Method for improving the thawing quality of quick-frozen fresh-cut potato

Abstract

The present application relates to a method for improving the thawing quality of quick-frozen fresh-cut potatoes, and belongs to the technical field of fruit and vegetable freezing processing. The main steps of the present application include: raw material selection; pretreatment; preparation of a composite osmotic dehydration color protection agent; variable-temperature negative pressure ultrasonic assisted enzyme inactivation; gradient magnetic field assisted quick freezing; packaging; frozen storage; pulse electric field assisted high-humidity low-temperature anaerobic thawing. The present application realizes effective preservation of the quality of quick-frozen fresh-cut potatoes after thawing by innovatively adopting vacuum ultrasonic assisted low-temperature enzyme inactivation, gradient magnetic field assisted quick freezing and pulse electric field assisted high-humidity low-temperature anaerobic thawing, so that the hardness of the thawed frozen fresh-cut potatoes is increased by more than 10%, the browning is reduced by more than 10%, and the juice loss rate is controlled within 11%.

Description

Technical Field

[0001] This invention relates to a method for improving the quality of quick-frozen fresh-cut potatoes after thawing, belonging to the field of fruit and vegetable freezing processing technology. Background Technology

[0002] Quick-frozen potatoes are widely used in the food industry due to their advantages such as long shelf life, high safety, high nutrient retention, convenient processing, and low food waste generation. They are often used in the production of fast food and convenience foods. However, the formation and growth of ice crystals during the freezing and thawing process of quick-frozen potatoes can lead to the following problems: 1) Ice crystal damage during freezing: Ice crystals formed during freezing often cause mechanical damage to the food matrix, resulting in a significant decline in food quality after thawing. Fresh fruits and vegetables with cell wall structures are more sensitive to freezing and thawing damage; 2) Significant quality decline after thawing: After thawing, the integrity of fruit and vegetable cells is lost, often resulting in juice loss, tissue softening, decreased water retention and tenderness, and increased enzyme activity, seriously affecting the edible quality and commercial value of the product; 3) Energy efficiency issues in freezing / thawing: Conventional freezing (freezer freezing) and thawing (air / static water thawing) are slow, have low energy efficiency, and result in rapid deterioration of thawed quality. While there are many existing methods to improve the freezing and thawing quality of quick-frozen potatoes, they have certain limitations. The processing steps for quick-frozen fruits and vegetables generally involve five major stages to improve their quality after thawing: pre-cooling, blanching and enzyme inactivation, quick-freezing, frozen storage, and thawing. However, for fresh-cut potatoes, the focus is on maintaining their frozen and thawed quality through three main processes: enzyme inactivation, quick-freezing, and thawing.

[0003] For potatoes with high starch content, traditional heat-induced enzyme inactivation processes, such as hot water blanching and steam blanching, can effectively reduce the activity of polyphenol oxidase in the material, thereby reducing browning after quick-freezing and thawing. However, they significantly damage the texture and taste of the potatoes. Therefore, in recent years, new hot blanching and non-hot blanching technologies have been developed. Hot blanching technologies include microwave blanching and ohmic blanching, while non-hot blanching technologies include ultrasonic-assisted blanching and pulsed electric field blanching. Microwave blanching works by using electromagnetic waves of a certain frequency to generate heat in fruit and vegetable raw materials, thereby inactivating enzymes and reducing enzymatic browning. However, it suffers from poor heating uniformity in actual production. Ohmic blanching utilizes the inherent resistance of fruits and vegetables by applying an electric field across their ends, converting electrical energy into heat energy while maintaining conductivity. This method is characterized by rapid heating, ease of operation, and good heating uniformity, but it is less suitable for materials with poor conductivity and low moisture content. Ultrasonic-assisted blanching utilizes the mechanical vibration and cavitation effect of ultrasound to enhance the thermosensitivity of enzymes in fruits and vegetables, effectively reducing blanching temperature and time. Furthermore, ultrasonic-assisted blanching has a low heating effect and can be performed at ambient temperature, effectively protecting thermosensitive compounds in fruits and vegetables. Pulsed electric field-assisted blanching applies short-duration high-voltage electric pulses to fruit and vegetable raw materials. This method can inactivate microorganisms and enzymes at normal ambient temperature without significantly affecting the original color, flavor, and nutritional value of the product.

[0004] In the quick-freezing process, traditional forced-air quick-freezing mainly relies on high-speed, low-temperature air convection to transfer cold energy. Its main advantage is uniform freezing rate of fruits and vegetables, but it also has problems such as high energy consumption of refrigeration equipment, evaporator frosting, product dehydration, color deterioration, and severe juice loss. In response, a large number of new quick-freezing technologies have been developed in recent years, including: forced-air quick-freezing, immersion quick-freezing, permeation dehydration-assisted quick-freezing, ultrasonic-assisted immersion quick-freezing, high-pressure assisted quick-freezing, and electric / magnetic field assisted quick-freezing. Among these technologies, immersion quick-freezing technology uses a liquid refrigerant as the heat transfer medium. By directly immersing fruit and vegetable raw materials in the low-temperature refrigerant, heat exchange is completed, rapidly cooling the core temperature of the food material to below -18°C, thus achieving quick-freezing. However, separating the immersion liquid from the material remains a challenge. Ultrasonic-assisted immersion quick-freezing technology incorporates appropriate high-intensity, low-frequency ultrasound during the freezing process to regulate the quick-freezing process. This technology can effectively increase the freezing rate, minimize the volume of ice crystals, thereby maintaining the integrity of cell structure and improving the quality of quick-frozen fruits and vegetables. High-pressure assisted quick-freezing controls the freezing process of fruit and vegetable raw materials under high pressure. The principle is that water's freezing point is very low under high pressure, dropping to -22°C at 200 MPa, making the sample less prone to crystallization and more easily vitrified during freezing. Under these conditions, the formed crystals have a higher density than water and tend not to expand in volume, reducing the mechanical damage caused by ice crystals to tissues. The application of magnetic fields can influence ice crystal formation by enhancing hydrogen bonds in water molecules and reducing the supercooling of water, thereby reducing the damage of ice crystals to fruit and vegetable cells.

[0005] In terms of the thawing process, traditional thawing methods include air thawing and water thawing, which mainly rely on the temperature difference between the medium and the frozen material as the driving force, thawing through heat transfer. Although these methods are low-cost, they have drawbacks such as long thawing time, susceptibility to microbial contamination, significant juice loss, and poor product quality. New thawing processes currently include ultrasonic thawing, microwave thawing, radio frequency (RF) thawing, and electric field thawing. Ultrasonic cavitation is believed to induce heat generation and promote heat transfer, thereby shortening thawing time; microwave-assisted freezing can induce water dipole rotation, thus affecting ice nucleation and formation during freezing; the mechanism of RF is similar to that of microwaves, attributed to dipole rotation and ionic conductivity induced by the electromagnetic field. Compared to microwaves, RF has the advantages of lower energy consumption and more uniform heating. Furthermore, due to its longer wavelength, RF can penetrate deeper, making it suitable for high-density and large-size products; the potential mechanism of electric field thawing is that under the influence of an electric field, the reorientation of water molecules and the formation of more ordered clusters lead to a decrease in free energy. In addition, the corona discharge of the electrodes ionizes the air, generating ionic momentum, which is transferred to the food, thereby accelerating thawing.

[0006] Some methods reduce cell damage from ice crystals by optimizing freezing speed, but they are not very effective in improving juice loss and texture changes during thawing. Other methods use a single thawing method, such as natural thawing or hot water thawing. Natural thawing takes a long time and is prone to microbial growth, while hot water thawing causes significant loss of nutrients and deterioration in texture. Therefore, developing a method to effectively improve the thawing quality of quick-frozen fresh-cut potatoes is of great significance.

[0007] Currently, maintaining the quality of fresh-cut potatoes after freezing and thawing mainly relies on color-protecting agents or specific technological innovations targeting freezing or thawing. Tan Xiaohong (Publication No.: CN111328872A) disclosed a potato processing technique and seasoning / coating method to prevent oxidation and greening after thawing, including steps such as screening, washing, peeling, secondary cleaning, draining, cooking, adding seasoning / coating powder, tumbling, freezing, and packaging. However, this primarily focuses on cooked potatoes, so its reference value is limited. Yuan Ye (Publication No.: CN205492363U) disclosed a potato thawing device, including a base plate, fan, thawing box, motor, and ventilation duct. This method mainly utilizes fan-blown air to thaw potatoes, which is easy to industrialize; however, it is difficult to avoid the browning problem caused by air thawing. Yu Haixia and Cai Yong (Publication No.: CN221634935U) disclosed an electrostatic field defrosting device. Compared with static electric field defrosting, this invention provides an electrostatic field defrosting device that can cause the material to tumble, increasing the effectiveness. Compared with this invention, this invention uses pulsed electric field combined with high humidity, low temperature, and anaerobic defrosting for the defrosting process. It innovatively combines pulsed electric field with a high humidity, low temperature, and anaerobic defrosting environment. The two work together to improve defrosting efficiency while effectively preventing browning on the surface of the material. Mi Si and Wang Xianghong et al. (Publication No.: CN118370340A) developed a method for preparing and defrosting quick-frozen pre-prepared vegetables. This invention combines soaking in CaCl2 solution and blanching in melatonin solution. After freezing, a low-pressure electrostatic field is used for defrosting, which can reduce the quality loss of the vegetables to a certain extent. Compared to the previous invention, this invention innovatively employs vacuum ultrasonic-assisted low-temperature enzyme inactivation in the enzyme inactivation process, and combines electric field thawing with a first-time combination of high-humidity, low-temperature, and anaerobic thawing. Furthermore, it incorporates gradient magnetic field-assisted quick-freezing, significantly improving enzyme inactivation and color protection, rapid freezing, and efficient thawing of fresh-cut potatoes. Ultimately, this effectively preserves the quality, texture, and color of quick-frozen fresh-cut potatoes. Zhang Yuanlu and Liu Guishan et al. (Publication No.: CN115644234A) disclosed a low-voltage electrostatic field combined with vacuum thawing device and method, using vacuum and humidity sensors to monitor the vacuum and humidity within the thawing chamber. Compared to that invention, this invention uses a pulsed electric field combined with specific high-humidity, low-temperature, and anaerobic treatment, which avoids irreversible electrical damage to cell membranes caused by continuous electric field action and the resulting increase in juice loss, as well as excessive swelling of potato starch under continuous electric field action and its impact on the texture after thawing. In addition, the pulsed electric field can increase nutrient retention and reduce energy consumption. Zhang Min et al. (Publication No.: CN118000411A) proposed a synergistic method for improving the quality of frozen edible mushrooms. They mentioned atmospheric pressure cold plasma (ACP) and high voltage electric field (HVEF) for pre-freezing pretreatment and static magnetic field-assisted quick-freezing technology, which reduced the damage to the tissue structure of edible mushrooms.Similarly, Zhang Min et al. (Publication No.: CN116268266A) also mentioned a composite method for controlling the color change of frozen fruit in acidic juice, which uses electromagnetic coupling-assisted quick-freezing technology to achieve a longer-term color retention of frozen fruit when directly used in acidic juice (pH < 3.5). In addition, Zhang Min (Publication No.: CN117837637A) also mentioned a composite method for improving the quality of frozen marinated apple slices, and a composite method for controlling the quality deterioration of pre-prepared steak during freezing / thawing (Publication No.: CN117837637A), all of which use static magnetic field-assisted quick-freezing technology. Compared with these inventions, this invention uses gradient magnetic field-assisted quick-freezing through gradual cooling and magnetic field action, rather than static magnetic field-assisted freezing technology, and combines high to low temperature conditions (from -10~-15℃ pre-freezing to -40~-45℃ deep freezing) and magnetic field action of increasing intensity (10~20mT). Raising the temperature to 30-40 mT allows uniform, fine ice crystal nuclei to form inside the potato cubes. Then, rapid freezing at low temperatures fixes the ice crystal size, reducing mechanical damage to cells. Compared to conventional static magnetic field-assisted freezing, this method causes less damage to the cell structure and has higher freezing efficiency. Xu Mingqiang and Ma Yan et al. (Publication No.: CN217429141U) disclosed an electromagnetic coupling-assisted freezing and thawing device for fruits and vegetables, primarily providing a device that maintains the shape and nutritional value of fruits and vegetables during thawing or freezing. However, it did not offer a complete solution for addressing juice loss, decreased hardness, and surface browning during the freezing and thawing process of fresh-cut fruits and vegetables. Compared to this invention, this invention, in addition to innovative synergistic treatment of individual stages of magnetic field-assisted freezing and power plant-assisted thawing, also proposes a comprehensive innovative solution: vacuum ultrasonic-assisted low-temperature enzyme inactivation synergistic gradient magnetic field-assisted rapid freezing, and pulsed electric field-assisted high-humidity low-temperature anaerobic thawing. This effectively ensures improved quality of quick-frozen fresh-cut fruits and vegetables, especially quick-frozen potatoes, after thawing.

[0008] In summary, existing technologies for color protection, quick-freezing, and thawing of fresh-cut potatoes mostly involve limited innovation in one stage. For example, the aforementioned studies and patents demonstrate methodological innovation in one or two stages of color protection, quick-freezing, or thawing to achieve beneficial effects during the freezing and thawing process. However, none of these technologies involve the development of techniques such as vacuum ultrasonic-assisted low-temperature enzyme inactivation, gradient magnetic field-assisted quick-freezing, or pulsed electric field-assisted high-humidity, low-temperature, anaerobic thawing. Furthermore, none address the synergistic effect of these three innovative technologies to improve the thawing quality of quick-frozen fresh-cut fruits and vegetables, especially potatoes. In contrast, this invention innovatively employs vacuum ultrasonic-assisted low-temperature enzyme inactivation, gradient magnetic field-assisted quick-freezing, and pulsed electric field-assisted high-humidity, low-temperature, anaerobic thawing to effectively maintain the quality of quick-frozen fresh-cut potatoes after thawing, significantly enhancing their commercial and edible value. The synergistic effect of these three technologies increases the hardness of thawed fresh-cut potatoes by more than 10%, reduces browning by more than 10%, and controls juice loss to within 11%. Summary of the Invention

[0009] The technical objective of this invention is to overcome the problems that traditional quick-freezing and thawing processing methods cannot solve, such as browning, softening, and high juice loss in frozen fresh-cut potatoes after thawing. The invention aims to develop a method for improving the thawing quality of quick-frozen fresh-cut potatoes.

[0010] The technical solution of the present invention: A method for improving the quality of quick-frozen fresh-cut potatoes after thawing mainly includes the following steps: (1) Raw material selection: Select fresh potatoes that are similar in size and shape, have no visible damage, have normal sensory qualities, and are suitable for freezing processing as raw materials; (2) Pre-treatment: The selected potato raw materials are washed, peeled, and cut into corresponding cubes, shreds, strips, and slices according to the shape and specifications required; (3) Preparation of composite permeation dehydration and color protection agent: The composite permeation dehydration and color protection agent is mainly composed of food-grade calcium chloride with a mass concentration of 0.2%-0.4%, food-grade citric acid with a mass concentration of 0.1%-0.3%, and food-grade sodium ascorbate with a mass concentration of 0.05%-0.15%. The mass ratio of fresh-cut potato raw material to composite permeation dehydration and color protection agent during soaking is 1:3-1:5. (4) Temperature- and negative pressure ultrasonic-assisted enzyme inactivation: The fresh-cut potatoes pretreated in step (2) are immersed in the composite permeation dehydration and color-protecting agent prepared in step (3). First, they are ultrasonically treated for 4-5 minutes at a vacuum of 0.01-0.06 MPa and 55-65℃. Then, the temperature is raised to 80-85℃ and ultrasonic treatment is continued for 1-2 minutes. Finally, they are drained in a vacuum device at 4℃ for 5-8 minutes to obtain enzyme-inactivated fresh-cut potatoes. (5) Gradient magnetic field-assisted quick freezing: Place the enzyme-inactivated fresh-cut potatoes from step (4) into a magnetoelectric coupling device, pre-freeze them for 20-30 minutes in a magnetic field environment of -10℃~15℃ and 10~20mT; then quick-freeze them for 40-60 minutes in a magnetic field environment of -30℃~35℃ and 30~40mT to ensure that the core temperature is ≤-18℃; (6) Packaging: The quick-frozen fresh-cut potatoes from step (5) are packaged in PE polyethylene + PET polyethylene terephthalate mixed material bags at a rate of 1kg / bag, vacuum-packed and sealed; (7) Freezing: The packaged quick-frozen fresh-cut potatoes are placed in a constant temperature cold storage at -20±0.5℃ for freezing; (8) Pulsed electric field coordinated high humidity low temperature anaerobic thawing: Quickly unseal the packaging of fresh-cut potatoes frozen in step (7) and spread them evenly on the plastic mesh tray in the magneto-electric coupling device. After closing the door of the magneto-electric coupling device, quickly fill the magneto-electric coupling device with N2 until all the air in the magneto-electric coupling device cavity is replaced. Control the temperature and humidity conditions in the cavity to 4~6℃ and 80%~90%RH respectively. Thaw the material to the center temperature of 4±0.1℃ under a pulsed electric field strength of 20~30kV / cm. The potatoes suitable for freezing processing in step (1) mainly refer to varieties with a reducing sugar content of 0.3% to 0.5% by fresh weight, a starch content of 16% to 24% by fresh weight, and that are not prone to browning, have a firm texture, and are resistant to steaming and cooking.

[0011] In step (2), the specifications of fresh-cut potatoes of different shapes are as follows: the side length of fresh-cut potato cubes is 1±0.2mm; the side length of fresh-cut potato shreds is 0.15±0.05mm and the length is 50±10mm; the side length of fresh-cut potato strips is 10±2mm and the length is 55±5mm; and the thickness of fresh-cut potato slices is 6±0.5mm.

[0012] In step (4), the two ultrasonic conditions used in the variable temperature negative pressure ultrasound-assisted enzyme inactivation technology are 20~40 kHz low frequency ultrasound with a power of 200~400 W. In step (8), the distance between the needle-shaped electrode plate in the magnetoelectric coupling device and the center of the flat material is 10-15cm. The time required between the frozen material being laid out and placed into the magnetoelectric coupling device and being filled with N2 and the temperature and humidity being controlled to a specific range is 1-2 minutes. In step (8), the pulse electric field frequency is 30s for processing and 15s for stopping, and the process is completed when the center temperature rises to 4±0.1℃.

[0013] The beneficial effects of this invention are as follows: (1) The enzyme inactivation process adopts negative pressure temperature change treatment combined with ultrasonic assistance: negative pressure can minimize the contact between fresh-cut potatoes and oxygen during processing, thereby ensuring oxidative browning throughout the enzyme inactivation process; in temperature change treatment, the low temperature stage can reduce texture damage, and the high temperature stage can rapidly inactivate enzymes such as polyphenol oxidase; calcium chloride in the compound enzyme inactivation solution enhances the cell wall structure, citric acid and sodium ascorbate synergistically inhibit browning, and the synergistic effect of ultrasonic waves on cell permeability and the heat effect generated by vibration jointly promote the penetration of color-protecting agent and improve enzyme inactivation efficiency; (3) Gradient magnetic field-assisted quick freezing: Through gradual cooling and magnetic field action, the magnetic field can change the hydrogen bond arrangement of water molecules, inhibit the formation of large ice crystals, lower the freezing point of potato cell sap, and make the freezing process proceed rapidly at a lower temperature, shortening the total freezing time. Combined with temperature conditions from high to low (from -10~-15℃ pre-freezing to -40~-45℃ deep freezing) and magnetic field action with increasing intensity (from 10~20mT to 30~40mT), uniform and fine ice crystal nuclei are first formed inside the potato cubes, and then the ice crystal size is fixed by low-temperature rapid freezing, reducing the mechanical puncture damage of ice crystals to cells; (4) Pulsed electric field synergistic high humidity low temperature anaerobic thawing technology: The pulsed electric field (20-30kV / cm) destroys the phospholipid bilayer of potato cell membranes through high-intensity electric field force, forming reversible microporous channels. This effect can promote the rapid melting and reabsorption of small ice crystals formed during freezing, reducing the loss of free juice; under high humidity conditions of 90%-95%, the evaporation of water on the surface of fresh-cut potato cubes during thawing can be effectively inhibited, avoiding hardening and shrinkage caused by surface dryness; the anaerobic low temperature environment can minimize the impact of enzymatic browning and non-enzymatic browning on the appearance quality of frozen fresh-cut potatoes. Through the four-dimensional synergistic mechanism of "electric field cell wall breaking to promote water reflux - high humidity to retain water and prevent drying - low temperature to inhibit enzymes and control texture - anaerobic to block oxygen and protect color", the overall quality of thawed fresh-cut potato cubes is significantly improved.

[0014] Through the synergistic effect of enzyme inactivation, quick-freezing, and thawing, compared with fresh-cut potatoes of the same processing specifications that have not undergone any of the enzyme inactivation, freezing, and thawing treatments of this invention but have only been subjected to ordinary freezing at -20 ℃ and air thawing at 4 ℃, the thawed frozen fresh-cut potatoes have a hardness that is more than 10% higher, browning that is more than 10% lower, and juice loss that is controlled to within 11%, significantly improving the commercial and edible value of quick-frozen fresh-cut potatoes. Detailed Implementation

[0015] The technical solution of the present invention will be described in detail below with reference to the embodiments: Example 1

[0016] A method for improving the quality of quick-frozen fresh-cut potato cubes during thawing, comprising the following steps: (1) Raw material selection: Select fresh Atlantic potatoes that are similar in size and shape, have no visible damage, have normal sensory quality, low reducing sugar content (0.3%~0.5% based on fresh weight), moderate starch content (16%~24% based on fresh weight), are not prone to browning, have a firm texture, and are resistant to steaming and cooking as raw materials. (2) Pretreatment: The selected potato raw materials are washed, peeled, and cut into fresh potato cubes with a side length of 1±0.2mm; (3) Enzyme inactivation: The enzymes of freshly cut potatoes are inactivated using variable temperature negative pressure ultrasonic-assisted technology. The pre-treated freshly cut potatoes are immersed in a compound permeation dehydration and color-protecting agent with a mass ratio of 1:4, consisting of 0.3% food-grade calcium chloride, 0.2% food-grade citric acid, and 0.1% food-grade sodium ascorbate. The vacuum degree is set to 0.05MPa, the ultrasonic power frequency is 30kHz, and the power is 300W. The potatoes are first treated at 60℃ for 4 minutes, then heated to 80℃ for 1 minute, and then drained in a vacuum device at 4℃ for 5 minutes. (4) Quick-freezing: Gradient magnetic field-assisted quick-freezing technology is used to process the enzyme-inactivated materials. The enzyme-inactivated fresh-cut potatoes are placed in a magnetoelectric coupling device and pre-frozen for 20 minutes in a magnetic field environment of -10℃ and 15mT; then quick-frozen for 40 minutes in a magnetic field environment of -30℃ and 35mT; and finally deep-frozen for 20 minutes at -40℃. (5) Packaging: For quick-frozen fresh-cut potatoes, they are packaged in PE polyethylene + PET polyethylene terephthalate mixed material bags at 1kg / bag, vacuum-packed and sealed; (6) Freezing: The packaged quick-frozen fresh-cut potatoes are placed in a constant temperature cold storage at -20±0.5℃ for freezing; (7) Thawing: The frozen fresh-cut potatoes were thawed using pulsed electric field combined with high humidity, low temperature and oxygen-free technology. The frozen fresh-cut potatoes were quickly unsealed and evenly spread on the plastic mesh tray in the magneto-electric coupling device. The distance between the needle electrode plate and the center of the spread material was required to be 12±0.5cm. After closing the door, N2 was quickly introduced from the magneto-electric coupling device until all the air in the chamber was replaced. The temperature and humidity conditions in the chamber were controlled by temperature and humidity control to be 5℃ and 85%RH respectively. Thawing was carried out under the conditions of electric field strength of 25kV / cm and pulse frequency of 30s processing and 15s stopping until the center temperature of the material reached 4±0.1℃.

[0017] Comparative Experiment 1 The following control experiment was set up to verify the effect of the entire technical solution of the present invention on improving the thawing quality of quick-frozen fresh-cut potato cubes; and to compare the effect of the present invention with the effect of applying a certain technology in the invention alone, so as to demonstrate the application effect of the combination of various technologies in the present invention.

[0018] Control 1: Freshly cut potato cubes of the same processing specifications that were not subjected to any of the enzyme inactivation, freezing, or thawing treatments of this invention, but were simply frozen at -20 ℃, served as Control 1; Control 2: Potato cubes treated only by the quick-freezing and thawing techniques of the present invention without undergoing the enzyme inactivation treatment of the variable temperature negative pressure ultrasonic assisted technology of the present invention; Control 3: Potato cubes that were not treated with the gradient magnetic field assisted quick-freezing technology of the present invention, but were treated with enzyme inactivation, ordinary freezing at -20 ℃, and thawing technology of the present invention, were used as Control 3; Control 4: Potato cubes that were not subjected to the pulsed electric field-assisted high humidity and low temperature anaerobic thawing treatment of the present invention, but were treated with the enzyme inactivation and quick-freezing technology of the present invention and thawed in conventional air at 4°C, served as Control 4.

[0019] Then, the hardness, browning index, and juice loss rate of the control sample and the potato cubes frozen and thawed using the method of this invention were tested under the same conditions. The experimental data are shown in Table 1.

[0020] Table 1. Quality results of frozen fresh-cut potato cubes after thawing compared with the control treatment and the treatment of the present invention. Different lowercase letters following the numbers in the same column indicate significant differences between different treatment groups for the same indicator. Experimental results showed that, compared with fresh-cut potato cubes without freezing and thawing treatment, the hardness of the samples decreased to varying degrees after freezing and thawing. This is because, during freezing and thawing, ice crystals generated during the freezing process damage the integrity of the cell structure. After thawing, the ice crystals melt into water, and the pores they occupied lead to an overall decrease in the hardness of the samples after thawing. The results showed that the overall hardness from high to low was: fresh-cut sample > Example 1 ≈ Control 2 > Control 4 > Control 3 > Control 1. This indicates that the enzyme inactivation, freezing, and thawing treatment of fresh-cut potato cubes using this invention can significantly improve the texture quality and effectively alleviate the destructive effect of freezing and thawing on the texture of the samples. In terms of color, the browning index was mainly used to reflect the browning of frozen fresh-cut potato cubes after air thawing. This browning process is mainly related to oxygen in the air. The browning process should be that the enzyme system in the potato cells is damaged and reacts with oxygen in the air to produce blackish-red quinone substances. The results showed that the overall browning index, from low to high, was: Fresh-cut sample < Example 1 ≈ Control 3 < Control 4 < Control 2 < Control 1. A higher browning index indicates more severe browning during processing. This demonstrates that the frozen-thawed fresh-cut potato cubes treated with this invention can effectively delay browning during the quick-freezing and thawing process. Juice loss rate is a key indicator for judging the commercial and edible quality of fresh-cut potatoes before and after freezing and thawing, and is closely related to the costs of production and downstream processing enterprises. The results showed that the juice loss rate of fresh-cut potato cubes after freezing and thawing was between 9% and 15%. The different treatment groups, in ascending order, were: Example 1 < Control 3 < Control 4 < Control 2 < Control 1. This result proves that the fresh-cut potato cubes treated with this invention can effectively maintain the total juice content of the sample after freezing and thawing, reducing the decline in edible and commercial quality during the freezing and thawing process.

[0021] Example 2

[0022] A method for improving the quality of quick-frozen fresh-cut potato shreds during thawing, comprising the following steps: (1) Raw material selection: Select fresh Atlantic potatoes that are similar in size and shape, have no visible damage, have normal sensory quality, low reducing sugar content (0.3%~0.5% based on fresh weight), moderate starch content (16%~24% based on fresh weight), are not prone to browning, have a firm texture, and are resistant to steaming and cooking as raw materials. (2) Pretreatment: The selected potato raw materials are washed, peeled, and cut into fresh potato shreds with a cross-sectional side length of 0.15mm and a length of 55mm; (3) Enzyme inactivation: The enzymes of freshly cut potatoes are inactivated using variable temperature negative pressure ultrasonic-assisted technology. The pre-treated freshly cut potatoes are immersed in a compound permeation dehydration and color-protecting agent with a mass ratio of 1:3, consisting of 0.2% food-grade calcium chloride, 0.3% food-grade citric acid, and 0.1% food-grade sodium ascorbate. The vacuum degree is set to 0.05 MPa, the ultrasonic power frequency is 30 kHz, and the power is 200 W. The potatoes are first treated at 60°C for 5 minutes, then heated to 85°C for 1 minute, and then drained in a vacuum device at 4°C for 5 minutes. (4) Quick-freezing: Gradient magnetic field-assisted quick-freezing technology is used to process the enzyme-inactivated materials. The enzyme-inactivated fresh-cut potatoes are placed in a magnetoelectric coupling device and pre-frozen for 25 minutes in a magnetic field environment of -15℃ and 10mT; then quick-frozen for 40 minutes in a magnetic field environment of -35℃ and 30mT; and finally deep-frozen for 25 minutes at -40℃. (5) Packaging: For quick-frozen fresh-cut potatoes, they are packaged in PE polyethylene + PET polyethylene terephthalate mixed material bags at 1kg / bag, vacuum-packed and sealed; (6) Freezing: The packaged quick-frozen fresh-cut potatoes are placed in a constant temperature cold storage at -20±0.5℃ for freezing; (7) Thawing: The frozen fresh-cut potatoes were thawed using pulsed electric field combined with high humidity, low temperature and oxygen-free technology. The frozen fresh-cut potatoes were quickly unsealed and evenly spread on the plastic mesh tray in the magneto-electric coupling device. The distance between the needle electrode plate and the center of the spread material was required to be 13±0.5cm. After closing the door, N2 was quickly introduced from the magneto-electric coupling device until all the air in the chamber was replaced. The temperature and humidity conditions in the chamber were controlled by temperature and humidity control to be 4℃ and 80%RH respectively. Thawing was carried out under the conditions of electric field strength of 20kV / cm and pulse frequency of 30s processing and 15s stopping until the center temperature of the material reached 4±0.1℃.

[0023] Comparative Experiment 2 The following control experiment was set up to verify the effect of the entire technical solution of the present invention on improving the thawing quality of quick-frozen fresh-cut potato shreds; and to compare the effect of the present invention with the effect of applying a certain technology in the invention alone, so as to demonstrate the application effect of the combination of various technologies in the present invention.

[0024] Control 1: Freshly cut potato shreds of the same processing specifications that were not subjected to any of the enzyme inactivation, freezing, or thawing treatments of this invention, but were simply frozen at -20 ℃, served as Control 1; Control 2: Potato shreds treated only by the quick-freezing and thawing techniques of the present invention without undergoing the enzyme inactivation treatment of the variable temperature negative pressure ultrasonic assisted technology of the present invention; Control 3: Potato shreds that were not treated with the gradient magnetic field assisted quick-freezing technology of the present invention, but were treated with enzyme inactivation, ordinary freezing at -20 ℃, and thawing technology of the present invention, were used as Control 3; Control 4: Potato shreds that were not subjected to the pulsed electric field synergistic high humidity low temperature anaerobic thawing treatment of the present invention, but were treated with the enzyme inactivation and quick-freezing technology of the present invention and thawed in conventional air at 4°C, served as Control 4.

[0025] Then, the hardness, browning index, and juice loss rate of the control sample and the potato shreds sample frozen and thawed using the method of this invention were tested under the same conditions. The experimental data are shown in Table 2.

[0026] Table 2. Quality results of frozen fresh-cut potato shreds after thawing compared with the control treatment and the treatment of the present invention. Different lowercase letters following the numbers in the same column indicate significant differences between different treatment groups for the same indicator. The experimental results showed that, compared with the fresh-cut potato shreds sample without freezing and thawing treatment, in terms of hardness, the overall hardness from high to low was: fresh-cut sample > Example 2 > Control 2 > Control 4 > Control 3 > Control 1. This indicates that the fresh-cut potato shreds treated with enzyme inactivation, freezing, and thawing according to the present invention can significantly improve the texture quality and effectively alleviate the destructive effect of freezing and thawing on the sample texture. In terms of color, the overall browning index from low to high was: fresh-cut sample < Example 2 ≈ Control 3 < Control 4 < Control 2 < Control 1. Therefore, it is proved that the frozen-thawed fresh-cut potato shreds treated with the present invention can effectively delay the browning problem during the quick-freezing and thawing process. The juice loss rate results showed that the juice loss rate of fresh-cut potato shreds after freezing and thawing treatment was between 10% and 16%. The different treatment groups in ascending order were: Example 2 < Control 2 < Control 4 < Control 3 < Control 1. This result proves that the fresh-cut potato shreds obtained by the present invention can effectively maintain the total juice content of the sample after freezing and thawing treatment, reducing the decline in the edible and commercial quality of fresh-cut potato shreds during the freezing and thawing process.

[0027] Example 3

[0028] A method for improving the quality of quick-frozen fresh-cut potato strips during thawing, comprising the following steps: (1) Raw material selection: Select fresh Atlantic potatoes that are similar in size and shape, have no visible damage, have normal sensory quality, low reducing sugar content (0.3%~0.5% based on fresh weight), moderate starch content (16%~24% based on fresh weight), are not prone to browning, have a firm texture, and are resistant to steaming and cooking as raw materials. (2) Pretreatment: The selected potato raw materials are washed, peeled, and cut into fresh potato strips with a cross-sectional side length of 10mm and a length of 55mm; (3) Enzyme inactivation: The enzymes of freshly cut potatoes are inactivated using variable temperature negative pressure ultrasonic-assisted technology. The pre-treated freshly cut potatoes are immersed in a compound permeation dehydration and color-protecting agent with a mass ratio of 1:3, consisting of 0.3% food-grade calcium chloride, 0.2% food-grade citric acid, and 0.1% food-grade sodium ascorbate. The vacuum degree is set to 0.05MPa, the ultrasonic power frequency is 30kHz, and the power is 400W. The potatoes are first treated at 60℃ for 5 minutes, then heated to 80℃ for 2 minutes, and then drained in a vacuum device at 4℃ for 5 minutes. (4) Quick-freezing: Gradient magnetic field-assisted quick-freezing technology is used to process the enzyme-inactivated materials. The enzyme-inactivated fresh-cut potatoes are placed in a magnetoelectric coupling device and pre-frozen for 30 minutes in a magnetic field environment of -15℃ and 20mT; then quick-frozen for 60 minutes in a magnetic field environment of -50℃ and 40mT; and finally deep-frozen for 30 minutes at -40℃. (5) Packaging: For quick-frozen fresh-cut potatoes, they are packaged in PE polyethylene + PET polyethylene terephthalate mixed material bags at 1kg / bag, vacuum-packed and sealed; (6) Freezing: The packaged quick-frozen fresh-cut potatoes are placed in a constant temperature cold storage at -20±0.5℃ for freezing; (7) Thawing: The frozen fresh-cut potatoes were thawed using pulsed electric field combined with high humidity, low temperature and oxygen-free technology. The frozen fresh-cut potatoes were quickly unsealed and evenly spread on the plastic mesh tray in the magneto-electric coupling device. The distance between the needle electrode plate and the center of the spread material was required to be 15±0.5cm. After closing the door, N2 was quickly introduced from the magneto-electric coupling device until all the air in the chamber was replaced. The temperature and humidity conditions in the chamber were controlled by temperature and humidity control to be 6℃ and 90%RH respectively. Thawing was carried out under the conditions of electric field strength of 30kV / cm and pulse frequency of 30s processing and 15s stopping until the center temperature of the material reached 4±0.1℃.

[0029] Comparative Experiment 3 The following control experiment was set up to verify the effect of the entire technical solution of the present invention on improving the thawing quality of quick-frozen fresh-cut potato strips; and to compare the effect of the present invention with the effect of applying a certain technology in the invention alone, so as to demonstrate the application effect of the combination of various technologies in the present invention.

[0030] Control 1: Fresh-cut potato strips of the same processing specifications that were not subjected to any of the enzyme inactivation, freezing, or thawing treatments of this invention, but were simply frozen at -20 ℃, served as Control 1; Control 2: Potato strips treated only by the quick-freezing and thawing techniques of the present invention, without undergoing the enzyme inactivation treatment of the variable temperature negative pressure ultrasonic assisted technology of the present invention, served as Control 2; Control 3: Potato strips that were not treated with the gradient magnetic field assisted quick-freezing technology of the present invention, but were treated with enzyme inactivation, ordinary freezing at -20 ℃, and thawing technology of the present invention, were used as Control 3; Control 4: Potato strips that were not subjected to the pulsed electric field-assisted high humidity and low temperature anaerobic thawing treatment of the present invention, but were treated with the enzyme inactivation and quick-freezing technology of the present invention and thawed in conventional air at 4°C, served as Control 4.

[0031] Then, the hardness, browning index, and juice loss rate of the control sample and the potato strips frozen and thawed using the method of this invention were tested under the same conditions. The experimental data are shown in Table 3.

[0032] Table 3. Quality results of frozen fresh-cut potato strips after thawing compared with the control treatment and the treatment of the present invention. Different lowercase letters following the numbers in the same column indicate significant differences between different treatment groups for the same indicator. The experimental results showed that, compared with the fresh-cut potato strips without freezing and thawing treatment, in terms of hardness, the overall hardness from high to low was: fresh-cut sample > Example 3 > Control 2 ≈ Control 4 > Control 3 > Control 1. This indicates that the fresh-cut potato strips treated with enzyme inactivation, freezing, and thawing according to the present invention can significantly improve the texture quality and effectively alleviate the destructive effect of freezing and thawing on the sample texture. In terms of color, the overall browning index from low to high was: fresh-cut sample < Example 3 ≈ Control 3 < Control 4 < Control 2 ≈ Control 1. Therefore, it is proved that the frozen-thawed fresh-cut potato strips treated with the present invention can effectively delay the browning problem during the quick-freezing and thawing process. The juice loss rate results showed that the juice loss rate of the fresh-cut potato strips after freezing and thawing treatment was between 9% and 15%. The different treatment groups in ascending order were: Example 3 < Control 2 < Control 4 < Control 3 < Control 1. This result proves that the fresh-cut potato strips obtained by the present invention can effectively maintain the total juice content of the sample after freezing and thawing treatment, reducing the decline in the edible and commercial quality of the fresh-cut potato strips during the freezing and thawing process.

[0033] Example 4

[0034] A method for improving the quality of quick-frozen fresh-cut potato slices during thawing includes the following steps: (1) Raw material selection: Select fresh Atlantic potatoes that are similar in size and shape, have no visible damage, have normal sensory quality, low reducing sugar content (0.3%~0.5% based on fresh weight), moderate starch content (16%~24% based on fresh weight), are not prone to browning, have a firm texture, and are resistant to steaming and cooking as raw materials. (2) Pre-treatment: The selected potato raw materials are washed, peeled, and cut into fresh potato slices with a thickness of 6mm; (3) Enzyme inactivation: The enzymes of freshly cut potatoes are inactivated using variable temperature negative pressure ultrasonic-assisted technology. The pre-treated freshly cut potatoes are immersed in a compound permeation dehydration and color-protecting agent at a mass ratio of 1:4, which is composed of 0.4% food-grade calcium chloride, 0.1% food-grade citric acid, and 0.15% food-grade sodium ascorbate. The vacuum degree is set to 0.05MPa, the ultrasonic power frequency is 30kHz, and the power is 400W. The potatoes are first treated at 55℃ for 5 minutes, then heated to 80℃ for 2 minutes, and then drained in a vacuum device at 4℃ for 5 minutes. (4) Quick-freezing: Gradient magnetic field-assisted quick-freezing technology is used to process the enzyme-inactivated materials. The enzyme-inactivated fresh-cut potatoes are placed in a magnetoelectric coupling device and pre-frozen for 30 minutes in a magnetic field environment of -15℃ and 20mT; then quick-frozen for 60 minutes in a magnetic field environment of -35℃ and 40mT; and finally deep-frozen for 30 minutes at -45℃. (5) Packaging: For quick-frozen fresh-cut potatoes, they are packaged in PE polyethylene + PET polyethylene terephthalate mixed material bags at 1kg / bag, vacuum-packed and sealed; (6) Freezing: The packaged quick-frozen fresh-cut potatoes are placed in a constant temperature cold storage at -20±0.5℃ for freezing; (7) Thawing: The frozen fresh-cut potatoes were thawed using pulsed electric field combined with high humidity, low temperature and oxygen-free technology. The frozen fresh-cut potatoes were quickly unsealed and evenly spread on the plastic mesh tray in the magneto-electric coupling device. The distance between the needle electrode plate and the center of the spread material was required to be 10±0.5cm. After closing the door, N2 was quickly introduced from the magneto-electric coupling device until all the air in the chamber was replaced. The temperature and humidity conditions in the chamber were controlled by temperature and humidity control to be 4℃ and 80%RH respectively. Thawing was carried out under the conditions of electric field strength of 25kV / cm and pulse frequency of 30s processing and 15s stopping until the center temperature of the material reached 4±0.1℃.

[0035] Comparative Experiment 4 The following control experiment was set up to verify the effect of the entire technical solution of the present invention on improving the thawing quality of quick-frozen fresh-cut potato slices; and to compare the effect of the present invention with the effect of applying a certain technology in the invention alone, so as to demonstrate the application effect of the combination of various technologies in the present invention.

[0036] Control 1: Freshly cut potato slices of the same processing specifications that were simply frozen at -20 ℃ without undergoing any of the enzyme inactivation, freezing, or thawing treatments of this invention were used as Control 1. Control 2: Potato slices treated only by the quick-freezing and thawing techniques of the present invention, without undergoing the enzyme inactivation treatment of the variable temperature negative pressure ultrasonic assisted technology of the present invention, served as Control 2; Control 3: Potato slices that were not treated with the gradient magnetic field assisted quick-freezing technology of the present invention, but were treated with enzyme inactivation, ordinary freezing at -20 ℃, and thawing technology of the present invention, were used as Control 3; Control 4: Potato slices that were not subjected to the pulsed electric field synergistic high humidity low temperature anaerobic thawing treatment of the present invention, but were treated with the enzyme inactivation and quick-freezing technology of the present invention and thawed in the air at 4°C were used as Control 4.

[0037] Then, the hardness, browning index, and juice loss rate of the control sample and the potato slices frozen and thawed using the method of this invention were tested under the same conditions. The experimental data are shown in Table 4.

[0038] Table 4. Quality results of frozen fresh-cut potato slices after thawing compared with the control treatment and the treatment of the present invention. Different lowercase letters following the numbers in the same column indicate significant differences between different treatment groups for the same indicator. The experimental results showed that, compared with fresh-cut potato slices without freezing and thawing treatment, in terms of hardness, the overall hardness from high to low was: fresh-cut sample > Example 4 ≈ Control 2 > Control 4 > Control 3 > Control 1. This indicates that the enzyme-inactivating, freezing, and thawing treatment of fresh-cut potato slices using the present invention can significantly improve the texture quality and effectively alleviate the destructive effect of freezing and thawing on the sample texture. In terms of color, the overall browning index from low to high was: fresh-cut sample < Example 4 < Control 3 < Control 4 < Control 2 ≈ Control 1. Therefore, it is proved that the frozen-thawed fresh-cut potato slices treated with the present invention can effectively delay the browning problem during the quick-freezing and thawing process. The juice loss rate results showed that the juice loss rate of fresh-cut potato slices after freezing and thawing treatment was between 9% and 15%. The different treatment groups in ascending order were: Example 4 < Control 2 < Control 4 < Control 3 < Control 1. This result proves that the fresh-cut potato slices obtained by the present invention can effectively maintain the total juice content of the sample after freezing and thawing treatment, reducing the decline in the edible and commercial quality of fresh-cut potato slices during the freezing and thawing process.

Claims

1. A method for improving the quality of quick-frozen fresh-cut potatoes after thawing, characterized in that, The main steps include the following: (1) Raw material selection: Select fresh potatoes that are similar in size and shape, have no visible damage, have normal sensory qualities, and are suitable for freezing processing as raw materials; (2) Pre-treatment: The selected potato raw materials are washed, peeled, and cut into corresponding cubes, shreds, strips, and slices according to the shape and specifications required; (3) Preparation of composite osmotic dehydration and color protection agent: The composite osmotic dehydration and color protection agent is mainly composed of 0.2%-0.4% food-grade calcium chloride, 0.1%-0.3% food-grade citric acid, and 0.05%-0.15% food-grade sodium ascorbate. The mass ratio of fresh-cut potato raw material to composite osmotic dehydration and color protection agent during soaking is 1:3-1:

5. (4) Temperature- and negative pressure ultrasonic-assisted enzyme inactivation: The fresh-cut potatoes pretreated in step (2) are immersed in the composite permeation dehydration and color-protecting agent prepared in step (3). First, they are ultrasonically treated for 4-5 minutes at a vacuum of 0.01-0.06 MPa and 55-65℃, then heated to 80-85℃ and ultrasonically treated for 1-2 minutes. Finally, they are drained in a vacuum device at 4℃ for 5-8 minutes to obtain enzyme-inactivated fresh-cut potatoes. (5) Gradient magnetic field-assisted quick freezing: Place the enzyme-inactivated fresh-cut potatoes from step (4) into a magnetoelectric coupling device, pre-freeze them for 20-30 minutes in a magnetic field environment of -10℃~15℃ and 10~20mT; then quick-freeze them for 40-60 minutes in a magnetic field environment of -30℃~35℃ and 30~40mT to ensure that the core temperature is ≤-18℃; (6) Packaging: The quick-frozen fresh-cut potatoes from step (5) are packaged in PE polyethylene + PET polyethylene terephthalate mixed material bags at a rate of 1kg / bag, vacuum-packed and sealed; (7) Freezing: The packaged quick-frozen fresh-cut potatoes are placed in a constant temperature cold storage at -20±0.5℃ for freezing; (8) Pulsed electric field coordinated high humidity and low temperature anaerobic thawing: Quickly unseal the fresh-cut potatoes in the frozen storage in step (7) and spread them evenly on the plastic mesh tray in the magneto-electric coupling device. After closing the door of the magneto-electric coupling device, quickly fill the magneto-electric coupling device with N2 until all the air in the magneto-electric coupling device cavity is replaced. Control the temperature and humidity conditions in the cavity to 4~6℃ and 80%~90%RH respectively. Thaw the material until the center temperature reaches 4±0.1℃ under the pulse electric field strength of 20~30kV / cm.

2. The method for improving the thawing quality of quick-frozen fresh-cut potatoes according to claim 1, characterized in that, The potatoes suitable for freezing processing in step (1) mainly refer to varieties with a reducing sugar content of 0.3% to 0.5% by fresh weight, a starch content of 16% to 24% by fresh weight, and that are not prone to browning, have a firm texture, and are resistant to steaming and cooking.

3. The method for improving the thawing quality of quick-frozen fresh-cut potatoes according to claim 1, characterized in that, In step (2), the specifications of fresh-cut potatoes of different shapes are as follows: the side length of fresh-cut potato cubes is 1±0.2mm; the side length of fresh-cut potato shreds is 0.15±0.05mm and the length is 50±10mm; the side length of fresh-cut potato strips is 10±2mm and the length is 55±5mm; and the thickness of fresh-cut potato slices is 6±0.5mm.

4. The method for improving the thawing quality of quick-frozen fresh-cut potatoes according to claim 1, characterized in that, In step (4), the two ultrasonic conditions used in the variable temperature negative pressure ultrasound-assisted enzyme inactivation technology are 20~40 kHz low frequency ultrasound with a power of 200~400 W.

5. The method for improving the thawing quality of quick-frozen fresh-cut potatoes according to claim 1, characterized in that, In step (8), the distance between the needle-shaped electrode plate in the magnetoelectric coupling device and the center of the flat material is 10-15cm. The time required for the frozen material to be placed in the magnetoelectric coupling device and filled with N2 and the temperature and humidity controlled to a specific range is 1-2 minutes.

6. The method for improving the thawing quality of quick-frozen fresh-cut potatoes according to claim 1, characterized in that, In step (8), the pulse electric field frequency is 30s for processing and 15s for stopping, and the core temperature rises to 4±0.1℃ for complete thawing.

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