Fresh fruits and vegetables are very perishable and susceptible to postharvest diseases which limit the storage period and marketing life of them. Moreover, postharvest decay results in substantial economic losses around the world. As is known, synthetic fungicide treatment has long been the main method for controlling postharvest diseases [1]. However, there is increasing international concern over the indiscriminate use of synthetic fungicides on crops because of the possible harmful effects on human health [2] and the emergence of pathogen resistance to fungicides [3]. Therefore, new alternatives for controlling postharvest diseases which have good efficacy, low residues, and little or no toxicity to non-target organisms are in urgent demand.

A great many alternative strategies, including biological control with antagonistic microorganisms, applications of plant bioactive compounds, refrigerated storage, heat treatment, high pressure processing and modified atmosphere storage [4,5], have been used to control postharvest diseases of fruits and inhibit growth of the pathogens. However, all these methods could not control postharvest diseases as effectively as synthetic fungicides. And some of the processes also have limitations, such as partial destruction of quality attributes of food products, especially heat-labile nutrients and sensory attributes [6].

Chitin, together with its derivative chitosan, has been reported as a promising alternative to control postharvest diseases. Chitin is the second most abundant biopolymer renewable source in nature after cellulose, which has a versatile application potential in the agriculture-food industry [7], for instance, as a biopesticide, which has been approved by the U.S. Environmental Protection Agency (EPA), and as a food additive, approved by the U.S. Food and Drug Administration (FDA). Similarly, chitosan has become a prospective alternative treatment for fruit and vegetables due to its natural character, antimicrobial activity, and elicitation of defense responses in plant tissue [8,9]. Chitin or chitosan has been used to control postharvest diseases of many fruits such as pear [10], strawberry [11,12], table grape [13], tomato [14], citrus [15], and longan [16].

This review summarizes the efficiency of chitin and chitosan on controlling postharvest diseases which consequently prolongs the shelf-life of fruits, and its possible mechanisms involved. New research approaches to fully understand the mechanism of chitin and chitosan against fungal pathogens are also suggested.

Chitin and chitosan are polysaccharides, chemically similar to cellulose, differing only by the presence or absence of nitrogen [17]. Chitin, a naturally abundant mucopolysaccharide, and the supporting material of crustaceans, insects, etc., is well known to consist of 2-acetamido-2-deoxy-β-d-glucose though a β (1→4) linkage. Chitin can be degraded by chitinase. Its immunogenicity is exceptionally low, in spite of the presence of nitrogen. It is a highly insoluble material resembling cellulose in its solubility and low chemical reactivity. It may be regarded as cellulose with hydroxyl at position C-2 replaced by an acetamido group. Like cellulose, it functions naturally as a structural polysaccharide. Chitin is a white, hard, inelastic, nitrogenous polysaccharide and the major source of surface pollution in coastal areas. Chitosan is the N-deacetylated derivative of chitin, although this N-deacetylation is almost never complete. A sharp nomenclature with respect to the degree of N-deacetylation has not been defined between chitin and chitosan [18,19]. The structures of cellulose, chitin and chitosan are shown in Figure 1 [20].

Chitin and its derivative chitosan have been of interest in the past few decades due to their potential broad range of industrial applications [21,22]. However, there has been limited attention paid to the food application of these versatile biopolymers. They offer a wide range of unique applications including bioconversion for the production of value-added food products [23–25], preservation of foods from microbial deterioration [26–30], formation of biodegradable films [31–36], recovery of waste material from food processing discards [37–44], purification of water [45–48] and clarification and deacidification of fruit juices [49–53]. In this text, we especially pay attention to the antimicrobial function of chitin and chitosan in fruits. Hernández-Lauzardo et al. have reported the significant effect of chitosan on inhibiting three isolates of Rhizopus stolonifer obtained from several fruits [54]. It has been reported that chitin and chitosan are effective in reducing postharvest diseases of fruits and vegetables by inhibiting spore germination, germ tube elongation, mycelial growth of fungal phytopathogens, enhancing the efficacy of antagonistic yeasts, and boosting the activity of defense-related enzymes or pertinent substances [55–60].

Recently, the method of using chitin and chitosan to control postharvest diseases of fruits was developed. Chitosan at low molecular weight (LMWC) has been reported to control postharvest diseases of citrus fruit [15]. The results indicated that LMWC significantly inhibited the decay of citrus fruit caused by Penicillium digitatum, Penicillium italicum, Botrydiplodia lecanidion, and Botrytis cinerea after 14 days storage at 25 °C, and is more effective than TBZ and high molecular weight chitosan (HMWC) (Table 1). Meanwhile, low molecular weight chitosan coating beneficially influenced firmness, total soluble solid content, titratable acidity, ascorbic acid content and water content of citrus fruit after 56 days of storage at 15 °C (Table 2). Bhaskara Reddy et al. [12] found that pre-harvest chitosan sprays effectively inhibited the postharvest decay of strawberry fruit caused by Botrytis cinerea during storage at 3 and 13 °C, and the decay decreased with increasing chitosan concentration (Figure 2). Furthermore, fruits from chitosan sprayed plants were firmer and ripened at a slower rate as indicated by anthocyanin content and titratable acidity than berries from non-treated plants (Figures 3 and 4).

In addition to being singularly applied, there are many reports on combined applications of chitosan with other antifungal compounds. Yu et al. [61] found that chitosan applied alone or with Cryptococcus laurentii could effectively inhibit the blue mold rot caused by Penicillium expansum in apple fruit after seven days of incubation at 20 °C. When applied alone, treatment with chitosan at the highest concentration (1%) and the lowest viscosity (12 cP) was the most effective. When used in combinations, treatment of Cryptococcus laurentii with chitosan at a concentration of 0.1% and lowest viscosity (12 cP) was the most effective (Table 3). Similar results were found when using chitosan coating with postharvest calcium on extending shelf-life of strawberries [62]; and chitosan with ethanol on controlling postharvest gray mold of table grapes caused by Botrytis cinerea [63].

Biological antagonists have already been shown to effectively inhibit the postharvest decay of fruit in recent years [64–67]. However, for biological control to be accepted as an economically viable option, consistency and efficacy of antagonistic yeasts in controlling postharvest diseases must be enhanced [68,69]. Many attempts have been proposed to improve the performance of postharvest biocontrol yeasts. Physiological manipulation may be a useful method [69]. Recently, enhancement of the biocontrol efficacy of antagonists to postharvest diseases of fruits by addition of chitin or chitosan to the growth medium was reported. Yu et al. [10] found that the disease incidence and lesion diameter of blue mold rot caused by Penicillium expansum in pear fruit was significantly inhibited by Cryptococcus laurentii which was cultivated in nutrient yeast dextrose broth (NYDB) media amended with chitin, especially at the optimal concentration (1.0%) (Figures 5 and 6). Similarly, our research team used Rhodotorula glutinis cultivated with NYDB amended with chitin or nutrient yeast chitin broth (NYCB: Chitin as the sole carbon source instead of dextrose in the media of nutrient yeast dextrose broth) to control the grey mold decay caused by Botrytis cinerea in strawberries [11], we found that the antagonistic activity of R. glutinis was greatly enhanced by chitin inducing incubation (0.5% chitin), which resulted in a significant reduction of the disease incidence (Figure 7).

Because of the positive charge on the C₂ of the glucosamine monomer below pH 6, chitosan is more soluble and has a better antimicrobial activity than chitin [70]. Therefore, the application on controlling postharvest decay of fruits and the possible mechanisms discussed mostly rest on chitosan. The exact mechanism of the antimicrobial action of chitin, chitosan, and their derivatives is still imperfectly known, but different mechanisms have been proposed [71].

In conlusion, chitin and its derivative chitosan have shown a great potential as natural biodegradable substances which have anti-microbial activities. Previous studies have indicated that chitin and chitosan could effectively inhibit postharvest diseases of fruits by direct inhibition on spore germination, germ tube elongation and mycelial growth of phytopathogens and indirect inducement of defense-related enzymes, such as POD, PPO, PAL, GLU. However, the mode of action for chitin and chitosan controlling postharvest diseases of fruits are still limited and unclear. Therefore, to fully understand the mechanism of chitin and chitosan against fungal pathogens and the function in inducing defense response of fruits to pathogen infection, new approaches at the molecular and proteomic level, including the separation and identification of differential expression genes and differential expression proteins, are really needed in further studies.