Preparation method of fresh purple sweet potato slice by radio frequency explosion puffing assisted by sugar boiling and freeze-thaw pretreatment

Abstract

The application discloses a kind of sugar boiling, freeze-thaw pretreatment auxiliary radio frequency explosion puffing fresh purple sweet potato slice preparation method: step 1, the preparation of fresh purple sweet potato slice: after washing and drying, fresh purple sweet potato is treated to obtain the fresh purple sweet potato round piece with uniform size and thickness;Step 2, the pretreatment of fresh purple sweet potato round piece: the fresh purple sweet potato round piece is boiled in maltitol solution for 0-6 min;Then freeze and thaw at room temperature, freeze-thaw frequency is 1-2 times;Then put into oven and bake to a certain moisture content, and then transferred to refrigerator and cold storage evenly overnight;Step 3, radio frequency explosion puffing treatment: the purple sweet potato round piece obtained in step 2 is placed in radio frequency explosion puffing device for radio frequency explosion puffing treatment.The preparation method of the application can significantly improve the puffing rate and texture characteristics of fresh purple sweet potato, and greatly retain its anthocyanin content, which has great significance for the processing field of oil-free puffed fresh potato.

Description

Technical Field

[0001] This invention relates to a method for preparing fresh purple sweet potato chips that undergoes sugar boiling, freeze-thaw pretreatment, and radio frequency explosion puffing, belonging to the field of food processing technology. Background Technology

[0002] Sweet potatoes are the world's sixth largest food crop, crucial for food security in developing countries and a valuable vegetable crop in developed countries. Purple sweet potatoes, in addition to the nutritional components of regular sweet potatoes, are rich in selenium and anthocyanins. However, purple sweet potatoes are one of the least developed food crops, with consumption declining year by year in Malaysia and Europe. Puffed foods are popular worldwide due to their crispness, deliciousness, and convenience. Therefore, puffed purple sweet potato snacks have become a research hotspot for expanding the purple sweet potato market. Puffed purple sweet potato flour snacks suffer from shrinkage and poor texture, while fresh purple sweet potatoes, with their firm cell and starch structures, are difficult to puff. Currently, no research has been found on the puffing of fresh purple sweet potatoes.

[0003] Traditional frying and puffing, while enabling rapid food cooking and providing satisfying sensory attributes such as color, aroma, and texture, suffers from high oil content and the formation of harmful substances (such as acrylamide), potentially leading to cardiovascular disease, hypertension, diabetes, and a range of other health problems. Non-frying puffing technologies, such as microwave puffing, microwave vacuum puffing, extrusion puffing, and explosive puffing, suffer from drawbacks including high puffing temperatures, long processing times, uneven heating, and unsuitability for fresh samples, significantly limiting the development of the puffing industry. Radio frequency explosive puffing is an emerging puffing technology that combines radio frequency heating with explosive puffing techniques. Radio frequency, as an electromagnetic wave (3 kHz to 300 MHz), can induce the oscillation and migration of polar particles and the rotation of dipoles in materials, converting electrical energy into heat energy, thus heating the material from the inside out. Explosive puffing involves maintaining a certain temperature for a period of time, then instantaneously reducing the pressure, causing the internal moisture of the material to evaporate rapidly, resulting in rapid volume expansion, followed by drying under vacuum. The combination of radio frequency (RF) and explosive expansion technology offers advantages over microwave expansion, including deeper penetration and better heating uniformity; faster heating rate and higher efficiency compared to explosive expansion; and better preservation of the original material morphology compared to extrusion expansion. Currently, research on RF explosive expansion is limited.

[0004] Pre-treatment of materials before puffing is beneficial for obtaining products with the required size and shape, improving puffing efficiency, and enhancing product quality. Freeze-thaw pretreatment disrupts the cellular structure of fruits and vegetables by forming ice crystals during freezing, thereby accelerating mass and heat transfer and improving the texture of the product. Sugar boiling pretreatment involves boiling fruits and vegetables in a sugar solution for a period of time, altering the basic composition (starch, protein) and cell morphology of the materials through high temperature, accelerating solute inflow, and preventing enzymatic browning. Furthermore, a necessary pre-drying process before puffing is the formation of a gas-barrier film on the material surface, which facilitates the accumulation of puffing pressure within the material. Freeze-thaw followed by sugar boiling pretreatment significantly improves the texture of carrots after explosive puffing and drying, and significantly inhibits shrinkage. However, the effect of sugar boiling and freeze-thaw assisted radiofrequency explosive puffing on the volume expansion of fresh purple sweet potatoes has not yet been reported. Therefore, the content of this invention application possesses a certain degree of novelty. Summary of the Invention

[0005] The purpose of this invention is to propose a method for preparing fresh purple sweet potato chips by sugar boiling, freeze-thaw pretreatment assisted by radio frequency explosion puffing, in order to overcome the problem that fresh purple sweet potatoes are difficult to puff without oil. This invention method is also applicable to other tuber crops.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for preparing fresh purple sweet potato chips that undergoes sugar boiling, freeze-thaw pretreatment, and radiofrequency explosion puffing includes the following steps:

[0008] Step 1, Preparation of fresh purple sweet potato slices: After washing and drying, fresh purple sweet potatoes are processed to obtain fresh purple sweet potato round slices of uniform size and thickness.

[0009] Step 2, Pre-treatment of fresh purple sweet potato slices: Boil fresh purple sweet potato slices in maltitol solution for 0-6 minutes, then rinse with water to remove any remaining sugar stains and dry the surface; then freeze and thaw at room temperature, i.e., perform freeze-thaw treatment, 1-2 times; then put them in an oven to dry to a certain moisture content, and then transfer them to a refrigerator to refrigerate overnight until they are completely moist.

[0010] Step 3, Radio Frequency Explosion Puffing Treatment: The purple sweet potato slices obtained in Step 2 are placed into a radio frequency explosion puffing device for radio frequency variable temperature pressure difference puffing treatment. The distance between the upper and lower electrode plates in the radio frequency explosion puffing device is 170~180 mm, the pressure of the puffing vessel is 0.18~0.22 MPa, the puffing temperature is 85~95℃, the residence time is 5~10 min, the radio frequency vacuum drying temperature is 65~75℃, the vacuum degree is -0.08 MPa, and the drying time is 1~1.5 h until the safe moisture content is reached.

[0011] Furthermore, in step (1), the fresh purple sweet potato slices obtained are round slices with a diameter of 2.8~3.2 cm and a thickness of 2.85~3.25 mm.

[0012] Furthermore, in step (2), the concentration of the maltitol solution is 20-40%.

[0013] Furthermore, in step (2), the freezing temperature is -20~-80℃.

[0014] Furthermore, in step (2), the purple sweet potato slices are pre-dried in an oven at 55~65℃ until the moisture content is 15~55%.

[0015] Furthermore, in step (2), the temperature at which the purple sweet potato slices are refrigerated overnight in the refrigerator is 4°C.

[0016] Furthermore, in step (3), the radio frequency explosion puffing device adopts a hot air-assisted radio frequency variable temperature pressure difference puffing and drying device.

[0017] Furthermore, the specific operation of step (3) is as follows:

[0018] Place the purple sweet potato slices obtained in step 2 into the second annular cavity of the double-layer hollow cylindrical container inside the puffing kettle, insert the temperature measuring fiber into the geometric center of the purple sweet potato slices, fix the upper cover plate and the lower bottom plate of the puffing kettle, and then place the puffing kettle on the lower electrode plate, controlling the distance between the upper electrode plate and the lower electrode plate to be 175 mm.

[0019] Compressed air is introduced into the puffing kettle using an air compressor. The pressure of the puffing kettle is adjusted to 0.2 MPa. Radio frequency heating is turned on until the temperature of the sweet potato chips reaches 90°C. The radio frequency generator is then turned off and the temperature is maintained for 5 minutes.

[0020] Start the vacuum pump to introduce the vacuum from the vacuum buffer tank into the puffing kettle to puff the purple sweet potato slices by instantaneous temperature and pressure difference. When the vacuum degree is -0.08 MPa, start the radio frequency generator to maintain the temperature at 70℃ and keep it at that temperature for 1 hour, so that the moisture content of the purple sweet potato slices reaches the safe moisture content (3~5%).

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] (1) By using fresh potato samples with high starch content for puffing, the problem of fresh potatoes being difficult to puff without oil is overcome, and the disadvantages of poor taste and wrinkled skin when puffing starch or whole powder samples are solved. This method is applicable to most potato crops.

[0023] (2) Maltitol used in sugar cooking is a sugar substitute. It has lower calories and sweetness than sucrose, and diabetics can consume it in moderation.

[0024] (3) By pre-treating fresh purple sweet potatoes with sugar boiling and freeze-thaw cycles, enzymatic browning is inhibited while altering their cell and starch structures. This allows the fresh purple sweet potatoes to puff up at lower temperatures, shortening the pre-drying time and better preserving their nutrients, thus extending their shelf life.

[0025] (4) The radio frequency explosive puffing method used in this invention overcomes the disadvantages of traditional deep-frying puffing, such as high oil content, long drying time of steam explosive puffing, and uneven heating and high temperature of microwave puffing. It can obtain high-quality products in a shorter time and improve work efficiency. The equipment used is easy to operate and has high work efficiency.

[0026] In summary, the method of this invention uses fresh purple sweet potatoes as raw material, cuts them into round slices of uniform size and thickness, and then uses sugar boiling, freeze-thaw, and pre-drying treatments assisted by radio frequency explosive puffing to obtain fresh-cut purple sweet potato puffed snacks with increased volume, crisp texture, and high nutritional value. Experiments show that the method of this invention effectively preserves the anthocyanin content of fresh purple sweet potatoes while causing them to expand in volume. Compared with the untreated puffed group, the pretreated group showed a 95.14% increase in volume, a 722.54% increase in anthocyanin retention (p<0.05), and significantly improved textural properties of the purple sweet potato chips, resulting in a product that is both delicious and nutritious. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the radio frequency explosive expansion device used in the method of the present invention.

[0028] Figure 2 This is a schematic diagram showing the position of the expansion vessel on the lower electrode plate of the radio frequency generator.

[0029] Figure 3 This is a cross-sectional view of an extrusion kettle containing purple sweet potato chips.

[0030] Figure 4 yes Figure 3 Top view.

[0031] The meanings of the labels in the diagram are as follows:

[0032] 1. Vacuum pump; 2. Vacuum buffer tank; 3. Vacuum gauge; 4. First manual ball valve; 5. Second manual ball valve; 6. Air compressor; 7. Pressure gauge; 8. High-temperature resistant tube; 9. Exhaust vent; 10. Upper electrode plate; 11. Screw; 12. Control panel; 13. Temperature measuring fiber optic probe; 14. Fiber optic temperature measuring device; 15. Computer; 16. Lower electrode plate; 17. Motor; 18. Expansion kettle; 19. Electric hot air blower; 20. Temperature measuring flange; 21. Air vent; 22. Purple sweet potato chips; 23. Gas block brick; 24. Outer hollow cylinder of container; 25. Inner hollow cylinder of container; 26. Top cover plate; 27. Bottom plate; 28. Vent; A. Variable temperature and pressure differential system; B. Hot air assisted radio frequency heating system; C. Computer fiber optic temperature measuring system. Detailed Implementation

[0033] The present invention will be further explained and described below with reference to the accompanying drawings and specific embodiments.

[0034] The present invention provides a method for preparing fresh purple sweet potato chips using sugar boiling and freeze-thaw assisted radio frequency explosive puffing, employing the aforementioned radio frequency explosive puffing device, and comprising the following steps:

[0035] (1) Preparation of fresh purple sweet potato slices: After washing and drying, fresh purple sweet potatoes are cut into cylinders using a corer with a blade of 2.8~3.2 cm, and then sliced ​​using a slicer to obtain purple sweet potato round slices with a thickness of 2.85~3.25 mm.

[0036] (2) Pretreatment of fresh purple sweet potato slices: Boil fresh purple sweet potato slices in a 20-40% maltitol solution for 0-6 minutes, then rinse off any remaining sugar residue with water and dry the surface. Freeze the boiled purple sweet potato slices in a freezer at -20 to -80°C, then remove them from the freezer and thaw at room temperature. Repeat this process 1-2 times. After thawing, place the purple sweet potato slices in an oven at 55-65°C until the moisture content is 15-55%, and then transfer them to a 4°C freezer to homogenize overnight.

[0037] (3) Radio frequency explosion puffing of fresh purple sweet potato slices: Place the purple sweet potato slices into a sealed puffing vessel and puff them with radio frequency temperature differential pressure. The distance between the upper and lower electrodes of the radio frequency is 170~180 mm, the puffing pressure is 0.18~0.22 MPa, the puffing temperature is 65~75℃, the residence time is 5~10 min, the radio frequency vacuum drying temperature is 65~75℃, the vacuum degree is -0.08 MPa, and the drying time is 1~1.5 h until the safe moisture content (3~5%) is reached.

[0038] Advantages of the above technical solution:

[0039] (1) Fresh potato samples with high starch content were used for puffing, which overcame the problem that fresh samples were difficult to puff without oil, and solved the disadvantages of poor taste and wrinkled skin when puffed starch or whole powder samples.

[0040] (2) Maltitol used in sugar cooking is a sugar substitute. It has lower calories and sweetness than sucrose, and diabetics can consume it in moderation.

[0041] (3) The present invention pre-treats fresh purple sweet potatoes by sugar boiling and freeze-thaw, which inhibits enzymatic browning and changes its cell structure and starch structure, thereby enabling fresh purple sweet potatoes to puff at a lower temperature, shortening the pre-drying time, and better preserving its nutrients.

[0042] (4) The radio frequency explosion puffing method used in this invention overcomes the disadvantages of traditional deep-frying puffing, such as high oil content, long drying time of steam explosion puffing, and uneven heating and high temperature of microwave puffing. It can obtain high-quality products in a short time and improve work efficiency.

[0043] The preferred fresh purple sweet potato is the "Violet" variety.

[0044] Preferably, the maltitol solution used in the sugar cooking process has a lower sweetness and caloric value than sucrose. It is a sweetener that can be consumed in moderation by diabetics.

[0045] Preferably, in step 3, a radio frequency explosion puffing device is used to puff the purple sweet potato chips. The radio frequency explosion puffing device is a hot air-assisted radio frequency variable temperature pressure difference puffing and drying device with patent number ZL 202221818948.4. The structure of the device is described in [reference needed]. Figures 1-3 .

[0046] The operation process of step 3 above:

[0047] (1) Open the box and place the sweet potato chips to be puffed into the second annular cavity of the double-layer hollow cylindrical container inside the puffing kettle 18. After fixing the upper cover plate 26 and the lower bottom plate 27 of the puffing kettle 18, place the puffing kettle 18 on the lower electrode plate 16 of the radio frequency heating system B. Connect the first annular cavity to the compressed air pipeline and the vacuum pipeline of the variable explosion puffing system A with two high-temperature resistant tubes 8 respectively. Insert the temperature measuring fiber probe 13 into the geometric center of the sweet potato chips. Connect the fiber to the temperature measuring device 14 through the temperature measuring flange 20. Connect the temperature measuring device 14 to the computer 15 and close the box door.

[0048] (2) The operator starts the explosive puffing system A through the independent power supply, closes the vacuum valve, and introduces compressed air into the puffing vessel 18 through the air compressor 6 for pressurization. The pressure gauge 7 shows that the pressure inside the puffing vessel 18 has reached the required level, and the second manual ball valve 5 is closed. The operator sets the distance between the upper electrode plate 10 and the lower electrode plate 16 and the processing time through the control panel 12; the control panel 12 controls the motor 17 to drive the four screws 11 to rotate and adjust the distance between the upper electrode plate 10 and the lower electrode plate 16; the operator turns on the radio frequency generator through the control panel 12 to perform radio frequency heating treatment; at the same time, the fiber optic temperature measurement software in the computer 15 is turned on to control the computer fiber optic temperature measurement system C to perform real-time temperature measurement of the material center.

[0049] (3) Pre-open vacuum pump 1 to evacuate vacuum buffer tank 2 to a certain vacuum level. After the material in expansion vessel 18 reaches the set temperature and is maintained for a period of time, heating is completed. Turn off the radio frequency generator, open the first manual ball valve 4, turn on vacuum pump 1, and the vacuum in vacuum buffer tank 2 is instantly input into expansion vessel 18 for instantaneous explosive expansion. Vacuum gauge 3 shows that the vacuum level in expansion vessel 18 has reached the requirement. Start the radio frequency generator and perform radio frequency vacuum drying at a certain temperature. After radio frequency vacuum drying for a period of time, turn off the radio frequency generator, open the pressure relief valve to release pressure, and restore the entire system to normal pressure. After reaching normal pressure, the system will automatically stop. At the same time, stop the relevant fiber optic temperature measurement software in computer 15. Computer 15 will automatically record real-time temperature measurement data. Open expansion vessel 18 and take out the material to measure relevant indicators.

[0050] Sweet potatoes are the world's sixth largest food crop, rich in starch, sugars, protein, vitamins, fiber, and various amino acids, making them an excellent source of nutrition. Purple sweet potatoes, in addition to the nutritional components of regular sweet potatoes, are also rich in selenium and anthocyanins. However, purple sweet potatoes are one of the least developed food crops, with consumption in Malaysia and Europe declining year by year. Puffed foods are loved and consumed worldwide for their crispness, deliciousness, and convenience. Therefore, puffed purple sweet potato snacks have become a research hotspot for expanding the purple sweet potato consumer market. Puffed purple sweet potato flour snacks suffer from shrinkage and poor taste, while fresh purple sweet potatoes, due to their firm cell and starch structures, are difficult to puff. The above-mentioned sugar-cooking and freeze-thaw assisted radiofrequency explosion puffing method was used to puff fresh purple sweet potatoes, including the following operations:

[0051] Cut fresh purple sweet potatoes into round slices 3 cm in diameter and 3 mm thick, then boil them in a 20-40% maltitol solution for 0-6 minutes. Rinse the boiled purple sweet potato slices with distilled water, wipe off excess water, and freeze them overnight at -18 to -80°C. Then remove the purple sweet potato slices and thaw them at room temperature. Repeat the freeze-thaw cycle 1-2 times. After thawing, pre-dry the purple sweet potato slices in a 60°C oven until the moisture content is 15-55%, then transfer them to a 4°C refrigerator and thaw overnight. The pre-treated purple sweet potato chips are placed between the layers of the double-layered hollow cylindrical container in the puffing kettle 18. The upper cover plate and lower bottom plate of the puffing kettle 18 are fixed, and then the puffing kettle 18 is placed on the lower electrode plate 16 of the radio frequency heating system B. Then, the distance between the upper electrode plate and the lower electrode plate is adjusted to 175 mm through the control panel 12. When puffing purple sweet potato chips in batches, the quantity is 6 to 8 chips. Compressed air is introduced into the puffing kettle 18 through the air compressor 6, the pressure of the puffing kettle 18 is adjusted to 0.2 MPa, the second manual ball valve 5 is closed, the radio frequency generator is turned on for heating, and the final temperature is set to 90℃. After maintaining the temperature for 5 minutes, the radio frequency generator was turned off, and vacuum pump 1 was started (the vacuum buffer tank 2 was pre-evacuated to a certain vacuum level, i.e., 0.08 MPa, using vacuum pump 1). The first manual ball valve 4 was opened to introduce the vacuum from the vacuum buffer tank 2 into the puffing kettle 18 to explosively puff the purple sweet potato chips 22. When the vacuum gauge 3 showed that the vacuum level in the puffing kettle 18 was 0.08 MPa, the radio frequency generator was started to perform radio frequency vacuum drying on the material in the puffing kettle 18, which was maintained for 1 hour. After the radio frequency explosive puffing was completed, the material was taken out and its hardness, color, degree of puffing, and anthocyanin retention rate were measured.

[0052] Experiment 1: Determination of the effect of sugar cooking concentration and time on the radiofrequency explosive puffing of fresh purple sweet potato slices

[0053] (1) After washing and drying the fresh purple sweet potatoes, cut them into cylinders with a corer with a blade diameter of 3 cm, and then cut them into round slices with a thickness of 3 mm using a slicer.

[0054] (2) Boil fresh purple sweet potato slices in 20%, 30%, and 40% maltitol solutions for 0, 1, 2, 3, 4, 5, and 6 minutes respectively. Then rinse the surface clean and wipe off excess water, and freeze overnight at -20℃. Then remove the purple sweet potato slices from the freezer and let them thaw at room temperature. After thawing, place the purple sweet potato slices in a 60℃ oven to pre-dry to a moisture content of 25%, and then transfer them to a 4℃ refrigerator to be uniformly moistened overnight.

[0055] (3) The pre-treated purple sweet potato slices are placed between the layers of the double-layer hollow cylindrical container in the puffing kettle 18. The upper cover plate and the lower bottom plate of the puffing kettle 18 are fixed. Then the puffing kettle 18 is placed on the lower electrode plate 16. The distance between the upper and lower electrode plates is adjusted to 175 mm through the control panel 12. When puffing yam slices in batches, the quantity is 6 to 8 slices. Compressed air is introduced into the puffing kettle 18 through the air compressor 6. The pressure of the puffing kettle 18 is adjusted to 0.2 MPa. The pressure valve is closed. The radio frequency generator is turned on to heat up. The final temperature is set to 90℃. The radio frequency generator is turned off and kept warm for 5 min. Then the vacuum pump is turned on and the vacuum valve is opened to introduce the vacuum in the vacuum buffer tank 2 into the puffing kettle 18 to explode and puff the purple sweet potato slices 22. When the vacuum gauge 3 shows that the vacuum degree in the puffing kettle 18 is 0.08 MPa, the radio frequency generator is started to perform radio frequency vacuum drying on the material in the puffing kettle 18 and is maintained for 1 h.

[0056] The puffing results are shown in Table 1. The puffing volume of fresh purple sweet potatoes increased with increasing sugar concentration and cooking time, with the best puffing effect observed at a concentration of 30-40% and a cooking time of 6 minutes. Compared with pre-drying only fresh samples, the puffing volume of purple sweet potato slices treated with a 40% sugar concentration and cooking for 6 minutes increased by 76.46%. p <0.05), anthocyanin retention increased by 322.53% ( p <0.05), and significantly improved the hardness, crispness and color of purple sweet potato chips.

[0057] Table 1

[0058]

[0059] Experiment 2: Determination of the effect of freeze-thaw temperature and number of freeze-thaw cycles on the radiofrequency explosive puffing of fresh purple sweet potato chips.

[0060] Purple sweet potato slices with a diameter of 3 cm and a thickness of 3 mm were prepared according to Example 1. The fresh purple sweet potato slices were treated under the optimal sugar-cooking conditions (40% sugar concentration, 6 min) obtained in Example 1, and then frozen overnight in refrigerators at 4°C, -20°C, -40°C, and -80°C to ensure complete freezing. The purple sweet potato slices were removed from the refrigerator and thawed at room temperature. The freezing process was then repeated. The thawed purple sweet potato slices were pre-dried in a 60°C oven until the moisture content reached 25%, and then transferred to a 4°C refrigerator for homogenization overnight. The pre-treated purple sweet potatoes were then subjected to radio frequency explosive puffing treatment according to Example 1.

[0061] The puffing results are shown in Table 2. Both the freeze-thaw temperature and the number of freeze-thaw cycles significantly affected the texture of the fresh purple sweet potato chips, but had no significant effect on the color. The purple sweet potato chips with the best puffing volume were obtained at freezing temperatures of -20 to -40℃ and one freeze-thaw cycle. Compared to the 4℃ treatment group, the puffing volume of the purple sweet potato chips obtained at -40℃ with one freeze-thaw cycle increased by 47.13% (…). p<0.05), anthocyanin retention increased by 61.30% ( p <0.05), and significantly improved the hardness and crispness of purple sweet potato chips.

[0062] Table 2

[0063]

[0064] Experiment 3: Determination of the effect of pre-dried moisture content on the radio frequency explosive puffing of fresh purple sweet potato chips.

[0065] Purple sweet potato slices with a diameter of 3 cm and a thickness of 3 mm were prepared according to Example 1. The fresh purple sweet potato slices were treated with the optimal sugar cooking conditions (40% sugar concentration, 6 min cooking) and freeze-thaw conditions (one freeze-thaw cycle at -40°C) obtained in Example 2. The thawed purple sweet potato slices were then pre-dried in a 60°C oven to a moisture content of 15%, 25%, 35%, 45%, and 55%, respectively, and then transferred to a 4°C refrigerator for overnight homogenization. The pre-treated purple sweet potatoes were then subjected to radio frequency explosive puffing treatment according to Example 1.

[0066] The puffing effect is shown in Table 3. When the moisture content is below 45%, the puffing volume and anthocyanin retention rate increase with increasing moisture content. When the moisture content exceeds 45%, the puffing volume and anthocyanin retention rate decrease slightly. Therefore, the optimal moisture content for pre-drying is 35%~55%, which provides higher puffing volume, textural properties, and anthocyanin retention rate.

[0067] Table 3

[0068]

[0069] In summary, the sugar-cooking and freeze-thaw assisted radio frequency explosive puffing technology employed in this invention can significantly improve the puffing volume and textural properties of fresh purple sweet potatoes, reduce color difference, and significantly increase the retention rate of anthocyanin content. This method solves the problem of oil-free puffing of fresh purple sweet potatoes, providing excellent sensory quality while greatly preserving nutritional components, which is of great significance for the processing and application of fresh purple sweet potatoes and the expansion of the consumer market.

Claims

1. A method for preparing fresh purple sweet potato chips using sugar boiling, freeze-thaw pretreatment assisted by radio frequency explosion puffing, characterized in that, Includes the following steps: Step 1, Preparation of fresh purple sweet potato slices: After washing and drying, fresh purple sweet potatoes are processed to obtain fresh purple sweet potato round slices of uniform size and thickness; the obtained fresh purple sweet potato slices are round slices with a diameter of 2.8~3.2 cm and a thickness of 2.85~3.25 mm; Step 2, Pre-treatment of fresh purple sweet potato slices: Boil fresh purple sweet potato slices in maltitol solution for 0-6 minutes, then rinse with water to remove any remaining sugar residue and dry the surface. After freezing, thaw at room temperature, i.e., perform freeze-thaw treatment, 1-2 times. Then, place them in an oven to dry to a certain moisture content, and then transfer them to a refrigerator for overnight refrigeration. The concentration of maltitol solution is 20-40%, the freezing temperature is -20 to -80℃, the purple sweet potato slices are pre-dried in an oven at 55-65℃ to a moisture content of 15-55%, and the purple sweet potato slices are refrigerated overnight at 4℃. Step 3, Radio Frequency Explosion Puffing Treatment: The purple sweet potato slices obtained in Step 2 are placed into a radio frequency explosion puffing device for radio frequency variable temperature pressure difference puffing treatment; the radio frequency explosion puffing device is a hot air-assisted radio frequency variable temperature pressure difference puffing and drying device; the specific operation is as follows: Place the purple sweet potato slices obtained in step 2 into the second annular cavity of the double-layer hollow cylindrical container inside the puffing kettle, insert the temperature measuring fiber into the geometric center of the purple sweet potato slices, fix the upper cover plate and the lower bottom plate of the puffing kettle, and then place the puffing kettle on the lower electrode plate, controlling the distance between the upper electrode plate and the lower electrode plate to be 175 mm. Compressed air is introduced into the puffing kettle using an air compressor. The pressure of the puffing kettle is adjusted to 0.2 MPa. Radio frequency heating is turned on until the temperature of the sweet potato chips reaches 90°C. The radio frequency generator is then turned off and the temperature is maintained for 5 minutes. Start the vacuum pump to introduce the vacuum from the vacuum buffer tank into the puffing kettle to puff the purple sweet potato slices by instantaneous temperature and pressure difference. When the vacuum degree is -0.08 MPa, start the radio frequency generator to maintain the temperature at 70℃ and keep it at that temperature for 1 hour to bring the moisture content of the purple sweet potato slices to a safe level.

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