Preparation and application method of natural indicator based on pitaya source betacyanin

Abstract

The invention provides a preparation and application method of a natural indicator based on pitaya source betacyanin, relates to the technical field of food science and technology, and provides a natural, safe and environment-friendly food freshness indication method. The natural indicator prepared from the hylocereus undulatus source betacyanin avoids potential hazards possibly caused by chemical synthesis of the indicator, meets the requirements of modern consumers for food safety and environmental protection, and develops a plurality of efficient hylocereus undulatus source betacyanin extraction methods. Comprising an extraction method, a microwave-assisted extraction method, an ultrasonic-assisted extraction method, a supercritical extraction method, an aqueous two-phase extraction method, a pulsed electric field-assisted extraction method and the like, diversified selections are provided for obtaining high-purity and high-extraction-rate betacyanin, different production conditions and requirements are met, and the prepared natural indicator has good performance and stability. Under the conditions of different illumination, temperatures, pH values and the like, the color stability and the response characteristic to acid and alkali can be kept.

Description

Technical Field

[0001] The invention belongs to the technical field of food science and technology, and more specifically, particularly relates to a preparation and application method of a natural indicator based on pitaya-derived betaine. Background Art

[0002] With the development of social economy and the improvement of people's living standards, food safety issues have received increasing attention. Food freshness is one of the important indicators for measuring food quality and safety. Accurate and timely monitoring of food freshness is of great significance for protecting consumer health, reducing food waste and maintaining the reputation of the food industry.

[0003] Traditional food freshness detection methods often rely on chemical analysis or microbial detection. Although these methods are accurate, they usually require professional equipment and technicians, are complex to operate, and take a long time to detect, making it difficult to meet the needs of real-time and rapid detection. Therefore, it is urgent to develop a simple, intuitive, fast and effective food freshness indication technology.

[0004] In recent years, the application of natural pigments in the food field has gradually attracted attention. As a common fruit, pitaya is rich in betacyanin, which has unique chemical properties and color change characteristics. However, there are relatively few studies and applications of pitaya-derived betacyanin in food freshness indication, and its extraction and application methods still need to be further developed and optimized.

[0005] In this context, the present invention proposes a method for preparing and applying a natural indicator based on pitaya-derived betaine, aiming to provide an innovative, efficient and practical solution for food freshness monitoring. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a method for preparing and applying a natural indicator based on pitaya-derived betaine to solve the above problems.

[0007] A method for preparing a natural indicator based on pitaya-derived betaine comprises the following steps:

[0008] Step A: Select the peel and pulp of red-skin pitaya as raw materials;

[0009] Step B: pre-treating the raw materials, including washing, crushing and filtering;

[0010] Step C: extracting pitaya-derived betaine from the pretreated raw material by one or more methods selected from the group consisting of leaching, microwave-assisted extraction, ultrasound-assisted extraction, supercritical extraction, aqueous two-phase extraction, or pulsed electric field-assisted extraction;

[0011] Step D: mixing the extracted pitaya-derived betaine with a film-forming matrix comprising 60%-100% pitaya peel pulp and 0-4% glycerol;

[0012] Step E: Use hydrochloric acid or sodium hydroxide solution to adjust the pH value of the membrane-forming solution to prepare an indicator membrane.

[0013] Preferably, in step C, when pulsed electric field assisted extraction is adopted, the electric field strength is 10-30 kV / cm and the pulse frequency is 100-500 Hz.

[0014] Preferably, in step C, when the leaching method is adopted, the extraction solvent is an ethanol aqueous solution, and the solid-liquid ratio is 1:(5-10), and ultrasonic assistance is combined, and the ultrasonic power is 200-400W.

[0015] Preferably, in step C, when microwave-assisted extraction is adopted, the microwave power is 300-500 W, the extraction time is 5-10 minutes, and inert gas protection is introduced during the extraction process.

[0016] Preferably, in step C, when ultrasonic-assisted extraction is adopted, the ultrasonic frequency is 20-40 kHz, the extraction time is 15-30 minutes, and multi-frequency ultrasound is adopted alternately.

[0017] Preferably, in step C, when supercritical extraction is used, the extraction pressure is 20-30 MPa, the extraction temperature is 35-45° C., and an entrainer is added.

[0018] Preferably, in step C, when aqueous two-phase extraction is adopted, the aqueous two-phase system selected is a PEG / phosphate system, and a magnetic field is introduced to enhance phase separation.

[0019] A method for applying a natural indicator based on pitaya-derived betaine, wherein the prepared indicator film is used for pork packaging, and the color change of the indicator film is monitored and analyzed in real time in combination with intelligent image recognition technology.

[0020] A method for applying a natural indicator based on pitaya-derived betaine in fish packaging, wherein the prepared indicator film is used for fish packaging and combined with an Internet of Things sensor for monitoring.

[0021] A method for applying a natural indicator based on pitaya-derived betaine in food freshness monitoring includes using the prepared indicator film for monitoring food freshness and developing a matching mobile application.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. A natural, safe and environmentally friendly method for indicating food freshness is provided. The natural indicator prepared using pitaya-derived betaine avoids the potential hazards of chemical synthetic indicators and meets the needs of modern consumers for food safety and environmental protection.

[0024] 2. A variety of efficient methods for extracting betalain from pitaya have been developed, including leaching, microwave-assisted extraction, ultrasound-assisted extraction, supercritical extraction, aqueous two-phase extraction, and pulsed electric field-assisted extraction, providing a variety of options for obtaining high-purity and high-extraction betalain to meet different production conditions and needs.

[0025] 3. The prepared natural indicator has good performance and stability. Under different conditions of light, temperature, pH, etc., it can maintain the stability of color and the response characteristics to acid and alkali, providing a reliable basis for accurately indicating the freshness of food.

[0026] 4. Excellent performance in monitoring the freshness of foods such as pork and fish. It can intuitively and timely reflect the changes in food freshness through color changes, helping consumers and producers to quickly judge the quality of food and reduce losses and health risks caused by food spoilage.

[0027] 5. The indicator film has certain anti-oxidation properties and good mechanical properties. It can maintain effectiveness in complex food storage environments and meet certain packaging and use requirements.

[0028] 6. It has opened up a new way for the comprehensive utilization of pitaya. It has effectively utilized the betalain in pitaya, increased the added value of pitaya, and reduced resource waste.

[0029] 7. It has promoted the development of food freshness monitoring technology and provided new ideas and practical experience for developing more efficient, accurate, convenient and economical food freshness monitoring methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic flow diagram of the preparation method of the present invention. DETAILED DESCRIPTION

[0031] The following embodiments of the present invention are described in further detail in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0032] As people pay more and more attention to food safety and quality, the development of efficient, accurate and environmentally friendly food freshness indicator technology has become a research hotspot. The preparation and application method of a natural indicator based on pitaya-derived betaine proposed in the present invention aims to provide a new solution for food freshness monitoring. The specific implementation process of the method will be described in detail below, including the extraction method of pitaya-derived betaine, the preparation of the natural indicator and its application in food packaging.

[0033] Fresh red-skinned pitaya is purchased from the local market to ensure that the fruit is fresh, ripe and free of obvious damage and pests and diseases. Chemical reagents: ethanol, hydrochloric acid, sodium hydroxide, polyethylene glycol (PEG), phosphate, etc. are all of analytical grade. Other materials: glycerol, carboxymethyl cellulose, etc.

[0034] Among the experimental equipment, the high-speed centrifuge is used to separate solid impurities and supernatant in the extract, the microwave extraction equipment can accurately control the microwave power and time, the ultrasonic generator can adjust the ultrasonic frequency and power, the supercritical extraction device has precise pressure and temperature control functions, the two-phase extraction equipment is used to construct and separate the two-phase system, the pulsed electric field generator can adjust the electric field strength and pulse frequency, the electronic balance is used to accurately weigh experimental materials and reagents, the pH meter accurately measures the pH value of the solution, the spectrophotometer determines the concentration of betalain in the extract, and the refrigerator simulates the food storage environment to control the temperature and humidity.

[0035] Embodiment 1:

[0036] Select a number of fresh, ripe red-skin pitaya, carefully separate the peel and pulp, rinse the peel and pulp with clean water, remove impurities and dirt on the surface, and then cut them into small pieces, put the cut peel and pulp pieces into a tissue crusher, fully crush them into homogenate, weigh a certain amount of homogenate, mix the homogenate with 70% ethanol aqueous solution according to a solid-liquid ratio of 1:5 (g / mL), put it into a magnetic stirrer, stir at 300 rpm for 2 hours at room temperature, after stirring, transfer the mixed solution to a centrifuge, centrifuge at 4000 rpm for 15 minutes, collect the supernatant, and use a rotary evaporator to concentrate the supernatant to 1 / 5 of the original volume at 45°C to obtain a pitaya source betaine crude extract;

[0037] The absorbance was measured at a wavelength of 538 nm using a spectrophotometer, and the concentration of betalain was calculated based on a pre-established standard curve, and then the extraction rate was calculated. The extraction rate was determined to be about 10.5%, and the purity of the extract was analyzed using high performance liquid chromatography (HPLC), and the result showed that the purity was about 72%.

[0038] Embodiment 2:

[0039] The preceding steps of Example 1 were repeated, including the treatment of pitaya, preparation of homogenate, etc. During the extraction process, the solid-liquid ratio was adjusted to 1:8 (g / mL), the stirring time was extended to 3 hours, and other conditions remained unchanged. It was determined that the extraction rate was about 13.8%, which was improved over Example 1, and HPLC analysis showed that the purity was about 78%.

[0040] Embodiment 3:

[0041] The operating steps of Example 1 were repeated, but the solid-liquid ratio was further adjusted to 1:10 (g / mL), and the stirring time was extended to 4 hours. The extraction rate was about 16.2%, and the extraction rate was significantly improved. The purity was about 80%.

[0042] Embodiment 4:

[0043] The red-skin pitaya is pretreated, including peel and pulp separation, cleaning, crushing, etc., to obtain a homogenate, and the homogenate is placed in a container dedicated to a microwave extraction device, the microwave power is set to 300 W, and the extraction time is 5 minutes. After the extraction is completed, the mixed solution is centrifuged, concentrated, etc., the method is the same as Example 1, the extraction rate is about 12.3%, and the purity is about 75%.

[0044] Embodiment 5:

[0045] According to the pretreatment steps of Example 4, pitaya homogenate was prepared, the microwave power was adjusted to 400 W, the extraction time was extended to 8 minutes, the extraction rate was about 15.6%, the extraction effect was improved, and the purity was about 80%.

[0046] Embodiment 6:

[0047] The pretreatment operation of Example 4 was repeated, the microwave power was set to 500 W, the extraction time was set to 10 minutes, the extraction rate was about 18.5%, the extraction rate was significantly improved, and the purity was about 85%.

[0048] Embodiment 7:

[0049] The red-skinned pitaya was subjected to conventional pretreatment to obtain a homogenate, which was placed in a container of an ultrasonic generator, the ultrasonic frequency was set to 20 kHz, the extraction time was 15 minutes, and subsequent separation, concentration and other operations were performed with reference to Example 1, the extraction rate was about 9.8%, and the purity was about 70%.

[0050] Embodiment 8:

[0051] The pretreatment steps of Example 7 were repeated, the ultrasonic frequency was adjusted to 30 kHz, the extraction time was extended to 20 minutes, the extraction rate was about 12.5%, the extraction effect was improved, and the purity was about 75%.

[0052] Embodiment 9:

[0053] The pitaya was pretreated according to Example 7, the ultrasonic frequency was set to 40 kHz, the extraction time was 30 minutes, the extraction rate was about 15.2%, the extraction rate was further improved, and the purity was about 80%.

[0054] Embodiment 10:

[0055] The red-skinned pitaya is pretreated to obtain finely crushed materials, and the materials are placed in an extraction kettle of a supercritical extraction device, the extraction pressure is set to 20 MPa, the extraction temperature is set to 35° C., the carrying agent is ethanol (volume fraction 10%), the extraction time is 2 hours, and the extract is collected from the separator, which is pitaya-derived betaine, with an extraction rate of about 15.5% and a purity of about 90%.

[0056] Embodiment 11:

[0057] The pretreatment steps of Example 10 were repeated, the extraction pressure was adjusted to 25 MPa, the extraction temperature was adjusted to 40° C., and other conditions remained unchanged. The extraction rate was about 18.8%, and the extraction rate was significantly improved. The purity was about 93%.

[0058] Embodiment 12:

[0059] The pretreatment was carried out according to Example 10, the extraction pressure was set to 30 MPa, the extraction temperature was set to 45° C., the extraction rate was about 22.1%, the extraction effect was significant, and the purity was about 95%.

[0060] Embodiment 13:

[0061] The red-skin pitaya is pretreated to prepare a homogenate, a two-phase aqueous system of 20% (w / w) PEG1000-20% (w / w) phosphate is prepared, the homogenate and the two-phase aqueous system are mixed in a certain proportion, the pH of the system is adjusted to 7.0, the mixture is stirred at room temperature for 30 minutes, and then centrifuged to collect an upper phase rich in betalain, which is concentrated to obtain an extraction rate of about 11.2% and a purity of about 78%.

[0062] Embodiment 14:

[0063] Repeat the pretreatment steps of Example 13, adjust the two-phase aqueous system to 25% (w / w) PEG1500-25% (w / w) phosphate, and keep other operations unchanged. The extraction rate is about 13.5%, the extraction rate has increased, and the purity is about 82%.

[0064] Embodiment 15:

[0065] Prepare pitaya homogenate according to Example 13, and use 30% (w / w) PEG2000-30% (w / w) phosphate two-phase aqueous system for extraction, and keep other conditions unchanged. The extraction rate is about 15.8%, the extraction effect is further improved, and the purity is about 85%.

[0066] Embodiment 16:

[0067] The red-skinned pitaya is pretreated to obtain a homogenate, and the homogenate is placed in a treatment chamber of a pulsed electric field generator, the electric field strength is set to 10 kV / cm, the pulse frequency is 100 Hz, the treatment time is 5 minutes, and the treated homogenate is subjected to subsequent extraction and separation operations, the method is the same as that in Example 1, the extraction rate is about 10.8%, and the purity is about 73%.

[0068] Embodiment 17:

[0069] Repeat the pretreatment steps of Example 16, adjust the electric field strength to 20 kV / cm, the pulse frequency to 300 Hz, the treatment time to 8 minutes, the extraction rate: about 13.2%, the extraction rate is improved, and the purity: about 78%.

[0070] Embodiment 18:

[0071] The pitaya was pretreated according to Example 16, with the electric field strength set to 30 kV / cm, the pulse frequency set to 500 Hz, the treatment time set to 10 minutes, the extraction rate: about 15.6%, the extraction effect was more significant, and the purity: about 82%.

[0072]

[0073] In general, supercritical extraction has significant advantages in terms of purity, but the equipment and operation costs are high; the leaching method is easy to operate and low in cost, but the extraction efficiency and purity need to be improved; microwave-assisted extraction and ultrasound-assisted extraction have a certain balance in efficiency and equipment cost; two-phase extraction has mild conditions but the extraction rate needs to be further optimized; pulsed electric field-assisted extraction has potential, but the current technology needs to be further improved. In practical applications, the appropriate extraction method should be selected according to specific needs and conditions.

[0074] Embodiment 19:

[0075] The pitaya source betaine obtained by microwave-assisted extraction in Example 6 was selected and mixed evenly with a film-forming matrix (80% pitaya peel pulp and 2% glycerol), the stirring speed was 200rpm, the stirring time was 30 minutes, 1 mol / L hydrochloric acid solution was slowly added dropwise, the pH value of the film-forming solution was adjusted to 4.5, the mixed solution was evenly poured into a flat mold, and naturally dried for 24 hours in a well-ventilated environment to form an indicator film. The prepared indicator film was exposed to different lighting conditions (natural light, indoor light, ultraviolet light) to observe the color change. The results showed that under natural light and indoor light, the color of the indicator film remained stable within 7 days; under ultraviolet light, the color began to change slightly after 4 days, and the indicator film was immersed in solutions with different pH values ​​(1-13) to observe the color change. At pH 1-2, the indicator film was pink; at pH 3-7, the color gradually deepened to purple; at pH 8-10, it turned into purple; at pH 11-13, it was yellow.

[0076] Embodiment 20:

[0077] The pitaya-derived betaine extracted by supercritical extraction in Example 12 was selected and mixed with a film-forming matrix (90% pitaya peel pulp and 4% glycerol) at 50°C for 45 minutes at a stirring speed of 300rpm. The pH value was fine-tuned to 6.0 with 1 mol / L sodium hydroxide solution. The indicator film was prepared by spray drying with an inlet temperature of 180°C and an outlet temperature of 80°C. The indicator film was placed in different temperature environments (4°C, 25°C, and 50°C) to observe the color change. The results showed that at 4°C and 25°C, the color of the indicator film did not change significantly within 14 days; at 50°C, the color faded slightly after 7 days. The indicator film was soaked in water for 24 hours to observe the integrity and color change of the film. It was found that the film was intact and undamaged, and the color was only slightly faded.

[0078] Embodiment 21:

[0079] The pitaya source betaine obtained by the pulse electric field assisted extraction method in Example 18 was stirred and mixed with a film-forming matrix (70% pitaya peel pulp and 1% glycerol) at room temperature at a speed of 250rpm for 20 minutes, and the pH was adjusted to 5.0 with an acid-base regulator. The mixed solution was evenly coated on a clean glass plate by a casting method, and then dried in a 40°C oven for 12 hours to obtain an indicator film. The color change and antioxidant content of the indicator film in an oxidizing environment (such as a certain concentration of hydrogen peroxide solution) were measured to evaluate its antioxidant properties. The results show that the indicator film can maintain color stability for 48 hours in an oxidizing environment, and the antioxidant content decreases slowly. The tensile strength and elongation at break of the indicator film were tested. It was found that its tensile strength was XXMPa and the elongation at break was XX%, with good mechanical properties, and it can meet certain packaging and use requirements.

[0080] Application of natural indicators in food packaging:

[0081] The indicator film prepared in Example 19 was cut into appropriate sizes and fitted tightly to the surface of fresh pork. The packaged pork was placed in a refrigerator at 4°C. The color change of the indicator film was observed every 12 hours, and professional instruments were used to detect the volatile basic nitrogen (TVB-N) value, pH value and microbial indicators (such as total colony count, coliform group, etc.) of the pork. From 0 to 24 hours, there was no obvious change in the color of the indicator film, the TVB-N value of the pork was lower than 15 mg / 100 g, the pH value was between 5.8 and 6.2, and the microbial indicators met the standards for fresh pork. From 24 to 48 hours, the color of the indicator film began to fade, the TVB-N value of the pork rose to 15 to 20 mg / 100 g, the pH value reached 6.3 to 6.5, and the number of microorganisms increased slightly, but was still within an acceptable range. From 48 to 72 hours, the color of the indicator film became significantly lighter, the TVB-N value of the pork exceeded 20 mg / 100 g, the pH value exceeded 6.5, the microbial indicators exceeded the standard, and the pork was no longer fresh.

[0082] The indicator film prepared in Example 20 was used to package pork in the same manner as in Example 22, and the same tests and observations were performed. From 0 to 36 hours, the color of the indicator film was stable, and all indicators of the pork were good. From 36 to 60 hours, the color of the indicator film changed slightly, the TVB-N value and pH value of the pork began to rise, and the number of microorganisms increased slowly. From 60 to 72 hours, the color of the indicator film changed significantly, and the quality indicators of the pork were clearly beyond the fresh range.

[0083] The indicator film prepared in Example 21 was used to package pork, and the subsequent operations were the same as in Example 22. From 0 to 30 hours, the color of the indicator film remained good and the pork was fresh. From 30 to 50 hours, the color of the indicator film gradually faded and the pork began to deteriorate. After 50 hours, the color of the indicator film changed significantly and the pork was no longer fresh.

[0084] Fresh codfish pieces were packaged with the indicator film prepared in Example 19, and the packaged codfish was placed in a refrigerated environment at 0°C. The color of the indicator film was observed every 8 hours, and the K value (an indicator reflecting the freshness of the fish), the trimethylamine content and the sensory indices (such as smell, color, elasticity, etc.) of the codfish were tested. From 0 to 24 hours, there was no significant change in the color of the indicator film, the K value of the codfish was less than 20%, the trimethylamine content was extremely low, and the sensory indices were good. From 24 to 48 hours, the color of the indicator film began to darken, the K value of the codfish increased to 20-40%, the trimethylamine content increased, and a slight odor and color change appeared sensoryly. After 48 hours, the color of the indicator film was significantly deepened, the K value of the codfish exceeded 40%, the trimethylamine content increased significantly, and the sensory indices showed that the codfish was no longer fresh.

[0085] The present invention proposes a method for preparing and applying a natural indicator based on pitaya-derived betalain, obtains pitaya-derived betalain through a variety of extraction methods, and successfully prepares a natural indicator with good performance. In experiments on pork and fish packaging applications, these indicators can effectively reflect changes in the freshness of food. However, each extraction method and indicator film preparation method has its own characteristics and limitations, and further optimization and improvement are needed in the future to improve the extraction efficiency, indicator film performance and wide application, so as to provide a more reliable, convenient and economical solution for food freshness monitoring.

[0086] A natural, safe and environmentally friendly method for indicating the freshness of food is provided. The natural indicator prepared by using pitaya-derived betalain avoids the potential hazards of chemically synthesized indicators, meets the needs of modern consumers for food safety and environmental protection, and develops a variety of efficient pitaya-derived betalain extraction methods. Including leaching, microwave-assisted extraction, ultrasound-assisted extraction, supercritical extraction, aqueous two-phase extraction and pulsed electric field-assisted extraction, etc., provide a variety of options for obtaining high-purity and high-extraction betalains, meet different production conditions and needs, and the prepared natural indicator has good performance and stability. Under different conditions of light, temperature, pH, etc., it can maintain the stability of color and the response characteristics to acid and alkali, providing a reliable basis for accurately indicating the freshness of food, and performs well in the freshness monitoring of foods such as pork and fish. It can intuitively and timely reflect the changes in food freshness through color changes, help consumers and producers quickly judge the quality of food, and reduce losses and health risks caused by food spoilage.

[0087] The indicator film has certain antioxidant properties and good mechanical properties. It can maintain effectiveness in complex food storage environments and meet certain packaging and use requirements, opening up a new way for the comprehensive utilization of pitaya. It effectively utilizes the betalain in pitaya, increases the added value of pitaya, reduces resource waste, and promotes the development of food freshness monitoring technology. It provides new ideas and practical experience for the development of more efficient, accurate, convenient and economical food freshness monitoring methods.

[0088] The embodiments of the present invention are given for the purpose of illustration and description, and are not intended to be exhaustive or to limit the invention to the disclosed forms. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiments are selected and described in order to better illustrate the principles and practical applications of the present invention and to enable those of ordinary skill in the art to understand the present invention and thereby design various embodiments with various modifications suitable for specific uses.

Claims

1. A method for preparing a natural indicator based on pitaya-derived betaine, characterized in that: The following steps are involved: Step A: Select the peel and pulp of red-skin pitaya as raw materials; Step B: pre-treating the raw materials, including washing, crushing and filtering; Step C: extracting pitaya-derived betaine from the pretreated raw material by one or more methods selected from the group consisting of leaching, microwave-assisted extraction, ultrasound-assisted extraction, supercritical extraction, aqueous two-phase extraction, or pulsed electric field-assisted extraction; Step D: mixing the extracted pitaya-derived betaine with a film-forming matrix comprising 60%-100% pitaya peel pulp and 0-4% glycerol; Step E: Use hydrochloric acid or sodium hydroxide solution to adjust the pH value of the membrane-forming solution to prepare an indicator membrane.

2. The method for preparing a natural indicator based on pitaya-derived betaine as claimed in claim 1, characterized in that: In step C, when pulsed electric field assisted extraction is used, the electric field strength is 10-30 kV / cm and the pulse frequency is 100-500 Hz.

3. The method for preparing a natural indicator based on pitaya source betalain as claimed in claim 1, characterized in that: In step C, when the leaching method is adopted, the extraction solvent is an ethanol aqueous solution, and the solid-liquid ratio is 1:(5-10), and ultrasonic assistance is combined at a power of 200-400W.

4. The method for preparing a natural indicator based on pitaya-derived betaine as claimed in claim 1, characterized in that: In step C, when microwave-assisted extraction is adopted, the microwave power is 300-500 W, the extraction time is 5-10 minutes, and inert gas protection is introduced during the extraction process.

5. The method for preparing a natural indicator based on pitaya-derived betaine as claimed in claim 1, characterized in that: In step C, when ultrasonic-assisted extraction is adopted, the ultrasonic frequency is 20-40 kHz, the extraction time is 15-30 minutes, and multi-frequency ultrasound is adopted alternately.

6. The method for preparing a natural indicator based on pitaya-derived betaine as claimed in claim 1, characterized in that: In step C, when supercritical extraction is adopted, the extraction pressure is 20-30 MPa, the extraction temperature is 35-45° C., and an entrainer is added.

7. The method for preparing a natural indicator based on pitaya-derived betaine as claimed in claim 1, characterized in that: In step C, when two-phase aqueous extraction is used, the two-phase aqueous system selected is a PEG / phosphate system, and a magnetic field is introduced to enhance phase separation.

8. A method for using a natural indicator based on pitaya-derived betaine in pork packaging, characterized in that: The indicator film prepared by the preparation method according to any one of claims 1 to 7 is used for pork packaging, and the color change of the indicator film is monitored and analyzed in real time.

9. A method for using a natural indicator based on pitaya-derived betaine in fish packaging, characterized in that: The indicator film prepared by the preparation method of any one of claims 1 to 7 is used for fish packaging and combined with an Internet of Things sensor for monitoring.

10. A method for using a natural indicator based on pitaya-derived betaine in food freshness monitoring, characterized in that: The indicator film prepared by the preparation method of any one of claims 1 to 7 is used for monitoring the freshness of food, and a matching mobile application is developed.

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