Drying method for improving quality of walnut kernels and walnut oil

Abstract

The invention belongs to the technical field of walnut processing, and particularly relates to a drying method for improving the quality of walnut kernels and walnut oil. The method comprises the following steps: (1) freezing and storing: peeling green-peel walnuts, storing for 2-4 months at-18-4 DEG C, and shelling to obtain fresh walnut kernels; and (2) hot air drying: drying the fresh walnut kernels in the step (1) at 40-50 DEG C for 8-20 hours. According to the method, freezing storage and hot air drying are combined, so that high-quality walnut kernels and walnut oil are obtained; for example, after the treatment, the lowest acid value is 0.521 mg / g, and the highest oil yield can reach 47.7%; the peroxide value is the lowest (0.0031 g / 100g), and the DPPH free radical scavenging rate is preferably 26.84%.

Description

Technical Field

[0001] This invention belongs to the field of walnut processing technology, specifically relating to a drying method for improving the quality of walnut kernels and walnut oil. Background Technology

[0002] Walnuts are a nutritious nut, rich in protein, unsaturated fatty acids, vitamins, and various minerals, and are widely used in food, health products, and other fields. Drying is a crucial step in walnut processing, directly affecting their quality and market value. However, due to their high moisture content, walnuts are prone to spoilage and rotting at room temperature, impacting their quality and shelf life. Proper drying methods can reduce the moisture content of walnuts, inhibit microbial growth and reproduction, minimize nutrient loss, and extend their shelf life, which is essential for ensuring the quality and market competitiveness of walnut products.

[0003] Currently, the main methods for drying walnuts are: hot air drying, segmented drying, and vacuum drying.

[0004] Hot air drying refers to a process in which the drying temperature is set and maintained at a fixed value, and the material is continuously heated to evaporate the moisture in the walnuts, thus achieving the purpose of drying. However, using hot air drying alone may cause adverse reactions such as oxidation and browning in the material due to the high temperature and oxygen, affecting the quality of the walnut kernels and walnut oil.

[0005] Segmented drying divides the drying process into multiple stages, each with different parameters such as temperature and time, which are adjusted according to the material characteristics to improve drying efficiency while maximizing material quality. This method requires precise control of hot air temperature, wind speed, and residence time based on material characteristics and drying stages, making operation and control relatively complex.

[0006] Vacuum drying technology is a process that utilizes a vacuum environment to dry materials. In a vacuum environment, the air pressure is significantly reduced, and the boiling point of water also decreases significantly. The moisture in the material does not need to reach the conventional boiling point of 100℃ to rapidly vaporize and evaporate, greatly accelerating the drying speed. Vacuum drying has a good protective effect on heat-sensitive components, but it also has disadvantages such as low efficiency, high energy consumption, and high equipment costs.

[0007] Therefore, it is necessary to improve the above methods and invent a method that can improve the quality of walnut kernels and walnut oil, especially walnut kernels and walnut oil that perform well in terms of indicators such as acid value, fat content, peroxide value and DPPH free radical scavenging rate. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention provides a method for improving the quality of walnut kernels and walnut oil. This method primarily involves peeling the walnuts, freezing them, and then drying them with hot air, thereby improving the walnut quality. Through this treatment, the peroxide value of the walnut oil is reduced to as low as 0.0031 g / 100 g, and the acid value is reduced to as low as 0.521 mg / g, resulting in optimal walnut oil quality.

[0009] The specific solution of the present invention is as follows: A drying method for improving the quality of walnut kernels and walnut oil includes the following steps: (1) Frozen storage Take green-skinned walnuts, remove the skin, store them at -18 to -4℃ for 2 to 4 months, remove the shells, and obtain fresh walnut kernels; (2) Hot air drying Place the fresh walnut kernels in (1) at 40-50℃ and dry for 8-20 hours.

[0010] Preferably, in (1), it is stored at -18°C for 3 months.

[0011] Preferably, in (2), the product is dried at 45°C for 16 hours.

[0012] Preferably, the above-mentioned indicators for improving the quality of walnut kernels and walnut oil are at least four of the following: walnut kernel color, walnut kernel reducing power, walnut acid value, walnut kernel fat content, walnut kernel oil yield, total antioxidant activity of walnut oil, and DPPH antioxidant activity of walnut oil.

[0013] Preferably, the above-mentioned indicators for improving the quality of walnut kernels and walnut oil are: acid value, fat content, peroxide value, and DPPH free radical scavenging rate.

[0014] In this application, the main focus for examining walnut kernels was color, fat content, peroxide value, acid value, and oil yield; for walnut oil, the total antioxidant activity and DPPH free radical scavenging activity were examined. The lower the acid value and peroxide value, the better the product quality; the higher the fat content and the higher the DPPH free radical scavenging rate, the better the product quality.

[0015] The beneficial effects of this invention are as follows: (1) In this application, freezing storage and hot air drying are combined, and the two work together to obtain high-quality walnut kernels and walnut oil; for example, after the above treatment, the acid value is as low as 0.521 mg / g, the oil yield is as high as 47.7%, the peroxide value is as low as 0.0031g / 100g, and the DPPH free radical scavenging rate is as high as 26.84%; (2) The method of this application is simply to obtain high-quality walnut kernels and walnut oil through freeze storage and hot air drying. Compared with other chemical reagent treatment methods, the method of this invention is not only simple, but also safe and environmentally friendly. (3) The walnut kernels and walnut oil processed by the method of the present invention are of high quality, providing a strong raw material guarantee for subsequent walnut processing products. Attached Figure Description

[0016] Figure 1 The effect of storage and drying methods on the moisture content of walnut kernels; Figure 2 The effect of storage and drying methods on the color of walnut kernels; Figure 3 The effect of storage and drying methods on the fat content of walnut kernels; Figure 4 The effect of storage and drying methods on the peroxide value of walnut kernels; Figure 5 The effect of storage and drying methods on the acid value of walnut kernels; Figure 6 The effect of storage and drying methods on the oil yield of walnut kernels; Figure 7 The effects of storage and drying methods on the total antioxidant activity of walnut oil; Figure 8 The effects of storage and drying methods on the antioxidant activity of DPPH in walnut oil. Detailed Implementation

[0017] To enable those skilled in the art to better understand the present invention, the present invention will now be further described in conjunction with specific embodiments.

[0018] Example 1 This embodiment mainly describes in detail the source of walnut kernels, the method of obtaining walnut oil, and the methods used to test the quality of both, as follows: 1. Materials and Methods 1.1 Experimental Materials and Instruments The green-skinned walnuts were provided by Changyi Dingli Walnut Planting Professional Cooperative. The drying equipment used was a Shanghai Jinghong DHG-9070A electric heating forced-air drying oven and a Shanghai Boxun DZF-6050MBE vacuum drying oven.

[0019] Drying method After peeling, green walnuts were stored in the refrigerator (4℃) and freezer (-18℃) for three months, respectively, to obtain fresh walnut kernels. The kernels were then dried using hot air drying, segmented drying, and vacuum drying, with different drying temperatures, times, and pressures set for each method. The dried kernels were then pressed for oil to investigate the effects of different storage and drying methods on the quality of walnuts and walnut oil. Specific drying conditions are as follows: (1) Hot air drying: The walnut kernels were placed in a 45℃ oven and dried for 0, 8, 12, 16 and 20 hours respectively, and then taken out for subsequent experiments.

[0020] (2) Segmented drying: The walnut kernels were placed in a 35℃ oven and dried for 4 hours. The temperature was then adjusted to 50℃ and dried for 8 hours. Finally, the temperature was adjusted to 30℃ and dried for 8 hours. The kernels were taken out at 0, 8, 12, 16 and 20 hours respectively for subsequent experiments.

[0021] (3) Vacuum drying: The walnut kernels were placed in a vacuum drying oven at 45℃ and dried for 0, 8, 12, 16 and 20 hours under a vacuum of 0.1 MPa, respectively, and then taken out for subsequent experiments.

[0022] Figure 1 In the process, the walnut kernels are dried by freezing hot air, that is, stored at -18℃ for three months, shelled and then processed by the hot air drying step in (1) above. Similarly, freeze-drying, that is, storing at -18℃ for three months, shelling to obtain fresh walnut kernels, and then processing them through the segmented drying steps in (2) above; Freeze-drying, that is, storing at -18℃ for three months, removing the shells to obtain fresh walnut kernels, and then processing them using the vacuum drying step in (3) above; Cold storage and hot air drying, that is, cold storage at 4°C for three months, shelling to obtain fresh walnut kernels, and then processing them through the hot air drying step in (1) above.

[0023] Figures 2-8 In this context, the freeze-hot air drying, freeze-segment drying, freeze-vacuum drying, and refrigerated hot air drying mentioned above are all the same as the above-described treatments.

[0024] Quality Analysis of Walnuts and Walnut Oil 1.3.1 Determination of moisture content in walnut kernels Take 3 g of walnut kernel sample and use a moisture analyzer to determine its moisture content. Perform three parallel measurements and take the average value.

[0025] 1.3.2 Walnut kernel color analysis The color difference of walnut kernels was measured using a colorimeter, and its L value was recorded. * a * b * Value. The reference point is the whiteboard (L).* =46.67, a * = 9.7, b * =16.54), L * The value represents the brightness of the color of the sample being tested; a * The value represents the degree of red or green, a * When >0, the color is reddish, a * When <0, the color is more greenish; b * The value represents the degree of orange or blue, b * When >0, the color is more orange, b * When <0, the color is bluish; ΔE * The larger the value, the greater the color difference. The color difference formula is as follows:

[0026] Note: , , The values ​​are for walnut green husk samples measured at 0 h. , , The measured value of walnut green husk sample at th

[0027] 1.3.5 Determination of fat content in walnut kernels The fat content of walnut kernels was determined using the first method, Soxhlet extraction, of GB 5009.6-2016, "National Food Safety Standard - Determination of Fat in Food".

[0028] 1.3.3 Determination of peroxide value in walnut kernels The peroxide value of walnut kernels was determined using the first method of indicator titration as specified in GB 5009.277-2023, "National Food Safety Standard - Determination of Peroxide Value in Food".

[0029] 1.3.4 Determination of acid value in walnut kernels The acid value of walnut kernels was determined using the second method of cold solvent automatic potentiometric titration, as specified in GB 5009.299-2016 "National Food Safety Standard - Determination of Acid Value in Food".

[0030] 1.3.6 Determination of oil yield from walnut kernels Take 25 ± 0.1 g of walnut kernels and put them into an oil press. Stir-fry them at 130℃ for 10 minutes and then press them to extract oil. Weigh the oil and record the weight of the extracted oil and the weight of the removed impurities. Calculate the oil yield.

[0031]

[0032] 1.3.7 Determination of total antioxidant activity of walnut oil Dissolve 2 mL of walnut oil in 15 mL of distilled water, then mix with 2.5 mL of 0.2 M phosphate buffer (pH 6.6) and 2.5 mL of 1% potassium ferricyanide. Incubate the mixture at 50 °C for 20 minutes. Add 2.5 mL of 10% trichloroacetic acid to the mixture. Then, mix 2.5 mL of this solution with 2.5 mL of distilled water and 0.5 mL of 0.1% FeCl3, and measure the absorbance at 700 nm using a spectrophotometer. An increase in the absorbance of the reaction mixture indicates an increase in reducing power.

[0033] 1.3.8 Determination of DPPH free radical scavenging activity of walnut oil Using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) kit, 0.5 mL of walnut oil and 2.5 mL of DPPH ethanol solution (20 μmol / L) were mixed thoroughly and then sealed and allowed to stand at room temperature in the dark for 30 min. Using ethanol as a blank, the absorbance was measured at 517 nm for detection.

[0034] 1.3.9 Determination of accompanying substances in walnut oil The sterol content was determined according to NY / T 3111-2017 Gas Chromatography-Mass Spectrometry; the vitamin E content was determined according to GB 5009.82-2016 Method I Gas Chromatography.

[0035] 1.3.10 Determination of 3-chloropropanol ester and glycidyl ester content in walnut oil The contents of 3-chloropropanol ester and glycidyl ester were determined according to the isotope dilution-alkaline hydrolysis-gas chromatography-mass spectrometry method in Part II of GB 5009.191-2024.

[0036] Example 2 Results and Analysis In this embodiment, the walnut kernels from Example 1 were dried using different methods, and the quality of the walnut kernels and walnut oil obtained by each drying method was tested and analyzed to obtain results that are more conducive to improving the quality of walnut kernels and walnut oil. Specifically, as follows: 2.1 Effects of storage and drying methods on the moisture content of walnut kernels The effects of storage and drying methods on the moisture content of walnut kernels are shown in Table 1 and... Figure 1 As shown: Table 1. Effects of storage and drying methods on the moisture content of walnut kernels.

[0037] Note: Each experiment was performed in triplicate, and the results are expressed as mean ± standard deviation. Duncan's difference analysis was performed using ANOVA, and P < 0.05 was considered statistically significant. ABCD indicates a significant difference when comparing different drying methods for the same drying time; abcd indicates a significant difference when comparing different drying times for the same drying method, and so on.

[0038] Depend on Figure 1 As shown in Table 1, the moisture content decreased with increasing drying time. During the drying process from 0 to 8 hours, the moisture in the walnut kernels was rapidly lost with increasing drying time, and the rate of moisture loss began to slow down from 8 to 16 hours. After 16 hours of drying, the trend of moisture content change in the walnut kernels became relatively flat. Among the three drying methods, the walnut kernels dried by hot air had the fastest rate of moisture loss, followed by segmented drying and vacuum drying. This indicates that the drying time of hot air drying has a greater impact on the moisture content of the walnuts; the longer the drying time, the slower the rate of moisture loss. In the two storage methods, the moisture content of walnut kernels that were frozen after hot air drying was consistently lower than that of walnut kernels stored under cold storage. This indicates that the rate of moisture loss in frozen-stored walnuts is faster, and the moisture content of walnuts can be reduced to the target level in a shorter time.

[0039] 2.2 Effects of storage and drying methods on the color of walnut kernels The effects of storage and drying methods on the color and color difference of walnut kernels are shown in Table 2 and 3. Figure 2 : Table 2. Effects of storage and drying methods on the color of walnut kernels

[0040] In Table 2, with increasing drying time, the L* value of walnut kernels dried using different methods showed a trend of first decreasing and then increasing, while the a* value showed a trend of first increasing and then decreasing. With increasing drying time, among the three drying methods, the b* value of walnut kernels dried by hot air drying and segmented drying showed a trend of first decreasing, then increasing, and finally decreasing; the b* value of vacuum-dried walnut kernels gradually decreased with increasing drying time. Among the two storage methods, the b* value of walnut kernels dried by cold air refrigeration showed an increasing trend with increasing drying time. The color changes of the walnut kernels all became more and more obvious with increasing drying time. Figure 2As shown, among the three drying methods, the color difference (ΔE) of walnut kernels dried by hot air drying and segmented drying showed a trend of first increasing and then decreasing, while the ΔE of walnut kernels dried by vacuum drying showed a trend of increasing, decreasing, and then increasing again. During vacuum drying, due to the lower pressure of the drying environment, the moisture evaporation rate is relatively fast, resulting in more stable color changes compared to hot air drying. Within a certain drying time range, the moisture content gradually decreases. As the moisture gradually decreases, the high temperature accelerates the oxidation rate of unsaturated fatty acids, leading to a darker color and an increase in the total color difference (ΔE). In both storage methods, the ΔE of walnut kernels reached its maximum value after 12 hours of drying, and at 12 hours, freeze-drying with hot air (ΔE = 21.55) > refrigerated hot air drying (ΔE = 7.19). The results indicate that the color difference between the inner seed coat and kernel of walnuts after vacuum drying is positively correlated with the drying temperature; that is, the higher the drying temperature, the darker the color. Unsaturated fatty acids are significantly affected by drying temperature; the longer the drying time and the higher the temperature, the more easily enzymatic browning occurs in the oxygen-containing drying medium, indirectly leading to poor color properties.

[0041] 2.3 Effects of storage and drying methods on the fat content of walnut kernels The effects of storage and drying methods on the fat content of walnut kernels are shown in Table 3 and Figure 3 As shown: Table 3. Effects of storage and drying methods on the fat content of walnut kernels.

[0042] Under different drying methods, the fat content of walnut kernels showed a similar trend with drying time. For example... Figure 3 As shown in Table 3, during the initial drying stage (0-16 hours), the fat content of walnut kernels gradually increased with increasing heating time, reaching its maximum after 16 hours and then stabilizing. The effect of the three drying methods on the fat content of walnut kernels was in the order of hot air drying > segmented drying > vacuum drying. In the two storage methods, the fat content of walnut kernels dried by hot air and then refrigerated was greater than that of walnut kernels stored by freezing, indicating that walnuts stored under refrigeration have a higher fat content, which may be related to the oxidation of fats in the walnut kernels.

[0043] 2.4 Effects of storage and drying methods on the peroxide value of walnut kernels The effects of storage and drying methods on the peroxide value of walnut kernels are shown in Table 4: Table 4. Effects of storage and drying methods on the peroxide value of walnut kernels

[0044] Depend on Figure 4As shown in Table 4, with increasing drying time, the peroxide value of walnut kernels showed a trend of first increasing and then decreasing within the drying period of 0-16 h, reaching its minimum value at 16 h. At 16 h of drying, among the three drying methods, hot air drying resulted in the lowest peroxide value (0.003 g / 100 g), followed by vacuum drying (0.004 g / 100 g), while segmented drying resulted in the highest peroxide value (0.005 g / 100 g). Comparing the two storage methods, it was found that the peroxide value of walnut kernels dried by cold-air drying was consistently higher than that of those dried by freeze-hot air drying. At 16 h of drying, the peroxide value of walnut kernels decreased significantly, with the peroxide value of cold-air dried walnut kernels at 0.005 g / 100 g, higher than that of freeze-hot air dried walnut kernels (0.003 g / 100 g), indicating that the degree of oxidation of cold-air dried walnut kernels was high and their quality was unstable. Controlling the storage conditions of walnuts and selecting appropriate extraction methods can improve oxidative stability and reduce peroxide value, which is beneficial for better controlling the quality of walnut kernels.

[0045] 2.5 Effects of storage and drying methods on the acid value of walnut kernels Table 5. Effects of storage and drying methods on the acid value of walnut kernels.

[0046] With increasing heating time, the acid value of walnut kernels decreased rapidly during the 0-8 h drying process, and then stabilized after 8-20 h. As drying time increased, the moisture content, lipid oxidation, and free fatty acid content of the walnut kernels decreased rapidly from 0-8 h, leading to a decrease in acid value and improved quality. Among the three drying methods, hot air drying for 16 h resulted in the best quality walnut kernels, with the lowest acid value at 0.521 mg / g. In terms of storage methods, the acid value of freeze-dried walnut kernels was consistently lower than that of refrigerated hot air dried kernels. This may be because the longer storage time of walnuts allows lipid molecules to hydrolyze under the influence of moisture, temperature, and various environmental factors, releasing free fatty acids and affecting the acid value of walnut oil. The decrease in moisture content, lipid oxidation, and free fatty acid content with increasing drying time leads to a decrease in acid value and improved quality. Fatty acids can affect the stability of oils and fats. The main reason for the increase in acid value is that the small molecules produced by the decomposition and breakdown of fatty acids, especially unsaturated fatty acids, are further oxidized to produce organic acids.

[0047] 2.6 Effects of storage and drying methods on the oil yield of walnut kernels Walnut oil is a natural plant oil, with walnuts containing 40%-72% oil. The effects of different drying methods on the oil yield of walnut kernels are as follows: Figure 6 As shown in Table 6: Table 6. Effects of storage and drying methods on the oil yield of walnut kernels

[0048] As drying time increased, the oil yield of walnut kernels gradually rose within 0-16 hours. At 0 hours, due to the high moisture content and soft texture of the kernels, their structure was not easily broken during pressing, making it difficult to release oil and resulting in a yield of 0%. After 16 hours of drying, the oil yield reached its maximum and then stabilized. At 16 hours, the oil yield of walnut kernels using the three drying methods, from highest to lowest, was: hot air drying (47.70%), vacuum drying (47.29%), and segmented drying (45.57%). In both storage methods, except at 16 hours, the oil yield of refrigerated hot air-dried walnut kernels was consistently higher than that of frozen hot air-dried walnut kernels. At 0 hours, due to the high moisture content of the kernels, they were difficult to extract oil through pressing, resulting in a yield of 0. The oil yield of walnuts gradually increased with the extension of drying time.

[0049] 2.7 Effects of storage and drying methods on the total antioxidant activity of walnut oil The curves showing the effects of storage and drying methods on the total antioxidant activity of walnut oil are as follows: Figure 7 As shown in Table 7, the total antioxidant activity of walnut oil is represented by the absorbance value at 700 nm.

[0050] Table 7. Effects of storage and drying methods on the total antioxidant activity of walnut oil

[0051] The effects of different drying methods on the total antioxidant activity of walnut oil after freeze storage showed similar changes. With increasing heating time, the total antioxidant activity of walnut oil initially increased and then decreased, reaching its maximum at 16 hours of drying. This may be because in the initial drying stage, some antioxidants in the walnuts increased, leading to an increase in reducing power, or the decrease in moisture increased the concentration of antioxidants, thus increasing the total antioxidant activity. However, as drying continued, antioxidants may be oxidized, leading to a decrease in total antioxidant activity. Among the three drying methods, the total antioxidant activity of walnut oil at 16 hours of drying, from strongest to weakest, was hot air drying (0.075), segmented drying (0.074), and vacuum drying (0.071). In terms of storage methods, the total antioxidant activity of refrigerated hot air-dried walnut oil showed an initial decrease and then an increase. At 16 hours of drying, the total antioxidant activity of refrigerated hot air-dried walnut oil (0.064) was lower than that of freeze-dried walnut oil (0.075).

[0052] 2.8 Effects of storage and drying methods on the antioxidant activity of walnut oil DPPH like Figure 8 Table 8 shows the effects of different storage and drying methods on the antioxidant activity of DPPH in walnut oil.

[0053] Table 8. Effects of storage and drying methods on the antioxidant activity of DPPH in walnut oil.

[0054] With increasing drying time, the DPPH free radical scavenging capacity of walnut oil dried by different methods showed a trend of first increasing and then decreasing, similar to the trend of changes in the total antioxidant activity of walnut oil. Among the three drying methods, the DPPH free radical scavenging rate of walnut oil from high to low was segmented drying, hot air drying, and vacuum drying, with the maximum value at 16 h of drying. At this point, the DPPH free radical scavenging rate of walnut oil from high to low was segmented drying (31.31%), hot air drying (31.11%), and vacuum drying (21.64%). In both storage methods, with the extension of drying time, the DPPH free radical scavenging rate of walnut oil dried by freeze-drying and hot air drying was consistently higher than that of walnut oil dried by refrigeration and hot air drying within 8-20 h.

[0055] In this application, the effects of hot air drying, segmented drying, and vacuum drying on the color difference, moisture content, oil yield, fat content, acid value, peroxide value, and oxidative stability of walnuts were compared.

[0056] After three drying methods, at the same temperature, the moisture content of walnuts decreased with increasing heating time, and the rate of decrease slowed down until it plateaued. The water loss rate of walnuts after the three drying methods was hot air drying > segmented drying > vacuum drying. Within 0-16 hours, the color of the walnut kernels changed more significantly with increasing drying time. The effect of the three drying methods on the color of walnuts was hot air drying > segmented drying > vacuum drying. With increasing heating time, the absorbance of walnuts dried after freeze-drying reached its maximum value at 16 hours. The reducing power of the three drying methods, from strongest to weakest, was hot air drying, vacuum drying, and segmented drying. After 16 hours of hot air drying following freeze-drying, the acid value was the lowest (0.521 mg / g), the fat content was the highest (58.04 g / 100g), the peroxide value was the lowest (0.0031 g / 100g), and the DPPH free radical scavenging rate was the best. After heating and drying using different methods, the oil yield of walnuts was hot air drying > segmented drying > vacuum drying. It is evident that walnut kernels dried for 16 hours using freeze-hot air drying are of the best quality. In general, walnut kernels from frozen-stored, shelled walnuts dried for 16 hours using hot air drying yield the best quality kernels.

[0057] 2.9 Effects of drying methods on walnut oil byproducts and risk factors In addition to the investigations in 2.1-2.8, Example 2.9 also investigated the effects of drying methods on walnut oil byproducts and risk factors, and the results are shown in Table 9. Analysis of the effects of drying methods on walnut oil byproducts and risk factors in Table 9 shows that glycidyl esters were not detected in walnut oils prepared by the three drying methods, and the content of 3-chloropropanol esters was low, all below 1.25 mg / kg, meeting the EU's recommended limit for the permissible content of 3-chloropropanol esters in edible vegetable oils. The freeze-dried walnut oil had the lowest content (0.49 mg / kg), indicating that the walnut oil prepared by this method is relatively safe.

[0058] Table 9. Effects of drying methods on walnut oil byproducts and risk factors.

[0059] Sterols are functional components in oils and fats, possessing physiological activities such as cholesterol-lowering. The sterol content showed no significant difference, indicating that the drying method has little impact on the sterol content in walnut oil. Vitamin E (VE) is an important antioxidant, beneficial for maintaining oil stability and improving its nutritional value. The VE content of the three drying methods, from highest to lowest, was: freeze-hot air drying (1.81 mg / kg), freeze-segmented drying (1.52 mg / kg), and freeze-vacuum drying (0.89 mg / kg). This indicates that freeze-hot air drying better preserves VE in walnut oil, while freeze-segmented drying leads to a significant loss of VE. Hot pressing has a high oil yield and has been widely used in walnut oil extraction, increasing the solubility of fat-soluble substances in walnuts.

[0060] By comprehensively comparing the three methods mentioned above and analyzing the results of hazardous substance control and beneficial component detection, the focus is on examining risk factors such as glycidyl esters and 3-chloropropanol esters, as well as lipid-associated compounds such as sterols and vitamins. E The results show that the walnut oil prepared by first freezing and then hot air drying in this invention has better quality.

Claims

1. A drying method for improving the quality of walnut kernels and walnut oil, characterized in that, Includes the following steps: (1) Frozen storage Take green-skinned walnuts, remove the skin, store them at -18 to -4℃ for 2 to 4 months, remove the shells, and obtain fresh walnut kernels; (2) Hot air drying Place the fresh walnut kernels in (1) at 40-50℃ and dry for 8-20 hours.

2. The drying method for improving the quality of walnut kernels and walnut oil as described in claim 1, characterized in that, (1) Store at -18℃ for 3 months.

3. The drying method for improving the quality of walnut kernels and walnut oil as described in claim 1, characterized in that, (2) Dry at 45°C for 16 hours.

4. The drying method for improving the quality of walnut kernels and walnut oil as described in claim 1, characterized in that, The indicators for improving the quality of walnut kernels and walnut oil are at least four of the following: walnut kernel color, walnut kernel reducing power, walnut kernel acid value, walnut kernel fat content, walnut kernel oil yield, total antioxidant activity of walnut oil, and DPPH antioxidant activity of walnut oil.

5. The drying method for improving the quality of walnut kernels and walnut oil as described in claim 1, characterized in that, The indicators for improving the quality of walnut kernels and walnut oil are: acid value, fat content, peroxide value, and DPPH free radical scavenging rate.

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