Research progress on storage and preservation technology of cut fruits and vegetables
In recent years, people's consumption patterns are constantly changing, which promotes the rapid development of fast food industry, and people's demand for fresh-cut fruits and vegetables that can be eaten directly, nutritious and hygienic is rapidly increasing. Fresh-cut fruits and vegetables not only have the advantages of freshness and convenience, but also have important environmental protection functions. Fresh-cut fruits and vegetables keep their flavor and nutrition well, but their storability is lower than that of intact fruits and vegetables. This paper mainly expounds the research progress of fresh-keeping technologies such as shortening storage period after cutting and processing fruits and vegetables.
Keywords: cutting fruits and vegetables; Preservation technology; study
1 Introduction
At present, in Europe, America, Japan and other developed countries and regions, fresh-cut fruits and vegetables have achieved systematic and standardized production, and a large number of products have entered food stores and supermarkets. It is reported that the consumption of fresh-cut fruits and vegetables in the United States and other western developed countries has accounted for 1/3 of the consumption of fruits and vegetables. In China, the production of fresh-cut fruits and vegetables has just started, and the processing scale is relatively small. The output and quality of fresh-cut fruits and vegetables in China can't meet the needs of social development. The main reason is that there are problems in the processing technology and preservation technology of fresh-cut fruits and vegetables, the price is high, the shelf life (about 7 days) can not be guaranteed, and the quality of fresh-cut fruits and vegetables has no testing standards. China is a big producer of fruits and vegetables, accounting for about13 of the world's total output. The backward production and technology of fresh-cut fruits and vegetables not only affects the improvement of farmers' income level, but also affects the strategic adjustment of China's agricultural and rural industrial structure. Therefore, it is of great economic and far-reaching social significance to study the preservation technology of fresh-cut fruits and vegetables.
Storage and fresh-keeping technology of 2-slice fruits and vegetables
2. 1 Cryopreservation
Low temperature treatment can effectively slow down the activities of enzymes and microorganisms, inhibit the respiration of fruits and vegetables, slow down the rate of various biochemical reactions, delay aging and inhibit browning. Because the temperature coefficient Q 10 of the enzyme activity chemical reaction is 2 ~ 3, the physiological and biochemical reaction decreases to 1/3 ~ 1/2 every time the temperature decreases. Therefore, when cutting materials, low temperature operation can minimize the deterioration of physiological metabolism such as ethylene and increased respiratory rate, and the shelf life can be greatly extended. Sun Wei, Ding Baolian and others [1] studied the respiration rate of potatoes, carrots, sweet peppers, radishes, lettuce, celery, cabbage, Chinese cabbage, broccoli, mushrooms, cauliflower and shiitake mushrooms after being cut at different temperatures of 10 ~ 30℃, and found that the suitable temperature for cutting vegetables should be/kloc-. After being processed, transported and sold at 5℃, the number of microorganisms on the surface of cut vegetables can be kept at least at 10d, while at 10d, the number of microorganisms on the surface of cut vegetables can only be basically stable for 3 days, and then it rises sharply. Different fruits and vegetables have different tolerance to low temperature, and each fruit and vegetable has its best processing and storage temperature.
2.2 Modified atmosphere preservation
As a pollution-free preservation technology, modified atmosphere preservation has attracted much attention all over the world. The shelf life of fruits can be greatly extended by modified atmosphere packaging after pretreatment. The combination of modified atmosphere packaging and cold storage can significantly improve the storage quality and prolong the storage period of cut fruits. Creating an environment with low O2 and high CO2 during storage can reduce respiration, inhibit ethylene production, delay senescence of cut fruits and vegetables and prolong storage time. While reducing the concentration of O2 and increasing the concentration of CO2, prevent the formation of anaerobic environment, because the formation of this environment will easily lead to the odor of fruits when they breathe without oxygen. Fruits and vegetables can obtain suitable gas environment through proper packaging and respiration, and can also artificially change the gas composition of storage environment. Generally, 2% ~ 5% O2 and 5% ~ 10% CO2 should be kept in the packaging of cut fruits and vegetables to maintain the quality. Bai [2] found that sealed packaging with polyethylene film with different permeability of CO2 and O2 can prolong the fresh-keeping period of sliced coarse melon from 6d when unpacked to 12d, and its quality is better than that of unpacked melon. The thickness and composition of packaging film also have great influence on the preservation effect. Zhou Tao et al [3] found that using high-density polyethylene film can better inhibit the lignification of water bamboo slices and keep their tenderness than using low-density polyethylene film packaging. Wang Qingzhang et al [4] used low-density polyethylene film with the thickness of 0 10mm and 0.08 mm respectively and PA/PE composite film with the thickness of 0.08 mm and 0.06 mm respectively to vacuum package lotus roots. The results showed that PA/PE could significantly inhibit the browning of lotus roots and retain more nutrients.
2.3 coating protection
Coating technology can coat edible film on the surface of cut fruits and vegetables to form a coating, which can maintain and improve the quality of products. Edible film can reduce water loss and prevent aromatic components from volatilizing; Preventing oxygen from entering, reducing the oxygen concentration on the surface of fruits and increasing the CO2 concentration, thus inhibiting respiration, delaying ethylene production, delaying the ripening and senescence of fruits and vegetables, and being beneficial to storage; Inhibiting the browning of fruits and vegetables, adding antioxidants and anti-browning agents to the film-forming agent can reduce the oxidative deterioration and discoloration of cut fruits and vegetables. Mei et al. [5] coated sliced carrots with a mixture of 5% calcium gluconate and calcium lactate containing 0.2% VE, which kept the quality and nutritional components of sliced products well.
Film coating agents can be divided into sugar, protein and compound. Sugar coating agents mainly include chitosan, sodium alginate, starch and konjac edible film. Protein can be divided into corn protein, soybean protein and whey protein. The composite membrane is a membrane formed by a variety of substances such as sugar, fat and protein after certain treatment. Because of their different properties and complementary functions, the films formed have more ideal properties. Peng et al. [6] treated horseshoe slices with 2% chitosan coating, which effectively inhibited browning.
3 Control microorganisms in sliced fruits and vegetables
Fresh-cut fruits and vegetables, especially cut fruits, are rich in nutrients such as sugar, which is beneficial to the growth of microorganisms and easy to lead to decay. At present, Japan, France and other countries have formulated strict microbial control standards for fresh-cut fruits and vegetables to ensure the hygiene and quality of fresh-cut products.
Fresh-cut fruits and vegetables mainly prevent microbial growth by controlling water activity (aw) and acidity (pH), using preservatives and cold storage. Bacteria are the main microorganisms on vegetables, and the number of mold and yeast is less; In addition to some bacteria, there are relatively many molds and yeasts on fruits. The microbial communities on different kinds of vegetables and fruits are very different. Common bacteria on fruits and vegetables include Erwinia, Pseudomonas, Xanthomonas, Corynebacterium, Bacillus, Clostridium, etc., especially Erwinia and Pseudomonas. Erwinia, some Pseudomonas (such as Pseudomonas marginata), Bacillus and Clostridium can cause bacterial softening and decay of fruits and vegetables. These bacteria can secrete pectinase, decompose pectin and soften plant tissues; Sometimes when water seeps out, it smells bad.
3. 1 chemical preservative
Acetic acid, benzoic acid, sodium hypochlorite, sorbic acid and its salts, H2O2, etc. It can effectively inhibit the growth and reproduction of microorganisms and effectively control spoilage bacteria and pathogenic bacteria that can still grow at low temperature. In production, preservatives are often added to the cleaning solution for cleaning. Chen [7] treated the cut lettuce with sodium hypochlorite, hydrogen peroxide and calcium chloride respectively, among which 100mg/kgNaClO soaked for 3 minutes had the best storage effect. But the shelf life of sodium hypochlorite is generally only one week. If you want to get a longer shelf life, you should use other preservatives such as potassium sorbate. The pH value of fresh-cut vegetable tissues is generally 4.5 ~ 7.0, which is suitable for the growth of various spoilage bacteria. Adding proper amount of acetic acid, citric acid and lactic acid can reduce the pH value of fruits and vegetables and inhibit the growth and reproduction of microorganisms. However, too much acid will destroy the flavor of fruits and vegetables. 3.2 Biological preservatives
Biological preservatives refer to a kind of antibacterial substances from plants, animals and microorganisms. Because fresh-cut fruits and vegetables are ready-to-eat products, the application of chemical preservatives is limited, so the research and application of natural preservatives from biology are paid more and more attention. In recent years, it has been found that bacteriocin or bacteriocin-like metabolites of lactic acid bacteria can effectively inhibit the growth of harmful microorganisms such as Aeromonas hydrophila and Listeria monocytogenes in fresh-cut fruits and vegetables. Vescovo et al [8] successfully applied lactic acid bacteria to preserve lettuce salad. Due to the high cost and single control effect of biological preservatives, their current application is greatly limited.
3.3 Physical methods
In recent years, irradiation, ozone, ultrasound, ultraviolet, ultra-high voltage, pulsed electric field and pulsed magnetic field have made great progress in controlling microorganisms in fruit and vegetable cutting. Compared with traditional heat treatment, these physical methods have little temperature change and will not cause obvious chemical changes or odor. They can not only keep the nutritional components of products, but also keep the freshness and good flavor of products. In recent years, they have been widely used in production. Gao Xiang et al. [9] irradiated fresh-cut celery, and the results showed that the irradiation dose of 1kGy could effectively control microbial reproduction and reduce the number of bacteria by two orders of magnitude; Mold and yeast decreased by an order of magnitude; Coliform bacteria were not detected; At the same time, the activity of polyphenol oxidase was greatly inhibited, and the activity of polyphenol oxidase was reduced by 60 units compared with the control. The nutritional index was good, and the content of Vc on the 6th day was higher than that in the control group (15%). Excellent sensory quality. However, it should be noted that different fruits and vegetables have different radiation tolerance when using irradiation to preserve fruits and vegetables. When the radiation dose exceeds a certain value, the cell membrane will be damaged
Ultraviolet irradiation can also control the microorganisms that cut fruits and vegetables, and has a significant killing effect on bacteria, molds, yeasts, viruses and other microorganisms. Ultraviolet rays can not only kill microorganisms on the surface of fruits and vegetables, but also induce the cutting of some secondary metabolites in fruits and vegetables, which has antibacterial effect, thus prolonging the fresh-keeping period of cut fruits and vegetables. Ultra-high pressure sterilization is to package food raw materials in a certain way, put them in a liquid medium, and act under the pressure of 100 ~ 1000 MPa for a period of time to make them meet the requirements of sterilization. Its basic principle is the lethal effect of pressure on microorganisms. A Japanese company, at 25℃, uses 606? 108Pa can kill all spores on potato salad within 20min minutes. Ultrasonic sterilization has also been applied in recent years. Ultrasonic sterilization can not achieve better sterilization effect when used alone, but it can be combined with other sterilization measures to achieve better results. At present, cut fruits and vegetables are generally cleaned by ultrasonic. The sterilization mechanism of pulsed electric field and pulsed magnetic field is not completely clear, but the sterilization time is short and good sterilization effect can be achieved.
4 Quality changes of cut fruits and vegetables
4. Browning and softening of1sliced products
Browning and whitening of fresh-cut fruits and vegetables are mainly controlled by enzyme inhibitors and antioxidants in production, or by reducing oxygen concentration to inhibit enzymatic browning. Traditionally, sodium sulfite and citric acid are used. In recent years, it has been found that zinc acetate, calcium chloride, isoascorbic acid and its sodium salt, L- cysteine and 4- substituted resorcinol have significant effects on the control of enzymatic reaction. Studies on potato chips, apple chips and fresh-cut carambola chips abroad show that the combination of various browning inhibitors is effective in controlling browning.
4.2 hardness is reduced and the structure is transparent.
Pan Yonggui et al. [10] studied pineapple slices and found that the hardness of pineapple slices decreased rapidly. The mechanism may be that ethylene injury and respiratory injury accelerate the senescence process of fruits and vegetables, especially mutant fruits, and induce some activities of pectinase, cellulase, lipase and peroxidase, which cause tissue cells to collapse and pulp to soften. Cell rupture caused by division leads to the activation of cell degrading enzymes or increased contact with substrates, which leads to the destruction of cells; The invasion of microorganisms will secrete pectinase and cellulase, which will destroy the tissues of fruits and vegetables. Tissue transparency is particularly serious in the segmentation of cantaloupe. After cutting cantaloupe, if the cutting temperature is too high or the cutting process is incorrect, the sliced cantaloupe will become transparent within a few hours, and the transparency rate can reach 60% of the whole sliced cantaloupe.
2065438+03 March Green Science and Technology Phase 3 5 Conclusion
After sorting, cleaning and cutting, fruits and vegetables no longer exist in a complete state, which leads to a series of physiological and biochemical changes, which will affect the quality of cut fruits and vegetables, and then affect the safety and shelf life of cut products. Therefore, the research on physiological and biochemical changes of cut fruits and vegetables has been widely concerned and needs further study.
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