A microwave-assisted thin-layer blanching method for inactivating enzymes in leafy green vegetables

Abstract

This invention discloses a novel enzyme-inactivating method using microwave blanching assisted by a thin layer of water, belonging to the field of agricultural product processing technology. The invention primarily involves immersing vegetables in a thin layer of water before microwave blanching. In this invention, the thin layer of water just covers each stem and leaf tissue, significantly reducing water consumption and avoiding excessive energy loss. Compared to traditional blanching, this blanching method shortens the time required to achieve less than 10% enzyme activity by 50%-83%, significantly reducing the degree of heat exposure to the vegetables and preserving their quality.

Description

Technical Field

[0001] This invention relates to a microwave-assisted thin-water blanching method for inactivating enzymes in leafy green vegetables, belonging to the field of agricultural product processing technology. Background Technology

[0002] Frozen green vegetables are prone to yellowing upon reheating. This is due to incomplete enzyme inactivation during pretreatment, leading to chlorophyll degradation into yellow-green substances, or browning caused by peroxidase and polyphenol oxidase. Currently, the industrial method for enzyme inactivation is still hot water blanching. This method is not ideal for non-uniform materials, as heat conduction in materials with varying thicknesses, such as the stems and stalks of leafy greens, cannot achieve the desired effect. Therefore, a processing method that can both ensure quality and quickly achieve enzyme inactivation is needed to replace the traditional blanching method.

[0003] Microwaves, as a highly efficient and clean energy source, can rapidly heat the interior and exterior of materials almost simultaneously, proving to be more effective than traditional heat conduction methods for enzyme inactivation. However, due to the rapid heating rate of microwaves, there is a risk of localized overheating. Therefore, to ensure the moisture content and sensory quality of high-moisture materials such as frozen fruits and vegetables after enzyme inactivation, industrial microwave blanching still requires a large amount of water as a medium to mitigate the direct effect of microwaves on the materials. However, this method of using a thick layer of water before using the microwave heating device, or using the microwave heating device and the thick layer of water simultaneously, has several drawbacks. First, it affects microwave penetration and material absorption, with more microwaves being absorbed by the water, leading to incomplete enzyme inactivation. Second, it generates a significant amount of wastewater and requires higher microwave energy, failing to meet the demands of energy conservation and environmental protection. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides an energy-saving and water-saving high-efficiency enzyme inactivation method suitable for leafy vegetables. The aim is to achieve more efficient enzyme inactivation while maintaining better vegetable quality, and to significantly reduce water consumption during the blanching process, ultimately achieving energy conservation and emission reduction.

[0005] The first technical solution provided by this invention is a microwave-assisted thin-layer blanching method for inactivating enzymes in leafy green vegetables, comprising the following steps:

[0006] S1, collect leafy vegetables and pre-process them to obtain stem and leaf tissues;

[0007] S2, Immerse each stem and leaf tissue from step S1 into a container containing a thin layer of water, the water layer height of which is (d-1mm) to (d+1mm); where d is the maximum thickness of the leafy vegetable stem and leaf tissue in step S1.

[0008] S3, the container from step S2 is microwave-sterilized using a microwave processing device to obtain leafy vegetable products with an enzyme inactivation rate of over 90%. It should be noted that during the enzyme inactivation process, d is taken as the maximum thickness of the leaf tissue of the same type of leafy vegetable. If different types of leafy vegetables exist, a different d value is assigned to each type of leafy vegetable, and the d value differs for different types of vegetables.

[0009] In some embodiments, the leafy vegetables include Shanghai bok choy, Suzhou bok choy, spinach, Chinese cabbage, etc.

[0010] In some embodiments, step S1, the pretreatment process for leafy vegetables includes vegetable washing and screening, removal of residual moisture, and harvesting of vegetable stems and leaves.

[0011] Furthermore, the vegetable washing and screening process is as follows: whole vegetables are washed with cold water to remove yellowed or rotten parts of the vegetables.

[0012] Optionally, the cold water cleaning can be performed by cold water spraying or soaking.

[0013] Optionally, the temperature of the cold water is controlled at 10-25℃ to ensure that the quality of the vegetables is not damaged by excessively cold or hot temperatures.

[0014] Optionally, if some leaves of a vegetable turn yellow or shrivel, remove that part; if the vegetable rots or spoils, exclude the entire vegetable.

[0015] Furthermore, the removal of residual moisture is specifically as follows: The surface moisture of the washed and screened vegetables is removed. That is, the washed and screened vegetables are taken out of the spraying or soaking environment and placed in a dehydration environment to remove surface moisture.

[0016] Optionally, the process of removing washed vegetables from the aquatic environment can be carried out manually, by conveyor belt, or by mechanical gripper.

[0017] Alternatively, surface moisture of vegetables can be removed by non-thermal dehydration methods such as natural air drying or mechanical spin drying.

[0018] Optionally, the environment for removing moisture from the vegetable surface is a naturally ventilated environment at 10-25℃.

[0019] Furthermore, the vegetable stems and leaves are harvested as follows: the roots of the vegetables are removed after surface moisture has been drained, and individual stem and leaf tissues are collected. After the vegetables are drained, their roots are removed, and the scattered stem and leaf tissues are collected and sorted by weight.

[0020] Furthermore, the harvesting of vegetable stems and leaves also includes a stem and leaf flattening process. Specifically, the stem and leaf flattening process involves applying pressure to flatten the stems and leaves and collecting them; the collected stems and leaves are flattened by applying a certain pressure, causing them to shed their natural curled shape and maintain a flat state, and then collected for subsequent processes.

[0021] Optionally, the stems and leaves can be flattened by rolling or flattening.

[0022] In some embodiments, the stem and leaf tissue is a leaf blade, petiole, or the entire stem and leaf.

[0023] In some embodiments, in step S2, the container is selected as a container made of non-wave-absorbing materials such as polytetrafluoroethylene.

[0024] In some embodiments, the temperature of the thin-layer water in step S2 is greater than 70°C. The temperature of the thin-layer water is related to the amount of stem and leaf structure processed in a single operation and the microwave power used in the subsequent microwave-assisted enzyme inactivation process. When the microwave power / processing volume (i.e., the microwave power density) is high, a lower preheating temperature can be used to reduce energy loss during the preheating conduction heating process; when the microwave power density is low, a higher preheating temperature is used to ensure that the stem and leaf structure can reach the enzyme inactivation temperature in a short time. Setting the initial water temperature of the thin-layer water above 70°C ensures that rapid enzyme inactivation of large batches of materials can be performed under commonly used low-power (kilowatt-level) microwave sources.

[0025] In some embodiments, in step S2, the stem and leaf tissues are arranged in a single layer in the spatial distribution of the thin layer of water. A single layer arrangement means that there is no longitudinal overlap between the stem and leaf tissues.

[0026] The stem and leaf tissues are placed independently in the container, and pressure can be applied to each stem and leaf tissue in the container to immerse it.

[0027] In some embodiments, in step S3, the microwave power density of the microwave processing device is greater than 8.8 W / g stem and leaf tissue. The heating time is related to the microwave power / stem and leaf structure weight; the higher the value of the microwave power / stem and leaf structure weight, the shorter the heating time.

[0028] In some embodiments, the microwave rinsing time in step S3 is 10 to 50 seconds.

[0029] In some embodiments, in step S3, the stem and leaf tissues after microwave blanching are cooled.

[0030] Furthermore, cooling methods include air cooling, water cooling, or low-temperature refrigeration.

[0031] The second technical solution provided by this invention is the application of the method described in the first technical solution in vegetable preservation.

[0032] Compared to existing technologies, this invention offers the following advantages: This invention primarily involves immersing vegetables in a thin layer of water before microwave blanching. In this invention, the thin layer of water just covers each stem and leaf tissue, significantly reducing water consumption and avoiding excessive energy loss. Compared to traditional blanching methods, this invention reduces the time required to reach enzyme activity levels below 10% by 50%-83%, significantly reducing the degree of heating on the vegetables and preserving their quality. Attached Figure Description

[0033] Figure 1 Temperature change curves of leaves and stems during the thin-layer microwave-assisted blanching process (heating mode: initial water temperature 70℃, power density 17.6w / g, water layer height 6mm) and water bath blanching process.

[0034] Figure 2 Changes in POD enzyme activity in leaves and stems during thin-layer microwave-assisted blanching and water bath blanching. This indicates the remaining enzyme activity in the leaf area under this enzyme inactivation mode; The parameters indicate the remaining enzyme activity in the stem and petiole under this enzyme inactivation mode; Water bath-leaf: the percentage of remaining enzyme activity in the leaf part when the water bath temperature is 99℃ and the water volume is 200mL; Water bath-petiole: the percentage of remaining enzyme activity in the leaf part when the water bath temperature is 99℃ and the water volume is 200mL; 8.8w / g-6mm-50℃: the parameters for microwave-assisted thin-layer water rinsing and enzyme inactivation when the microwave power is 8.8w / g, the water layer height is 6mm, and the initial water temperature is 50℃.

[0035] Figure 3 Water consumption for different blanching methods. Detailed Implementation

[0036] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0037] The leafy vegetables involved in the embodiments and comparative examples of this invention are fresh Shanghai bok choy purchased from the market and placed at room temperature (25°C) for 4 hours before testing.

[0038] The enzyme inactivation container is a polytetrafluoroethylene cylindrical box with no top. The inner diameter of the cylindrical container is 120mm and the height is 15mm.

[0039] The microwave-assisted enzyme inactivation device is a commercial microwave oven (NE-1753, Panasonic Corporation), with a maximum power of 1800W.

[0040] Traditional hot blanching equipment is a water bath (DF-101KS, Zhengzhou Hengyan Instrument Co., Ltd.), with a maximum heating temperature of 150℃.

[0041] The test methods involved in the embodiments and comparative examples of this invention are as follows:

[0042] 1. Blanching process temperature: For the complete stem and leaf structure, a thermocouple is inserted into the main vein of the leaf to represent the overall temperature of the leaf; for the leaf blade, the thermocouple is fixed to the surface; for the stem, the thermocouple is inserted into the geometric center of the stem. The other end of the thermocouple is connected to a computer to record the temperature rise curve of the leaf.

[0043] 2. Stem and Leaf Structure Quality Determination: A colorimeter was used to measure the color of the leaves before and after the blanching treatment. The results are expressed as L, a, and b values. For leaf measurements, the main vein was avoided, and 5-10 locations were measured. For petiole measurements, the petiole was flattened before measurement (to fully cover the light-transmitting opening). The highest and lowest values ​​were discarded in the final color calculation, and the average value of the remaining values ​​was taken.

[0044] 3. Determination of peroxidase (POD) activity:

[0045] Extraction: pH 6.5 0.1M phosphate buffer PBS, material-to-liquid ratio (1:5), grind in an ice bath until homogenized, centrifuge twice to remove residue, and take the supernatant as crude enzyme extract.

[0046] Measurement: Spectrophotometric determination. Add 0.1 mL of the above crude enzyme extract to 2.9 mL of substrate (1 mL 0.2% guaiacol, 0.9 mL 0.3% H2O2, 1 mL PBS), shake well for 10 s, and immediately place in a spectrophotometer to measure the absorbance change at 470 nm. Record the value every 20 s for a total of 2 min. The enzyme activity is defined as an increase of 0.01 μL in absorbance per minute as 1 unit of POD activity.

[0047] Example 1

[0048] The specific method is as follows:

[0049] S1. Freshly purchased Shanghai bok choy was placed at room temperature for 4 hours to equilibrate. It was then soaked and washed in cold water (18℃) to remove yellowed tissue and discard any whole vegetables with rotten structures. The vegetables were then air-dried in a well-ventilated environment at room temperature (25℃) to remove surface moisture. The roots were removed to separate the leaves and stems for collection. 3.0±0.3g of leaves and 2.0±0.1g of stems were collected. The naturally curled leaves were flattened with a plastic sheet without damaging their structure. The longest side of the selected leaves was 80mm, and the shortest side was 60mm. The selected stems were 15mm long, 15mm wide, and 5mm thick, and were placed in room temperature water for subsequent measurements.

[0050] S2, the maximum thickness of the stem and leaf structure is 5mm, and the water depth is set based on this. The three depths are: 3mm for partial immersion, 6mm for full immersion, and 12mm for complete immersion. Add water at 50℃ to the enzyme inactivation container and adjust to the three water depths (3mm, 6mm, 12mm). Immediately place the flattened leaves or stems into the enzyme inactivation container, quickly adjust their position using a glass rod, and immediately begin the subsequent microwave-assisted enzyme inactivation process.

[0051] S3. Turn on the microwave heating equipment and set different microwave power densities (4.4W / g, 8.8W / g, 13.2W / g, 17.6W / g) according to the weight of the leaves or stems. Set the heating time to 10s, 20s, 30s, 40s, and 50s, and perform heating while measuring the temperature in real time. After heating is complete, immediately place the sample in ice water (4℃) to cool it down and maintain its state after heating. Perform enzyme activity and quality determination on the obtained samples.

[0052] Example 2

[0053] The specific method is as follows:

[0054] S1. Freshly purchased Shanghai bok choy was placed at room temperature for 4 hours to equilibrate. It was then soaked and washed in cold water (18℃) to remove yellowed tissue and discard any whole vegetables with rotten structures. The vegetables were then air-dried in a well-ventilated environment at room temperature (25℃) to remove surface moisture. The roots were removed to separate the leaves and petioles for collection. 3.0±0.3g of leaves and 2.0±0.1g of petioles were collected. The naturally curled leaves were flattened with a plastic sheet without damaging their structure. The longest side of the selected leaves was 80mm, and the shortest side was 60mm. The selected petioles were 15mm long, 15mm wide, and 5mm thick, and were placed in room temperature water for subsequent measurements.

[0055] S2, the maximum thickness of the stem and leaf structure is 5mm, and the water depth is set based on this. The depths are: 3mm for partial immersion, 6mm for full immersion, and 12mm for complete immersion. Add 70℃ water to the enzyme inactivation container and adjust to the three water depths (3mm, 6mm, 12mm). Immediately place the flattened leaves or stems into the enzyme inactivation container, quickly adjust their position using a glass rod, and immediately begin the subsequent microwave-assisted enzyme inactivation process.

[0056] S3. Turn on the microwave heating equipment and set different microwave power densities (4.4W / g, 8.8W / g, 13.2W / g, 17.6W / g) according to the weight of the leaves or stems. Set the heating time to 10s, 20s, 30s, 40s, and 50s, and perform heating while measuring the temperature in real time. After heating is complete, immediately place the sample in ice water (4℃) to cool it down and maintain its state after heating. Perform enzyme activity and quality determination on the obtained samples.

[0057] Example 3

[0058] The specific method is as follows:

[0059] S1. Freshly purchased Shanghai bok choy was placed at room temperature for 4 hours to equilibrate. It was then soaked and washed in cold water (18℃) to remove yellowed tissue and discard any whole vegetables with rotten structures. The vegetables were then air-dried in a well-ventilated environment at room temperature (25℃) to remove surface moisture. The roots were removed to separate the leaves and petioles for collection. 3.0±0.3g of leaves and 2.0±0.1g of petioles were collected. The naturally curled leaves were flattened with a plastic sheet without damaging their structure. The longest side of the selected leaves was 80mm, and the shortest side was 60mm. The selected petioles were 15mm long, 15mm wide, and 5mm thick, and were placed in room temperature water for subsequent measurements.

[0060] S2, the maximum thickness of the stem and leaf structure is 5mm, and the water depth is set based on this. The three depths are: 3mm for partial immersion, 6mm for full immersion, and 12mm for complete immersion. Add 90℃ water to the enzyme inactivation container and adjust to the three water depths (3mm, 6mm, 12mm). Immediately place the flattened leaves or stems into the enzyme inactivation container, quickly adjust their position using a glass rod, and immediately begin the subsequent microwave-assisted enzyme inactivation process.

[0061] S3. Turn on the microwave heating equipment and set different microwave power densities (4.4W / g, 8.8W / g, 13.2W / g, 17.6W / g) according to the weight of the leaf or stem. Set the heating time to 10s, 20s, 30s, 40s, and 50s, and perform heating while measuring the temperature in real time. After heating is complete, immediately place the sample in ice water (4℃) to cool it down and maintain its state after heating.

[0062] Comparative Example 1: Traditional water bath blanching to inactivate enzymes

[0063] After step S1 of Example 1 is completed, the leaves or stems are placed in a 99°C water bath for heating. Heating times are set to 10s, 20s, 30s, 40s, and 50s, and the temperature is measured in real time. After heating is complete, the samples are immediately placed in ice water (4°C) to cool them down and maintain their heated state. The obtained samples are then subjected to enzyme activity and quality determination.

[0064] Test Case

[0065] The blanching process parameters for each group in Examples 1-3 and Comparative Example 1 are shown in Table 1. At a water bath temperature of 99℃, it took 20 seconds for the residual enzyme activity of the leaves to reach 10% because the material is thin and heat transfer is faster. However, for materials with a certain thickness, such as the stem, inactivation required 60 seconds (Group 37 in Table 1), and the water volume was 200 mL. When the power density (8.8 W / g) was constant, the inactivation time of materials with an initial water temperature >70℃ was shortened by 50% or more, and 90℃ shortened the time by 67%. Regarding the immersion method (water layer thickness), it was found that semi-immersion (water layer thickness 3 mm) and full immersion (water layer thickness 12 mm) were not conducive to enzyme inactivation, which may be related to the penetration depth of microwaves in water. By monitoring the heating process of Group 23 in Table 1, [the following data was obtained]. Figure 1 It is known that microwaves accelerate the internal heating of materials, taking only about 20 seconds for the entire material to reach 100°C, while a water bath takes about 180 seconds, especially in the final stage of heating (90-100°C). This also means that microwaves accelerate the internal heating of materials, thereby rapidly inactivating enzymes.

[0066] Table 1. Heating mode settings for the thin-layer microwave-assisted blanching process and the time required to achieve 10% residual peroxidase activity.

[0067]

[0068]

[0069] At a water layer thickness of just-submerged (6 mm), different power densities (4.4, 8.8, 13.2, 17.6 w / g) were compared to determine their effect on enzyme inactivation rate. Figure 2 It was found that water baths required 20s and 60s respectively for the remaining enzyme activity in leaves and petioles to reach 10% of their initial activity. Under conditions of 70℃, 17.6w / g power density, and a 6mm water depth, most POD enzymes in leaves and petioles could be rapidly inactivated within 10s, significantly shortening the enzyme inactivation time compared to traditional water bath scalding and reducing the degree of heating. Higher power density resulted in faster inactivation; the fastest (70℃+17.6w / g, 90℃+13.2w / g, 90℃+17.6w / g) allowed the time required for the remaining enzyme activity to fall below 10% to be less than 10s, a reduction of 83% compared to water baths. In summary, to achieve better enzyme inactivation efficiency in a faster time, the initial water temperature should be >70℃ and the power density >8.8w / g for stem and leaf tissues when the water depth is 6mm, which can achieve ideal enzyme inactivation results (time reduction of 50%-83%). Figure 3It is evident that, compared to the traditional water bath rinsing method for enzyme inactivation, the water consumption is only half that of the traditional method, achieved with a water layer thickness of just 6mm for immersion. In summary, the thin-layer water-assisted microwave enzyme inactivation method provided by this invention significantly reduces enzyme inactivation time while saving substantial amounts of water.

[0070] Leaves and petioles from group 23 in Table 1 were microwave-treated for 10 seconds (initial water temperature 70℃, power density 17.6w / g, water level 6mm), and leaves and petioles treated in a 99℃ water bath for 10 seconds and 60 seconds were quick-frozen, stored at -18℃ for 7 days, and then reheated by steaming at 100℃ for 15 minutes. The color difference was then measured, and the results are shown in Table 2. When the degree of enzyme inactivation was insufficient (taking a 10-second water bath as an example), the green value (-a) decreased more than that of the sample at the inactivation endpoint, indicating a greater degree of yellowing. On the other hand, this also shows that the color difference between microwave treatment for 10 seconds and water bath treatment for 60 seconds under this microwave heating mode is not significant, proving that this bleaching method can achieve an appearance color almost identical to that of water bath treatment in a short time.

[0071] Table 2. Color changes of leaves and stems during the thin-layer microwave-assisted blanching process (heating mode: initial water temperature 70℃, power density 17.6w / g, water layer height 6mm) and water bath blanching process.

[0072]

[0073] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Anyone skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.

Claims

1. A microwave-assisted thin-layer blanching method for inactivating enzymes in leafy green vegetables, characterized in that, Includes the following steps: S1, collect leafy vegetables and pre-process them to obtain stem and leaf tissues; S2, each stem and leaf tissue from step S1 is immersed in a container containing a thin layer of water, the water layer height of which is d ± 1 mm; where d is the maximum thickness of the leafy vegetable stem and leaf tissue in step S1; wherein, the initial water temperature of the thin layer of water is above 70°C, and the stem and leaf tissue is arranged in a single layer in the thin layer of water. S3, the container from step S2 is subjected to microwave blanching using a microwave processing device. The microwave power density of the microwave processing device is greater than 8.8W / g of stem and leaf tissue, and the microwave blanching time is 10~50s, resulting in leafy vegetable products with an enzyme inactivation rate of over 90%.

2. The method according to claim 1, characterized in that, In step S1, the pretreatment process for leafy vegetables includes vegetable washing and screening, removal of residual moisture, and harvesting of vegetable stems and leaves.

3. The method according to claim 2, characterized in that, The specific steps for cleaning and screening vegetables are as follows: wash whole vegetables with cold water to remove yellowed or rotten parts of the vegetables. The removal of residual moisture is specifically as follows: removing moisture from the surface of the washed and screened vegetables; The vegetable stems and leaves are harvested as follows: the roots of the vegetables, after the surface moisture has been removed, are removed, and individual stem and leaf tissues are collected.

4. The method according to any one of claims 1 to 3, characterized in that, The stem and leaf tissues are leaf blades, petioles, or the entire stem and leaf.

5. The method according to claim 1, characterized in that, In step S3, the stem and leaf tissues after microwave blanching are cooled.

6. The application of the method according to any one of claims 1 to 5 in vegetable preservation.

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