CN114009489A

CN114009489A - Fresh-keeping method for fresh-cut apples

Info

Publication number: CN114009489A
Application number: CN202111254066.XA
Authority: CN
Inventors: 刘翠华; 王涣灵; 李岑; 任小林; 田林平
Original assignee: Northwest A&F University
Current assignee: Northwest A&F University
Priority date: 2021-10-27
Filing date: 2021-10-27
Publication date: 2022-02-08

Abstract

The invention relates to a fresh-cut apple preservation method, belonging to the technical field of fruit preservation, comprising the following steps: (1) soaking the fresh-cut apples in an inulin solution; (2) sealing the fresh-cut apples in a fresh-keeping box and storing at 4°C. The results showed that when the concentration of inulin was 1.25%, the preservation effect of fresh-cut apples was the best. On this basis, combined with 50℃, 2min heat treatment or 1.0% vitamin C treatment, the fresh-cut apples could be improved to a certain extent. The preservation effect of apples.

Description

Fresh-keeping method for fresh-cut apples

Technical Field

The invention relates to a fresh-cut apple preservation method, and belongs to the technical field of fruit preservation.

Background

Under the current fast-paced life style, fresh-cut fruits and vegetables are a fruit and vegetable processing mode which is very popular with wide consumers. The fruit and vegetable juice has portability and the characteristics of keeping the original taste, flavor and nutritive value of the fruit and vegetable, and is very in line with the modern market value. However, many fruits are easy to brown after being cut, and the commodity value of the fruits is seriously influenced. The reason is that after the fruits and vegetables are subjected to fresh-cut treatment, tissues of the fruits and vegetables are damaged, cell membranes and subcellular structures are damaged, and cell walls and tissues of the fruits and vegetables are degraded and browned by hydrolase, oxidase and the like, so that the original quality of the fruits and vegetables is influenced, nutrition loss is caused, even peculiar smell is generated, and the edible value and the commercial value of the fruits and vegetables are finally lost.

The apple (Malus domestica) and Malus of Maloideae of Rosaceae are rich in minerals and vitamins, and have high nutritive value. However, when the apple is cut, the flesh is easily browned, because mechanical injury causes damage to cell structures including cell membranes and cell partitions, a large amount of hydrolytic enzymes, oxidative enzymes and the like in cells are released, and active substances such as phenolic substances and the like are easily oxidized under the action of the enzymes, so that the flesh is browned. Briefly, the plasma membrane is a barrier between living cells and the outside, and the rupture of the plasma membrane caused by fresh cutting leads to the disappearance of the barrier, which leads to the imbalance of the active oxygen metabolism of the cells and the disturbance of normal physiological metabolism.

Inulin is a plant natural polysaccharide, mainly originating from the family of Compositae and Campanulaceae among dicotyledonous plants, and the family of Gramineae and Liliaceae among monocotyledonous plants. The inulin is rich in dietary fiber and prebiotics, has low calorie, and can be used for preventing obesity, regulating blood sugar, reducing blood lipid, and promoting mineral absorption. With the increasing living standard of the public, the diet concept not only requires taste and safety, but also pays attention to foods beneficial to the body, such as low sugar, low fat, high dietary fiber, and the like. In 2003, inulin was confirmed by the U.S. food and drug administration as a generally recognized safe substance, and the daily effective intake amount reached 5g, and the recommended intake amount could not exceed 20 g. Inulin is formally introduced into China in 5 months in 2009, is approved as a new resource food in the sanitation law of the people's republic of China and the safety management method of new resource food, can replace fatty sugar for food processing, and has good physiological functions, wide sources and simple production process. Inulin has good application prospect and development potential, and as a functional component, the inulin can be added into various health-care foods except infant foods, but the main consumer groups and target clients of fresh-cut fruits are office workers, students and other groups with fast life rhythm, and do not include infants. Currently, inulin is widely used in milk tablets, milk powder, daily drinks and herbal tea for the middle-aged and the elderly, and various imported milk powder is also added with the inulin.

The existing fresh-cut fruit and vegetable mode comprises:

1. fresh-keeping coating film

The fresh-keeping means that a layer of macromolecular liquid film is coated on the outer side of the fruit, so that the exchange of gas between the liquid film and the fruit and the outside is limited, the water evaporation is inhibited, and a low-O film is created₂And CO₂The micro environment of the fresh-cut fruits and vegetables can store the fruits and prevent the normal respiratory propagation of microorganisms, thereby having good protection effect on the color, texture, smell and nutritive value of the fresh-cut fruits and vegetables. The material can be composed of protein, polysaccharide, fat and other functional active ingredient materials, and can be prepared by taking xanthan gum and the like as a thickening agent and matching with glycerol and other plasticizers. Compared with expensive protein materials, polysaccharide coating materials are more popular, such as chitosan, sodium alginate, konjac glucomannan and the like, and the research is the most at present. The coating preservation treatment is carried out on chitosan or sodium alginate with proper concentration, the phenomenon of dehydration and shrinkage of the pulp of the fruit during the storage period can be effectively inhibited, and simultaneously, the browning and the nutrient content reservation are effectively inhibited, the hardness of the fruit is favorably maintained, so that the effect of prolonging the storage period is achieved, and the like. For example, it has been found that the best results are obtained when 1.3% sodium alginate, 1.99% glycerol and 0.26% tea tree oil are combined to treat fresh-cut apples.

2. Cold preservation and fresh preservation

The method is the most important and common method, and can effectively inhibit the respiratory intensity of fresh-cut fruits, control the physiological and biochemical reaction rate, delay aging, inhibit browning and inhibit the growth and reproduction of microorganisms.

3. Heat treatment for fresh-keeping

The heat treatment is a common treatment method for controlling the rot of the picked fruits, the proper treatment time and treatment temperature do not influence various quality indexes of the fruits, and the effect of green prevention and control on the diseases after picking can be achieved.

The method usually treats the fruits under a high-temperature condition for a short time or a low-temperature condition for a long time so as to kill or inactivate various pathogenic bacteria causing rot, control the activity of the enzymes, regulate the metabolism of the fruits in the aspects of physiology and biochemistry, promote the stress resistance of the fruits and finally achieve the effects of storage and fresh keeping.

4. Moisture preserving box

Water loss is one of the major causes of fruit loss and waste. In the storage, shelf life and consumption links, the water loss rate can be greatly reduced by using the preservation box for packaging, the quality of the fresh fruits is kept, and the storage life of the fruits is prolonged. It is worth noting that steam is easy to condense in the preservation box from refrigeration to room temperature, and the condensed water drops are usually weakly acidic and are beneficial to the growth and propagation of microorganisms, so that the condensation phenomenon needs to be prevented.

5. Modified Atmosphere Package (MAP)

The principle of modified atmosphere packaging is to create an environment with low oxygen and high carbon dioxide in the package, so that the respiration of the product and the air permeability of the film are balanced. The controlled atmosphere packaging requires attention to the selection of materials, and in addition, the size and thickness of the box also influence the physiological metabolism of the fruits, thereby influencing the length of the storage period.

6. Preservatives and preservatives

Common preservative preservatives include citric acid, sulfite, potassium sorbate, vitamin C, zinc chloride, Ethylene Diamine Tetraacetic Acid (EDTA), sodium benzoate, calcium lactate, and the like. Research shows that some compounds such as kojic acid, ascorbic acid and the like can effectively inhibit the browning of fresh-cut apples. Adults should ingest 100mg of vitamin C every day, but too much easily causes damage to blood, nerves, kidneys, digestive system, etc., and thus cannot abuse it. Natural preservatives have also begun to be used in the processing of fresh cut fruits such as herbs, essential oils, spices and the like from plants, lysozyme and the like from animals, nisin, bacteriocins and the like from microorganisms.

Fresh fruits and vegetables are an essential part in healthy diet, and can provide necessary vitamins and trace elements for human bodies. However, after the fruits are cut and peeled, the flesh tissues of the fruits can be damaged to induce respiration damage, and the ethylene release, the synthesis and conversion of metabolites such as phenols and the like can also be influenced, such as polyphenol oxidase (PPO) for catalyzing the oxidation of phenols, and the activity of cellulase for catalyzing the decomposition of cell walls can be obviously improved, so that the browning of tissues, the generation of peculiar smell, the destruction of cell membranes, the degradation of cell walls and the like can be caused. At the same time, the content of some nutrient components, especially vitamins, is remarkably reduced, which seriously damages the nutritional value of the fruits. Of course, the damage of the pulp tissue and the outflow of nutrients also provide favorable conditions for the growth and propagation of microorganisms, resulting in the occurrence of infectious diseases, ultimately resulting in economic losses and potential health hazards.

On the other hand, with the continuous improvement of the living standard of people, the rhythm of life and work is continuously accelerated, and the demand of consumers for fresh-cut fruits which are rich in nutrition and convenient to carry is increasing. The fresh-cut fruits are subjected to the treatments of raw material selection, cleaning and disinfection, peeling, cutting and trimming, rinsing and color protection, draining, film coating, packaging, storage and the like, and can be directly eaten without being treated after being purchased by a consumer; the fresh-cut fruits have less nutrition damage and convenient eating, and are deeply favored by consumers in countries and regions with fast pace of life, such as Europe, America, Japan, and the like as an important form of modern fruit sales.

The survey of the early market price shows that the chitosan is 278 and 440 yuan/kg, the sodium alginate is about 98 yuan/kg, the konjac glucomannan is 90-150 yuan/kg, and the inulin is only 32-54 yuan/kg. And inulin has better health care effect on human body than other materials, and if the fresh-keeping effect of inulin is equivalent to that of the traditional coating fresh-keeping method, inulin plays a great role in the field of coating fresh-keeping.

At present, fresh-cut fruits are still in the beginning stage in China, and the fresh-cut fruit industry is in the spotlight. In order to solve the contradiction between the postharvest problem of the fresh-cut fruits and the demand of the people in China on the fresh-cut fruits, the invention aims to prolong the postharvest shelf life of the fresh-cut fruits by combining the inulin film coating treatment technology with heat treatment and vitamin C treatment.

Disclosure of Invention

The technical problem to be solved by the invention is to provide a technology for prolonging the postharvest shelf life of fresh-cut apples and solving the contradiction between the postharvest shelf life of the fresh-cut apples and the demand of people in China on the fresh-cut fruits by combining heat treatment and vitamin C treatment with an inulin film coating treatment technology.

The technical scheme for solving the technical problems is as follows: a fresh-cut apple preservation method comprises the following steps: (1) soaking fresh cut apples in an inulin solution; (2) sealing with a fresh-keeping box and storing at 4 deg.C.

The invention has the beneficial effects that:

on the basis of the technical scheme, the invention can be further improved as follows.

Further, the mass percent of the inulin solution is 0.5% -1.5%.

Further, the mass percent of the inulin solution is 1.05% -1.35%.

Further, the mass percentage of the inulin solution is 1.25% or 1.15%.

Further, before the step (1), a heat treatment step is further performed: soaking fresh-cut apples in hot water at 30-60 deg.C for 1-2 min.

Further, the heat treatment step specifically comprises: soaking fresh-cut fructus Mali Pumilae in 50 deg.C hot water for 2 min.

Further, the inulin solution contains 0.5 to 2 mass percent of vitamin C.

Further, the inulin solution in the step (1) contains 1% by mass of vitamin C.

The invention has the beneficial effects that: the inulin has good fresh-keeping effect on fresh-cut apples, and is lower in price compared with the existing preservatives and preservatives, so that the cost can be saved, and the inulin is harmless to human bodies.

The result shows that the fresh-keeping effect of the fresh-cut apples is best when the inulin concentration is 1.25%, and on the basis, the fresh-keeping effect of the fresh-cut apples can be improved to a certain extent by combining heat treatment at 50 ℃ for 2min or treatment with 1.0% of vitamin C.

Drawings

FIG. 1 shows the change of weight loss rate of fresh-cut apples with time after inulin treatment with different concentrations;

FIG. 2 is a graph showing the effect of hardness on the flesh of freshly cut apples after treatment with different concentrations of inulin;

FIG. 3 is a graph of the effect of different concentrations of inulin treatment on the soluble solids content of fresh-cut apples;

FIG. 4 shows the trend of PPO activity (A420) of fresh-cut apples with time after different concentrations of inulin treatment;

FIG. 5 is a graph showing the POD activity (A470) of fresh-cut apples after treatment with different concentrations of inulin over time;

FIG. 6 is a graph showing the effect of different concentrations of inulin treatment on the color difference of the surface of fresh-cut apples;

FIG. 7 shows browning and wilting of fresh-cut apples in inulin coating test 4 d;

FIG. 8 is a graph of the effect of different concentrations of inulin treatment on sensory evaluation of fresh-cut apples;

FIG. 9 shows the effect of different heat treatment combinations on the weight loss of fresh-cut apples after composite treatment with a 1.25% inulin coating film;

FIG. 10 is a graph showing the effect of different heat treatment combinations on the hardness of fresh-cut apple pulp after combined treatment with 1.25% inulin coating;

FIG. 11 is a graph of the effect on soluble solids content of fresh-cut apples after different heat treatment combinations and a 1.25% inulin coating combined treatment;

FIG. 12 shows the effect of different heat treatment combinations on the surface color difference of fresh-cut apples after composite treatment with a 1.25% inulin film;

FIG. 13 is a graph of the effect of different heat treatment combinations on PPO activity (A420) of fresh-cut apples after composite treatment with a 1.25% inulin coating film;

FIG. 14 is a graph of the effect on POD activity (A470) in fresh-cut apples after different heat treatment combinations in combination with a 1.25% inulin film coating;

FIG. 15 shows browning and wilting after 72h of heat treatment combined with inulin coating treatment;

FIG. 16 is a graph of the impact on sensory evaluation of fresh-cut apples after different heat treatment combinations and a 1.25% inulin film coating compounding treatment;

FIG. 17 shows the weight loss of fresh-cut apples after film coating composite treatment with different vitamin C contents and 1.25% inulin as a function of time;

FIG. 18 is a study of the effect of composite treatment of different vitamin C contents and 1.25% inulin films on the hardness of fresh-cut apple pulp by heat treatment/vitamin C treatment in combination with inulin film on the fresh-keeping of fresh-cut apples;

FIG. 19 shows the effect of coating composite treatment of different vitamin C contents and 1.25% inulin on the soluble solids content of fresh-cut apples;

FIG. 20 shows the change trend of PPO activity (A420) of fresh-cut apples subjected to coating film composite treatment with different vitamin C contents and 1.25% inulin with time;

FIG. 21 shows the POD activity (A470) of freshly cut apples after coating and compounding treatment with different vitamin C contents and 1.25% inulin as a function of time;

FIG. 22 shows browning and wilting after 72h of vitamin C combined inulin coating treatment;

FIG. 23 is a graph showing the effect of different vitamin C contents and 1.25% inulin film coating combination treatment on sensory evaluation of fresh-cut apples;

FIG. 24 shows the effect of coating composite treatment of different vitamin C contents and 1.25% inulin on the color difference of the surface of fresh-cut apples.

Detailed Description

The principles and features of this invention are described below in conjunction with examples which are set forth to illustrate, but are not to be construed to limit the scope of the invention.

The following experimental procedure was carried out to evaluate the freshness-retaining effect of inulin on fresh-cut apples:

1. materials and instruments

1.1 test reagent Material

The mature Xinjiang red Fuji apple which is sold in the market, neat and undamaged is purchased from the starling fruit wholesale market. High-quality pure natural inulin, pure natural konjac powder, water-soluble chitosan and food-grade isolated soybean protein powder are purchased from Zhejiang spring biotechnology. Ascorbic acid (C)₆H₈O₆) Agar, tryptone, yeast extract, acetic acid buffer solution, glucose, guaiacol solution, potassium dihydrogen phosphate, catechol, acetic acid-sodium acetate buffer solution, and hydrogen peroxide (H)₂O₂) For analytical purification, purchaseIn Shanghai national drug Agents.

1.2 instruments and implements for testing

A portable colorimeter of Konica MINOLTA (Konica MINOLTA) model CR-400, a constant temperature water bath, a fruit hardness tester, an electronic balance, a soluble solid measuring instrument, a Shimadzu CORPORATION microplate reader, a small scale, a THERMO electric measuring CORPORATION Mini-p25 centrifuge, an optical microscope, and a Heil refrigerator.

2. Test treatment and method

2.1 inulin film coating preliminary test

Different inulin solution concentration gradients of 0% (blank control group), 0.5%, 1%, 1.5%, 2%, 2.5% (mass percentage, the same applies below), and 7 treatment groups were set with the prior art coating formula (such as konjac glucomannan-chitosan-soy protein isolate composite coating) as the CK control group (marked as CK control in the figure). Each treatment group takes 4 Xinjiang red Fuji apples with the same size and maturity, namely 4 biological replicates, each apple is cut into about 28 cuboid small blocks with the length and width of 1.0cm and the height of 1.5cm, all the apple blocks are divided into 4 parts, after the apples are soaked in the solution for 5min, the apples are respectively sealed by a preservation box and stored at 4 ℃, and the hardness, the soluble solid content, the weight loss rate, the surface microorganism quantity, the polyphenol oxidase activity, the surface color difference value and the peroxidase activity are measured on one part of each group at 1d, 2d, 3d and 4d, and a 6-8 human sensory evaluation group is established for sensory evaluation according to the table 1 to select the optimal inulin concentration range.

2.2 inulin film coating test

Two inulin concentrations with better fresh-keeping effect are obtained in a pre-test, and inulin concentration gradients are drawn according to the range, and inulin with different concentrations is taken as a substrate for 1.05%, 1.15%, 1.25%, 1.35%, 1.45% and 1.55%, and the total amount is 6 treatment groups. Taking 4 Xinjiang red Fuji apples with the same size and maturity in each treatment group, namely 4 biological replicates, cutting each apple into about 28 cuboid small blocks with the length and width of 1.0cm and the height of 1.5cm, dividing all the apple blocks into 4 parts, soaking the apple blocks in a solution for 5min, respectively sealing the apple blocks by using a preservation box at 4 ℃ for storage, measuring the hardness, the soluble solid content, the weight loss rate, the surface microorganism quantity, the polyphenol oxidase activity, the surface color difference value and the peroxidase activity of one part of each group in 1d, 2d, 3d and 4d, establishing a 6-8 human sensory evaluation group for sensory evaluation according to the table 1, and selecting the most suitable inulin treatment concentration according to relevant indexes.

2.3 Heat shock treatment in combination with inulin coating preservation Pre-test

On the basis of the treatment of the inulin coating solution with the optimal concentration, different heat treatment combinations are set, and the inulin coating solution is soaked in hot water at 40 ℃, 50 ℃ and 60 ℃ for 1min and 2min at constant temperature respectively, and a control group (marked as CK in the figure) is not subjected to heat treatment, so that 7 treatment groups are totally arranged. Taking 4 Xinjiang red Fuji apples with the same size and maturity in each treatment group, namely 4 biological repetitions, cutting each apple into about 28 cuboid small blocks with the length and width of 1.0cm and the height of 1.5cm, dividing all the apple blocks into 4 parts, performing constant-temperature water bath heat treatment on each group, soaking in solution for 5min, respectively sealing with a preservation box at 4 ℃ for storage, measuring the hardness, the soluble solid content, the weight loss rate, the surface microbial quantity, the polyphenol oxidase activity, the surface color difference value and the peroxidase activity of one part of each group at 1d, 2d, 3d and 4d, establishing a 6-8 human sensory evaluation group for sensory evaluation according to the table 1, and selecting the most suitable heat treatment combination on the basis of the optimal inulin coating concentration.

2.4 Heat treatment in combination with inulin coating preservation test

The inulin coating solution was treated with hot water at 35 deg.C, 40 deg.C, 45 deg.C, 50 deg.C, 55 deg.C and 60 deg.C for 2 minutes, and the control group (CK in the figure) was not heat-treated, and 7 treatment groups were used in total. Each treatment group takes 4 Xinjiang red Fuji apples with the same size and maturity, namely 4 biological replicates, each apple is cut into about 28 cuboid small blocks with the length and width of 1.0cm and the height of 1.5cm, all the apple blocks are divided into 4 parts, after the apples are soaked in the solution for 5min, the apples are respectively sealed by a preservation box and stored at 4 ℃, in the 1d, 2d, 3d and 4d, one part of each group is measured for hardness, soluble solid content, weight loss rate, surface microorganism quantity, polyphenol oxidase activity, surface color difference value and peroxidase activity, and a 6-8 human sensory evaluation group is established for sensory evaluation according to the table 1, so that the most suitable temperature and time treatment combination is obtained finally.

2.5 vitamin C treatment in combination with inulin coating fresh-cut preservation test

On the basis of the treatment with the inulin coating solution of the optimum concentration, 0.5%, 1%, 1.5%, and 2% (mass%, the same applies hereinafter) of vitamin C was added, and the control group (denoted as CK in the figure) was not added with vitamin C, for a total of 5 treatment groups. Taking 4 Xinjiang red Fuji apples with the same size and maturity in each treatment group, namely 4 biological replicates, cutting each apple into about 28 cuboid small blocks with the length and width of 1.0cm and the height of 1.5cm, dividing all the apple blocks into 4 parts, soaking the apple blocks in the solution for 5min, respectively sealing the apple blocks by using a preservation box at 4 ℃ for storage, measuring the hardness, the soluble solid content, the weight loss rate, the surface microorganism quantity, the polyphenol oxidase activity, the surface color difference value and the peroxidase activity of one part of each group at 1d, 2d, 3d and 4d, and establishing a 6-8 human sensory evaluation group for sensory evaluation according to the table 1 to select the optimum vitamin C adding concentration.

3. Correlation index measuring method

3.1 weight loss ratio

Weighing the net weight of the preservation box by adopting a small electronic balance weighing method, and respectively taking out and weighing one sample before and after the preservation box is placed into a cold storage on the 2 nd day, the 3 rd day and the 4 th day after the preservation box is placed into the cold storage. Finally, a weight loss rate calculation formula is obtained, namely the weight loss rate is (the weight of the sample after storage-the weight of the preservation box)/(the weight of the sample before storage-the weight of the preservation box) multiplied by 100%.

3.2 pulp firmness

The computer, the fruit hardness meter and related measurement software are started, a probe with a cylinder and a diameter of 0.3cm is adopted, and the puncture distance is 1 cm. And taking 7 parts of each fruit block, measuring the hardness, deleting the maximum value and the minimum value, and finally taking the hardness of the sample as the average value of the rest data.

3.3 soluble solids content

And (4) determining the content of soluble solids in the fresh-cut apples by using a brix meter. First, the measuring instrument was calibrated and the joint was wiped clean with distilled water. Secondly, taking pulp tissues of different parts of the apple, grinding and mashing the pulp tissues, filtering filtrate, sucking 2-3 drops of the filtrate by a dropper to a receiving position, and measuring to obtain data. This process was repeated 7 times, with the maximum and minimum removed, and the average of the other 5 data was found to be the soluble solids content value for this group of samples.

3.4 Polyphenol oxidase Activity

1.0g of apple pulp was weighed, ground, added to a 10ml centrifuge tube containing 5.0ml of 0.1mol/L pH5.5 acetate buffer (containing 1mmol of PEG, 4% of pvpp and 1% of Triton X-100), mixed at low temperature, shaken several times, extracted for 20min, centrifuged at 12000 Xg at 4 ℃ for 20min, and the supernatant was collected and stored at low temperature. Then, the measurement was carried out by taking 2.5ml of 0.05mol/L acetic acid-sodium acetate buffer solution +0.1ml of distilled water +0.6ml of catechol as a control tube, measuring the 420nm position of the reaction tube of 2.5ml of acetic acid-sodium acetate buffer solution +0.1ml of crude enzyme solution +0.6ml of catechol, adding all reagents to represent the start of the reaction, recording the absorbance after 15s, and recording the absorbance every 1min for 7 times. The calculation formula is A₄₂₀＝(A_{Final (a Chinese character of 'gan')}-A_{Starting point})/(t_{Final (a Chinese character of 'gan')}-t_{Starting point}) (ii) a PPO Activity (A)₄₂₀/min×g)＝(A₄₂₀×V)/(V_S×m)。

In the formula A₄₂₀Represents absorbance; a. the_{Final (a Chinese character of 'gan')}Representing the absorbance termination value of the reaction mixed liquor; a. the_{Starting point}Expressing the initial value of the absorbance of the reaction mixed liquor; t is t_{Final (a Chinese character of 'gan')}Represents the reaction termination time (min); t is t_{Starting point}Represents the reaction start time (min); v represents the total volume (mL) of the sample extract; v_SRepresents the volume (mL) of the sampled extract at the time of measurement; m represents the sample mass (g). The increase in absorbance change per minute per gram of pulp by 1 PPO activity unit.

3.5 peroxidase Activity

Weighing 1.0g apple pulp, grinding, adding into 5.0mL 0.1mol/L pH5.5 acetic acid buffer solution (containing 1mmol PEG, 4% pvpp and 1% Triton X-100) 10m centrifuge tube, shaking at low temperature for several times, mixing, extracting for 20min, centrifuging at 12000 Xg and 4 deg.C for 20min, collecting supernatant, and storing at low temperature for use. Then, the actual measurement is carried out,3mL of guaiacol solution, 0.1mL of distilled water and 0.2mL of H₂H₂The solution was used as a control tube, and 3mL of guaiacol solution +0.1mL of crude enzyme solution +0.2mL of H was measured₂O₂The absorbance of the solution reaction tube at 470nm was recorded 15s after all reagents were added, and the absorbance was recorded every 30s for 7 times. The calculation formula is A₄₇₀＝(A_{Final (a Chinese character of 'gan')}-A_{Starting point})/(t_{Final (a Chinese character of 'gan')}-t_{Starting point}) (ii) a POD Activity (A)₄₇₀/min*g)＝(A₄₂₀×V)/(V_S×m)

In the formula A₄₇₀Represents absorbance; a. the_{Final (a Chinese character of 'gan')}Representing the absorbance termination value of the reaction mixed liquor; a. the_{Starting point}Expressing the initial value of the absorbance of the reaction mixed liquor; t is t_{Final (a Chinese character of 'gan')}Represents the reaction termination time (min); t is t_{Starting point}Represents the reaction start time (min); v represents the total volume (mL) of the sample extract; v_SRepresents the volume (mL) of the sampled extract at the time of measurement; m represents the sample mass (g). The increase in absorbance change per minute per gram of pulp by 1 POD activity unit was determined.

3.6 measurement of surface color difference

Measurement was carried out with a portable color difference meter of the Japanese Konica MINOLTA CR-400 model, with reference to the latest color difference formula: CIEDE2000 performs the calculation of the color difference values.

Immediately after the apple was cut on day 1, the initial Lab value was recorded as a standard value by a portable colorimeter, and then Lab value measurement was performed on each of the fruit pieces taken out of the refrigerator on day 2, day 3 and day 4, and 7 times of measurement was performed for each sample, and then the average value was obtained to calculate the color difference. Where L denotes illuminance (luminance), a denotes a range from red to green, and b denotes a range from blue to yellow.

3.7 surface microbial count assay

The determination of the surface microbial quantity is carried out by taking GB 4789.2-2016 (food safety national standard food microbiology detection colony total number determination) written by Qingdao Haibo biology (2017) as a standard.

3.8 sensory evaluation

The test was conducted using the sensory evaluation table shown in table 1 as a standard, and was conducted mainly from several aspects of vision, smell, taste and flavor, wherein vision and smell are the most intuitive evaluation methods and are important components in sensory evaluation, and the vision and smell largely affect appetite, and each account for 20% by weight. But mouthfeel and flavour are most important for food, and whether it is good or not, determines the final purchase rate and the repurchase rate of the consumer, so that mouthfeel and flavour are each weighted 30%. Consumers usually prefer and purchase high-quality cut fruits, and the cut fruits are characterized by sufficient moisture, bright and fresh appearance, rich fruit flavor, appetites at a glance, crisp and juicy mouthfeel, excellent flavor and rich fragrance. And the cut fruits which cannot be sold usually shrink into small pieces, so that the browning is serious, obvious peculiar smell cannot be tolerated, and people cannot swallow the food.

TABLE 1 sensory evaluation of fresh-cut apples

4. Results and analysis

4.1 Effect of inulin coating test on fresh-keeping of freshly cut apples

(1) Weight loss ratio

As can be seen from FIG. 1, the overall weight loss rate gradually increases with the shelf life, the inulin with the concentration of 0.5% -1.5% can better keep the original water content after being treated, the weight loss rate does not exceed 0.5%, and especially at 2d and 3d, the fresh-cut apples treated by the inulin with the concentration of 1.15% and 1.25% have no obvious water loss. The CK control group had the best moisturizing effect, and the '1.25% inulin' group was inferior to the blank control group without treatment (0% inulin) ((P < 0.05)).

(2) Hardness of pulp

The data in fig. 2 represents the difference between the nd and 1d hardnesses of the respective treatment groups, i.e., the hardness change amount (the change amount of the different index mentioned later is also the index difference between the nd and 1 d). When the treatment concentration of inulin is 1.25%, the hardness of the fresh-cut apples can be maintained most effectively, and the CK control group has the same effect.

(3) Soluble solids content

The data in FIG. 3 represent the variation in soluble solids, which are better maintained within 4 days when the inulin concentration is between 1.05% and 1.35%.

(4) Polyphenol Oxidase (PPO) activity

Fig. 4 shows that polyphenol oxidase (PPO) activity increases significantly with time, which is one of the major enzymes affecting browning of fruits and vegetables. At 4d, the PPO activity was highest in the untreated control blank ("0% inulin") whereas the CK control was significantly lower than the others (P < 0.05). In the inulin-treated group, the effect of inhibiting PPO activity was best at a concentration of 1.25%, which was significantly different from that of the untreated blank group ("0% inulin") (P < 0.05).

(5) Peroxidase (POD) Activity

Fig. 5 shows that, as shelf life is prolonged, Peroxidase (POD) activity is increased, the CK control group has the best effect and has no significant difference from the "1.25% inulin" group, and the "1.35% inulin" effect has the second effect, and has a significant difference (P <0.05) from the untreated blank control group ("0% inulin"), and the increase in POD activity means that senescence of fresh-cut apples is delayed.

(6) Determination of surface color difference

FIG. 6 shows that the intensity (L value) of the inulin-treated apples at a concentration of 1.35% was not substantially changed at the fresh-cut 4 d; the a values (representing that the apples are changed from green to red) of 1.25 percent of the fresh-cut apples after inulin treatment are not obviously changed; the b values (representing flesh yellowing and browning) of the fresh-cut apples of the inulin-treated group and the CK control group with the concentration of 1.25% have no significant change. After calculation, the color difference value (Δ E) of the CK control group was significantly lower than that of the other treatment groups.

(7) Surface microbial count assay

As shown in Table 2, the inulin-treated group having a concentration of 1.45% after fresh-cutting for 24 hours had the least amount of surface microorganisms of 0CFU/g, and the next inulin having a concentration of 1.35%; the inulin-treated group with a concentration of 1.15% after 72h had a minimum amount of microorganisms on the surface of the fresh-cut apples of 50 CFU/g; the surface microbial load of fresh-cut apples in the inulin-treated group at a concentration of 1.15% after 140h was at a minimum of 90 CFU/g.

TABLE 2 influence of inulin treatment of different concentrations on the microbial count on the surface of freshly cut apples (unit: CFU/g)

Treatment method	24h	72h	140h
				0% inulin	130c	300c	4000g
0.5% inulin	70b	150b	180b
				1.0% inulin	50a	60a	110a
1.05% inulin	80c	110b	130a
				1.15% inulin	30a	50a	90a
1.25% inulin	70b	110b	120a
				1.35% inulin	20a	220c	400c
1.45% inulin	0a	90a	300c
				1.5% inulin	30a	80a	800e
2.0% inulin	280d	700d	1300f
				2.5% inulin	100c	250c	500d

Note: the difference of the lower case letters in the same column indicates that the difference is significant (P <0.05)

(8) Sensory evaluation

Fig. 7 shows browning and wilting of the fresh-cut apples in the inulin coating test 4d, and it can be seen that at 4d, the inulin-treated group with a concentration of 1.25% had the best sensory quality of the fresh-cut apples, no significant browning of the flesh tissue, and the untreated blank control group ("0% inulin") had severe surface browning and poor sensory quality.

As can be seen from FIG. 8, sensory evaluation results of the treatment groups other than the non-treated blank control group ("0% inulin"), the "0.5% inulin" and the "2.5% inulin" groups exceeded 8 points. At 4d, the inulin-treated group at a concentration of 1.25% gave the highest overall sensory evaluation score for fresh-cut apples, followed by the CK control group. While the untreated control blank ("0% inulin") showed significant browning at day 4, a strong cider flavor and a minimum overall sensory evaluation score.

In conclusion, the fresh-keeping effect is evaluated from the weight loss rate, the pulp hardness, the soluble solid content, the PPO activity, the POD activity and the sensory evaluation, when the inulin concentration is 1.25%, the fresh-keeping effect on the fresh-cut apples is the best, and when the inulin concentration is 1.15%, the effect is the second; the evaluation is carried out by the indexes of surface color difference value and surface microorganism quantity, the effect is best when the inulin concentration is 1.15%, and the inulin concentration is not significantly different from 1.25% (P is less than 0.05). In conclusion, when the inulin concentration is 1.25%, the whole fresh-keeping effect on the fresh-cut apples is optimal.

4.2 Effect of Heat treatment in combination with inulin coating on the preservation of freshly cut apples

(1) Weight loss ratio

As can be seen from fig. 9, on the basis of the optimal inulin treatment (inulin concentration of 1.25%), the water loss rate of the fresh-cut apples subjected to the compound treatment at 60 ℃ is higher than that of the control group (only the film coating treatment with the optimal inulin concentration is adopted, the heat treatment group is not adopted, and the subsequent mark is the heat treatment group), but the water loss rate of the fresh-cut apples subjected to the compound treatment at 50 ℃ for 2min is not lost in 4d, and the weight loss rate is always 0%, which is significantly better than that of the other treatment groups (P < 0.05).

(2) Hardness of pulp

The data in fig. 10 are the variation of pulp hardness for the "60 ℃, 1 min" group and the "55 ℃, 2 min" combined treatment group, which have the best effect on maintaining pulp hardness, and have no significant difference (P <0.05) from the "40 ℃, 2 min" and "45 ℃, 2 min" treatment groups.

(3) Soluble solids content

The data in fig. 11 is the amount of change in soluble solids, with the "60 ℃, 1 min" combination treatment group having significantly higher amount of change than the other treatment groups as shelf life increased.

(4) Determination of surface color difference

Fig. 12 shows that the brightness (L value) of the fresh-cut apples subjected to the heat treatment at 60 ℃ has the largest change at 4d compared with the control group (the non-heat-treated group), and has a significant difference (P < 0.05); the a value (representing that the apple is changed from green to red) in the combined treatment groups of 40 ℃ and 50 ℃ has no significant change; after being treated for 2min at 50 ℃ and 55 ℃, the change of the b value (representing the flesh yellowing and browning) is obviously inhibited. In addition, the color difference (Δ E value) of the pulp after the heat treatment at 60 ℃ was significantly higher than that of the control group (P <0.05), while the color difference was the least in the test group after 2min treatment at 55 ℃ and then in the treatment group at 50 ℃ for 2 min.

(5) Polyphenol Oxidase (PPO) activity

The PPO activity of the polyphenol oxidase (PPO) is gradually increased (figure 13), the PPO activity of the fresh-cut apples after heat treatment is slightly higher than that of a control group (a non-heat treatment group), wherein the PPO activity of the fresh-cut apples after heat treatment is slightly higher than that of the control group (the non-heat treatment group), and the PPO activity of the fresh-cut apples after heat treatment is obviously different from that of the control group (the non-heat treatment group) (P is less than 0.05) between the compound treatment group with the temperature of 60 ℃ and 2 min.

(6) Peroxidase (POD) Activity

As can be seen in fig. 14, Peroxidase (POD) activity was significantly improved (P <0.05) after heat treatment at 50 ℃ for 2min in combination with inulin coating treatment.

(7) Sensory evaluation

As can be seen in FIG. 15, after 72 hours of treatment, the composite treatment group of "50 ℃, 2 min" has the best fresh-keeping effect, no obvious browning on the surface and the best color protection effect.

The comprehensive scores of the sensory evaluation group (fig. 16) show that the fresh-cut apples of the other compound treatment groups have good sensory properties except that the fresh-cut apples have obvious browning, stronger apple vinegar taste and obvious water stain after being subjected to heat treatment at 60 ℃, so that the final comprehensive scores are lower, and the comprehensive scores are all over 8.5.

(8) Surface microbial count assay

Compared with the control group, the number of surface microorganisms can be effectively reduced after the treatment for 2min at the temperature of 45-60 ℃, wherein the bacteriostatic effect is the best after the treatment for 2min at the temperature of 50 ℃, and is obviously higher than that of other treatment groups (table 3).

TABLE 3 number of microorganisms (unit: CFU/g) on the surface of freshly cut apples after coating with 1.25% inulin in different heat treatment groups

In conclusion, the weight loss rate, POD activity, surface microorganism amount and other detection indexes are analyzed, the inulin with the concentration of 1.25% is used for treatment for 2min, and the heat treatment at 50 ℃ can improve the fresh-keeping effect; the indexes such as pulp hardness, soluble solid content and surface color difference value are analyzed, the inulin with the concentration of 1.25% is used for treatment for 2min, the heat treatment at 55 ℃ can improve the fresh-keeping effect, and the effect is better than 55 ℃ in combination with other indexes at 50 ℃. Therefore, in order to further improve the fresh-keeping effect of the fresh-cut apples, a composite treatment combining the coating treatment of 1.25% of inulin and the treatment at 50 ℃ for 2min is recommended.

4.3 Effect of vitamin C treatment in combination with inulin coating on the preservation of freshly cut apples

(1) Weight loss ratio

As shown in fig. 17, the VC-compounded fresh-cut apples with a compound concentration of 0.5% to 1.5% on the basis of 1.25% inulin coating had no water loss, and had a weight loss rate of 0% in 4d, which was significantly different from that of the control group (only 1.25% inulin coating, VC concentration of 0%, and subsequent label of only 1.25% inulin coating) (P < 0.05). When the content of the vitamin C reaches 2.0%, the weight loss rate of fresh cut apples is slightly higher than that of a control group (only 1.25% of inulin film coating treatment), but no significant difference exists (P is less than 0.05).

(2) Surface microbial count assay

The bacteriostatic effect of the vitamin C is not obvious, and the bacteriostatic effect of adding 1.0 percent of the vitamin C is slightly improved on the basis of only 1.25 percent of inulin treatment at 72h and 140 h; however, at 24h, the combination concentration of the 1.25% inulin was 1.0%, and the Vc treatment of 1.5% and 2.0% had significantly better bacteriostatic effects than the control group (only 1.25% inulin film coating treatment) and the composite treatment containing 0.5% Vc (Table 4).

TABLE 4 influence of different vitamin C contents and 1.25% inulin film coating on the microbial count (unit: CFU/g) on the surface of fresh-cut apples

Treatment method	24h	72h	140h
				0％Vc	70a	110b	120c
0.5％Vc	90a	200a	400a
				1.0％Vc	50b	100b	110c
1.5％Vc	40b	100b	200b
				2.0％Vc	30b	80b	210b

(3) Hardness of pulp

The data in fig. 18 represents the amount of hardness change. The pulp hardness decreased with the increase of shelf life, and was well maintained when the vitamin C content was 1.0% as compared with the control group (only 1.25% inulin coating treatment). When the content of the vitamin C reaches more than 1.5 percent, the pulp hardness is greatly reduced, namely, compared with the control group, the pulp hardness of the compound test group of '1.5 percent VC' and '2.0 percent VC' is obviously reduced (P is less than 0.05).

(4) Soluble solids content

The data in fig. 19 represent the amount of change in soluble solids, which was reduced in the control group (only 1.25% inulin film coated) for fresh-cut apples treated with 1.25% inulin combined with various concentrations of vitamin C.

(5) Polyphenol Oxidase (PPO) activity

As can be seen from fig. 20, there was no significant difference (P <0.05) between the polyphenol oxidase (PPO) activity and the control group when the vitamin C concentration was 2.0%, but all were significantly higher than the other treatment groups (only 1.25% inulin film-coated treatment).

(6) Peroxidase (POD) Activity

As can be seen from fig. 21, the Peroxidase (POD) activity was slightly significant in the treatment groups having vitamin C contents of 1.0% and 2.0% (P <0.05) over the other treatment groups and the control group (only 1.25% inulin film-coated treatment).

(7) Sensory evaluation

Fig. 22 shows that vitamin C concentrations of 1.0% and 1.5% both have very good color-protecting and freshness-retaining effects, with no apparent browning on the surface.

FIG. 23 shows that the scores of all treatments were higher, and the total score reached 9 points or more, except that the "2.0% VC" treatment group showed severe browning.

(8) Determination of surface color difference

FIG. 24 shows that the 4d internal brightness (L value), a value (representing that the apple turns from green to red) and b value of the fresh-cut apple treated with 1.5% of vitamin C added to the treatment solution with inulin content of 1.25% did not change significantly; the color difference (delta E value) of the treated group was significantly lower than that of the control group (P < 0.05).

In conclusion, the compound treatment of adding 1.0% of vitamin C into 1.25% of inulin solution has certain promotion effect on the insurance of fresh-cut apples, especially on the aspect of water retention.

5. Conclusion and discussion

5.1 discussion of application prospect of fresh-cut fruit coating preservation

The fresh-cut fruit box has important significance for improving the commodity value of the fruit, and the fresh-cut fruit also meets the requirements of consumers under the background that the life rhythm is increasingly accelerated. However, the fruits such as fresh-cut apples are easy to brown and rot. Therefore, finding a commercial form of post-harvest treatment suitable for fresh-cut fruits is of great practical significance.

5.2 conclusion of the test

When the inulin concentration is 1.25%, the fresh-keeping effect of the fresh-cut apples is optimal, and on the basis, the fresh-keeping effect of the fresh-cut apples can be improved to a certain extent by combining heat treatment at 50 ℃ for 2min or treatment with 1.0% of vitamin C.

Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made to the above embodiments by those of ordinary skill in the art within the scope of the present invention.

Claims

1. A method for preserving fresh-cut apples, comprising the following steps: (1) soaking the fresh-cut apples with an inulin solution; (2) using a fresh-keeping box to seal and store at 4°C.

2. A fresh-cut apple preservation method according to claim 1, characterized in that, in the step (1), the mass percentage of the inulin solution is 0.5%-1.5%, and the soaking time is 5min.

3. A kind of fresh-cut apple preservation method according to claim 2, is characterized in that, the mass percentage of described inulin solution is 1.05%-1.35%.

4 . The fresh-cut apple preservation method according to claim 3 , wherein the mass percentage of the inulin solution is 1.25% or 1.15%. 5 .

5. according to a kind of fresh-cut apple preservation method described in any one of claim 1 to 4, it is characterized in that, before described step (1), also carry out heat treatment step: fresh-cut apple is heated at 30 ℃-60 ℃ Soak in hot water at a constant temperature for 1min-2min.

6 . The fresh-cut apple preservation method according to claim 5 , wherein the heat treatment step is specifically: soaking the fresh-cut apples in hot water of 50° C. for 2 minutes at a constant temperature. 7 .

7. A fresh-cut apple preservation method according to any one of claims 1 to 4, wherein the inulin solution in the step (1) contains 0.5%-2% of vitamin C by mass percentage.

8 . The fresh-cut apple preservation method according to claim 7 , wherein the inulin solution contains 1% of vitamin C by mass. 9 .