A composite method for controlling the discoloration of frozen fruits in acidic juice

Abstract

The present invention relates to a composite method for controlling the discoloration of frozen fruits in acidic juice, and belongs to the technical field of fruit freezing processing. The main steps of the present invention include: 1) raw material selection; 2) pretreatment; 3) negative pressure ultrasonic penetration and pre-dehydration; 4) acid-resistant hydrogel impregnation; 5) non-thermal enzyme treatment; 6) electromagnetic coupling assisted impregnation quick freezing and acid-resistant hydrogel freezing coating; 7) acid protective film plating; 8) packaging. The frozen fruits processed by this process can be directly used in acidic juice (pH < 3.5) to keep the color unchanged for a long time, meeting the actual production needs; the developed composite color preservative can avoid the shortcomings of the use of single color preservatives and dyes, and has the characteristics of safety, greenness and high efficiency.

Description

Technical Field

[0001] The invention relates to a composite method for controlling the discoloration of frozen fruit in acidic juice, belonging to the technical field of fruit freezing processing. Background Art

[0002] Frozen fruit is a quick-frozen product made from fresh fruit after a series of processing, which can maintain the original flavor, color and nutrients of the fruit. Frozen fruit prolongs the storage period of fruit, facilitates long-distance transportation, and meets the consumption needs of different regions and seasons. In recent years, adding "visible" fruit to juice drinks has gradually become a juice processing method that attracts consumers. Frozen fruit is used as a raw material for adding fruit to juice because of its convenience. However, frozen fruit tissue cells are often irreversibly damaged. On the one hand, the hard ice crystals formed by freezing can pierce the fruit cells and cause mechanical damage, and the fruit will lose juice after thawing; on the other hand, freezing will produce a concentration effect. During the freezing process, liquid water gradually turns into solid ice, and the free water content decreases, resulting in an increase in the ion concentration in the cell. This will increase the rate of biochemical reactions in the cell and accelerate the deterioration of the product. Under these adverse effects, the most intuitive change phenomenon of frozen fruit added to juice is color deterioration, such as fading and browning. In addition, juices on the consumer market are often acidic, which is particularly unfavorable for maintaining the green color of frozen fruits. Because under acidic conditions, the magnesium in the porphyrin ring of the green chlorophyll structure is easily oxidized by H + Replacement, thus turning into brown pheophytin, accelerating the deterioration of fruit color.

[0003] The traditional method of controlling discoloration of frozen fruits often uses soaking in a color-protecting liquid. Chen Zhuliang and Lin Yonglu (patent application number: CN201110001447.7) disclosed a frozen preservation process for lychees, in which the lychees are blanched and color-protected before freezing. The blanching treatment destroys the enzyme activity on the surface of the lychee shell, preventing the lychee shell from changing color; the color protection treatment is performed by placing a mixed color-protecting liquid of 5% citric acid + 2% salt with a pH value of 2 to 3, making the appearance of the lychees more bright red. Gou Xiaoqin and Sang Zuolin (patent application number: CN201910909205.4) disclosed a fruit freezing processing method using 0.2%-0.4% SO 2Solution or 2% saline solution or 0.3%-0.5% citric acid solution to protect the color of the fruit. Luo Genfu (patent application number: CN201810684321.6) disclosed a preparation process for rock sugar fruit assortment, in which strawberries, yellow peaches and other fruits are treated with 0.1% and 0.3% sodium chloride color protection solution before freezing. However, the color protection method for frozen fruits disclosed in the above patents is relatively simple, and cannot effectively inhibit the destruction of chlorophyll under acidic conditions, and does not involve a method for controlling the discoloration of frozen fruits in acidic juice. The use of sulfites is also prone to residues, which is not conducive to the production of "green" frozen fruits.

[0004] Some studies specifically focus on greening during fruit and vegetable processing, and develop greening agents. Li Jilan (2008) et al. used kiwifruit as a test material to study the effects of different types of greening agents on the chlorophyll preservation rate of kiwifruit pulp. The results showed that 0.04% EDTA-2Na, 0.1% sodium sulfite, and 0.12% zinc acetate treatment for 1 hour can better preserve the chlorophyll of kiwifruit. Hua Changhong (patent application number: CN201510203979.7) disclosed a method for greening and preserving water celery, which uses an aqueous solution composed of 160mg / L zinc acetate and 10mg / L salicylic acid to protect the greenness of water celery, which can effectively extend the shelf life of water celery and delay the yellowing and browning of water celery. Wang Yi, Li Yongcai, Bi Yang, et al. (patent application number: CN201410656657.3) disclosed a green coating agent for early pear and its preparation method to solve the problem that the peel of early pear fruit is very easy to yellow under normal temperature storage conditions. The raw material composition of the green coating agent is 3.0%-4.0% oxidized acetate starch, 0.5%-1.0% palmitic acid, 1.0% β-cyclodextrin, 2.0%-3.0% glycerol, 0.5%-1.0% monoglyceride, 25mg / L gibberellin, which can effectively inhibit the degradation of chlorophyll when used in early pear. However, the greening agents developed by the above research and patents are all aimed at unfrozen fruit and vegetable raw materials, and do not involve methods for controlling the discoloration of frozen fruits in acidic juice. Although the prior art involves the preparation technology of packaging films and coating agents, it does not involve the preparation technology of acid-resistant sustained-release hydrogels that can be used for frozen film formation. These color protection methods using coating agents are not suitable for film formation on the surface of frozen fruits. The materials and processes used in the technology of the present invention are completely different from those of the prior art.

[0005] In view of the damage caused by freezing to fruit tissue cells and the acceleration of browning, quick freezing is an effective way to improve the quality of frozen products. Regarding the quick freezing of food, liquid immersion quick freezing is favored because of its fast freezing, convenient operation and low energy consumption. On the basis of quick freezing, in recent years, in order to obtain a better freezing effect, more and more research has focused on using physical field technologies to assist freezing operations, such as ultrasound, microwaves, far infrared, high voltage, electric fields, magnetic fields, radio frequency, etc. Compared with simple quick freezing, effective physical field assistance or synergistic application further improves the product quality of quick-frozen food, which is conducive to the formation of more uniform and fine ice crystals during the freezing process. In recent years, static magnetic fields have gradually been valued because of their mild effect and convenient application. Antimagnetic materials such as water in food will be magnetized in a magnetic field, which will affect the melting point, thermal conductivity and other properties of these materials. Studies have shown that the application of magnetic fields can affect the formation of ice crystals by strengthening hydrogen bonds in water molecules and reducing the supercooling of water. With the deepening of research, the potential advantages of using static magnetic fields to assist quick freezing of food have gradually emerged. Mok et al. (2015) studied the effect of static magnetic field on the freezing of 0.9% NaCl solution from the perspective of ice crystals. The results showed that the static magnetic field shortened the phase change time and made the ice crystals smaller. Tang et al. (2020) studied the effects of different magnetic fields on the freezing parameters of cherries and blueberries. The results showed that the application of magnetic field can reduce juice loss and ice crystal size to a certain extent. The above studies and patents show the beneficial role of magnetic field in assisted freezing. However, none of them involve the development of technology using static magnetic field coupled with electrostatic field to assist color protection and impregnation of quick-frozen fruits, let alone the method of using this freezing technology in combination with color protection agent treatment to control the discoloration of frozen fruits in acidic juice. Summary of the invention

[0006] The technical purpose of the present invention is to overcome the problem that frozen fruits are very easy to change color when used in acidic juice and traditional color protection agents and color protection methods are not effective, and to develop a composite method for controlling the color change of frozen fruits in acidic juice.

[0007] The technical solution of the present invention:

[0008] A composite method for controlling the discoloration of frozen fruit in acidic juice mainly comprises the following steps:

[0009] (1) Raw material selection: Select fresh fruits of uniform size, free from pests and diseases, mechanical damage, rot and deterioration, normal color, moderate maturity, and suitable for freezing processing as raw materials;

[0010] (2) Pretreatment: Wash, peel and cut the selected fruit raw materials so that the fruit pieces can be directly used in juice without thawing after subsequent freezing treatment;

[0011] (3) Negative pressure ultrasonic penetration and pre-dehydration: The pre-treated fruit pieces are immersed in a composite osmotic dehydration greening agent at 30°C for negative pressure ultrasonic penetration and pre-dehydration for 0.5 to 1 h;

[0012] (4) Acid-resistant hydrogel impregnation: The pre-dehydrated fruit pieces are immersed in the acid-resistant hydrogel mixture at 0-5°C according to the material-liquid ratio of 1:10 and then immersed in negative pressure ultrasonic immersion for 1-2 hours to achieve rapid cooling and hydrogel impregnation;

[0013] (5) Non-thermal enzyme inactivation treatment: During the hydrogel impregnation process, 50-100 kV high-voltage electrostatic field treatment is intermittently turned on, and negative pressure ultrasound is used to inactivate the enzyme in the fruit pieces.

[0014] (6) Electromagnetic coupling assisted immersion quick freezing and acid-resistant hydrogel freezing coating: electromagnetic coupling assisted color protection immersion quick freezing technology is used to quickly freeze the fruit pieces. After the hydrogel is frozen, a uniform hydrogel film is formed on the surface and inner layer of the fruit to protect the color of the fruit pieces after freezing;

[0015] (7) Acid color protection film coating: transfer the frozen fruit pieces to the color protection film liquid and immerse them for 5 to 20 seconds, then remove them to allow the surface of the fruit to condense and be coated with a layer of acid color protection film ice;

[0016] (8) Packaging: Take out the frozen fruit pieces, vacuum pack them, and then store them in a -20℃ freezer.

[0017] The composite osmotic dehydration greening agent in step (3) is composed of a greening agent composed of a mixture of 0.01% to 0.025% (w / v) zinc lactate and calcium lactate and a composite osmotic dehydration agent composed of a mixture of 30% to 60% sucrose and maltose.

[0018] The greening agent and the penetrating dehydrating agent are prepared separately and mixed together when used; the mass ratio of zinc lactate to calcium lactate in the greening agent is 1:1.

[0019] The vacuum degree used in the negative pressure ultrasonic osmotic pre-dehydration technology in step (3) is 0.02-0.05 MPa absolute pressure, and the ultrasound used is 28 kHz low-frequency ultrasound with a power of 300-600 W.

[0020] The acid-resistant hydrogel mixture in step (4) is composed of hydrogel and water-soluble acid-resistant composite embedded microparticles; the hydrogel is composed of: 1% to 2% gelatin, 0.1% to 0.5% (w / v) corn starch, and 0.1% to 0.5% (w / v) ferulic acid; the water-soluble acid-resistant composite microparticles are calcium bicarbonate sustained-release particles wrapped with a mixed wall material of glyceride, gelatin and corn starch.

[0021] In the negative pressure ultrasonic impregnation process of the hydrogel in step (4), the low frequency ultrasound is 28kHz, 300-600W, the vacuum pump is turned on, and the negative pressure ultrasound is performed for 1-2 hours under the condition of 0.02-0.05MPa absolute pressure.

[0022] The ultrasound-assisted working mode is intermittent working, working for 20 seconds and stopping for 10 seconds, so as to avoid the heat generation caused by long-term continuous working of ultrasound.

[0023] In the step (6), the electromagnetic coupling-assisted color-protection immersion quick freezing can achieve rapid freezing of the fruit blocks, with an overall freezing time of less than 10 minutes and a time of passing through the maximum ice crystal formation zone of less than 150 seconds; the immersion quick freezing liquid comprises 50% (v / v) ethanol, 10% (v / v) glycerol, 2% (sucrose), 0.1% sodium isoascorbate, and 0.05% citric acid, and the quick freezing temperature is set at -60°C to -70°C.

[0024] The static magnetic field strength of the equipment used for electromagnetic coupling-assisted color-protection immersion quick freezing is 6-10 mT, the electrostatic field strength is 1-5 kV / cm, and the electromagnetic field coupling action temperature range is -5°C to 4°C.

[0025] The acidic color-protecting film in step (7) is composed of 1% to 1.5% (w / v) gelatin, 0.5% to 1% (v / v) glacial acetic acid and 0.2% to 0.5% (v / v) glycerol.

[0026] The beneficial effects of the present invention are:

[0027] (1) The present invention provides a freezing color protection processing method that effectively improves the greening and color protection effect of the color protection agent, and adopts negative pressure ultrasonic treatment to effectively improve the mass transfer efficiency and significantly promote osmotic dehydration and the penetration of the color protection agent.

[0028] (2) The present invention provides a composite color protection agent (an osmotic dehydration green protection agent, an acid-resistant hydrogel mixture, and an acidic color protection film mixture) that can be used for fruit freezing, which has a controlling effect on the enzymatic browning that is easy to occur after the frozen fruit is thawed and the magnesium-free browning of chlorophyll under acidic conditions; the composite color protection agent is green, safe, and highly effective.

[0029] (3) The acid-resistant hydrogel freezing coating method provided by the present invention can integrate the film-forming process into the freezing process, and the preparation method is simple and effective. In addition to directly protecting the color of frozen fruits, the acid-resistant hydrogel freezing coating can also achieve long-term and stable color protection for strongly acidic fruits through the sustained release of color protectants and antacids; the hydrogel infiltrated into the fruit also has a cryoprotective effect.

[0030] (4) The present invention uses high-voltage electrostatic field treatment before freezing the fruit, which can inhibit bacteria and inactivate enzymes, thereby reducing the content of microorganisms in the frozen fruit, inhibiting enzyme activity, and controlling browning, thereby improving the overall quality of the frozen fruit. As a non-thermal processing method, high-voltage electric field treatment can also effectively avoid the damage to the quality of frozen fruit caused by traditional blanching treatment.

[0031] (5) The fruit quick freezing method provided by the present invention can achieve rapid freezing of fruit pieces. This method can make the center temperature of the frozen material within 3 cm from the outermost layer pass through the maximum ice crystal formation zone (-1 to -5°C) in less than 2.5 minutes. The circulating freezing liquid immersion quick freezing greatly reduces the ice crystal size of the frozen fruit. Electromagnetic coupling assistance further reduces the size of the formed ice crystals. Combined with the pre-osmosis dehydration process, the frozen fruit is less damaged by ice crystals, thereby effectively reducing the discoloration of the fruit. Electromagnetic coupling assisted quick freezing also makes the gel layer formed on the surface and inner layer of the fruit thin and uniform.

[0032] (6) The method for controlling the discoloration of frozen fruits in acidic juice provided by the present invention is not only applicable to the problem of discoloration of frozen fruits in acidic juice, but also suitable for the processing of frozen fruits for other purposes. After the frozen products obtained by this method are thawed, the physical, chemical and nutritional indicators are maintained well, and the overall quality of the material is improved.

[0033] (7) When freezing fruits using the freezing method of the present invention, the energy consumption of liquid immersion freezing is low. Depending on the frozen materials, the freezing cost is reduced by 10 to 30% compared with the traditional quick freezer, and the practical application feasibility is high. DETAILED DESCRIPTION

[0034] The technical solution of the present invention will be specifically described below in conjunction with embodiments:

[0035] Example 1

[0036] A composite method for controlling the discoloration of frozen lime fruit in acidic juice comprises the following steps:

[0037] (1) Raw material selection: Select fresh lime fruits of uniform size, free from pests and diseases, mechanical damage, rot and deterioration, normal color, moderate maturity, and suitable for freezing processing as raw materials.

[0038] (2) Pretreatment: Wash and cut the selected lime fruit raw materials. Specifically, wash the fresh fruit first, remove dirt and impurities, drain and set aside, and then cut the washed fruit into slices.

[0039] (3) Negative pressure ultrasonic osmotic pre-dehydration: The pretreated lime was immersed in a composite osmotic dehydration greening agent at 30°C for 0.5 h of negative pressure ultrasonic osmotic dehydration. The osmotic dehydration greening agent consisted of a 0.01% (w / v) mixture of zinc lactate and calcium lactate (1:1) and an osmotic dehydration agent consisting of a 50% mixture of sucrose and maltose. The vacuum degree used in the negative pressure ultrasonic immersion dehydration technology was 0.02 MPa (absolute pressure), and the ultrasound used was 28kHz low-frequency ultrasound with a power of 300W. Osmotic dehydration makes the greening effect of zinc salts and copper salts significant, and the chlorophyll changes in the lime fruit can effectively chelate with metal ions. At the same time, this helps to reduce cell turgor pressure, so that the ice crystals formed in the subsequent freezing process significantly reduce the damage to cells.

[0040] (4) Acid-resistant hydrogel impregnation: The pre-dehydrated lime was transferred into the 4°C hydrogel mixture and continued to be immersed in negative pressure ultrasound for 1 h to achieve rapid cooling and hydrogel impregnation. During this process, low-frequency ultrasound (28 kHz, 300 W) was turned on, and the vacuum pump was turned on, and negative pressure ultrasound was impregnated for 1 h under 0.05 MPa (absolute pressure).

[0041] (5) High-voltage electrostatic field treatment: During the lime fruit hydrogel impregnation process, the fruit was treated under a high-voltage electrostatic field of 80 kV for 1 h.

[0042] (6) Quick freezing and hydrogel freezing coating: The above-treated lime slices are quick frozen using electromagnetic coupling-assisted color protection immersion quick freezing technology. After the hydrogel is frozen, a uniform hydrogel film is formed on the surface and inner layer of the lime to protect the color of the frozen lime fruit. The ice crystals formed by the frozen lime using this technology are small, causing little damage to the lime fruit cells. The quick freezing temperature is set at -70°C, and the fruit layer thickness is 30 to 50 mm. Electromagnetic coupling is used to assist liquid immersion freezing. The static magnetic field strength is 8 mT, the electrostatic field strength is 3 kV / cm, and the electromagnetic field coupling temperature range is -5°C to 4°C. When the center temperature of the sample drops to -20°C, the freezing is completed.

[0043] (7) Acid color-protecting film coating: Transfer the frozen fruit pieces to the color-protecting film liquid and immerse them for 10 seconds, then remove them so that the surface is coated with a layer of acid color-protecting film ice.

[0044] (8) Packaging: Take out the frozen lime slices, vacuum pack them, and then store them in a -20°C freezer.

[0045] Comparative experiment 1

[0046] The following control experiment was set up to test the effect of the entire technical solution of the present invention in controlling the discoloration of frozen lime fruit in acidic juice; and the effect of the present invention was compared with the effect of a certain technology in the single application invention to reflect the application effect of the combination of various technologies in the present invention.

[0047] Control 1: without any color-protecting freezing treatment of the present invention, only limes frozen at -20°C were used as control 1;

[0048] Control 2: The limes were not subjected to any color protection treatment of the present invention, but were only quick-frozen by the quick-freezing technology of the present invention as control 2;

[0049] Control 3: limes that were pre-dehydrated by negative pressure ultrasonic osmosis and then quickly frozen by the quick freezing technology of the present invention were used as control 3;

[0050] Control 4: limes that were pre-treated by high-voltage electrostatic field and then quickly frozen by the quick-freezing technology of the present invention were used as control 4;

[0051] Control 5: limes that were coated with acid-resistant hydrogel and then quickly frozen using the quick freezing technology of the present invention were used as control 5;

[0052] Then the control sample and the lime slice sample frozen by the method of the present invention were placed in acidic juice respectively to test the discoloration of different samples in acidic juice. The number of lime slice samples added was 4, the volume of acidic juice was 330 mL, pH = 3.2, and the initial temperature was 65 ° C. The experimental data are shown in Tables 1 to 5. The experimental results show that the control samples 1 and 2 had obvious color changes 30 minutes after adding the acidic juice, and the control samples 3, 4, and 5 had obvious color changes 60 minutes after adding the acidic juice. The color parameters L, a, and b values ​​changed significantly, while the lime slice sample frozen by the inventive method can be added to the acidic juice without discoloration for 2 hours, and the color parameters L, a, and b values ​​have no significant changes within 120 minutes. This shows that the present invention can significantly control the discoloration of frozen lime fruit in acidic juice, and the effect is significantly better than the ordinary freezing method and the single invention technology.

[0053] Table 1 Comparison of color change of control 1 and frozen lime of the present invention in acidic juice

[0054]

[0055] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

[0056] Table 2 Comparison of color change of control 2 and frozen lime of the present invention in acidic juice

[0057]

[0058] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

[0059] Table 3 Comparison of color change of control 3 and frozen lime of the present invention in acidic juice

[0060]

[0061] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

[0062] Table 4 Comparison of color change of control 4 and frozen lime of the present invention in acidic juice

[0063]

[0064] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

[0065] Table 5 Comparison of color change of control 5 and frozen lime of the present invention in acidic juice

[0066]

[0067] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

[0068] Example 2

[0069] A composite method for controlling the discoloration of frozen green grapes in acidic juice comprises the following steps:

[0070] (1) Raw material selection: Select fresh green grapes that are uniform in size, free of pests and diseases, mechanical damage, rot and deterioration, normal in color, moderately mature, and suitable for freezing processing as raw materials.

[0071] (2) Pretreatment: The selected green grapes are washed, peeled, and cut. Specifically, the fresh fruits are first washed, dirt and impurities are removed, and the fruits are drained for later use. Then the washed green grapes are peeled and cut in half.

[0072] (3) Negative pressure ultrasonic osmotic pre-dehydration: The pretreated green grapes were immersed in a composite osmotic dehydration greening agent at 30°C for negative pressure ultrasonic osmotic dehydration for 0.5 h. The composite osmotic dehydration greening agent consists of a greening agent composed of a 0.02% (w / v) mixture of zinc lactate and calcium lactate (1:1) and an osmotic dehydrating agent composed of a 40% mixture of sucrose and maltose. The vacuum degree used in the negative pressure ultrasonic immersion dehydration technology is 0.03 MPa (absolute pressure), and the ultrasound used is a 28kHz low-frequency ultrasound with a power of 300W. Osmotic dehydration makes the greening effect of zinc salts and copper salts significant, and the chlorophyll changes in the green grape fruit can effectively chelate with metal ions. At the same time, this helps to reduce cell turgor pressure, so that the ice crystals formed in the subsequent freezing process significantly reduce the damage to cells.

[0073] (4) Acid-resistant hydrogel impregnation: The pre-dehydrated green grapes were transferred into the hydrogel mixture at 4°C and continued to be immersed in negative pressure ultrasound for 1 h to achieve rapid cooling and hydrogel impregnation. During this process, low-frequency ultrasound (28 kHz, 300 W) was turned on, and the vacuum pump was turned on, and negative pressure ultrasound was impregnated for 1 h at 0.03 MPa (absolute pressure).

[0074] (5) High-voltage electrostatic field treatment: During the hydrogel impregnation process of green grape fruits, a 50 kV high-voltage electrostatic field was turned on for 1 h.

[0075] (6) Quick freezing and hydrogel freezing coating: The green grapes treated as above are quick frozen using electromagnetic coupling assisted color protection immersion quick freezing technology. After the hydrogel is frozen, a uniform hydrogel film is formed on the surface and inner layer of the green grapes to protect the color of the green grapes after freezing. The ice crystals formed by the green grapes frozen using this technology are small, causing little damage to the green grape fruit cells. The quick freezing temperature is set at -60°C, and the thickness of the fruit layer is 30 to 50 mm. Electromagnetic coupling is used to assist liquid immersion freezing. The static magnetic field strength is 6mT, the electrostatic field strength is 5kV / cm, and the electromagnetic field coupling temperature range is -5°C to 4°C. When the center temperature of the sample drops to -20°C, the freezing is completed.

[0076] (7) Acid color-protecting film coating: Transfer the frozen fruit pieces to the color-protecting film liquid and immerse them for 10 seconds, then remove them so that the surface is coated with a layer of acid color-protecting film ice.

[0077] (8) Packaging: Extract the frozen green extract, vacuum pack it, and then store it in a -20℃ freezer.

[0078] Comparative experiment 2

[0079] The following control experiment was set up to test the effect of the entire technical solution of the present invention in controlling the discoloration of frozen green grapes in acidic juice; and the effect of the present invention was compared with the effect of a certain technology in the single application invention to reflect the application effect of the combination of various technologies in the present invention.

[0080] Control 1: Green grapes frozen at -20°C were used as control 1 without any color-protecting freezing treatment of the present invention;

[0081] Control 2: Green grapes that were not subjected to any color protection treatment of the present invention and were only quick-frozen using the quick-freezing technology of the present invention were used as control 2;

[0082] Control 3: Green grapes that were pre-dehydrated by negative pressure ultrasonic osmosis and then quick-frozen by the quick-freezing technology of the present invention were used as control 3;

[0083] Control 4: Green grapes that were pretreated by high-voltage electrostatic field and then quick-frozen by the quick-freezing technology of the present invention were used as control 4;

[0084] Control 5: Green grapes that were coated with acid-resistant hydrogel and then quick-frozen using the quick-freezing technology of the present invention were used as control 5;

[0085] Then the control sample and the green grape sample frozen by the method of the present invention were placed in acidic juice respectively to test the discoloration of different samples in acidic juice. The number of green grape samples added was 2 petals, the volume of acidic juice was 330mL, pH=3.2, and the initial temperature was 65°C. The experimental data are shown in Tables 6 to 10. The experimental results show that the control samples 1, 2, and 4 changed color 30 minutes after adding the acidic juice, and the control samples 3 and 5 changed color 60 minutes after adding the acidic juice. The color parameters L, a, and b values ​​changed significantly, while the green grape sample frozen by the inventive method can be added to the acidic juice without discoloration for 2 hours, and the color parameters L, a, and b values ​​have no significant changes within 120 minutes. This shows that the present invention can significantly control the discoloration of frozen green grape fruits in acidic juice, and the effect is significantly better than ordinary freezing methods and single invention technologies.

[0086] Table 6 Comparison of color change of control 1 and frozen green grapes of the present invention in acidic juice

[0087]

[0088] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

[0089] Table 7 Comparison of color change of control 2 and frozen green grapes of the present invention in acidic juice

[0090]

[0091] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

[0092] Table 8 Comparison of color change of control 3 and frozen green grapes of the present invention in acidic juice

[0093]

[0094]

[0095] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

[0096] Table 9 Comparison of color change of control 4 and frozen green grapes of the present invention in acidic juice

[0097]

[0098] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

[0099] Table 10 Comparison of color change of control 5 and frozen green grapes of the present invention in acidic juice

[0100]

[0101] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

[0102] Example 3

[0103] A composite method for controlling the discoloration of frozen kiwi fruit in acidic juice comprises the following steps:

[0104] (1) Raw material selection: Select fresh kiwifruit as raw materials that are uniform in size, free of pests and diseases, mechanical damage, rot and deterioration, normal in color, moderately mature, and suitable for freezing processing.

[0105] (2) Pretreatment: Wash, peel and cut the selected kiwifruit raw materials. Specifically, wash the fresh kiwifruits, remove dirt and impurities, drain and set aside. Then peel and slice the washed fruits. The fruits can be cut into slices or blocks.

[0106] (3) Negative pressure ultrasonic osmotic pre-dehydration: The pretreated kiwifruit was immersed in a composite osmotic dehydration greening agent at 30°C for 0.5h of negative pressure ultrasonic osmotic dehydration. The composite osmotic dehydration greening agent consists of a greening agent composed of a mixture of 0.015% (w / v) zinc lactate and calcium lactate (1:1) and an osmotic dehydrating agent composed of a mixture of 60% sucrose and maltose. The vacuum degree used in the negative pressure ultrasonic immersion dehydration technology is 0.03MPa (absolute pressure), and the ultrasound used is a 28kHz low-frequency ultrasound with a power of 400W. Osmotic dehydration makes the greening effect of zinc salts and copper salts significant, and the chlorophyll changes in kiwifruit can effectively chelate with metal ions. At the same time, this helps to reduce cell turgor pressure, so that the ice crystals formed during the subsequent freezing process significantly reduce the damage to cells.

[0107] (4) Acid-resistant hydrogel impregnation: The pre-dehydrated kiwifruit was transferred into the hydrogel mixture at 4°C and continued to be immersed in negative pressure ultrasound for 1.5 h to achieve rapid cooling and hydrogel impregnation. During this process, low-frequency ultrasound (28 kHz, 400 W) was turned on, and the vacuum pump was turned on, and negative pressure ultrasound was impregnated for 1.5 h at 0.03 MPa (absolute pressure).

[0108] (5) High-voltage electrostatic field treatment: During the hydrogel impregnation process of kiwifruit fruit, a 60 kV high-voltage electrostatic field was turned on for 1.5 h.

[0109] (6) Quick freezing and hydrogel freezing coating: The kiwifruit treated above is quick frozen using electromagnetic coupling assisted color protection immersion quick freezing technology. After the hydrogel is frozen, a uniform hydrogel film is formed on the surface and inner layer of the kiwifruit to protect the color of the kiwifruit after freezing. The ice crystals formed by kiwifruit frozen using this technology are small, causing little damage to the kiwifruit fruit cells. The quick freezing temperature is set at -80°C, and the fruit layer thickness is 30 to 50 mm. Electromagnetic coupling is used to assist liquid immersion freezing. The static magnetic field strength is 10 mT, the electrostatic field strength is 1 kV / cm, and the electromagnetic field coupling temperature range is -5°C to 4°C. When the center temperature of the sample drops to -20°C, the freezing is completed.

[0110] (7) Acid color-protecting film coating: Transfer the frozen fruit pieces to the color-protecting film liquid and immerse them for 10 seconds, then remove them so that the surface is coated with a layer of acid color-protecting film ice.

[0111] (8) Packaging: Take out the frozen kiwifruit, vacuum pack it, and then store it in a -20℃ freezer.

[0112] Comparative experiment 3

[0113] The following control experiment was set up to test the effect of the entire technical solution of the present invention in controlling the discoloration of frozen kiwi fruit in acidic juice; and the effect of the present invention was compared with the effect of a certain technology in the single application invention to reflect the application effect of the combination of various technologies in the present invention.

[0114] Control 1: kiwifruits frozen at -20°C were used as control 1 without any color-protecting freezing treatment of the present invention;

[0115] Control 2: kiwi fruit that was not subjected to any color protection treatment of the present invention and was only quick-frozen using the quick-freezing technology of the present invention was used as control 2;

[0116] Control 3: kiwi fruit quick-frozen by the quick-freezing technology of the present invention after being pre-dehydrated by negative pressure ultrasonic osmosis is used as control 3;

[0117] Control 4: kiwi fruit quick-frozen by the quick-freezing technology of the present invention after being pre-treated by high-voltage electrostatic field is used as control 4;

[0118] Control 5: Kiwi fruit that was quick-frozen using the quick-freezing technology of the present invention after being coated with acid-resistant hydrogel was used as control 5;

[0119] Then the control sample and the kiwi slice samples frozen by the method of the present invention were placed in acidic juice respectively to test the discoloration of different samples in acidic juice. The number of kiwi slice samples added was 4, the volume of acidic juice was 330 mL, pH = 3.2, and the initial temperature was 65 ° C. The experimental data are shown in Tables 11 to 15. The experimental results show that the control samples 1, 2, 3, 4, and 5 changed color 30 minutes after adding acidic juice, and the color parameters L, a, and b values ​​changed significantly, while the kiwi slice samples frozen by the method of the invention can be added to the acidic juice without discoloration for 2 hours, and the color parameters L, a, and b values ​​have no significant changes within 120 minutes. This shows that the present invention can significantly control the discoloration of frozen kiwi fruit in acidic juice, and the effect is significantly better than the ordinary freezing method and the single invention technology.

[0120] Table 11 Comparison of color change of control 1 and frozen kiwi fruit of the present invention in acidic juice

[0121]

[0122] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

[0123] Table 12 Comparison of color change of control 2 and frozen kiwi fruit of the present invention in acidic juice

[0124]

[0125] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

[0126] Table 13 Comparison of color change of control 3 and frozen kiwi fruit of the present invention in acidic juice

[0127]

[0128] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

[0129] Table 14 Comparison of color change of control 4 and frozen kiwi fruit of the present invention in acidic juice

[0130]

[0131] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

[0132] Table 15 Comparison of color change of control 5 and frozen kiwi fruit of the present invention in acidic juice

[0133]

[0134] Different lowercase letters after the numbers in the same column indicate significant differences (P<0.05)

Claims

1. A composite method for controlling the discoloration of frozen fruit in acidic juice, characterized in that: The main steps include: (1) Raw material selection: Select fresh fruits of uniform size, free from pests and diseases, mechanical damage, rot and deterioration, normal color, moderate maturity, and suitable for freezing processing as raw materials; (2) Pretreatment: Wash, peel and cut the selected fruit raw materials so that the fruit pieces can be directly used in juice without thawing after subsequent freezing treatment; (3) Negative pressure ultrasonic penetration and pre-dehydration: The pre-treated fruit pieces are immersed in a composite osmotic dehydration greening agent at 30°C for negative pressure ultrasonic penetration and pre-dehydration for 0.5 to 1 h; The composite osmotic dehydration greening agent is composed of a greening agent composed of a mixture of 0.01% to 0.025% (w / v) zinc lactate and calcium lactate and an osmotic dehydrating agent composed of a mixture of 30% to 60% sucrose and maltose; the greening agent and the osmotic dehydrating agent are prepared separately and mixed together when used; the mass ratio of zinc lactate to calcium lactate in the greening agent is 1:1; The vacuum degree used in the negative pressure ultrasonic osmotic pre-dehydration technology is 0.02-0.05 MPa absolute pressure, and the ultrasound used is 28 kHz low-frequency ultrasound with a power of 300-600 W; (4) acid-resistant hydrogel impregnation: the pre-dehydrated fruit pieces are immersed in an acid-resistant hydrogel mixture at 0-5° C. according to a material-liquid ratio of 1:10 and continue to be immersed in negative pressure ultrasound for 1-2 hours to achieve rapid cooling and hydrogel impregnation; the acid-resistant hydrogel mixture is composed of hydrogel and water-soluble acid-resistant composite embedded microparticles; the hydrogel composition is: 1%-2% gelatin, 0.1%-0.5% (w / v) corn starch, 0.1%-0.5% (w / v) ferulic acid; the water-soluble acid-resistant composite microparticles are calcium bicarbonate sustained-release particles wrapped with a mixed wall material of glyceride, gelatin and corn starch; During the hydrogel negative pressure ultrasonic impregnation process, the low frequency ultrasound is 28kHz, 300-600W, the vacuum pump is turned on, and the negative pressure ultrasound is performed for 1-2h under the condition of 0.02-0.05MPa absolute pressure; (5) Non-thermal enzyme inactivation treatment: During the hydrogel impregnation process, 50-100 kV high-voltage electrostatic field treatment is intermittently turned on, and negative pressure ultrasound is used to inactivate the enzyme in the fruit pieces; (6) Electromagnetic coupling assisted immersion quick freezing and acid-resistant hydrogel freezing coating: The fruit pieces are quick frozen by electromagnetic coupling assisted color protection immersion quick freezing technology. After the hydrogel is frozen, a uniform hydrogel film is formed on the surface and inner layer of the fruit to protect the color of the frozen fruit pieces. The immersion quick freezing liquid is composed of 50% (v / v) ethanol, 10% (v / v) glycerol, 2% sucrose, 0.1% sodium isoascorbate, and 0.05% citric acid. The quick freezing temperature is set at -60°C to -70°C. The static magnetic field strength of the electromagnetic coupling-assisted color-protection immersion quick freezing equipment is 6 to 10 mT, the electrostatic field strength is 1 to 5 kV / cm, and the electromagnetic field coupling temperature range is -5°C to 4°C; (7) coating with an acidic color protection film: transferring the frozen fruit pieces to an acidic color protection film liquid and immersing them for 5 to 20 seconds, then taking them out to allow the surface of the fruit to be condensed and coated with a layer of acidic color protection film; the acidic color protection film is composed of 1% to 1.5% (w / v) gelatin, 0.5% to 1% (v / v) glacial acetic acid and 0.2% to 0.5% (v / v) glycerol; (8) Packaging: Take out the frozen fruit pieces, vacuum pack them, and then store them in a -20℃ freezer.

2. A composite method for controlling the discoloration of frozen fruit in acidic juice according to claim 1, characterized in that: The ultrasound-assisted working mode in steps (3), (4) and (5) is intermittent working, working for 20 seconds and stopping for 10 seconds, so as to avoid heat generation due to long-term continuous operation of ultrasound.

3. A composite method for controlling the discoloration of frozen fruit in acidic juice according to claim 1, characterized in that: In the step (6), electromagnetic coupling-assisted color-protection immersion quick freezing can achieve rapid freezing of the fruit pieces, with the overall freezing time being less than 10 minutes and the time to pass through the maximum ice crystal formation zone being less than 150 seconds.

Images