A purple yam powder and a preparation method thereof

Abstract

The application provides a purple yam powder and a preparation method thereof. In view of the problem of incomplete browning control and low anthocyanin and polysaccharide retention rate in the existing purple yam powder preparation process, the preparation method of the purple yam powder provided by the application realizes the improvement of color protection and retention performance under the condition of not setting a hot water treatment process and not adding artificial synthetic pigments, preservatives and antioxidants.

Description

Technical Field

[0001] This invention belongs to the field of food processing, specifically the field of deep processing technology of agricultural products, and more specifically relates to a purple yam powder and its preparation method. Background Technology

[0002] Purple yam is the tuber of a plant in the genus Dioscorea of ​​the family Dioscoreaceae. Its flesh is rich in anthocyanins, a key characteristic distinguishing it from common yam. However, fresh purple yam is not suitable for long-term storage, and processing it into powder is its primary deep-processing method. Purple yam is prone to enzymatic browning after peeling and slicing, and the anthocyanins in the flesh are easily degraded during drying and heating. Color preservation is a core issue in the powdering process. Existing technologies, blanching for color preservation causes the dissolution and loss of anthocyanins and polysaccharides; sulfite color preservation has residual limitations; and organic acid soaking only acts on the surface of the slices, while the anthocyanins inside the flesh still significantly degrade under prolonged heating, resulting in a light and dark color in the finished product. Regarding drying, conventional hot air drying has a long heating time, while vacuum freeze drying is energy-intensive and time-consuming. Therefore, how to control the degree of browning and improve the retention rate of anthocyanins and polysaccharides simultaneously without blanching or adding exogenous ingredients is a problem that needs to be solved in this field. Summary of the Invention

[0003] To address the problems of incomplete browning control and low anthocyanin and polysaccharide retention rates in existing purple yam powder production processes, this invention provides a method for preparing purple yam powder that simultaneously improves color protection and retention performance without requiring a blanching process or adding artificial pigments, preservatives, or antioxidants.

[0004] The technical solution of the present invention is as follows: A method for preparing purple yam powder, comprising the following steps:

[0005] S1. Wash fresh purple yam, peel it to a thickness of 0.8-1.5mm, and collect the yam peel for later use;

[0006] S2. Slice the peeled purple yam into slices 2-4 mm thick, and immerse the resulting potato chips in a color-protecting solution. The color-protecting solution is an aqueous solution containing citric acid, L-cysteine, and yam peel polyphenol extract, wherein the amount of yam peel polyphenol extract used is 0.01%-0.10% of the mass of the color-protecting solution based on the mass of the total phenols. The yam peel polyphenol extract is obtained by extracting and concentrating the yam peel obtained in S1 with an ethanol aqueous solution.

[0007] S3. Freeze the soaked potato chips along with the color-protecting solution at -25--35℃, so that the core temperature of the potato chips drops to below -18℃ within 90 minutes, and keep them at below -18℃ for 1-3 hours. Then thaw them at 0-4℃, and the potato chips are always submerged in the color-protecting solution during the thawing process.

[0008] S4. After thawing, drain the potato chips and dry them in sections at a temperature not exceeding 52°C until the moisture content of the potato chips is no more than 8%.

[0009] S5. The dried potato chips are crushed, sieved, and impurities are removed at a temperature not exceeding 25°C to obtain the purple yam powder.

[0010] Anthocyanins in potato flesh are located within the vacuoles of thin-walled cells, making it difficult for soaked color-protecting components to penetrate deep into the tissue. This invention involves freezing and thawing potato chips soaked in a color-protecting solution, along with the solution itself: during freezing, ice crystals grow inside and outside the cells, forming microporous channels that penetrate the material; during thawing, the ice crystals melt from the outside in and shrink in volume, propelling the color-protecting solution along these channels to penetrate deep into the tissue, allowing the color-protecting and auxiliary color-protecting components to reach the internal cells. The yam peel polyphenols and anthocyanins entering the vacuoles stabilize the color-developing structure; simultaneously, the microporous channels accelerate drying and dehydration, shorten heating time, reduce anthocyanin thermal degradation, and preserve the porous structure of the powder, facilitating water penetration during reconstitution and improving dispersibility. The polyphenols required for auxiliary coloring are derived from the yam peel produced during the peeling process of this technology, eliminating the need for external ingredients. It should be noted that the present invention does not include a blanching process; the yam peel polyphenol extract is derived from the purple yam itself; apart from the purple yam tissue and its extract, the citric acid and L-cysteine ​​in the color-protecting liquid are conventional color-protecting ingredients permitted for use in food processing; the present invention does not add artificial synthetic pigments, preservatives, antioxidants and anti-caking agents.

[0011] Preferably, in step S1, the peeled purple yam is sliced ​​within 5 minutes and then immersed in a color-protecting solution; the thickness deviation of the slices is no more than 0.5 mm.

[0012] Preferably, in step S2, the color-protecting solution contains 0.3%-1.0% citric acid and 0.01%-0.1% L-cysteine; the pH value of the color-protecting solution is 3.0-3.6. More preferably, the color-protecting solution contains 0.5%-0.8% citric acid and 0.03%-0.07% L-cysteine, and the amount of the yam peel polyphenol extract is 0.03%-0.07% of the total phenols in the extract. A pH of 3.0-3.6 represents the overlapping range where polyphenol oxidase activity is significantly inhibited, anthocyanin conformation is stable, and flavor is acceptable. Preferably, 0.1%-0.3% calcium chloride can also be added to the color-protecting solution; calcium ions cross-link with pectin in the cell walls of potato flesh, which can reduce juice loss during the thawing stage after freeze-thaw pore-forming.

[0013] Preferably, in step S2, the immersion temperature is 10-25℃, the time is 20-60 min, and the mass ratio of potato chips to color-protecting liquid is 1:3-1:6.

[0014] Preferably, in step S2, the yam peel polyphenol extract is prepared as follows: Yam peel is washed and dried until the moisture content is no more than 10%, pulverized through a 40-mesh sieve, and extracted with a 50%-70% (v / v) ethanol aqueous solution at a material-to-liquid ratio of 1:10-1:20 g / mL at 50-60℃ for 60-120 min. The extract is then filtered and concentrated under reduced pressure to remove the ethanol. The polyphenols in yam peel contain both highly polar phenolic acids and less polar condensed tannins. A moderate concentration of ethanol aqueous solution can achieve a good extraction balance between the two, and the subsequent alcohol removal burden is relatively light. More preferably, the reduced pressure concentration temperature is 50-55℃, and the concentration is carried out until the solid content is 5%-15%; the total phenol content of the yam peel polyphenol extract, calculated as gallic acid, is no less than 30%. Specifying the concentration endpoint and the lower limit of the total phenol content ensures that the extract quality falls within a repeatable range, thereby supporting the control of feed dosage based on total phenols.

[0015] Preferably, in step S3, the freezing and thawing are repeated 2-3 times; the freezing cooling rate is 1.0-3.0℃ / min, and the freezing temperature is -28 to 32℃. A faster cooling rate results in more and smaller ice crystal nuclei, forming a dense and interconnected microporous network; too slow a cooling rate results in the formation of coarse ice crystals, with tissue damage concentrated in a localized area, and fluid leakage during thawing. Repeated freeze-thaw cycles allow for more thorough penetration, but too many cycles cause repeated stress on the tissue, increased fluid loss, and the release of anthocyanins; 2-3 cycles are the optimal value.

[0016] Preferably, in step S3, the thawing temperature is 1-3℃ and the time is 2.5-3.5h.

[0017] Preferably, in step S4, the segmented drying includes: drying the first segment at 40-45℃ for 2-3 hours, and drying the second segment at 48-52℃ for 4-6 hours.

[0018] Preferably, in step S5, the pulverization temperature is 18-22℃, and the sieving is done through an 80-120 mesh sieve.

[0019] The present invention also provides purple yam powder prepared by the above preparation method.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] (1) The color protection effect is stable, the anthocyanin retention rate is 80.3%-88.7%, and the browning index is 31.6-40.5, which is significantly better than the conventional hot air drying process;

[0022] (2) Eliminating the blanching process avoids the dissolution and loss of anthocyanins and polysaccharides, with a polysaccharide retention rate of 88.0%-92.5%;

[0023] (3) The co-color complex has a continuous stabilizing effect on the anthocyanin structure. After the finished product is sealed and protected from light for 30 days at 40℃ and 75% relative humidity, the anthocyanin retention rate is still not less than 75%.

[0024] (4) The powder retains a porous structure, with a mixing and dispersing time of 15-24s and an agglomeration rate of 2.6%-5.1%, and no need to add maltodextrin or anti-caking agents;

[0025] (5) The auxiliary coloring component is taken from the yam peel produced by this process, which reduces the amount of waste material to be processed; the whole process uses conventional hot air drying and pulverizing equipment, and the conditions for the newly added freeze-thaw process are clearly defined, making it suitable for continuous production. Attached Figure Description

[0026] Figure 1 The image shows the appearance of the purple yam powder prepared in Example 1 of this invention. Detailed Implementation

[0027] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features described therein can be combined with each other.

[0028] Example 1

[0029] (1) Preparation of yam peel polyphenol extract: Take 1.0 kg of fresh purple yam, rinse with running water, peel mechanically to a thickness of 1.2 mm, and complete the peeling operation within 4 min, collecting about 145 g of yam peel. After washing, dry the yam peel with hot air at 55℃ to a moisture content of 8.2%, pulverize and pass through a 40-mesh sieve to obtain yam peel powder. Take the yam peel powder and add it to a 60% ethanol aqueous solution at a material-to-liquid ratio of 1:15 g / mL, extract at 55℃ for 90 min, repeat the extraction twice, combine the filtrates, and concentrate under reduced pressure at 52℃ to remove ethanol until the solid content is 11.4%, obtaining 12.6 g of yam peel polyphenol extract, with a total phenol content of 34.7% calculated as gallic acid.

[0030] (2) Slicing and Color-Protecting Marinating: Within 3 minutes of peeling, the peeled purple yam was sliced ​​into 3.0 mm thick slices, yielding 852 g of potato chips. A color-protecting solution was prepared, containing 0.6% citric acid and 0.05% L-cysteine. The above-mentioned yam peel polyphenol extract was added to make the total phenols account for 0.05% of the color-protecting solution by mass, with the remainder being water. The pH was adjusted to 3.3, and the total volume of the color-protecting solution was 2.6 kg. The potato chips were immediately immersed in the color-protecting solution and marinated at 18°C ​​for 40 min.

[0031] (3) Freeze-thaw penetration: The soaked potato chips, together with the color-protecting solution, are placed in a quick-freezing device and cooled to -30°C at a rate of 2.2°C / min. The core temperature of the potato chips reaches -18°C at 52 min. After cooling to -30°C, the temperature is kept warm for 2.0 h. Then, the potato chips, together with the color-protecting solution, are transferred to a 2°C refrigeration environment to allow them to thaw slowly in the color-protecting solution for 3.0 h. The freeze-thaw cycle is repeated once.

[0032] (4) Segmented low-temperature drying: After the potato chips are taken out, they are naturally drained for 8 minutes and then laid in a single layer in the hot air drying equipment. The first stage is dried at 42℃ for 2.5 hours and the second stage is dried at 50℃ for 5.0 hours. The moisture content was measured to be 7.3% when the chips were taken out of the box. The total drying time was 7.5 hours.

[0033] (5) Low-temperature grinding and grading: The dried potato chips were ground at 22°C, passed through a 100-mesh sieve, and air-separated to remove coarse particles and impurities, yielding 198g of purple yam powder, which was then vacuum-sealed.

[0034] Example 2

[0035] (1) Preparation of yam peel polyphenol extract: Take 1.0 kg of fresh purple yam, rinse with running water, peel mechanically to a thickness of 0.8 mm, and complete the peeling operation within 3 min, collecting about 98 g of yam peel. After washing, dry the yam peel with hot air at 50℃ to a moisture content of 9.5%, and pulverize it through a 40-mesh sieve. Add 50% ethanol aqueous solution at a material-to-liquid ratio of 1:10 g / mL, extract at 50℃ for 60 min, extract once, filter, and concentrate the filtrate under reduced pressure at 50℃ to a solid content of 5.8%, obtaining 7.9 g of yam peel polyphenol extract with a total phenol content of 31.2%.

[0036] (2) Slicing and Color-Protecting Marinating: Peeled purple yam was sliced ​​into 2.0 mm thick slices with a thickness deviation of 0.4 mm, yielding 868 g of potato chips. A color-protecting solution was prepared with 0.3% citric acid and 0.05% L-cysteine. The above extracts were added to make the total phenols account for 0.01% of the color-protecting solution by mass. The pH was adjusted to 3.6, and the total volume of the color-protecting solution was 2.6 kg. The potato chips were immersed in the solution and marinated at 10°C for 60 min.

[0037] (3) Freeze-thaw penetration: The temperature was lowered to -30℃ at a rate of 1.0℃ / min, and the core temperature of the potato chips reached -18℃ at 85 min. The chips were kept warm for 2.0 h. Then, they were slowly thawed in the color-protecting solution at 2℃ for 3.0 h. The freeze-thaw cycle was repeated once.

[0038] (4) Segmented low-temperature drying: After draining, spread out flat, dry at 42℃ for 2.5h in the first segment and at 50℃ for 5.3h in the second segment. The moisture content after drying is 7.8%, and the total time is 7.8h.

[0039] (5) Low-temperature pulverization and grading: Pulverize at 24℃, pass through a 100-mesh sieve, and air-separate to obtain 205g of purple yam powder.

[0040] Example 3

[0041] (1) Preparation of yam peel polyphenol extract: Take 1.0 kg of fresh purple yam, peel it mechanically to a thickness of 1.5 mm, and complete the peeling operation within 4 min, collecting about 186 g of yam peel. After washing, dry the yam peel with hot air at 60℃ to a moisture content of 7.6%, and pulverize it through a 40 mesh sieve. Add 70% ethanol aqueous solution at a material-to-liquid ratio of 1:20 g / mL, and extract at 60℃ for 120 min, extracting twice. Combine the filtrates and concentrate under reduced pressure at 54℃ to a solid content of 14.2%, obtaining 21.3 g of yam peel polyphenol extract with a total phenol content of 33.5%.

[0042] (2) Slicing and Color-Protecting Marinating: Peeled purple yam was sliced ​​into 4.0 mm thick slices with a thickness deviation of 0.5 mm, yielding 840 g of potato chips. A color-protecting solution was prepared with 1.0% citric acid and 0.05% L-cysteine. The above extracts were added to make the total phenols account for 0.10% of the color-protecting solution by mass. The pH was adjusted to 3.0, and the total volume of the color-protecting solution was 2.6 kg. The potato chips were immersed in the solution and marinated at 25°C for 20 min.

[0043] (3) Freeze-thaw penetration: the temperature was lowered to -30℃ at a cooling rate of 3.0℃ / min, the core temperature reached -18℃ in 38min, and the temperature was maintained for 2.0h; then it was slowly thawed in the color protection solution at 2℃ for 3.0h, and the freeze-thaw cycle was repeated once.

[0044] (4) Segmented low-temperature drying: After draining, spread out flat, dry the first segment at 42℃ for 2.5h, the second segment at 50℃ for 5.1h, the moisture content after drying is 7.4%, and the total time is 7.6h.

[0045] (5) Low-temperature pulverization and grading: Pulverize at 20℃, pass through a 100-mesh sieve, and air-separate to obtain 196g of purple yam powder.

[0046] Example 4

[0047] (1) Preparation of polyphenol extract from yam peel: Take 1.0 kg of fresh purple yam, peel it mechanically to a thickness of 1.0 mm, and collect about 122 g of yam peel. Dry the yam peel at 55℃ to a moisture content of 8.8%, pulverize it through a 40-mesh sieve, add 60% ethanol aqueous solution at a material-to-liquid ratio of 1:15 g / mL, extract at 55℃ for 90 min, extract twice, and concentrate under reduced pressure at 52℃ to a solid content of 10.7%, to obtain 10.4 g of extract with a total phenol content of 32.8%.

[0048] (2) Slicing and color-protecting immersion: Slice into thin slices with a thickness of 3.0 mm to obtain 858 g of potato chips. Prepare a color-protecting solution with a citric acid mass concentration of 0.6% and an L-cysteine ​​mass concentration of 0.01%. Add the above extracts to make the total phenol content 0.05%, adjust the pH to 3.3, and the total amount of color-protecting solution is 2.6 kg. Immerse at 18°C ​​for 40 min.

[0049] (3) Freeze-thaw penetration: the temperature was lowered to -25℃ at a rate of 1.2℃ / min, the core temperature reached -18℃ at 78 min, and the temperature was maintained for 1.0 h; then it was slowly thawed in the color protection solution at 2℃ for 3.0 h, and the freeze-thaw cycle was repeated once.

[0050] (4) Segmented low-temperature drying: After draining, spread out flat, dry at 42℃ for 2.5h in the first segment and at 50℃ for 5.4h in the second segment. The moisture content after drying is 7.6%, and the total time is 7.9h.

[0051] (5) Low-temperature pulverization and grading: Pulverize at 22℃, pass through a 100-mesh sieve, and air-separate to obtain 199g of purple yam powder.

[0052] Example 5

[0053] (1) Preparation of polyphenol extract from yam peel: Take 1.0 kg of fresh purple yam, peel it mechanically to a thickness of 1.3 mm, and collect about 158 ​​g of yam peel. Dry the yam peel at 55℃ to a moisture content of 8.0%, pulverize it through a 40-mesh sieve, add 60% ethanol aqueous solution at a material-to-liquid ratio of 1:15 g / mL, extract at 55℃ for 90 min, extract twice, and concentrate under reduced pressure at 52℃ to a solid content of 12.1%, to obtain 13.7 g of extract with a total phenol content of 35.1%.

[0054] (2) Slicing and color-protecting immersion: Slice into thin slices with a thickness of 3.0 mm to obtain 849 g of potato chips. Prepare a color-protecting solution with a citric acid mass concentration of 0.6% and an L-cysteine ​​mass concentration of 0.10%. Add the above extracts to make the total phenol content 0.05%, adjust the pH to 3.3, and the total amount of color-protecting solution is 2.6 kg. Immerse at 18°C ​​for 40 min.

[0055] (3) Freeze-thaw penetration: the temperature was lowered to -35℃ at a rate of 2.8℃ / min, and the core temperature reached -18℃ in 34 min. The temperature was then maintained for 3.0 h. The temperature was then slowly thawed in the color-protecting solution at 2℃ for 3.0 h. The freezing and thawing process was repeated for a total of 3 freeze-thaw cycles.

[0056] (4) Segmented low-temperature drying: After draining, spread out flat, dry at 42℃ for 2.5h in the first segment and at 50℃ for 3.8h in the second segment. The moisture content after drying is 7.1%, and the total time is 6.3h.

[0057] (5) Low-temperature pulverization and grading: Pulverize at 22℃, pass through a 100-mesh sieve, and air-separate to obtain 194g of purple yam powder.

[0058] Example 6

[0059] (1) Preparation of polyphenol extract from yam peel: Take 1.0 kg of fresh purple yam, peel it mechanically to a thickness of 1.2 mm, and collect about 146 g of yam peel. Dry the yam peel at 55℃ to a moisture content of 8.3%, pulverize it through a 40-mesh sieve, add 60% ethanol aqueous solution at a material-to-liquid ratio of 1:15 g / mL, extract at 55℃ for 90 min, extract twice, concentrate under reduced pressure at 52℃ to a solid content of 11.2%, and obtain 12.8 g of extract with a total phenol content of 34.9%.

[0060] (2) Slicing and color-protecting immersion: Slice into thin slices with a thickness of 3.0 mm to obtain 853 g of potato chips. Prepare a color-protecting solution with a citric acid mass concentration of 0.6% and an L-cysteine ​​mass concentration of 0.05%. Add the above extracts to make the total phenol content 0.05%, adjust the pH to 3.3, and the total amount of color-protecting solution is 2.6 kg. Immerse at 18°C ​​for 40 min.

[0061] (3) Freeze-thaw penetration: the temperature was lowered to -30℃ at a rate of 2.2℃ / min, and the core temperature reached -18℃ at 53 min. The temperature was maintained for 2.0 h. Then, the temperature was slowly thawed in the color protection solution at 0℃ for 4.0 h. The above freezing and thawing process was repeated for a total of 2 freeze-thaw cycles.

[0062] (4) Segmented low temperature drying: After draining, spread out flat, dry at 42℃ for 2.5h in the first segment and at 50℃ for 4.3h in the second segment. The moisture content after drying is 7.2%, and the total time is 6.8h.

[0063] (5) Low-temperature pulverization and grading: Pulverize at 22℃, pass through a 100-mesh sieve, and air-separate to obtain 192g of purple yam powder.

[0064] Example 7

[0065] (1) Preparation of polyphenol extract from yam peel: Take 1.0 kg of fresh purple yam, peel it mechanically to a thickness of 1.1 mm, and collect about 134 g of yam peel. Dry the yam peel at 55℃ to a moisture content of 8.5%, pulverize it through a 40-mesh sieve, add 60% ethanol aqueous solution at a material-to-liquid ratio of 1:15 g / mL, extract at 55℃ for 90 min, extract twice, concentrate under reduced pressure at 52℃ to a solid content of 11.0%, and obtain 11.8 g of extract with a total phenol content of 34.1%.

[0066] (2) Slicing and color-protecting immersion: Slice into thin slices with a thickness of 2.5 mm to obtain 861 g of potato chips. Prepare a color-protecting solution with a citric acid mass concentration of 0.6% and an L-cysteine ​​mass concentration of 0.05%. Add the above extracts to make the total phenol content 0.05%, adjust the pH to 3.3, and the total amount of color-protecting solution is 2.6 kg. Immerse at 15°C for 45 min.

[0067] (3) Freeze-thaw penetration: the temperature was lowered to -30℃ at a cooling rate of 2.0℃ / min, the core temperature reached -18℃ at 57 min, and the temperature was maintained for 2.0 h; then it was slowly thawed in the color protection solution at 4℃ for 2.0 h, and the freeze-thaw cycle was repeated once.

[0068] (4) Segmented low temperature drying: After draining, spread out flat, dry at 40℃ for 3.0h in the first segment and at 48℃ for 6.0h in the second segment. The moisture content after drying is 7.6%, and the total time is 9.0h.

[0069] (5) Low-temperature pulverization and grading: Pulverize at 22℃, pass through a 100-mesh sieve, and air-separate to obtain 201g of purple yam powder.

[0070] Example 8

[0071] (1) Preparation of polyphenol extract from yam peel: Take 1.0 kg of fresh purple yam, peel it mechanically to a thickness of 1.2 mm, and collect about 143 g of yam peel. Dry the yam peel at 55℃ to a moisture content of 8.4%, pulverize it through a 40-mesh sieve, add 60% ethanol aqueous solution at a material-to-liquid ratio of 1:15 g / mL, extract at 55℃ for 90 min, extract twice, and concentrate under reduced pressure at 52℃ to a solid content of 11.3%, to obtain 12.4 g of extract with a total phenol content of 34.4%.

[0072] (2) Slicing and color-protecting immersion: Slice into thin slices with a thickness of 3.5 mm and a thickness deviation of 0.4 mm to obtain 847 g of potato chips. Prepare a color-protecting solution with a citric acid mass concentration of 0.6% and an L-cysteine ​​mass concentration of 0.05%. Add the above extracts to make the total phenol content 0.05%, adjust the pH to 3.3, and the total amount of color-protecting solution is 2.6 kg. Immerse at 20°C for 30 min.

[0073] (3) Freeze-thaw penetration: the temperature was lowered to -30℃ at a rate of 2.2℃ / min, the core temperature reached -18℃ at 60min, and the temperature was maintained for 2.0h; then it was slowly thawed in the color protection solution at 2℃ for 3.0h, and the freeze-thaw cycle was repeated once.

[0074] (4) Segmented low temperature drying: After draining, spread out flat, dry at 45℃ for 2.0h in the first segment and at 52℃ for 4.0h in the second segment. The moisture content after drying is 7.4%, and the total time is 6.0h.

[0075] (5) Low-temperature pulverization and grading: Pulverize at 18℃, pass through a 120-mesh sieve, and air-separate to obtain 190g of purple yam powder.

[0076] Comparative Example 1 (Conventional Process)

[0077] Take 1.0 kg of fresh purple yam from the same batch as in Example 1, rinse with running water and peel mechanically to a thickness of 1.2 mm, cut into thin slices with a thickness of 3.0 mm, spread directly on a hot air drying device without color protection treatment, and dry at 65°C for 6.0 h in one go, with a moisture content of 7.5% upon exiting the drying chamber; pulverize the dried potato slices at room temperature (about 26°C), pass through a 100-mesh sieve, and obtain 203 g of purple yam powder.

[0078] Comparative Example 2 (color protection but no freeze-thaw cycle)

[0079] Yam peel polyphenol extract was prepared according to step (1) of Example 1, with a total phenol content of 34.5%. Yam slices were prepared and color-protecting impregnation was completed according to step (2) of Example 1. After impregnation, the potato slices were directly drained without freezing or thawing. Then, they were subjected to segmented low-temperature drying according to step (4) of Example 1. The first stage was dried at 42℃ for 2.5 hours, and the second stage at 50℃ for 5.0 hours. The moisture content was still 14.2%. Drying was continued at 50℃ until the moisture content reached 7.8%, with a total drying time of 13.5 hours. Yam powder was pulverized through a 100-mesh sieve according to step (5) of Example 1, yielding 204g of purple yam powder.

[0080] Comparative Example 3 (freeze-thaw but color-protecting solution does not contain yam peel polyphenol extract)

[0081] Take 1.0 kg of fresh purple yam, peel it mechanically to a thickness of 1.2 mm, slice it to a thickness of 3.0 mm, and immediately immerse it in a color-protecting solution. The color-protecting solution contains 0.6% citric acid and 0.05% L-cysteine, without adding yam peel polyphenol extract. Adjust the pH to 3.3. The total amount of color-protecting solution is 2.6 kg. Soak it at 18°C ​​for 40 min. Then, perform freeze-thaw penetration (freeze-thaw once) according to step (3) of Example 1, and dry it to a moisture content of 7.4% according to step (4) of Example 1, for a total time of 7.6 h. Then, pulverize it through a 100-mesh sieve according to step (5) of Example 1 to obtain 199 g of purple yam powder.

[0082] Comparative Example 4 (Slow Freezing)

[0083] Take 1.0 kg of fresh purple yam, prepare yam peel polyphenol extract according to step (1) of Example 1, slice and complete color protection soaking (851 g of potato chips) according to step (2). Place it together with the color protection solution in a freezing environment of -5℃ for slow freezing. The core temperature of the potato chips reaches -18℃ after 8.5 h, and keep warm for 2.0 h; then thaw in the color protection solution at 2℃ for 3.0 h. Dry to 7.5% moisture content according to step (4) of Example 1, for a total time of 9.8 h, and pulverize through a 100 mesh sieve according to step (5) to obtain 200 g of purple yam powder.

[0084] Comparative Example 5 (Thawed at room temperature)

[0085] Take 1.0 kg of fresh purple yam, prepare yam peel polyphenol extract according to step (1) of Example 1, slice and complete color protection soaking according to step (2). After quick freezing and heat preservation according to step (3) of Example 1, place the potato chips together with the color protection solution in a room temperature environment of 25°C to thaw. The center temperature of the potato chips rises above 0°C after 1.6 h and is left to stand for a total of 3.0 h. Dry to 7.3% moisture content according to step (4) of Example 1, for a total time of 7.7 h. Crush and pass through a 100-mesh sieve according to step (5) to obtain 197 g of purple yam powder.

[0086] Comparative Example 6 (freeze-thaw cycle using pure water instead of color-protecting solution)

[0087] Take 1.0 kg of fresh purple yam, peel it mechanically to a thickness of 1.2 mm, slice it to a thickness of 3.0 mm, and immediately immerse it in the same amount of drinking water as in Example 1 (without adding citric acid, L-cysteine ​​and yam peel polyphenol extract) at 18°C ​​for 40 min. Then, perform freeze-thaw permeation (freeze-thaw once) according to step (3) of Example 1, and dry it to a moisture content of 7.5% according to step (4), with a total time of 7.6 h. Then, pulverize it through a 100-mesh sieve according to step (5) to obtain 202 g of purple yam powder.

[0088] Comparative Example 7 (with added hot stamping process)

[0089] Take 1.0 kg of fresh purple yam, prepare yam peel polyphenol extract according to step (1) of Example 1, and complete slicing according to step (2). Blanch the potato slices in boiling water for 3 minutes, remove them and cool them quickly to room temperature. Then prepare the color-protecting solution according to step (2) of Example 1 and soak them at 18°C ​​for 40 minutes. Then perform freeze-thaw penetration (freeze-thaw once) according to step (3) of Example 1, dry them to 7.4% moisture content according to step (4), and take a total of 9.8 hours. Then pulverize them through a 100-mesh sieve according to step (5) to obtain 189 g of purple yam powder.

[0090] [Performance Testing]

[0091] (a) Anthocyanin content (pH differential method, calculated as cyanidin-3-glucoside)

[0092] Weigh 0.50 g of the pulverized sample into a stoppered conical flask, add 25 mL of hydrochloric acid-acidified ethanol solution (95% ethanol and 1.5 mol / L hydrochloric acid mixed at a volume ratio of 85:15), and extract ultrasonically at 40℃ and 240W for 30 min. After cooling, centrifuge at 8000 r / min for 10 min, collect the supernatant, and repeat the extraction once with the residue. Combine the supernatants and dilute to 50 mL. Take two 1.0 mL aliquots of the extract, and dilute to 10 mL with pH 1.0 potassium chloride buffer (0.025 mol / L) and pH 4.5 sodium acetate buffer (0.4 mol / L), respectively. Equilibrate in the dark for 30 min, using distilled water as a blank, and measure the absorbance at 510 nm and 700 nm. (The formula is: A = (A...)) 510 -A 700 pH 1.0 − (A 510 -A 700 At pH 4.5, the absorbance difference was calculated. Anthocyanin content was converted using the formula C = A × M × DF × 1000 / (ε × l), where M is the molar mass of cyanidin-3-glucoside (449.2 g / mol), DF is the dilution factor, ε is the molar extinction coefficient (26900 L / (mol·cm), and l is the path length of the cuvette (1 cm). Results are expressed as milligrams of cyanidin-3-glucoside per 100 g dry basis sample.

[0093] (ii) Color values ​​L*, a*, b* and browning index

[0094] The colorimeter was used for measurement, with a D65 light source and an observation angle of 10°. Calibration was performed using a standard white plate before measurement. The powder sample was spread evenly in a sample dish and leveled to a thickness of at least 5 mm. Measurements were taken at five different locations, and the average value was calculated. The browning index was calculated using the following formula: BI = 100×(x−0.31) / 0.172, where x = (a*+1.75L*) / (5.645L*+a*−3.012b*).

[0095] (III) Polysaccharide retention rate (phenol-sulfuric acid method)

[0096] Weigh 2.00 g of sample, add petroleum ether and reflux twice to defatt it. After evaporation, reflux with 80% ethanol for 1 h to remove monosaccharides and oligosaccharides. Add 50 mL of water to the residue and extract at 80 °C for 2 h, repeating twice. Combine the extracts, concentrate under reduced pressure, and remove protein using the Sevag method. Add 4 times the volume of anhydrous ethanol and let stand overnight at 4 °C. Centrifuge to collect the precipitate, wash successively with anhydrous ethanol and acetone, dry, and then obtain crude polysaccharide product and weigh it. Take 1.0 mL of polysaccharide solution, add 1.0 mL of 5% phenol solution, quickly add 5.0 mL of concentrated sulfuric acid, mix well, and let stand at room temperature for 30 min. Measure the absorbance at 490 nm. Use glucose as a standard to prepare a standard curve and calculate the total sugar content. The polysaccharide retention rate is calculated as: polysaccharide content in the product (dry basis) / polysaccharide content in the raw material (dry basis) × 100%.

[0097] (iv) Accelerated anthocyanin retention rate after storage

[0098] Take 30g of sample, place it in an aluminum foil composite bag, vacuum seal it, and place it in a constant temperature and humidity chamber at 40±1℃ and 75±3% relative humidity for 30 days in the dark. After taking it out, determine the anthocyanin content according to test method (I). The retention rate is calculated as (content after storage / content before storage) × 100%.

[0099] Table 1. Key Process Points of Each Embodiment and Comparative Example

[0100] Example 1 1 time -30℃ 2.2℃ / min 2℃ 3.0h 0.05% 0.05% 0.6% / 3.3 Example 2 1 time -30℃ 1.0℃ / min 2℃ 3.0h 0.01% 0.05% 0.3% / 3.6 Example 3 1 time -30℃ 3.0℃ / min 2℃ 3.0h 0.10% 0.05% 1.0% / 3.0 Example 4 1 time -25℃ 1.2℃ / min 2℃ 3.0h 0.05% 0.01% 0.6% / 3.3 Example 5 3 times -35℃ 2.8℃ / min 2℃ 3.0h 0.05% 0.10% 0.6% / 3.3 Example 6 2 times -30℃ 2.2℃ / min 0℃ 4.0h 0.05% 0.05% 0.6% / 3.3 Example 7 1 time -30℃ 2.0℃ / min 4℃ 2.0h 0.05% 0.05% 0.6% / 3.3 Example 8 1 time -30℃ 2.2℃ / min 2℃ 3.0h 0.05% 0.05% 0.6% / 3.3 Comparative Example 1 — — — — — — Comparative Example 2 none — — 0.05% 0.05% 0.6% / 3.3 Comparative Example 3 1 time -30℃ 2.2℃ / min 2℃ 3.0h 0 0.05% 0.6% / 3.3 Comparative Example 4 1 time Slow freezing at -5℃ 2℃ 3.0h 0.05% 0.05% 0.6% / 3.3 Comparative Example 5 1 time -30℃ 2.2℃ / min 25℃ 3.0h 0.05% 0.05% 0.6% / 3.3 Comparative Example 6 1 time -30℃ 2.2℃ / min 2℃ 3.0h pure water — — Comparative Example 7 1 time -30℃ 2.2℃ / min 2℃ 3.0h 0.05% 0.05% 0.6% / 3.3 (preheating for 3 minutes)

[0101] Table 2 Summary of Test Results

[0102] Example 1 206.1 86.6 68.5 21.8 12.3 35.2 91.2 82.4 Example 2 191 80.3 66.3 19.1 12.9 40.5 88.4 76.3 Example 3 204.8 86.1 68.9 22.4 12.1 34.1 90.6 83.1 Example 4 194 81.5 67.1 20.2 12.6 38.3 88.9 77.5 Example 5 201 84.5 68 21.1 12.5 36.4 89.3 80.2 Example 6 211.1 88.7 70.1 23.4 11.8 31.6 92.5 85.6 Example 7 207.2 87 69.2 22.6 12 33.5 91.8 83.8 Example 8 198 83.2 66.8 20.5 12.7 38.9 88 78.9 Comparative Example 1 118 49.6 51.2 10.4 18.6 78.4 68.5 51.3 Comparative Example 2 152 63.9 59.4 15.2 15.1 55.3 84.1 62.7 Comparative Example 3 178 74.8 63.8 18.6 13.4 44.2 87.6 69.4 Comparative Example 4 185 77.7 65.2 19.3 13.1 41.8 86.2 71.8 Comparative Example 5 176 74 62.5 17.4 14.2 48.6 85 68.2 Comparative Example 6 131 55 55.1 13 16.8 65.7 82.3 55.9 Comparative Example 7 163 68.5 61.3 17.9 13.9 41 72.3 65.1

[0103] As shown in Table 2, the anthocyanin retention rates of Examples 1-8 were 80.3%-88.7%, and the browning index was 31.6-40.5, both significantly better than Comparative Example 1 (49.6% retention rate, 78.4 browning index) using the conventional process. Comparative Example 2, omitting the freeze-thaw step, showed a decrease in anthocyanin retention rate of approximately 23%, indicating that the microporous channels formed by freeze-thaw are both a condition for increasing the dehydration rate and a necessary prerequisite for color-protecting components to enter the potato flesh. Comparative Example 3, after removing yam peel polyphenols, showed a decrease in anthocyanin retention rate of approximately 12%, and the difference in retention rate after accelerated storage further widened, indicating that the auxiliary color-protecting components have a continuous structural stabilizing effect on anthocyanins. Comparative Examples 4 and 5 respectively show that the cooling rate determines the ice crystal morphology, and thawing needs to be completed slowly at low temperatures; Comparative Example 6 shows that having only channels without color-protecting components cannot achieve a color-protecting effect; Comparative Example 7 shows that although blanching can inhibit browning, it results in a large loss of polysaccharides, prolonged drying time, and poor reconstitution properties. Example 6, which underwent two freeze-thaw cycles, showed the best overall performance.

[0104] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope of the present invention.

Claims

1. A method for preparing purple yam powder, characterized in that, Includes the following steps: S1. Wash fresh purple yam, peel it to a thickness of 0.8-1.5mm, and collect the yam peel for later use; S2. Slice the peeled purple yam into slices 2-4 mm thick, and immerse the resulting potato chips in a color-protecting solution. The color-protecting solution is an aqueous solution containing citric acid, L-cysteine, and yam peel polyphenol extract, wherein the amount of yam peel polyphenol extract used is 0.01%-0.10% of the mass of the color-protecting solution based on the mass of the total phenols. The yam peel polyphenol extract is obtained by extracting and concentrating the yam peel obtained in S1 with an ethanol aqueous solution. S3. Freeze the soaked potato chips along with the color-protecting solution at -25--35℃, so that the core temperature of the potato chips drops to below -18℃ within 90 minutes, and keep them at below -18℃ for 1-3 hours. Then thaw them at 0-4℃, and the potato chips are always submerged in the color-protecting solution during the thawing process. S4. After thawing, drain the potato chips and dry them in sections at a temperature not exceeding 52°C until the moisture content of the potato chips is no more than 8%. S5. The dried potato chips are crushed, sieved, and impurities are removed at a temperature not exceeding 25°C to obtain the purple yam powder.

2. The preparation method according to claim 1, characterized in that, In step S2, the color-protecting solution has a citric acid concentration of 0.3%-1.0% and an L-cysteine ​​concentration of 0.01%-0.1%; the pH value of the color-protecting solution is 3.0-3.

6.

3. The preparation method according to claim 1, characterized in that, In step S2, the immersion temperature is 10-25℃ and the time is 20-60 min, and the mass ratio of potato chips to color-protecting liquid is 1:3-1:

6.

4. The preparation method according to claim 1, characterized in that, In step S2, the yam peel polyphenol extract is prepared as follows: the yam peel is washed and dried until the moisture content is no more than 10%, pulverized and sieved, and then extracted with a 50%-70% ethanol aqueous solution at a material-liquid ratio of 1:10-1:20 g / mL at 50-60℃ for 60-120 min. After filtration and concentration under reduced pressure to remove ethanol, the extract is obtained.

5. The preparation method according to claim 4, characterized in that, The vacuum concentration temperature is 50-55℃, and the concentration is carried out until the solid content is 5%-15%.

6. The preparation method according to claim 1, characterized in that, In step S3, the freezing and thawing cycle is repeated 2-3 times; the freezing cooling rate is 1.0-3.0℃ / min.

7. The preparation method according to claim 1, characterized in that, In step S3, the thawing time is 2.5-3.5 hours.

8. The preparation method according to claim 1, characterized in that, In step S4, the segmented drying includes: the first segment is dried at 40-45℃ for 2-3 hours, and the second segment is dried at 48-52℃ for 4-6 hours.

9. The preparation method according to claim 1, characterized in that, In step S2, the citric acid concentration in the color-protecting solution is 0.5%-0.8%, the L-cysteine ​​concentration is 0.03%-0.07%, and the amount of the yam peel polyphenol extract is 0.03%-0.07% of the mass of the color-protecting solution, calculated based on the mass of the total phenols therein.

10. A purple yam powder, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 9.

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