Abstract Fresh-cut fruits and vegetables are favored by consumers because of their ready-to-eat, convenient, nutritious, fresh and safe characteristics. However, the mechanical damage caused by minimal processing is not conducive to the maintenance of the quality of fresh-cut fruits and vegetables, which limits the development of fruit and vegetable processing industry. Therefore, how to effectively maintain quality and extend shelf life is a key problem that needs to be solved in the processing of fresh-cut fruits and vegetables. Proper and precise packaging can improve the surrounding environment of fresh-cut fruits and vegetables, reduce the number of microorganisms, and achieve the effect of extending shelf life and maintaining quality. However, different types of fresh-cut fruits and vegetables often require different packaging techniques due to their own characteristics. In thi

s paper, the research progress of fresh-cut fruits and vegetables classification and their applicable packaging technologies are reviewed, and the preservation principles of low-temperature storage, modified atmosphere packaging, ozone water treatment, vacuum packaging, ultra-high pressure packaging, edible coating, nano-packaging and microcapsule packaging technologies and their applications in fresh-cut fruits and vegetables are systematically analyzed.

Fresh-cut fruits and vegetables, also known as semi-processed fruits and vegetables or lightly processed fruits and vegetables, are freshly washed, peeled and cored, cut, preserved, packaged and other treatments to make them fresh, nutritious, convenient, green and healthy ready-to-eat products. Fresh-cut fruits and vegetables are favored by consumers for their freshness, convenience, and speed, and have been widely supplied to the catering and retail industries around the world [1]. However, minimal processing will change the integrity of fruits and vegetables, reduce the quality of fresh-cut fruits and vegetables, and limit the development of their processing industry [2]. Therefore, how to effectively maintain the quality and extend the storage period is the main problem that needs to be solved in the processing of fresh-cut fruits and vegetables. Proper packaging can slow down the deterioration of fresh-cut fruits and vegetables by reducing microbial infection, delaying enzymatic and non-enzymatic browning, and reducing tissue structure metabolism and respiration intensity, thereby extending the shelf life of fresh-cut fruits and vegetables [3]. However, different types of fresh-cut fruits and vegetables usually need to be packaged in different hardcover fresh-keeping packaging due to their different respiratory strength, nutrients, browning degree, etc. Therefore, it is of great significance to choose the appropriate hardcover packaging technology for the production of fresh-cut fruits and vegetables and the development of its industry. This paper classifies fruits and vegetables, summarizes the fresh-keeping packaging technology suitable for different types of fresh-cut fruits and vegetables, and introduces the hardcover fresh-keeping packaging technology used in fresh-cut fruits and vegetables at home and abroad, in order to provide a certain reference and reference for the further research and promotion of hardcover fresh-cut packaging technology of fresh-cut fruits and vegetables in China.

1] 1 Precise fresh-keeping packaging technology for different types of fresh-cut fruits and vegetables

2] Different types of fresh-cut fruits and vegetables usually require different precision packaging technologies to maintain their appearance, nutrient content, freshness and other qualities due to their different respiratory strength, water content, nutrient content, browning speed and texture. Leafy vegetables (celery, lettuce, spinach, etc.) are fresh-cut vegetables due to their soft texture and easy damage to the leaves, which makes them unable to be preserved by packaging technologies with high pressure such as ultra-high pressure and vacuum treatment, and usually uses ozone treatment [4], low-temperature storage [5], modified atmosphere packaging [6] and other technologies for fresh-keeping packaging. Browning of cut potatoes, apples, and pears is highly susceptible, and packaging technologies such as heat treatment [7], modified atmosphere packaging [7], and edible coating film [8] can significantly inhibit browning, so the application of these technologies to fresh-cut fruits and vegetables that are prone to browning can maintain the good sensory quality of fruits and vegetables. Fresh-cut fruits and vegetables (apples, pears) are easy to store due to their low respiration intensity, and are usually stored and packaged by edible coating [8] and heat treatment [7]. The fruits and vegetables of stone fruits (peaches, plums, apricots, etc.) are not easy to store due to their high respiration intensity. Ultra-high pressure combined with vacuum [9] and nano-ZnO packaging technology [10-11] can be used for preservation. It can be seen that different types of fresh-cut fruits and vegetables need to be preserved by different packaging technologies due to their own characteristics. Table 1 summarizes the precision preservation packaging technologies applicable to different types of fresh-cut fruits and vegetables.

3] 2. Fresh-cut fruits and vegetables preservation and packaging technology

4] 2.1 Low temperature preservation technology

5] Temperature is an important factor affecting the quality deterioration of fresh-cut fruits and vegetables during storage, and low-temperature storage is the most common method for fresh-cut fruits and vegetables to preserve their freshness [2]. Low temperature can inhibit the activity of various oxidases, reduce the respiratory intensity and enzymatic browning reaction rate of fresh-cut fruits and vegetables, thereby controlling the physiological and biochemical reactions in fruits and vegetables, delaying aging and spoilage, and inhibiting the growth and reproduction of spoilage bacteria, thereby achieving a good preservation effect [13]. However, when the temperature is lower than a certain degree, it will have a chilling effect, which is not conducive to the storage of fresh-cut fruits and vegetables, so the use of low-temperature storage and preservation needs to pay attention to temperature control. Low-temperature storage is a basic form of preservation, which is usually used in combination with technologies such as vacuum packaging and edible coating. However, not all fresh-cut fruits and vegetables are suitable for low-temperature storage, such as bananas, pears and other fresh-cut fruits and vegetables that are prone to cold damage, which are not easy to be stored at a lower temperature for freshness, and are usually packaged by heat treatment [15], edible coating [2] and other technologies.

6] Table 1 Precise fresh-keeping packaging technology for different types of fresh-cut fruits and vegetables

7] Table 1 Precise fresh-keeping packaging technology for different types of fresh-cut fruits and vegetables

8] 

9] 1] Using fresh-cut lettuce as raw material, the quality of vacuum-packed fresh-cut lettuce during low-temperature storage was studied. The results showed that vacuum packaging combined with low-temperature storage could effectively alleviate the weight loss rate, titratable acid content and strong respiration of fresh-cut lettuce

2.2 Modified atmosphere packaging

Modified atmosphere packaging (MAP) is a method of extending the shelf life of fresh-cut fruits and vegetables by adjusting and controlling the proportion of gases [7]. It is a preservation technology that puts fresh-cut fruits and vegetables into sealed packaging bags on the basis of low-temperature storage, and changes the proportion of storage environment gases at the same time. The principle of preservation is to reduce the fermentation and decay caused by the respiration of fresh-cut fruits and vegetables by reducing the O2 concentration and increasing the CO2 concentration, and at the same time reduce the growth and reproduction rate of microorganisms and the amount of ethylene produced [13], so as to prolong the shelf life of fresh-cut fruits and vegetables. Modified atmosphere packaging can be divided into two types: self-initiated modified atmosphere packaging (MAP) and controlled modified atmosphere packaging (CAP).

2.2.1 Spontaneous modified atmosphere packaging

Spontaneous modified atmosphere packaging uses the respiration of fresh-cut fruits and vegetables to consume O2 and produce CO2, which gradually forms a modified atmosphere environment with low O2 and high CO2 in the packaging bag, so as to achieve the purpose of fresh-cut fruits and vegetables [3]. GHIDELLI ET AL. [38] EVALUATED THE EFFECT OF THE COMBINATION OF SOYBEAN PROTEIN EDIBLE COATING AND TRADITIONAL AND SPECIAL MODIFIED ATMOSPHERE PACKAGING ON THE QUALITY OF FRESH-CUT EGGPLANT DURING STORAGE, AND FOUND THAT TRADITIONAL MODIFIED ATMOSPHERE PACKAGING COULD MAINTAIN FRUIT HARDNESS AND FRUIT WEIGHT, AND THE STORAGE PERIOD OF FRESH-CUT EGGPLANT COULD REACH 6 D, WHILE THE COMMERCIAL SHELF LIFE OF FRESH-CUT EGGPLANT COULD BE EXTENDED TO 8~9 D WITH SPECIAL MODIFIED ATMOSPHERE (80 kPa O2). In addition, self-initiated modified atmosphere packaging is also commonly used in the fresh-keeping packaging of fresh-cut cantaloupe, apples, carrots, lettuce, broccoli and other fruits and vegetables, and has achieved good preservation results [3].

2.2.2 Control of gas packaging

Controlled gas packaging refers to artificially establishing the proportion of O2, CO2, N2 and other gases required for the modified atmosphere packaging of fresh-cut fruits and vegetables, that is, selecting gases that can better maintain the quality of fresh-cut fruits and vegetables to replace the gas components in the original packaging, so as to extend the shelf life of fresh-cut fruits and vegetables [3]. MAP usually needs to maintain the internal content of 2%~5% O2 and 5%~10% CO2 in fresh-cut fruits and vegetables to maintain the quality of fresh-cut fruits and vegetables [13]. LI et al. [17] studied the effects of low-temperature storage at 4 °C combined with 64 μm PP film modified atmosphere packaging on the quality and shelf life of fresh-cut kiwifruit, and showed that modified atmosphere packaging with 10% CO2 and 5% O2 could maintain fruit firmness, total phenols and chlorophyll content, and that 10% CO2 or 40% CO2 modified atmosphere packaging could reduce microbial infection. However, in the process of storage and preservation of fresh-cut fruits and vegetables, the O2 concentration is easy to be too low, which will cause damage to fresh-cut fruits and vegetables due to anaerobic respiration, which is not conducive to storage. Nitrox, Ar, and N2O modified atmosphere packaging (MAP) can solve the disadvantages of ALC, and HEC is an alternative form of LEC and is suitable for fresh-cut fruits and vegetables such as mushrooms, celery, and kiwifruit that are prone to browning and fermentation [3].

2.3 Ozone water treatment

Ozone water treatment is a physical preservation technique [16]. Due to its significant antimicrobial activity against bacteria, fungi, viruses, and fungal spores, it has attracted extensive attention from the food preservation industry in recent years [35]. It acts on the lipoproteins of the outer shell mainly by penetrating the cell wall

2.5 Ultra-high pressure fresh-keeping packaging technology

Ultra-high pressure preservation technology is a non-thermal processing technology, which refers to the high-pressure treatment of fresh-cut fruits and vegetables at room temperature or lower temperature of more than 100 MPa, which deforms the protein and passivates the activity of enzymes in fresh-cut fruits and vegetables under high pressure, and kills most of the bacteria, molds and yeasts at the same time[41]。

The principle of ultra-high pressure preservation mainly affects the physiological activities of microorganisms by causing changes in the morphology of microbial cells and destroying the structure of their cell walls, cell membranes, and intercellular spaces, which in turn leads to the death of microorganisms [42]. In addition, the non-covalent bonds and tertiary structure of proteins in bacterial proteins were destroyed after ultra-high pressure treatment, resulting in protein coagulation and reduced catalytic activity of quality enzymes contained in fresh-cut fruits and vegetables. Therefore, ultra-high pressure treatment can achieve sterilization and enzyme inactivation, thereby reducing the risk of foodborne diseases and prolonging the storage period of fresh-cut fruits and vegetables. However, due to the high pressure, the ultra-high pressure fresh-cutting packaging technology is not suitable for fresh-cut fruits and vegetables with soft texture such as bananas, lettuce, spinach, etc., but is often used for fresh-cut fresh-cut fruits and vegetables with hard texture such as apples and cantaloupe.

2.6 Edible coating film preservation packaging technology

Coating preservation technology is usually used in the inner packaging of fresh-cut fruits and vegetables, which refers to soaking fresh-cut fruits and vegetables in a non-toxic liquid polymer edible coating agent to form a semi-breathable film with pathway and barrier properties on the surface of fresh-cut fruits and vegetables, so as to prolong the shelf life of fresh-cut fruits and vegetables [1].

After minimal processing, a large amount of surface area is exposed to air, which causes the phenolic substances in the tissues of fruits and vegetables to come into contact with a large amount of oxygen, and under the catalysis of polyphenol oxidase (PPO), the accumulation of quinone occurs, and the quinone is further oxidized and polymerized into brown pigments, i.e., enzymatic browning occurs [1]. After edible film packaging, it can effectively prevent a large amount of oxygen in the air from entering the fresh-cut fruits and vegetables, effectively inhibit the activity of polyphenol oxidase, slow down the browning of fruits and vegetables, and achieve the effect of improving the storage quality of fruits and vegetables [45]. Fruit and vegetable cutting causes tissue structure damage, which destroys the original protective system, and the outflow of juice provides good nutrients for microbial growth and reproduction [1]. Film-forming substances such as chitosan have certain antibacterial and bactericidal effects, and the treatment of fresh-cut fruits and vegetables with them can reduce the chance of microbial infection. At the same time, the coating treatment can also inhibit the respiration of fresh-cut fruits and vegetables, delay the formation of ethylene and reduce the metabolic rate, so as to maintain the quality and stability of the product. However, edible coating film will destroy the shape and structure of vegetable leaves, causing them to wilt, so it is not suitable for fresh-cut leafy vegetables.

2.7 Nano fresh-keeping packaging technology

Nano fresh-keeping packaging is a new research field, and nano packaging materials mainly refer to the sum of a class of food packaging materials that use nanotechnology to nano-add, nano-modify, and nano-synthesize packaging products to make them have certain characteristics or functions [48]. Nanoparticles can significantly improve raw materials