CN115558140A

CN115558140A - Method for preparing freshness indicating film based on different heat treatment purple sweet potato powder

Info

Publication number: CN115558140A
Application number: CN202211249553.1A
Authority: CN
Inventors: 刘俊; 李晨晨; 金昌海; 阚娟; 千春录; 陈丹
Original assignee: Yangzhou University
Current assignee: Yangzhou University
Priority date: 2022-10-12
Filing date: 2022-10-12
Publication date: 2023-01-03
Anticipated expiration: 2042-10-12
Also published as: CN115558140B

Abstract

The invention relates to a method for preparing a freshness indicating film based on different heat treatment purple sweet potato powder, belonging to the field of food packaging and comprising the following steps: (1) Respectively carrying out heat treatment of steaming, boiling and baking on the fresh purple sweet potatoes, peeling, cutting into blocks, freeze-drying, crushing and sieving to obtain heat-treated purple sweet potato powder; (2) Suspending the heat-treated purple sweet potato powder in water, homogenizing by a high-speed disperser, and gelatinizing by a water bath to obtain a purple sweet potato powder suspension; (3) Adding a certain amount of sodium alginate into the purple sweet potato powder suspension obtained in the step (2) to obtain a purple sweet potato powder-sodium alginate mixed solution; (4) Adding glycerol into the mixed solution obtained in the step (3), and mixing by a magnetic stirrer to obtain a film forming solution; (5) And pouring the obtained film forming solution into an organic glass mold, controlling humidity, air-drying and forming a film. The freshness indicating film prepared by the invention has the advantages of green preparation process and low production cost, and has application potential in the field of intelligent food packaging.

Description

Method for preparing freshness indicating film based on different heat treatment purple sweet potato powder

Technical Field

The invention relates to a method for preparing a freshness indicating film based on different heat treatment purple sweet potato powder, and belongs to the field of food packaging.

Background

In the food intelligent packaging field, the freshness indication film receives extensive attention because of its freshness that can real-time supervision food. Freshness indicating films are typically prepared by adding natural pigments (such as anthocyanin, betalain, curcumin, and the like) to a polysaccharide or protein matrix. Among them, anthocyanin is the most commonly used natural pigment, which can change molecular structure under different acid-base conditions and exhibit color change. Since foods are often accompanied by a change in ph during spoilage, anthocyanin-containing films can indicate freshness of foods by a change in color exhibited under different ph environments. However, the existing anthocyanin film has high preparation cost, and the industrialization of the anthocyanin film is limited. On one hand, in the preparation process of the anthocyanin film, organic solvents such as ethanol, methanol and the like are generally required to be used for extracting the anthocyanin from peels, pulps and petals of vegetables, fruits and flowers, and the adverse effects on the environment and the food safety are realized; on the other hand, in the production process of an anthocyanin film, polysaccharides, proteins and the like are generally required to be additionally added as a substrate of the film, which increases the production cost. Therefore, there is a need to find a green, economical method for preparing anthocyanin films. In addition, anthocyanin is easy to change color under the action of heat, light and oxygen, and the stability of anthocyanin is low, so that the practical application of anthocyanin films is limited. It is noted that the stability of anthocyanins is closely related to their chemical structure, and acylated anthocyanins have higher stability than non-acylated anthocyanins due to intramolecular co-coloration, and thus naturally occurring acylated anthocyanins are more suitable for use in the preparation of freshness indicator films.

The purple sweet potato is rich in acylated anthocyanin and is a suitable raw material for preparing the freshness indicating film. Currently, many researchers have extracted anthocyanins from purple sweet potatoes and compounded them with different polysaccharide matrices to produce freshness-indicating films. However, a large amount of organic solvent is needed in the anthocyanin extraction process, and other purple sweet potato components (such as starch, dietary fiber and the like) except anthocyanin are not effectively utilized, so the existing film-making method is not economical and environment-friendly; meanwhile, the stability of purple sweet potato anthocyanin cannot be improved through the existing research.

Disclosure of Invention

Aiming at the problems in the prior art, the invention provides a method for preparing a freshness indicating film based on different heat treatment purple sweet potato powder, aiming at overcoming the defects of high production cost, insufficient utilization of raw materials, poor anthocyanin stability and the like in the preparation process of the conventional anthocyanin film.

The purpose of the invention is realized as follows: a method for preparing a freshness indicating film based on different heat treatment purple sweet potato powder is characterized by comprising the following steps:

(1) Respectively carrying out heat treatment on the fresh purple sweet potatoes by steaming, boiling and baking, peeling, cutting into blocks, freezing, drying, crushing and sieving to obtain heat-treated purple sweet potato powder;

(2) Suspending the heat-treated purple sweet potato powder in water, homogenizing by a high-speed disperser, and gelatinizing by a water bath to obtain a purple sweet potato powder suspension;

(3) Adding a certain amount of sodium alginate into the purple sweet potato powder suspension obtained in the step (2) to obtain a purple sweet potato powder-sodium alginate mixed solution;

(4) Adding a certain amount of glycerol into the mixed solution obtained in the step (3), and mixing by using a magnetic stirrer to obtain a film forming solution;

(5) And pouring the obtained film forming solution into an organic glass mold, controlling humidity, air-drying and forming a film.

In the step (1), the steaming heat treatment is to put the purple sweet potatoes into a steamer to steam for 20 to 40 min; the boiling heat treatment is to put the purple sweet potatoes into boiling water to be boiled for 20 to 40 min; the baking heat treatment is to bake the purple sweet potatoes in an oven at 200 to 250 ℃ for 20 to 40 min; the cutting is to cut the peeled purple sweet potatoes into squares of 1 cm multiplied by 1 cm; the temperature of the freeze drying is-45 to-50 ℃, and the freeze drying time is 24 to 48 hours; the grinding speed is 1500 to 2000 rpm, and the grinding time is 1 to 2 min; the mesh number of the sieved screen is 100 to 150 meshes.

In the step (2), the homogenizing speed of the high-speed disperser is 3000 to 5000 rpm, and the homogenizing time is 2 to 4 min; the gelatinization temperature of the water bath kettle is 80-100 ℃, and the gelatinization time is 10-20 min.

In the step (3), the mass ratio of the purple sweet potato powder to the sodium alginate in the purple sweet potato powder-sodium alginate mixed solution is 5 to 9, and the concentration of the purple sweet potato powder is 30 to 50 mg/mL.

In the step (4), the addition amount of the glycerol is 0.5-0.8% of the total volume of the film forming solution; the rotating speed of the magnetic stirrer is 800 to 1000 r/min, and the stirring time is 60 to 90 min.

In the step (5), the size of the organic glass mold is 20-30 cm multiplied by 20-30 cm; when the film forming solution is subjected to humidity control air drying, the humidity control air drying condition is that the relative humidity is 40% -50%, the temperature is 30-40 ℃, and the humidity control air drying time is 24-48 h.

The purple sweet potato powder freshness indicating film based on different heat treatments is prepared by the method.

The method is advanced and scientific, the fresh purple sweet potatoes are respectively subjected to heat treatment of steaming, boiling and baking, then are peeled, cut into blocks, freeze-dried, crushed and sieved to obtain heat treatment purple sweet potato powder, and then the freshness indicating film is prepared on the basis of the heat treatment purple sweet potato powder. According to the invention, the stability of anthocyanin in the purple sweet potatoes is improved by carrying out heat treatment on the fresh purple sweet potatoes, and meanwhile, the starch, dietary fiber and anthocyanin in the purple sweet potato powder are comprehensively utilized to prepare the intelligent packaging film with the food freshness indication function, so that the intelligent packaging film has the advantages of green preparation process and low production cost, and has application potential in the field of intelligent food packaging.

The invention has the beneficial effects that: the inherent color of the purple sweet potato is kept through heat treatment such as steaming, boiling and baking, and the purple sweet potato anthocyanin has a stabilizing effect; meanwhile, the purple sweet potato is subjected to heat treatment, the tissue of the purple sweet potato is softened, and starch is gelatinized, so that a good matrix material is provided for the preparation of the anthocyanin film; the method directly utilizes the heat-treated purple sweet potato powder to prepare the freshness indicating film, avoids the fussy anthocyanin extraction and purification process, and is favorable for reducing the production cost and protecting the environment; meanwhile, other components (such as starch and dietary fiber) of the purple sweet potatoes are comprehensively utilized, and the production cost of the anthocyanin film is further reduced.

Drawings

FIG. 1 is a graph showing the analysis of the total phenol content in different purple sweet potato flours used in examples 1, 2, 3 and 1 of the present invention;

FIG. 1B is a graph showing the total anthocyanin content analysis in different purple sweet potato flours used in example 1, example 2, example 3 and comparative example 1 of the present invention;

FIG. 1C is a graph of the antioxidant activity analysis of different purple potato flours used in examples 1, 2, 3 and comparative example 1 of the present invention;

FIG. 1D is a graph showing the color rendering properties of anthocyanin in different purple sweet potato flours used in example 1, example 2, example 3 and comparative example 1 of the present invention;

FIG. 1E is a graph showing the film forming effect of different purple sweet potato powders used in example 1, example 2, example 3 and comparative example 1;

FIG. 2A is a surface scanning electron micrograph of examples 1, 2, 3 and comparative example 1 according to the present invention;

FIG. 2B is a sectional scanning electron micrograph of example 1, example 2, example 3 and comparative example 1 according to the present invention;

FIG. 3A is an infrared spectrum of example 1, example 2, example 3 and comparative example 1 of the present invention;

FIG. 3B is an X-ray diffraction pattern of example 1, example 2, example 3 and comparative example 1 of the present invention;

FIG. 4A is a graph showing UV-visible light transmittance analysis of example 1, example 2, example 3 and comparative example 1 according to the present invention;

FIG. 4B is a water vapor transmission rate analysis chart of example 1, example 2, example 3 and comparative example 1 of the present invention;

FIG. 4C is an oxygen transmission rate analysis chart of example 1, example 2, example 3 and comparative example 1 of the present invention;

FIG. 5 is a graph of the mechanical properties of example 1, example 2, example 3 and comparative example 1 of the present invention; in FIG. 5, A is a tensile strength analysis chart and B is an elongation at break analysis chart;

FIG. 6 is a thermogravimetric analysis of examples 1, 2, 3 and 1 according to the invention; in FIG. 6, A is a TGA analysis chart and B is a DTG analysis chart;

FIG. 7 is a graph showing antioxidant activities of examples 1, 2, 3 and 1 of the present invention;

FIG. 8 is a graph showing sensitivity analysis of example 1, example 2, example 3 and comparative example 1 in different pH (3-12) buffers and ammonia gas; a is a sensitivity analysis chart in buffers with different pH values (3-12), and B is a sensitivity analysis chart to ammonia gas;

FIG. 9 is a color stability analysis chart of examples 1, 2, 3 and comparative example 1 of the present invention at different temperatures (4 ℃ and 25 ℃); a is a color stability analysis chart at 4 ℃ and B is a color stability analysis chart at 25 ℃;

FIG. 10 is a graph of the delta values at different temperatures (4 ℃ and 25 ℃) of example 1, example 2, example 3 and comparative example 1 of the present inventionEA graph of value changes; in FIG. 10, A is Δ at 4 ℃EThe change graph B is delta at 25 DEG CEA graph of value changes;

fig. 11 is an application example of the freshness monitoring of the prawns in example 1, example 2, example 3 and comparative example 1 of the present invention.

Detailed Description

The invention is described in further detail below with reference to the following figures and examples:

example 1

A freshness indicating film prepared based on steaming purple sweet potato powder;

cleaning fresh purple sweet potatoes, putting the fresh purple sweet potatoes into a steamer filled with boiling water, steaming the purple sweet potatoes for 30 min, peeling the purple sweet potatoes, cutting the purple sweet potatoes into square blocks of 1 cm multiplied by 1 cm, freeze-drying the purple sweet potatoes for 48 h at the temperature of-50 ℃, crushing the purple sweet potatoes for 1 min at 2000 rpm, and sieving the purple sweet potatoes with a 100-mesh sieve to obtain the steaming heat treatment purple sweet potato powder.

Putting 6.8 g of steaming heat treatment purple sweet potato powder into a beaker, adding 170 mL of distilled water, homogenizing for 2 min at the room temperature by using a high-speed disperser at the speed of 5000 rpm, and then stirring for 15 min at 90 ℃ in a water bath kettle so as to completely gelatinize, thereby obtaining a purple sweet potato powder suspension; adding 1.36 g of sodium alginate into the purple sweet potato powder suspension, and magnetically stirring at 800 rpm at room temperature for 30 min to obtain a purple sweet potato powder-sodium alginate mixed solution; dropwise adding 1.22 g of glycerol into the purple sweet potato powder-sodium alginate mixed solution, and magnetically stirring at 800 rpm at room temperature for 60 min to obtain a film forming solution; pouring the film-forming solution into a plastic glass plate mold (24X 24 cm) ² ) And controlling the relative humidity to be 50%, the drying temperature to be 30 ℃ and the drying time to be 48 h to obtain the freshness indicating film prepared on the basis of steaming and heat treatment of the purple sweet potato powder, and naming the freshness indicating film as S-PSP film.

Example 2

A freshness indicating film prepared based on boiling heat treatment of purple sweet potato powder;

the heat treatment mode of the purple sweet potato in the embodiment 1 is changed into the mode that the fresh purple sweet potato is cleaned and then is put into a pot filled with boiling water to be boiled for 30 min, and the other preparation conditions are not changed, so that the freshness indicating film prepared based on the purple sweet potato powder subjected to boiling heat treatment is obtained and named as B-PSP film.

Example 3

A freshness indicating film prepared based on roasting heat treatment of purple sweet potato powder;

the heat treatment mode of the purple sweet potatoes in the embodiment 1 is changed into the mode that the fresh purple sweet potatoes are washed and then are placed into an oven preheated to 230 ℃, the purple sweet potatoes are baked for 30 min at 230 ℃, and the other preparation conditions are unchanged, so that the freshness indicating film prepared based on the purple sweet potato powder subjected to baking heat treatment is obtained and named as R-PSP film.

Comparative example 1

A freshness indicating film prepared based on non-heat treated purple sweet potato flour;

the processing method of the purple sweet potatoes in the embodiment 1 is changed into the method that after the fresh purple sweet potatoes are washed, the fresh purple sweet potatoes are directly peeled without being subjected to heat treatment, cut into square blocks of 1 cm multiplied by 1 cm, frozen and dried at-50 ℃ for 48 h, crushed at 2000 rpm for 1 min, and sieved by a 100-mesh sieve to obtain raw purple sweet potato powder, and the rest preparation conditions are unchanged to obtain the freshness indicating film prepared based on the purple sweet potato powder which is not subjected to heat treatment, and the freshness indicating film is named as PSP film.

1. The total phenol content, total anthocyanin content, antioxidant activity, anthocyanin color developing property and film forming effect of the steamed heat-treated purple sweet potato powder (S-PSP), the boiling heat-treated purple sweet potato powder (B-PSP), the roasting heat-treated purple sweet potato powder (R-PSP) and the non-heat-treated purple sweet potato powder (PSP) used in example 1, example 2, example 3 and comparative example 1, respectively, were measured to obtain the following results.

Adding 1 g of PSP, S-PSP, B-PSP and R-PSP powder into 40 mL of 75% ethanol solution, and extracting for 60 min under the conditions of ultrasonic power of 200W and frequency of 40 KHz; then 10000gCentrifuging for 15 min; repeating the above steps to extract residues again, and collecting supernatant; the supernatants were then analyzed for total phenol content, total anthocyanin content, antioxidant activity and color change in different buffers (pH 3-12). Determining the total phenol content by a forskolin phenol method; measuring the content of total anthocyanin by a pH differential method; taking 1 mL of supernatant to react with 3 mL of 100 mu mol/L DPPH methanol solution, and measuring the antioxidant activity of the supernatant; 0.5 mL of the supernatant and 0.5 mL of different buffer solutions (pH 3-12) were mixed at 20 ℃ for 1 min and the color change of the solution was recorded with a digital camera.

As shown in fig. 1, the total phenol content, the total anthocyanin content and the antioxidant activity in the purple sweet potatoes are improved by the steaming and boiling heat treatment. However, the roasting heat treatment reduces the total phenol content, the total anthocyanin content and the antioxidant activity in the purple sweet potatoes. The different heat treatments had no significant effect on the color change ability of the PSP anthocyanins compared to the PSPs. However, the film-forming solutions of the gelatinized PSP and R-PSP are reddish brown, while the film-forming solutions of the gelatinized S-PSP and B-PSP are dark purple. The two heat treatments of steaming and boiling are helpful for maintaining the color stability of the purple sweet potato anthocyanin in the gelatinization film-forming process. After the film is formed, the PSP and R-PSP films are reddish brown, and the S-PSP and B-PSP films are dark purple. The S-PSP and B-PSP films well keep the inherent color of the purple sweet potatoes, namely, the purple sweet potato powder subjected to steaming and boiling heat treatment can be directly used for preparing the films without anthocyanin extraction.

2. The microstructures, the ultraviolet-visible light transmittances, the water vapor transmittances, the oxygen transmittances, the mechanical properties, the thermogravimetric properties, the antioxidant activities, the pH sensitivities, the ammonia sensitivities, and the color stabilities of example 1, example 2, example 3, and comparative example 1 were tested, and the results were obtained as follows.

The surface and cross-sectional microstructures of the films were analyzed by scanning electron microscopy.

The surface and cross-sectional microstructures of the PSP, S-PSP, B-PSP and R-PSP films are shown in FIGS. 2A and 2B, respectively. The PSP film has a non-uniform surface and cross-section. However, films prepared based on heat-treated purple potato flour, particularly S-PSP and B-PSP films, have relatively uniform surfaces and cross-sections. It is shown that both steaming and boiling heat treatments significantly improve the uniformity of the film.

Using a Fourier transform infrared spectrometer with an attenuated total reflection mode at 400-4000 cm ⁻¹ The infrared spectrum of the film was measured in the range.

As shown in FIG. 3A, the PSP, S-PSP, B-PSP and R-PSP films have similar IR spectra. It is shown that the different heat treatments do not significantly change the functional groups in the film.

The X-ray diffraction pattern of the film was measured using a polycrystalline X-ray diffractometer, scanning from 5 to 75 at a speed of 2/min.

As shown in fig. 3B, the PSP film was in an amorphous state, while the films prepared based on heat-treated purple sweet potato flour, especially the S-PSP and B-PSP films, still partially retained the crystallization peaks of native starch at 15 ° and 17-18 °. The S-PSP and B-PSP films have higher stability.

The light blocking capability of the film samples was determined by scanning the film samples in the 200-800 nm range using an ultraviolet/visible spectrophotometer.

As shown in FIG. 4A, the light transmittance of the PSP, S-PSP, B-PSP and R-PSP films is less than 20% in the range of 200-800 nm, indicating that the films have excellent light blocking ability. At 370-600 nm, the PSP and R-PSP films have relatively high light blocking capabilities, while the S-PSP and B-PSP films exhibit similar light blocking capabilities.

The water vapor transmission rate (WVP) of the film was determined gravimetrically.

As shown in FIG. 4B, the WVP of the PSP, S-PSP, B-PSP and R-PSP films is at 26.9−30.1 ×10 ⁻¹¹ g m ⁻¹ s ⁻¹ Pa ⁻¹ Indicating that the film has a certain water vapor barrier ability. Meanwhile, S-PSP and B-PSP films have relatively low WVP.

The oxygen transmission rate (OP) of the film was measured by a gas permeability tester.

As shown in FIG. 4C, the OP of the PSP, S-PSP, B-PSP and R-PSP films was 0.55-0.73 cm ³ mm m ⁻² day ⁻¹ atm ⁻¹ Indicating that the film has a certain oxygen barrier ability. Meanwhile, S-PSP and B-PSP films have relatively low OP.

The Tensile Strength (TS) and Elongation At Break (EAB) of the films were measured by a universal tensile tester.

As shown in FIG. 5, TS of the S-PSP and B-PSP films is significantly higher than that of the PSP and R-PSP films; EAB of S-PSP, B-PSP and R-PSP films prepared based on different heat treatment purple sweet potato powder is obviously higher than that of the PSP films. The S-PSP and B-PSP films have better mechanical properties.

TGA of the thin film was determined using a thermogravimetric analyzer. TGA and Derivative of TGA (DTG) curves were recorded using a high purity nitrogen blanket with a flow rate of 20 mL/min and a temperature range of room temperature to 700 ℃.

As shown in FIG. 6, there are 3 characteristic thermal decomposition stages in the TGA and DTG curves of the PSP, S-PSP, B-PSP and R-PSP films. The second weight loss stage (170-390 ℃) is the main pyrolysis process. The PSP and R-PSP films had maximum decomposition rates at 240 ℃ and 236 ℃, respectively. In contrast, the temperature for the maximum decomposition rate of S-PSP and B-PSP films is higher (about 254 ℃). It was shown that the S-PSP and B-PSP films have higher thermal stability than the PSP and R-PSP films.

Using Trolox as a positive control, the radical scavenging rate of 1 mg of membrane sample after reaction with 3 mL of 100. Mu. Mol/L DPPH methanol solution at 20 ℃ in the dark for 30 min was determined to calculate the antioxidant activity, and the result was expressed as. Mu. Mol of Trolox equivalent/g of membrane.

As shown in FIG. 7, the S-PSP and B-PSP films showed higher antioxidant activity.

Immersing the film in different buffers (pH 3-12) for 1 min, and recording the color change of the film; in addition, the film was placed in an ammonia (1 mol/L ammonia solution) environment for 90 min and the film color change was recorded.

As shown in FIG. 8, the color of the S-PSP and B-PSP films was significantly changed in different pH buffers and ammonia environments. The S-PSP and B-PSP films have better color change capability and have greater application potential in intelligent packaging.

The films were stored at different temperatures (4 ℃ and 25 ℃) and a relative humidity of 50% for 30 days, and the Δ of the film sample was measured every three daysEThe value is obtained.

As shown in fig. 9A and 9B, no significant color change was observed for all films at different temperatures. As shown in FIGS. 10A and 10B, all films showed only small Δ values under different storage conditionsEVariation of value (Delta at 4 ℃)E <Delta at 2,25 ℃E <3) Indicating that all films had better storage stability.

3. Example 1, example 2, example 3 and comparative example 1 were used to monitor the freshness of shrimp and the results were obtained as follows.

The film sample is pasted on the inner top of a polypropylene container (diameter is 12 cm, height is 4 cm); then, about 170 g of fresh shrimp were placed at the bottom of the container and stored at 4 ℃ for 6 days; the color of the film and the total volatile basic nitrogen (TVB-N) content of the shrimp were measured every 24 h.

The TVB-N value is a common shrimp freshness evaluation index, and when the TVB-N value is lower than 20 mg/100 g, the shrimps are fresh. During the refrigeration process of the shrimp, the TVB-N value of the shrimp is increased from the initial 5.10 mg/100 g to the final 45.75 mg/100 g; on the third day the TVB-N value of the shrimp reached 23.11 mg/100 g, indicating that the shrimp was not edible. As shown in FIG. 11, the PSP, S-PSP, B-PSP and R-PSP films all showed color change within 6 days. The reddish brown color of the PSP and R-PSP films became progressively darker with increasing storage time. The color of the S-PSP and B-PSP films changed from initially purple to bluish-purple on the third day, and thus the color change of the S-PSP and B-PSP films was more easily perceived by consumers. The results show that the freshness of the shrimps can be effectively monitored by observing the color change of the S-PSP and B-PSP films.

In conclusion, the film prepared by steaming and boiling heat treatment of the purple sweet potato powder is preferably selected, shows a more compact microstructure, and has good water vapor and oxygen barrier capability, mechanical properties, thermal stability, antioxidant activity, pH sensitivity and ammonia sensitivity. Experimental results of the film application show that the film prepared based on the purple sweet potato powder subjected to steaming and boiling heat treatment can effectively monitor the freshness of shrimps, and has application potential in the field of intelligent food packaging.

The above contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention should not be limited thereby, and any modification made on the basis of the technical idea proposed by the present invention falls within the protection scope of the claims of the present invention.

Claims

1. A method for preparing a freshness indicating film based on different heat treatment purple sweet potato powder is characterized by comprising the following steps:

(1) Respectively carrying out heat treatment of steaming, boiling and baking on the fresh purple sweet potatoes, peeling, cutting into blocks, freeze-drying, crushing and sieving to obtain heat-treated purple sweet potato powder;

2. The method of preparing a freshness indicating film based on different heat treated purple potato flour of claim 1, wherein: in the step (1), the steaming heat treatment is to put the purple sweet potatoes into a steamer to steam for 20 to 40 min; the step of the boiling heat treatment is to put the purple sweet potatoes into boiling water to be boiled for 20 to 40 min; the baking heat treatment is to bake the purple sweet potatoes in an oven at 200 to 250 ℃ for 20 to 40 min; the cutting is to cut the peeled purple sweet potatoes into square blocks of 1 cm multiplied by 1 cm; the temperature of the freeze drying is-45 to-50 ℃, and the freeze drying time is 24 to 48 hours; the crushing speed is 1500 to 2000 rpm, and the crushing time is 1 to 2 min; the mesh number of the screened screen is 100 to 150 meshes.

3. The method of preparing a freshness indicating film based on different heat treatments of purple potato flour according to claim 1, wherein: in the step (2), the homogenizing speed of the high-speed disperser is 3000 to 5000 rpm, and the homogenizing time is 2 to 4 min; the gelatinization temperature of the water bath kettle is 80-100 ℃, and the gelatinization time is 10-20 min.

4. The method of preparing a freshness indicating film based on different heat treated purple potato flour of claim 1, wherein: in the step (3), the mass ratio of the purple sweet potato powder to the sodium alginate in the purple sweet potato powder-sodium alginate mixed solution is 5 to 9, and the concentration of the purple sweet potato powder is 30 to 50 mg/mL.

5. The method of preparing a freshness indicating film based on different heat treatments of purple potato flour according to claim 1, wherein: in the step (4), the addition amount of the glycerol is 0.5-0.8% of the total volume of the film forming solution;the above-mentionedThe rotating speed of the magnetic stirrer is 800 to 1000 rpm, and the stirring time is 60 to 90 min.

6. The method of preparing a freshness indicating film based on different heat treated purple potato flour of claim 1, wherein: in the step (5), the size of the organic glass mold is 20-30 cm multiplied by 20-30 cm; when the film forming solution is subjected to humidity control air drying, the humidity control air drying condition is that the relative humidity is 40% -50%, the temperature is 30-40 ℃, and the humidity control air drying time is 24-48 h.

7. Different heat treatment based purple potato flour freshness indicating film prepared by the method of any one of claims 1-6.