freshly picked vegetables, either cut or leave whole. After washing, the vegetables are blanched in weakly alkaline electrolyzed water, cooled, and then soaked in a color-protecting soaking solution. After draining, quick-freezing, and packaging, they are finally frozen.

[0007] As an example, the method for preparing the weakly alkaline electrolyzed water is as follows: using any one or more of sodium chloride, potassium chloride, calcium chloride solution or tap water as the electrolyte, the weakly alkaline electrolyzed water is prepared using a water electrolysis device with a diaphragm, and the pH of the weakly alkaline electrolyzed water is 7 to 9.

[0008] As an example, the pH of weakly alkaline electrolyzed water is 7.5–8.5.

[0009] To prevent chlorophyll from losing magnesium and turning yellow under acidic conditions, the higher the concentration of sodium chloride, potassium chloride, calcium chloride, and magnesium sulfate solutions in the preparation of weakly alkaline electrolyzed water, the higher the pH of the produced weakly acidic electrolyzed water. This necessitates the use of more water for dilution and adjustment, or the electrolysis equipment should have a built-in pH adjustment function to obtain weakly alkaline electrolyzed water with a preset pH.

[0010] As an example, the blanching temperature is 95–100℃, and the blanching time is 3–5 minutes. The purpose of blanching is to inactivate the peroxidase that causes browning. Because peroxidase is heat-resistant, and because excessive blanching will cause vegetables to become mushy, currently, moderate blanching is mainly used. This inactivates the peroxidase to a negative or slightly positive state, but some peroxidase activity will still remain.

[0011] As an example, cooling after blanching is achieved using either weakly alkaline electrolyzed water or cold air cooling.

[0012] As an example, the color-protecting soaking solution includes an endogenous peroxidase inhibitor and weakly alkaline electrolyzed water with a pH of 7–9.

[0013] As an example, the preparation method of endogenous peroxidase inhibitor is as follows: remove the leaves and stems of vegetables, leaving only the leaves, and pulp them into a slurry. Add bentonite and pectinase to the slurry, then clarify and filter to obtain clear juice.

[0014] The mass fraction of bentonite in the slurry is 0.2-0.5% (mass percentage), and the concentration of pectinase added to the slurry is 20-50 mg / L (i.e., 20-50 mg of pectinase is added per liter of slurry).

[0015] During the freezing and storage of leafy green vegetables, the discoloration mainly occurs in the stems and leaves, while the leaves themselves remain relatively unchanged. This is primarily because leaves contain higher levels of endogenous peroxidase inhibitors, such as polyphenols and ascorbic acid. Protecting the green color and preventing browning is more significant for the stems and leaves. Extracting the endogenous peroxidase inhibitors, such as polyphenols and ascorbic acid, from the leaves using processing aids and using them to inhibit peroxidase activity in the stems and leaves, thereby preventing browning, is a natural, endogenous method of color protection that better meets the current demand for green and pollution-free vegetables.

[0016] As an example, the preparation method of the color-protecting soaking solution is as follows: add clear juice to weakly alkaline electrolyzed water to prepare the color-protecting soaking solution; The volume of the clear juice is 10-20% of the total volume of the color-protecting soaking solution.

[0017] As an example, after cooling, vegetables are soaked in a color-protecting soaking solution for 5-15 minutes, which helps the vegetables' own peroxidase inhibitors suppress the activity of residual peroxidase.

[0018] When soaking for color protection, the leaves and stems can be separated, and the stems should be treated separately for color protection soaking.

[0019] This method of processing leaves and stems separately can save on the amount of vegetable leaf juice used.

[0020] As an example, the freezing temperature is -30 to -40°C, allowing green vegetables to quickly pass through the zone of maximum ice crystal formation.

[0021] The storage period is 12 months when frozen at -18℃.

[0022] The present invention has the following beneficial effects: This invention provides a method for protecting the green color and preventing browning of frozen vegetables. This method improves chlorophyll stability and inhibits residual peroxidase activity by using a dual synergistic effect of blanching with weakly alkaline electrolyzed water and soaking in endogenous peroxidase inhibitors to prevent enzymatic browning of frozen vegetables. This allows the vegetables to maintain their green color and quality even after long-term frozen storage.

[0023] This invention provides a method for protecting the green color and preventing browning of frozen vegetables. It uses weakly alkaline electrolyzed water instead of adding alkaline substances such as sodium bicarbonate and calcium hydroxide, and combines various chemical greening agents such as zinc or copper ions. Endogenous peroxidase inhibitors in vegetable leaves, such as polyphenols and ascorbic acid, are extracted using processing aids and used to inhibit peroxidase in leaf and stem tissues. This is a natural, endogenous color-protecting method that aligns with green and healthy concepts and is easy to promote and apply. Attached Figure Description

[0024] Figure 1 The process flow diagram of the method for protecting the green color and preventing browning of frozen vegetables in this application is shown. Detailed Implementation

[0025] To make the technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0026] The various instruments, equipment, raw materials or reagents used in the embodiments of this invention are not subject to any special restrictions on their source. They are all conventional products that can be purchased through regular commercial channels and can be prepared according to conventional methods known to those skilled in the art.

[0027] In addition, unless otherwise stated, “room temperature” in this instruction manual means 25±2℃.

[0028] The following is a detailed description of a method for blanching and preserving the green color of frozen vegetables according to this application.

[0029] Experiment Example 1: Investigating the effect of blanching time on POD activity and leaf and stem brittleness of frozen spinach. Step 1, Washing: Wash the freshly picked spinach; Step 2, Blanching: Blanching solution is tap water, blanch at 98℃ for 1-10 minutes to deactivate peroxidase that causes browning.

[0030] Step 3, Cooling: Immerse in water to cool until it reaches near room temperature.

[0031] Step 4, Quick-freezing: After draining the water, quick-freeze at -35°C to allow the green vegetables to quickly pass through the zone of maximum ice crystal formation. For draining, place the cooled spinach on a perforated stainless steel tray and leave it for 10-30 minutes until no water drips. The draining method in the following examples and comparative examples is the same as in this example.

[0032] Step 5, Packaging: Packaging bags are used for individual packaging.

[0033] Step 6: Freeze-freeze at -18℃ for 12 months.

[0034] Step 7, POD activity detection: The residual activity of peroxidase was detected using the method of the Chinese entry-exit inspection and quarantine industry standard "Inspection Procedures for Imported and Exported Quick-Frozen Vegetables" (SN / T 0626-2011). At the same time, the fragility of spinach leaves and stems was evaluated. The results are shown in Table 1.

[0035] Table 1. Effects of blanching time on POD guaiacol color development and stem breakage in spinach leaves and stems.

[0036] Note: In Table 1 above, "+" indicates a positive result, "±" indicates a small number of positive results, and "-" indicates a negative result.

[0037] As shown in Table 1 above, when blanching time is 1–3 minutes, POD activity shows a positive reaction; when blanching time is 4–6 minutes, POD activity shows a small positive reaction; and when blanching time exceeds 7 minutes, POD activity shows a negative reaction. However, combined with the results on the fragility of leaf stems, it can be seen that as the blanching time is extended, the spinach leaf stems become increasingly fragile, losing their chewiness and thus their commercial value.

[0038] Therefore, moderate blanching is more in line with actual needs. Thus, even after moderate blanching (3-5 minutes), spinach stems still retain a certain amount of peroxidase activity, and there is still a risk of enzymatic browning.

[0039] Example 2: Investigating the effect of weakly alkaline electrolyzed water prepared with different electrolytes on the color of spinach leaves and stems during frozen storage. A method for blanching frozen vegetables to preserve their green color includes the following steps: Step 1, Washing: Wash the freshly picked spinach; Step 2: Preparation of weakly alkaline electrolyzed water: Using tap water and a 0.02% salt solution (sodium chloride, potassium chloride, calcium chloride, or magnesium sulfate) as the electrolyte, weakly alkaline electrolyzed water is prepared using a water electrolysis device with a diaphragm. Then, water (tap water or factory-prepared soft water) is added for dilution. The final weakly alkaline electrolyzed water has a pH of 8.0.

[0040] Step 3, Blanching: The blanching solution is the weakly alkaline electrolyzed water with pH 8.0 prepared in Step 2 above. Blanch at 98℃ for 4 minutes to passivate the peroxidase that causes browning.

[0041] Step 4, Cooling: Immerse in water to cool. The water is the weakly alkaline electrolyzed water prepared in Step 2 above. Cool to near room temperature.

[0042] Step 5, quick freezing: Freezing temperature of -35℃ allows green vegetables to quickly pass through the zone of maximum ice crystal formation.

[0043] Step 6, Packaging: Packaging bags are used for individual packaging.

[0044] Step 7, Freezing: Store at -18℃ for 12 months. The results of the freezing test are shown in Table 2.

[0045] Table 2. Effects of different electrolytes on the color of spinach leaves and stems during frozen storage.

[0046] As shown in Table 2 above, electrolytes containing potassium chloride, calcium chloride, and magnesium sulfate can achieve a similar color-protecting effect to sodium chloride, maintaining the light green color of spinach leaves and stems for four months. However, using tap water as the electrolyte has a relatively weaker color-protecting effect.

[0047] Example 3: Investigating the effect of weakly alkaline electrolyzed water with different pH values ​​on the color of spinach leaves and stems during frozen storage. A method for blanching frozen vegetables to preserve their green color includes the following steps: Step 1, Washing: Wash the freshly picked spinach; Step 2: Preparation of weakly alkaline electrolyzed water: Using 0.02% sodium chloride as the electrolyte, weakly alkaline electrolyzed water is prepared using a water electrolysis device with a diaphragm. The pH of the weakly alkaline electrolyzed water is adjusted to 6.5, 7.0, 7.5, 8.0, 8.5, and 9.0.

[0048] Step 3, Blanching: The blanching solution is the weakly alkaline electrolyzed water prepared in step 2. Blanch at 98°C for 4 minutes to passivate the peroxidase that causes browning.

[0049] Step 4, Cooling: Immerse in water to cool. The water is the weakly alkaline electrolyzed water prepared in Step 2 above. Cool to near room temperature.

[0050] Step 5, quick-freeze: After draining the water, quick-freeze at -35℃ for 30 minutes to allow the green vegetables to quickly pass through the zone of maximum ice crystal formation. The draining method is the same as in Example 1 above.

[0051] Step 6, Packaging: Packaging bags are used for individual packaging.

[0052] Step 7, Freezing: Store at -18℃ for 12 months. The results of the freezing test are shown in Table 3.

[0053] Table 3. Effects of weakly alkaline electrolyzed water at different pH values ​​on the color of spinach leaves and stems during frozen storage.

[0054] As shown in Table 3 above, weakly alkaline electrolyzed water at a pH above 7.0 can prevent the leaves and stems from turning yellow during the blanching process. The treatment differences between pH 7.5 and 9.0 are relatively small. However, higher pH values ​​mean higher electrolyte usage, i.e., higher costs. Therefore, a pH range of 7.5–8.0 is considered the most suitable pH range for the blanching solution.

[0055] Example 4: Effects of different amounts of endogenous peroxidase inhibitor on the color development, breakage, and leaf and stem color of spinach leaves and stems (POD). A method for blanching frozen vegetables to preserve their green color includes the following steps: Step 1, Washing: Wash the freshly picked spinach; Step 2: Preparation of weakly alkaline electrolyzed water: Using a 0.02% sodium chloride solution as the electrolyte, weakly alkaline electrolyzed water is prepared using a water electrolysis device with a diaphragm. The pH of the weakly alkaline electrolyzed water is adjusted to 8.0 to prevent chlorophyll from losing magnesium and turning yellow under acidic conditions.

[0056] Step 3, Blanching: The blanching solution is the prepared weakly alkaline electrolyzed water. Blanch at 98℃ for 4 minutes to passivate the peroxidase that causes browning.

[0057] Because peroxidase is heat-resistant, and because excessive blanching will make vegetables mushy, the current practice is mainly to blanch them moderately. This can deactivate peroxidase activity to negative or a small amount of positive, but some peroxidase activity will still remain.

[0058] Step 4, Cooling: Immerse in water to cool. The water should be weakly alkaline electrolyzed water. Cool until it reaches near room temperature.

[0059] Step 5: Preparation of endogenous peroxidase inhibitor: Remove the leaves and stems from the spinach, leaving only the leaves. Pulverize the pulp, add 0.3% bentonite and 30 mg / L pectinase, then clarify and filter. The upper filtrate is the spinach leaf juice.

[0060] During the freezing and storage of leafy vegetables, the discoloration mainly occurs in the stems and leaves, while the leaves themselves do not change color easily. This is because leaves contain higher levels of endogenous peroxidase inhibitors, such as polyphenols and ascorbic acid.

[0061] Step 6, Color-protecting soaking: Prepare a solution of 0-25% (by volume) spinach leaf extract using pH 8.0 weakly alkaline electrolyzed water. Specifically, for example, to prepare a solution of 15% spinach leaf extract using pH 8.0 weakly alkaline electrolyzed water, add 15L of spinach leaf extract to 85L of weakly alkaline electrolyzed water. Then, soak the spinach in the color-protecting soaking solution for 10 minutes.

[0062] Endogenous peroxidase inhibitors are extracted from leaves and used to protect the green color of frozen vegetables. This allows the vegetables' own peroxidase inhibitors to suppress residual peroxidase activity, thereby preventing browning.

[0063] Step 7, quick-freeze: After draining the water, quick-freeze at -35℃ for 30 minutes to allow the green vegetables to quickly pass through the zone of maximum ice crystal formation.

[0064] Step 8, Packaging: Packaging bags are used for individual packaging.

[0065] Step 9: Freeze-freeze at -18℃ for 12 months.

[0066] Step 10, POD activity detection: The residual activity of peroxidase was detected using the method of the Chinese entry-exit inspection and quarantine industry standard "Inspection Procedures for Imported and Exported Quick-Frozen Vegetables" (SN / T 0626-2011). At the same time, the fragility of spinach leaves and stems was evaluated. The results are shown in Table 4.

[0067] Table 4. Effects of different amounts of spinach leaf extract on POD color development and breakage of spinach leaves and stems.

[0068] As shown in Table 4 above, after blanching at 98℃ for 4 minutes, the addition of 0-5% spinach leaf juice resulted in a small amount of positive POD colorimetric reaction. However, when the amount added increased to over 10%, the POD colorimetric reaction turned negative. This indicates that the endogenous peroxidase inhibitors contained in the spinach leaf juice can effectively inhibit residual peroxidase activity by changing the colorimetric reaction from a small amount of positive to negative.

[0069] Meanwhile, the effects of different amounts of added spinach leaf juice on the color of spinach leaves and stems during frozen storage were investigated, and the specific results are shown in Table 5 below.

[0070] Table 5. Effects of different amounts of spinach leaf juice added on the color of spinach leaves and stems during frozen storage.

[0071] As shown in Table 5, consistent with the results, adding 0-5% spinach leaf extract has a limited inhibitory effect on peroxidase, which explains the light browning of the spinach stems after 8 months of frozen storage. However, adding at least 10% spinach leaf extract effectively inhibits peroxidase activity in the stems and leaves, thus preventing yellowing and browning for up to 12 months. Since higher concentrations of spinach leaf extract increase costs, a concentration of 10-20% is generally recommended. Of course, if it is necessary to extend the storage time, the dosage can be increased.

[0072] Example 5 This embodiment provides a method for blanching and preserving the green color of frozen vegetables, the specific process of which is as follows: Figure 1 As shown, it includes the following steps: Step 1, Washing: Wash the freshly picked spinach; Step 2: Preparation of weakly alkaline electrolyzed water: Using a 0.02% sodium chloride solution as the electrolyte, weakly alkaline electrolyzed water is prepared using a water electrolysis device with a diaphragm. The final weakly alkaline electrolyzed water has a pH of 8.0 to prevent chlorophyll from losing magnesium and turning yellow under acidic conditions.

[0073] Step 3, Blanching: The blanching solution is the weakly alkaline electrolyzed water prepared in step 2 above. Blanch at 98°C for 4 minutes to passivate the peroxidase that causes browning.

[0074] Because of the heat resistance of peroxidase, and because excessive blanching can actually make vegetables mushy, moderate blanching is currently necessary. This can deactivate peroxidase activity to negative or a small amount of positive activity. However, blanched spinach will still retain some peroxidase activity.

[0075] Step 4, Cooling: Immerse in water for cooling. The water is composed of weakly alkaline electrolyzed water. Cool until it reaches near room temperature.

[0076] Step 5: Preparation of endogenous peroxidase inhibitor: Remove the leaves and stems from the spinach, leaving only the leaves. Pulverize the pulp, add 0.3% bentonite and 30 mg / L pectinase, then clarify and filter to obtain spinach leaf juice.

[0077] Peroxidase is one of the key enzymes causing browning. It has high heat resistance and is generally used as an indicator of the degree of blanching in vegetables. Spinach leaf pulp is originally cloudy. By adding bentonite and pectinase, it causes the pulp to settle and becomes clear. Endogenous peroxidase inhibitors can then be extracted and prepared.

[0078] During the frozen storage of leafy vegetables, the discoloration mainly occurs in the stems and leaves, while the leaves themselves do not change color easily. This is because the leaves contain higher levels of endogenous peroxidase inhibitors, such as polyphenols and ascorbic acid.

[0079] Step 6, Color Protection Soaking: Using the weakly alkaline electrolyzed water with pH 8.0 prepared in Step 2 above, add 15% (the percentage here is a volume fraction; the preparation here is 15L of spinach leaf juice added to 85L of water) of spinach leaf juice. Then, soak the vegetables in the color protection soaking solution for 10 minutes to allow the peroxidase inhibitors in the vegetables to inhibit the activity of residual peroxidase.

[0080] The essence of this step is to extract endogenous peroxidase inhibitors from the leaves, which can be used to protect the green color of frozen vegetables, inhibit the activity of residual peroxidase after blanching, and thus prevent browning.

[0081] Step 7, quick-freeze: After draining the water, quick-freeze at -35℃ for 30 minutes to allow the green vegetables to quickly pass through the zone of maximum ice crystal formation.

[0082] Step 8, Packaging: Packaging bags are used for individual packaging.

[0083] Step 9: Freeze-freeze at -18℃ for 12 months.

[0084] Comparative Example 1 A method for blanching frozen vegetables to preserve their green color includes the following steps: Step 1, Washing: Wash the freshly picked spinach; Step 2, Preparation of weakly alkaline electrolyzed water: Using 0.02% sodium chloride solution as the electrolyte, weakly alkaline electrolyzed water is prepared using a water electrolysis device with a diaphragm, and the pH of the rinsing solution is adjusted to 8.0.

[0085] Step 3, Blanching: The blanching solution is the prepared weakly alkaline electrolyzed water. Blanch at 98℃ for 4 minutes to passivate the peroxidase that causes browning.

[0086] Step 4, Cooling: Immerse in water to cool. The water is composed of weakly alkaline electrolyzed water. Cool to near room temperature.

[0087] Step 5, quick-freeze: Freeze at -35℃ for 30 minutes to allow the green vegetables to quickly pass through the zone of maximum ice crystal formation.

[0088] Step 6, Packaging: Packaging bags are used for individual packaging.

[0089] Step 7: Freeze-freeze at -18℃ for 12 months.

[0090] Comparative Example 2 A method for blanching frozen vegetables to preserve their green color includes the following steps: Step 1, Washing: Wash the freshly picked spinach; Step 2, Blanching: The blanching solution is tap water. Blanch at 98℃ for 4 minutes to deactivate the peroxidase that causes browning.

[0091] Step 3, Cooling: Immerse in water to cool until it reaches near room temperature.

[0092] Step 4, quick-freeze: After draining the water, quick-freeze at -35 ℃ for 30 minutes to allow the green vegetables to quickly pass through the zone of maximum ice crystal formation.

[0093] Step 5, Packaging: Packaging bags are used for individual packaging.

[0094] Step 6: Freeze-freeze at -18℃ for 12 months.

[0095] The difference between Comparative Example 1 and Comparative Example 2 is that Comparative Example 1 only used weakly alkaline electrolyzed water for blanching and did not undergo soaking with endogenous peroxidase inhibitors for color protection; while Comparative Example 2 did not use either weakly alkaline electrolyzed water for blanching or soaking with endogenous peroxidase inhibitors for color protection. After quick-freezing, the three treatment groups were stored at -18℃, and samples were taken every 4 months to observe the color changes of spinach leaves and stems. The specific color change results are shown in Table 6 below.

[0096] Table 6. Effects of different treatments on the color of spinach leaves and stems during frozen storage.

[0097] As shown in Table 6 above, the color changes of spinach during frozen storage mainly occurred in the leaf stems, while the color stability of the leaves was relatively high. This is partly because the leaves have a relatively higher chlorophyll content, resulting in a darker color, and thus the impact of chlorophyll degradation and enzymatic browning on their color is relatively small. On the other hand, the leaves contain higher levels of endogenous peroxidase inhibitors, such as polyphenols and ascorbic acid, than the leaf stems, which prevents enzymatic browning.

[0098] Example 5 used weakly alkaline electrolyzed water rinsing and color-protecting soaking with endogenous peroxidase inhibitors. After 12 months of frozen storage, the leaves were bright green and the leaf stems were light green, similar to the initial color. Comparative Example 1 used weakly alkaline electrolyzed water rinsing, and the color remained good for the first 4 months, but the color change in the 8th month showed that the leaf stems turned light brown. Comparative Example 2 did not use weakly alkaline electrolyzed water rinsing or color-protecting soaking with endogenous antioxidant enzyme inhibitors. Its leaf stems initially showed a light yellow color instead of a light green color. This is mainly because the chlorophyll in the leaf stems degrades when exposed to heat and acid. Using tap water or softened water will cause it to fade from green to yellow. Therefore, the leaf stems initially appear light yellow. This is not the case with weakly alkaline electrolyzed water rinsing, which resulted in the leaf stems showing a light green color.

[0099] As can be seen from the data in Table 2 of Comparative Example 2 in Table 1 compared to Table 2 of Example 2, the color of Comparative Example 2 after rinsing with tap water electrolysis is light green, and the color turns light brown after being frozen for 4 months; while Comparative Example 2 here uses tap water without electrolysis, and the leaves and stems are initially light yellow, and the color turns yellowish-brown after being frozen for 4 months. It can be seen that rinsing with tap water electrolysis has a certain color-preserving effect.

[0100] Example 6 like Figure 1 As shown, a method for blanching frozen vegetables to preserve their green color includes the following steps: Step 1, Washing: Wash the freshly picked komatsuna; Step 2: Preparation of weakly alkaline electrolyzed water: Using a 0.02% sodium chloride solution as the electrolyte, weakly alkaline electrolyzed water is prepared using a water electrolysis device with a diaphragm. The pH of the weakly alkaline electrolyzed water is 8.0 to prevent chlorophyll from losing magnesium and turning yellow under acidic conditions.

[0101] Step 3, Blanching: The blanching solution is the prepared weakly alkaline electrolyzed water. Blanch at 98℃ for 4 minutes to passivate the peroxidase that causes browning.

[0102] Step 4, Cooling: Immerse in water to cool. The water should be weakly alkaline electrolyzed water. Cool until it reaches near room temperature.

[0103] Step 5: Preparation of endogenous peroxidase inhibitor: Remove the leaves and stems of Komatsuna, leaving only the leaves, and pulp them. Add 0.3% bentonite and 30 mg / L pectinase, then clarify and filter to obtain clear juice from Komatsuna leaves.

[0104] Step 6, Color protection soaking: Prepare a clear juice of Komatsuna leaves with 15% (volume fraction, preparation method is the same as in Example 4) using weakly alkaline electrolyzed water with pH 8.0, and then soak the leaf stems in the color protection soaking solution for 10 min.

[0105] Step 7, quick-freeze: After draining the water, quick-freeze at -35℃ for 30 minutes to allow the green vegetables to quickly pass through the zone of maximum ice crystal formation.

[0106] Step 8, Packaging: Packaging bags are used for individual packaging.

[0107] Step 9: Freeze-freeze at -18℃ for 12 months.

[0108] Comparative Example 3 A method for blanching frozen vegetables to preserve their green color includes the following steps: Step 1, Washing: Wash the freshly picked komatsuna; Step 2, Blanching: Blanch in tap water at 98℃ for 4 minutes to deactivate the peroxidase that causes browning.

[0109] Step 3, Cooling: Immerse in water to cool until it reaches near room temperature.

[0110] Step 4, quick-freeze: After draining the water, quick-freeze at -35 ℃ for 30 minutes to allow the green vegetables to quickly pass through the zone of maximum ice crystal formation.

[0111] Step 5, Packaging: Packaging bags are used for individual packaging.

[0112] Step 6: Freezing: Store at -18℃ for 12 months. The results of testing the color of Komatsuna leaves and stems during freezing are shown in Table 7 below.

[0113] Table 7. Effects of different treatments on the color of Komatsuna leaves and stems during frozen storage.

[0114] As shown in Table 7 above, the color changes of Komatsuna leaves and stems during frozen storage are similar to those of spinach. The color changes are mainly observed in the stems, while the leaf color remains relatively stable and can maintain a bright green color throughout frozen storage. The blanching treatment using the weakly alkaline electrolyzed water of this invention can improve the chlorophyll stability of Komatsuna. Furthermore, by pulping the Komatsuna leaves to produce a clear juice, the endogenous peroxidase inhibitor in the leaves can inhibit browning of the Komatsuna stems and leaves, thus preserving the light green color of the stems and leaves, protecting the green color, preventing browning, and extending the frozen storage shelf life of Komatsuna.

[0115] In summary, the method for protecting the green color and preventing browning of frozen vegetables in this invention improves chlorophyll stability and inhibits residual peroxidase activity by blanching with weakly alkaline electrolyzed water and soaking in endogenous peroxidase inhibitors, thereby inhibiting enzymatic browning of frozen vegetables and maintaining the color quality of frozen green vegetables.

[0116] This document uses specific examples to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the methods and core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A method for protecting the green color and preventing browning of frozen vegetables, characterized in that, Includes the following steps: Wash freshly picked vegetables, either cut or leave whole. After washing, blanch the vegetables in weakly alkaline electrolyzed water, and then cool them. After cooling, the vegetables are soaked in a color-protecting solution, drained, quick-frozen, packaged, and finally frozen.

2. The method for protecting the green color and preventing browning of frozen vegetables according to claim 1, characterized in that, The method for preparing the weakly alkaline electrolyzed water is as follows: using one or more of sodium chloride, potassium chloride, calcium chloride, magnesium sulfate solution or tap water as the electrolyte, and preparing the weakly alkaline electrolyzed water using an electrolyzing water device with a diaphragm. The pH of weakly alkaline electrolyzed water is 7–9.

3. The method for protecting the green color and preventing browning of frozen vegetables according to claim 2, characterized in that, The pH of weakly alkaline electrolyzed water is 7.5–8.

5.

4. The method for protecting the green color and preventing browning of frozen vegetables according to claim 1, characterized in that, The blanching temperature is 95-100℃, and the blanching time is 3-5 minutes. After blanching, the water is cooled using weakly alkaline electrolyzed water or cold air.

5. The method for protecting the green color and preventing browning of frozen vegetables according to claim 1, characterized in that, The color-protecting soaking solution includes an endogenous peroxidase inhibitor and weakly alkaline electrolyzed water with a pH of 7.5–8.

5.

6. The method for protecting the green color and preventing browning of frozen vegetables according to claim 5, characterized in that, The preparation method of endogenous peroxidase inhibitor is as follows: remove the leaves and stems of vegetables, leaving only the leaves, and pulp them into a slurry. Add bentonite and pectinase to the slurry, then clarify and filter to obtain clear juice.

7. The method for protecting the green color and preventing browning of frozen vegetables according to claim 6, characterized in that, The preparation method of the color-protecting soaking solution is as follows: add clear juice to weakly alkaline electrolyzed water to prepare the color-protecting soaking solution; The volume of the clear juice is 10-20% of the total volume of the color-protecting soaking solution.

8. The method for protecting the green color and preventing browning of frozen vegetables according to claim 1, characterized in that, After cooling, soak the vegetables in the color-protecting soaking solution for 5-15 minutes.

9. The method for protecting the green color and preventing browning of frozen vegetables according to claim 1, characterized in that, The freezing temperature is -30 to -40°C; the storage period is 12 months under freezing conditions at -18°C.

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