Preparation method of freeze-dried radish crisp chips through combined drying

Abstract

The invention discloses a preparation method of freeze-dried radish crisp chips through combined drying. The method comprises the following steps: selecting fresh green radishes which are free of damage and rot and deterioration; cleaning the surface with clean water to remove dust and impurities; performing slicing treatment on the cleaned green radishes; the green radish slices are placed in a composite aqueous solution for color protection and seasoning, and the treatment time is 1-2 h; then the green radish slices are subjected to hot air pre-drying, and the water content of the pre-dried green radish slices is 30%-70%; and performing vacuum freeze drying on the green radish slices pre-dried by the hot air, so that the water content of the final product is below 5%. According to the preparation method of the freeze-dried radish crisp chips through combined drying, the appearance and taste of the finally formed green radish crisp chips are improved, nutritional ingredients are fully preserved, browning of the green radish crisp chips is delayed, and the shelf life of the green radish crisp chips is prolonged.

Description

Technical Field

[0001] The invention relates to the technical field of agricultural product processing, and in particular to a method for preparing combined drying and freeze-dried radish crisp slices. Background Art

[0002] Radish is rich in essential vitamins, mineral elements, amino acids and raphanin, a functional active ingredient rarely found in other vegetables. It has the effects of moistening the lungs and removing dryness, relieving cough and reducing phlegm, and enhancing immunity. It also plays an important role in cancer prevention and anti-cancer. It is known as "little ginseng" among the Chinese people. Studies have shown that the content of 8 mineral elements and raphanin in green-skinned and green-fleshed radishes is the highest, which is 2-3 times that of other radishes. In addition, raphanin has a significant effect on lung cancer, esophageal cancer, and forestomach cancer. Its mechanism of action is to induce the production of Phase II (carcinogenic factor detoxification) enzymes, further acting on the body's own anti-cancer system and regulating metabolism. Raphanin is also a natural plant antibiotic, containing an antiviral active substance - "interferon inducer", which can stimulate human cells to produce interferon and kill viruses. 1mg / mL raphanin has a significant inhibitory effect on a variety of bacteria, and can inhibit the growth of streptococci, pus cocci, pneumococci and Escherichia coli. Radish seed extract can eliminate oxidative free radicals, such as hydrogen peroxide, nitrite and other oxidative free radicals, which can cause lipid peroxidation to form a variety of products. These products are related to human aging, cancer, autoimmune diseases, anemia and other diseases.

[0003] Green radish has the disadvantages of being easy to rot and deteriorate, so its shelf life is relatively short. Drying is a common and effective way to extend the shelf life of fruits and vegetables. Drying fruits and vegetables can not only reduce the water activity of the material, but also inhibit the growth of microorganisms and the activity of enzymes. And green radish chips have the characteristics of low fat and low heat, crisp taste, low sugar and low salt, and unique flavor. At present, the technologies used for drying agricultural products mainly include vacuum freeze drying, hot air drying, microwave drying and microwave vacuum drying. Among them, the advantages of hot air drying are low equipment investment, strong adaptability, simple operation and control, and good sanitary conditions. However, the hot air drying temperature is high, and long-term contact with oxygen may produce bad odors, and the physiologically active ingredients in fruits and vegetables will also be greatly damaged, thereby seriously reducing the quality of the product. And long-term hot air drying causes the surface of fruits and vegetables to shrink severely during drying, the surface becomes hard and crusty, but the internal moisture cannot be removed in time, which will cause problems such as blackening and shrinkage of the appearance and internal corruption, greatly reducing the quality of food. Traditional vacuum freeze drying takes a long time to dry, but the food quality after drying is better, with less color change, less loss of aroma and nutrients, loose texture and dense taste. Although some auxiliary freeze drying products have good quality, they are only used for small-scale drying operations of high-value foods (such as fruits and vegetables) due to problems such as high energy consumption and difficulty in equipment manufacturing. However, since the vacuum freeze combined drying technology equipment is relatively simple and can combine the advantages of different drying technologies, it has broad development prospects. Summary of the invention

[0004] Based on previous research and existing problems, the present invention proposes a method for preparing combined drying and freeze-dried radish chips after further research and analysis. The process is safe, time-saving, easy to operate, reduces costs and can improve the taste of green radish chips. At the same time, it solves the problems of large loss of volatile aroma substances and poor appearance in the existing methods.

[0005] To achieve the above object, the present invention provides the following technical solution: a method for preparing combined drying and freeze-dried radish chips, the method steps are as follows:

[0006] S1. Select fresh, undamaged, and non-rotten green radishes;

[0007] S2. Wash the surface with clean water to remove dust and debris;

[0008] S3, slicing the cleaned green radish;

[0009] S4, placing the green radish slices after step S3 in the composite aqueous solution for color protection and seasoning, the treatment time is 1 to 2 hours;

[0010] S5, pre-drying the green radish slices after step S4 with hot air, so that the moisture content of the pre-dried green radish slices is 30% to 70%;

[0011] S6, vacuum freeze-drying the green radish slices after step S5, so that the moisture content of the final product is below 5%.

[0012] Preferably, after step S4, vacuum freeze drying pre-freezing process parameters are screened, and the freezing temperature and freezing time are selected. The freezing temperature is -20°C to -80°C, and the freezing time is 3h to 15h. By adjusting the freezing temperature and time in the freezing stage, the nutritional quality of the material can be retained to the greatest extent, and energy consumption can be reduced and resources can be saved.

[0013] The vacuum drying conditions were as follows: vacuum degree 2.1 mbar, cold trap temperature -40°C, and partition temperature 20°C.

[0014] Preferably, the temperature of hot air pre-drying in step S5 is 55° C. to 65° C. Using hot air as a pre-drying method can quickly reduce the moisture content of the material, and hot air drying is the simplest and most economical drying method. Appropriate hot air pre-drying temperature and drying time can change the organizational structure and composition matrix of the material, thereby improving the edible quality of the radish chips, while also greatly retaining the color and appearance of the fresh green radish, ensuring a high degree of retention of nutrients in the material.

[0015] Preferably, in step S6, the green radish is first frozen to a temperature of 10° C. or more below the eutectic point, and then further vacuum dried.

[0016] Preferably, in step S3, the thickness of the green radish slices is 3 mm to 1 cm. Slicing the green radish to 3 mm to 1 cm can ensure that the dried radish chips have a certain crispness, a non-soft taste, and a high heat transfer efficiency.

[0017] Preferably, in step S3, the thickness of the green radish slices is 5 mm; the vacuum freeze drying pre-freezing time is 9 hours, and the freezing temperature is -40°C; the water content of the green radish slices after hot air pre-drying is 50%, and controlling the water content of the freeze-dried product to be below 5% can reduce the growth of microorganisms in the product during storage, thereby ensuring the quality of the freeze-dried product, and the dehydration is thorough, the weight is light, and the product is convenient to transport. It can be stored and preserved at room temperature for a long period of time, and is particularly suitable for long-distance transportation and field operations, such as food supply in special environments such as military, navigation, and exploration.

[0018] Preferably, the composite aqueous solution in step S4 is a composite solution of sodium D-isoascorbate, citric acid, calcium chloride, salt and fructose, wherein the amount of sodium D-isoascorbate added in the composite aqueous solution is 0.1% to 0.3%, the amount of citric acid added is 1.3% to 1.7%, the amount of calcium chloride added is 0.5% to 2%, the amount of salt added is 0% to 2%, and the amount of fructose added is 0% to 5%. Protecting the color and seasoning the green radish can ensure the taste of the green radish crisps, and can also delay the browning of the green radish crisps and extend the shelf life of the green radish crisps.

[0019] Compared with the existing technology, the present invention provides a method for preparing combined drying and freeze-dried radish chips, which has the following beneficial effects:

[0020] (1) The present invention selects low-cost green radish as raw material, and the content of radish rasin is relatively high. The composite liquid of sodium D-isoascorbate, citric acid, calcium chloride, salt, and fructose is used for color protection and seasoning, which can effectively reduce the loss of nutrients in green radish and improve the nutritional value of green radish chips. The internal voids of green radish are filled with salt, sugar, and calcium chloride to reduce the shrinkage rate of green radish, thereby improving the retention of color, taste, and application quality of green radish chips. Hot air drying technology is used as pre-drying and vacuum freeze drying in series at different moisture conversion points. After reducing the initial moisture content, the internal moisture is completely evaporated during the post-drying process. This combined drying method not only makes the dried product have good quality, but also significantly reduces the drying energy consumption.

[0021] (2) The present invention adopts a suitable color protection and seasoning ratio to maintain the original color and flavor to a large extent; and selects suitable pre-freezing parameters for freezing, which can not only ensure that the nutritional quality of the material is retained to the greatest extent, but also reduce energy consumption and save resources. The hot air temperature and time are optimized, the texture and rehydration characteristics of the radish chips are improved, and the color, appearance and nutrients of the fresh green radish are greatly retained.

[0022] (3) The present invention breaks with the conventional thinking. The preparation method aims to ensure that the nutritional quality of the material is retained to the greatest extent, improve the texture and rehydration properties, and reduce energy consumption and save resources. The main goal is to provide a theoretical reference for the current combined drying and freeze-dried products. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a hardness diagram of the radish crisps prepared in Example 1 of the present invention;

[0024] Figure 2 This is a crispness diagram of the radish chips prepared in Example 1 of the present invention;

[0025] Figure 3 This is a diagram of the rehydration ratio of the radish crisps prepared in Example 1 of the present invention;

[0026] Figure 4 This is a hardness diagram of the radish chips prepared in Example 2 of the present invention;

[0027] Figure 5 This is a crispness diagram of the radish chips prepared in Example 2 of the present invention;

[0028] Figure 6 This is a diagram of the rehydration ratio of the radish chips prepared in Example 2 of the present invention;

[0029] Figure 7 This is a moisture state diagram of the radish chips prepared in Example 2 of the present invention;

[0030] Figure 8 This is a thermal stability diagram of the radish chips prepared in Example 2 of the present invention;

[0031] Fig. 9 This is a graph showing the Vc content of radish chips prepared in Example 2 of the present invention;

[0032] Fig.10 This is a graph showing the raphanin content of radish chips prepared in Example 2 of the present invention;

[0033] Fig.11 This is a sensory evaluation diagram of the radish chips prepared in Example 2 of the present invention. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0035] Embodiment 1

[0036] This embodiment proposes a method for preparing freeze-dried radish crisps, firstly, fresh, undamaged, and non-rotten green radishes are selected, then the surface is washed with clean water, dust and debris are removed, and then sliced ​​into 3mm to 1cm. The obtained green radish slices are subjected to color protection and seasoning, wherein the color protection liquid is a composite aqueous solution of D-sodium isoascorbate, citric acid, calcium chloride, salt, and fructose, wherein the amount of D-sodium isoascorbate added to the composite aqueous solution is 0.3%, the amount of citric acid added is 1.5%, the amount of calcium chloride added is 1%, the amount of salt added is 1.5%, and the amount of fructose added is 3%; the radish slices prepared above are pre-frozen at -20℃ to -80℃ for 3h to 15h, and vacuum freeze-dried after the pre-freezing, and the vacuum drying conditions are: vacuum degree 2.1mbar, cold trap temperature -40℃, partition temperature 20℃; dry to a water content below 5%, take out and immediately measure the hardness value, crispness value and rehydration ratio. The hardness value is as follows Figure 1 As shown in the figure (A is frozen at -20℃, B is frozen at -40℃, and C is frozen at -80℃), the brittleness value is as follows Figure 2 As shown (in the figure, A is frozen at -20°C, B is frozen at -40°C, and C is frozen at -80°C), rehydration is as follows Figure 3 (As shown in the figure, A is a -20°C freezing treatment, B is a -40°C freezing treatment, and C is a -80°C freezing treatment).

[0037] Experimental results: Figure 1 , Figure 2 It can be seen that when the slice thickness is 5mm, the hardness is the smallest, and the radish chips have a certain crispness. When the slice thickness is greater than 5mm, the hardness begins to increase, the brittleness decreases, and the surface begins to shrink. Thicker slices are prone to excessively large ice crystals, which pierce the cell wall and squeeze the surrounding tissue. After freeze-drying, the radish chips show a shrinkage phenomenon; the lower the pre-freezing temperature, the smaller the hardness and the greater the brittleness. Because when the pre-freezing temperature is low, the specific shapes formed after freezing are smaller and more numerous, while the internal voids of the freeze-dried product are smaller and more numerous, and the tissue is more The loose and uniform distribution increases the brittleness. Too low pre-freezing temperature increases the energy consumption. The pre-freezing temperature of -40℃ is below the eutectic point of green radish -20℃ (eutectic point of green radish -20℃), which can freeze the radish quickly. When the pre-freezing time is 9-12h, the hardness is the smallest and the brittleness is the largest. When the pre-freezing time is short, the temperature quickly reaches the eutectic point temperature. Under the condition of unbalanced internal and external pressures, water migrates from high concentration to low concentration, driving the movement of solutes. The sugar substances in the cells leak to the surface, causing surface hardening, increased hardness and reduced brittleness.

[0038] Depend on Figure 3 It can be seen that the lower the pre-freezing temperature, the greater the influence of slice thickness and freezing time on the rehydration ratio. When the freezing temperature is lower than -40℃ and the thickness is 5-7mm, the freezing time has little effect on the rehydration ratio, and the rehydration ratio does not change significantly within this range. When the slice thickness is less than 5mm, the rehydration ratio increases greatly, and the downward trend accelerates when it exceeds 7mm. In addition, the rehydration ratio increases with the increase of freezing temperature. It can be explained that higher freezing temperature is conducive to the formation of larger ice crystals, thereby forming larger pores after sublimation, while lower pre-freezing temperature forms small ice crystals and leaves uniform and small pores. When water enters the rehydration process, more layers of water vapor escape channels are required, so the rehydration rate is small. When the pre-freezing time increases, the rehydration ratio also shows a trend of first decreasing and then increasing, and there is a significant change.

[0039] In summary, the optimal prefreezing parameters are: slice thickness 5 mm, prefreezing temperature -40 °C, and prefreezing time 9 h.

[0040] Embodiment 2

[0041] This embodiment provides a method for preparing a combined drying and freeze-dried radish crisp, wherein green radish is selected and sliced ​​into 5 mm in the same manner as in the first embodiment, and the obtained green radish slices are subjected to color protection and seasoning, wherein the color protection liquid is a composite aqueous solution of sodium D-isoascorbate, citric acid, calcium chloride, salt, and fructose, wherein the amount of sodium D-isoascorbate added to the composite aqueous solution is 0.3%, the amount of citric acid added is 1.5%, the amount of calcium chloride added is 1%, the amount of salt added is 1.5%, and the amount of fructose added is 1. The amount added is 3%. The green radish slices after color protection and seasoning are placed in a blast drying oven for pre-drying. The pre-drying temperature is 55℃~65℃. The moisture content of the green radish after pre-drying is 30%~70% (the moisture content is 30%, which is recorded as H3F). The green radish slices after pre-drying are vacuum freeze-dried. The pre-freezing temperature is -40℃, the pre-freezing time is 9h, and the vacuum drying conditions are: vacuum degree 2.1mbar, cold trap temperature -40℃, partition temperature 20℃, and the moisture content of the final product is controlled below 5%. Table 1 is the color diagram of the radish crisps. Figure 4 This is the hardness chart of the radish crisp. Figure 5 This is a picture of the crispness of the radish crisp. Figure 6 is the rehydration ratio diagram, Figure 7 This is the moisture status diagram of Radish Crisp. Figure 8 This is the thermal stability diagram of radish chips. Fig. 9 This is the Vc content of radish chips. Fig.10 This is a chart showing the raphanin content in radish chips.

[0042] Table 1

[0043]

[0044]

[0045] As shown in Table 1, the △E of green radish chips is the smallest and the color is best preserved when they are pre-dried with hot air at 60℃ until the moisture conversion point is 0.5. Figure 4 , Figure 5 It can be seen that the hardness of green radish chips increased by 4 times and the crispness remained unchanged when pre-dried at 60℃ to a moisture conversion point of 0.5. The hardness of green radish chips can be significantly improved by pre-drying combined with freeze-drying, and the higher the pre-drying temperature and the longer the time, the higher the hardness. This may be related to the shrinkage degree of the material, that is, the longer the pre-drying time, the lower the moisture ratio of the material, the greater the shrinkage degree of the material, the denser the tissue, and the greater the hardness of the final product. This shows that hot air combined with vacuum freeze drying combined with vacuum freeze drying of green radish can effectively solve the problems of too low hardness of vacuum freeze drying, which is not conducive to later packaging and other processing, and the taste is not dense enough.

[0046] Depend on Figure 6It can be seen that the rehydration ratio of the processed products after combined drying is generally higher than that of the direct vacuum freeze-dried products, and the phenomenon that the lower the moisture content after pre-drying, the higher the rehydration ratio. The rehydration rate of the radish chips pre-dried at 60℃ hot air to a moisture conversion point of 0.5 is 19.5% higher than that of the direct vacuum freeze-dried chips. During the rehydration process, the surface layer material will absorb water in the gaps and expand, and these expanded waters will also delay the capillary suction by reducing the pore space. Therefore, the hot air pre-drying process may also be accompanied by the decomposition process of the surface layer material. Therefore, compared with the freeze-dried products, the combined air drying products have a smaller pressure for water to enter, so the rehydration ratio is generally higher than that of the freeze-dried products.

[0047] Depend on Figure 7 It can be seen that all drying methods can remove free water, and the strongly bound water content of the combined drying methods with different hot air time and moisture conversion point is greater than that of the control (33.9%); and with the increase of pre-drying time, the proportion of weakly bound water decreases. When pre-dried at 60℃ to a moisture conversion point of 0.5, the weakly bound water decreased from 38.4% to 34.7%; and for the combined drying methods with different hot air temperatures, the higher the pre-drying temperature and the longer the time, the greater the proportion of strongly bound water, which may be due to the degradation of some nutrients during processing, resulting in a decrease in the proportion of strongly bound water.

[0048] Depend on Figure 8 It can be seen that the weight loss rate of the radish crisps dried by vacuum freeze-drying is the highest. When the heating temperature is 200°C, the weight loss rate of the sample pre-dried by 60°C hot air to a moisture conversion point of 0.5 is 77.51%, and the weight loss rate of the sample dried by vacuum freeze-drying is 81.37%; the higher the hot air temperature and the longer the time, the smaller the weight loss rate of the sample, mainly due to the degradation and volatilization of the chemical components (bound water, volatile compounds, carbohydrates, etc.) in the radish crisp samples. The weight of all radish crisp samples gradually decreases with the increase of temperature. This reflects that the samples after combined drying have higher thermal stability than the samples after vacuum freeze-drying.

[0049] Depend on Fig. 9 It can be seen that after the combined drying treatment, the Vc content first increased and then decreased with the decrease of moisture content, and showed a decreasing trend with the increase of hot air temperature. The Vc retention rate was 64% when the moisture conversion point was 0.5 at 60℃ hot air pre-drying. With the extension of drying time and the increase of hot air temperature, Vc began to gradually degrade when exposed to high temperature. The greater the degree of radish tissue disintegration, and the longer the contact with oxygen, the greater the Vc loss in the later stage of drying, and the less the preservation amount.

[0050] Depend on Fig.10It can be seen that the content of raphanin without combined drying treatment is 443.24 mg / 100g. When the water ratio is 0.5 at 60℃ hot air, the content of raphanin increases to a maximum of 556.13 mg / 100g, an increase of 25.47%; but as the hot air time increases further and the hot air temperature increases, the content of raphanin decreases continuously. When the water ratio is 0.5 at 65℃ hot air treatment, the content of raphanin is lower than that of radish crisp without combined drying treatment. This shows that within a certain range, the increase of preheating temperature is conducive to the conversion of glucosinolate to raphanin; when the optimum temperature is exceeded, the increase of preheating temperature reduces the content of raphanin. At this time, the further increase of temperature affects the activity of myrosinase in the reaction system, reducing the content of raphanin.

[0051] Table 2

[0052]

[0053]

[0054] As shown in Table 2, the drying time of all the combined dried radish chips is shorter than that of the radish chips directly subjected to vacuum freeze drying. Under the same material amount, the drying time is shortened by about 27% and the drying energy consumption is reduced by drying at 60°C until the moisture conversion point is 0.5 and then low-temperature vacuum drying compared with vacuum freeze drying alone.

[0055] In summary, the method provided by the present invention adopts the best pre-freezing parameters: slice thickness of 5mm, pre-freezing temperature of -40℃, pre-freezing time of 9h; combined drying parameters: 60℃ hot air to 50% moisture content (point conversion point of 0.5) followed by low-temperature vacuum drying, and the moisture content of the final product is controlled below 5%. Combined drying significantly increases the hardness and rehydration ratio of radish crisps. The hardness of green radish crisps dried at 60℃ to a moisture conversion point of 0.5 and then low-temperature vacuum drying increases by 4 times and the crispness increases by 26.25%; the rehydration ratio increases by 19.5%; △E is the smallest, and the color is best maintained; the Vc retention rate is 84.37%, the raphanin content is 487.99mg / 100g DW~556.13mg / 100g DW, and the energy consumption is reduced by 15.9%~37.3%.

[0056] The combined drying green radish crisp preparation method provided by the invention can ensure that the appearance and taste of the green radish crisps finally formed are improved and the nutrients are fully preserved, and can also delay the browning of the green radish crisps and extend the shelf life of the green radish crisps.

[0057] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for a person skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to replace some of the technical features therein with equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions claimed to be protected by the present invention.

Claims

1. A method for preparing combined drying and freeze-dried radish chips, characterized in that: The steps are as follows: S1. Select fresh, undamaged, and non-rotten green radishes; S2. Wash the surface with clean water to remove dust and debris; S3, slicing the cleaned green radish; S4, placing the green radish slices after step S3 in the composite aqueous solution for color protection and seasoning, the treatment time is 1 to 2 hours; S5, pre-drying the green radish slices after step S4 with hot air, so that the moisture content of the pre-dried green radish slices is 30% to 70%; S6, vacuum freeze-drying the green radish slices after step S5, so that the moisture content of the final product is below 5%.

2. The method for preparing the combined drying and freeze-dried radish chips according to claim 1, characterized in that: After step S4, vacuum freeze drying pre-freezing process parameters are screened, and freezing temperature and freezing time are selected as two factors, the freezing temperature is -20°C to -80°C, and the freezing time is 3h to 15h; The vacuum drying conditions were as follows: vacuum degree 2.1 mbar, cold trap temperature -40°C, and partition temperature 20°C.

3. The method for preparing the combined drying and freeze-dried radish chips according to claim 2, characterized in that: The temperature of hot air pre-drying in step S5 is 55°C to 65°C.

4. The method for preparing the combined drying and freeze-dried radish chips according to claim 2 or 3, characterized in that: In step S6, the green radish is first frozen to a temperature of 10° C. or more below the eutectic point, and then further vacuum dried.

5. The method for preparing the combined drying and freeze-dried radish chips according to claim 4, characterized in that: In step S3, the thickness of the green radish slices is 3 mm to 1 cm.

6. The method for preparing the combined drying and freeze-dried radish chips according to claim 5, characterized in that: In step S3, the thickness of the green radish slices is 5 mm; the vacuum freeze drying pre-freezing time is 9 hours, and the freezing temperature is -40°C; the water content of the green radish slices after hot air pre-drying is 50%.

7. The method for preparing the combined drying and freeze-dried radish chips according to claim 1, characterized in that: The composite aqueous solution in step S4 is a composite solution of sodium D-isoascorbate, citric acid, calcium chloride, salt and fructose, wherein the amount of sodium D-isoascorbate added in the composite aqueous solution is 0.1% to 0.3%, the amount of citric acid added is 1.3% to 1.7%, the amount of calcium chloride added is 0.5% to 2%, the amount of salt added is 0% to 2%, and the amount of fructose added is 0% to 5%.

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