Introduction
============

Chitosan is the linear polysaccharide of glucosamine and N-acetylglucosamine units joined by β-1,4-glycosidic links and it is obtained by deacetilation of chitin through exposure to NaOH solutions or to the enzyme chitinase. Chitosan and chitin are naturally occurring polymers. For their biocompatibility and biosafety, their applications are widespread in many industries, such as cosmetology, food, biotechnology, pharmacology, medicine, and agriculture ([@B20]; [@B48]). In particular, chitosan has increasing interest in plant protection as a natural fungicide and plant defense booster, and meets the interest of many researchers, that used it to prolong the storage of an array of fruit and vegetables worldwide. Chitosan was the first compound in the list of basic substances approved in the European Union for plant protection purposes (Reg. EU 66 2014/563), for both organic agriculture and integrated pest management. A comprehensive review on the available data on the effectiveness of chitosan was published recently, for its preservation of fruit and vegetables, both alone and in combination with other treatments, and its mechanisms of action ([@B63]). However, the increasing knowledge of this biopolymer (Figure [1](#F1){ref-type="fig"}) and the fast advances in basic and applied research in this field require a more focused and schematic update based on the last 5 years of investigations (2013--2018). The reader can then focus on specific aspects from the long list of other reviews that have appeared on the subject, among which some have focused on the applications of chitosan to fruit and vegetables ([@B8]; [@B9]; [@B83]). When applied to plants, chitosan shows triple activity: (i) elicitation of host defenses; (ii) antimicrobial activity; and (iii) film formation on the treated surface. We will cover the recent information on these issues in the following sections, which is also listed comprehensively in the Tables, with examples of these applications.

![Number of documents available on Scopus through searches with keywords 'chitosan' and 'postharvest' in 'Article title, Abstract, and Keywords' or in 'All fields' published over the last 30 years (accessed on 6 November 2018).](fmicb-09-02745-g001){#F1}

Effectiveness of Chitosan in the Control of Postharvest Decay of Fruit
======================================================================

The potential effectiveness of chitosan as a coating for fresh fruit was first proposed by [@B57]. The first *in-vivo* application of chitosan on fruit was in the Josep Arul Laboratory, by Ahmed El Ghaouth, who produced a list of papers through the last decade of the last century. These included [@B22], where they applied chitosan to strawberries and other fruit, both alone and in combinations with other potential biocontrol agents, which then contributed to the develop of some commercial formulations. Following these promising investigations, and with the growing need for alternatives to the use of synthetic fungicides, chitosan use became popular, and it was proposed to be part of a new class of plant protectants ([@B8]). Chitosan coatings have now been applied to numerous temperate and subtropical fruit, both alone and in combination with other treatments (Tables [1](#T1){ref-type="table"}--[3](#T3){ref-type="table"}), with generally additive, and in some cases synergistic, effectiveness ([@B65]).

###### 

Postharvest chitosan treatments with other applications for storage decay of temperate fruit.

  Fruit          Decay agent                                                                        Combination with chitosan                                                                                               Reference
  -------------- ---------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------------------------------- ----------------
  Table grapes   *Botrytis cinerea*                                                                 Salicylic acid                                                                                                          [@B74]
                 General decay                                                                      Glucose complex                                                                                                         [@B30]
                 *Aspergillus niger, Rhizopus stolonifer*                                           --                                                                                                                      [@B16]
                 *Fusarium oxysporum*                                                               --                                                                                                                      [@B36]
                 General decay                                                                      --                                                                                                                      [@B27]
                 General decay                                                                      Ultraviolet-C                                                                                                           [@B29]
                 General decay                                                                      --                                                                                                                      [@B2]
                 *Aspergillus niger, Botrytis cinerea, Penicillium expansum, Rhizopus stolonifer*   Menta essential oil                                                                                                     [@B33]
                 *Botrytis cinerea*                                                                 Salvia officinalis essential oil                                                                                        [@B39]
  Strawberry     *Botrytis cinerea*                                                                 Lavander and thyme essential oil                                                                                        [@B71]
                 General decay                                                                      Poeny extract                                                                                                           [@B58]
                 *Penicillium expansum, Rhizopus stolonifer*                                        Olive oil processing waste                                                                                              [@B41]
                 Total microbial load                                                               Natamycin, nisin, pomegranate, grape seed extract                                                                       [@B21]
                 Total microbial load                                                               Quinoa protein-chitosan and quinoa protein-chitosan-sunflower oil                                                       [@B77]
                 Total microbial load                                                               Sodium benzoate and potassium sorbate                                                                                   [@B76]
                 *Botrytis cinerea*                                                                 Zataria multiflora essential oil                                                                                        [@B54]
                 *Rhizopus stolonifer*                                                              Cinnamon leaf essential oil containing oleic acid                                                                       [@B61]
                 General decay                                                                      --                                                                                                                      [@B10]
                 General decay                                                                      Geraniol and thymol                                                                                                     [@B6]
                 General decay                                                                      Carboxymethyl cellulose, hydroxypropylmethyl cellulose                                                                  [@B32]
                 *Botrytis cinerea*                                                                 Nanosized silver-chitosan composite                                                                                     [@B55]
                 General decay                                                                      Beeswax                                                                                                                 [@B78]
                 *Botryosphaeria* sp.                                                               --                                                                                                                      [@B80]
  Pear           General decay                                                                      Cellulose nanocrystals                                                                                                  [@B19]
                 General decay                                                                      Acylated soy protein isolate and stearic acid                                                                           [@B81]
  Apple          General decay                                                                      Olive waste extracts                                                                                                    [@B42], [@B41]
                 *Penicillium expansum*                                                             --                                                                                                                      [@B15]
                 *Venturia inaequalis*                                                              --                                                                                                                      [@B25]
                 *Penicillium expansum*                                                             --                                                                                                                      [@B49]
                 Calyx senescence                                                                   V                                                                                                                       [@B18]
  Citrus         *Penicillium digitatum, Penicillium italicum*                                      Silver nanoparticles                                                                                                    [@B3]
                 Colletotrichum gloeosporioides                                                     *Pichia membranaefaciens*                                                                                               [@B85]
                 *Penicillium digitatum, Penicillium italicum*                                      Cress and/or pomegranate extracts                                                                                       [@B75]
                 *Penicillium digitatum*                                                            Clove oil                                                                                                               [@B73]
                 *Penicillium digitatum*                                                            Cyclic lipopeptide antibiotics from *Bacillus subtilis*                                                                 [@B79]
                 General decay                                                                      Carboxymethyl cellulose                                                                                                 [@B4]
                 Total microbial load                                                               Silver and zinc oxide nanoparticles                                                                                     [@B40]
  Peach          *Monilinia laxa*                                                                   Polyethylene terephthalate punnets containing thyme oil and sealed with chitosan/boehmite nanocomposite lidding films   [@B13]
                 General decay                                                                      γ-ray                                                                                                                   [@B24]
                 *Monilinia fructicola*                                                                                                                                                                                     [@B53]
                 *Monilinia laxa, Botrytis cinerea, Rhizopus stolonifer*                            --                                                                                                                      [@B28]
  Sweet cherry   General decay                                                                      --                                                                                                                      [@B60]
                 --                                                                                 Hydroxypropyl methylcellulose                                                                                           [@B72]
  Plum           General decay                                                                      Ascorbic acid                                                                                                           [@B50]
                                                                                                                                                                                                                            

###### 

Postharvest chitosan treatments with other applications for storage decay of subtropical fruit.

  Fruit         Decay agent                                                                              Combination with chitosan                            Reference
  ------------- ---------------------------------------------------------------------------------------- ---------------------------------------------------- -----------
  Mango         Anthracnose (*Colletotrichum gloeosporioides*)                                           Spermidine                                           [@B38]
                Anthracnose (*Colletotrichum gloeosporioides*), stem-end rot (*L. theobromae* strains)   Lactoperoxidase system incorporated chitosan films   [@B45]
                Anthracnose                                                                              *Mentha piperita* L. essential oil                   [@B17]
                Anthracnose (*Colletotrichum gloeosporioides*), stem-end rot (*L. theobromae* strains)   Lactoperoxidase system incorporated chitosan films   [@B45]
                Anthrcanose                                                                              *Mentha piperita* L. essential oil                   [@B17]
  Citrus        Green mold (*Penicillium digitatum*)                                                     *Bacillus subtilis* ABS-S14                          [@B79]
                Anthracnose (*Colletotrichum gloeosporioides*)                                           *Pichia membranifaciens*                             [@B85]
  Avocado       Anthracnose (*Colletotrichum gloeosporioides*)                                           Thyme oil                                            [@B11]
  Tomato        *Alternaria alternata*                                                                   Methyl jasmonate                                     [@B12]
                *Aspergillus niger, Rhizopus stolonifer*                                                 Essential oil from *Origanum vulgare* L              [@B7]
  Pomegranate   *Penicillium* spp., *Pilidiella granati*                                                 Lemongrass film                                      [@B56]
                                                                                                                                                              

###### 

Preharvest chitosan treatments with other applications for storage decay of temperate fruit.

  Fruit          Decay                                                           Combination with chitosan      Reference
  -------------- --------------------------------------------------------------- ------------------------------ ----------------
  Citrus         *Penicillium digitatum*                                         *Rhodosporidium paludigenum*   [@B52]
  Peach          General decay                                                   Calcium chloride               [@B31]
  Jujube fruit   *Alternaria alternata*                                          --                             
  Table grapes   *Botrytis cinerea*                                              Salicylic acid                 [@B74]
                 *Botrytis cinerea*                                              --                             [@B27]
  Strawberry     *Botrytis cinerea* and *Rhizopus stolonifer*                    --                             [@B64]; [@B26]
                 *Botrytis cinerea*                                              --                             [@B51]
                 General decay                                                   --                             [@B70]
  Sweet cherry   *Monilinia laxa, Botrytis cinerea*, and *Rhizopus stolonifer*   --                             [@B27]
                                                                                                                

Chitosan Eliciting Activity
===========================

Chitosan is known to elicit plant defences against several classes of pathogens, including fungi, viruses, bacteria and phytoplasma ([@B23]). Moreover, in some studies, its eliciting activity was reported to be effective on pests ([@B5]). Based on our experience, the eliciting activity of chitosan accounts for 30 to 40% of its effectiveness in the control of postharvest decay of fresh fruit (Figure [2](#F2){ref-type="fig"}). The extent of this eliciting activity depends on the reactivity of the fruit tissues, and it is well known that fruit responses to stress decline with ripening ([@B67]). This eliciting activity of chitosan has been studied since the 1990's, which started with monitoring of the activities of enzymes linked to the defense mechanisms (e.g., chitinase) in different fruit (e.g., strawberries) ([@B22]). This was followed by investigations on other berries, citrus fruit and table grapes, among others. More recently, tools such as qRT-PCR and in recent years RNA-Seq (RNA-Sequencing) have allowed important information to be gained, first at the level of single gene expression, and then later at the level of global gene expression ([@B82]). This has provided good understanding of the multiple actions of chitosan applications and how they affect a number of physiological changes in fruit. As an example, the application of chitosan to strawberries at different times before harvest can affect the expression of a thousand or more genes ([@B46]). Some examples that have become available in the literature over the last 5 years are listed in Table [4](#T4){ref-type="table"}, which deal with the physiological changes that can occur in chitosan-treated fruit, both when the biopolymer is applied alone, and when it is combined with other treatments. The eliciting activity of chitosan is particularly effective toward latent infections, as a more reactive fruit can stop the infection process, through a balance that resembles quorum sensing, which is well known for bacterial infections ([@B59]).

![Proportion of antimicrobial, eliciting, and film-forming properties of chitosan.](fmicb-09-02745-g002){#F2}

###### 

Physiological changes that can occur in fresh fruit after chitosan treatment, alone or in combination with other applications.

  Fruit          Physiological change                                                                                                                                                                                                                                    Combination with chitosan                                                                                               Reference
  -------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------------------------------- -----------
  Apple          20 genes involved in defence responses, metabolism, signal transduction, transcription factors, protein biosynthesis, cytoskeleton.                                                                                                                     --                                                                                                                      [@B49]
                 Total phenolic, flavonoids, antioxidants, pigments, weight loss                                                                                                                                                                                         Olive waste extract                                                                                                     [@B42]
  Peach          Malondialdehyde content                                                                                                                                                                                                                                 γ-ray                                                                                                                   [@B24]
                 Catalase, peroxidase, β-1,3-glucanase and chitinase                                                                                                                                                                                                     --                                                                                                                      [@B53]
                 Total soluble solids, weight loss, ascorbic acid content                                                                                                                                                                                                Silver and zinc oxide nanoparticles                                                                                     [@B40]
                 Color and fruit firmness                                                                                                                                                                                                                                Polyethylene terephthalate punnets containing thyme oil and sealed with chitosan/boehmite nanocomposite lidding films   [@B13]
                 Fruit firmness, weight loss, total soluble solids, total phenolic content, and titratable acidity                                                                                                                                                       Calcium chloride                                                                                                        [@B31]
  Plum           Fruit firmness, respiration rate, fruit color, polygalacturonase, superoxide dismutase, peroxidase, catalase, polyphenol oxidase, phenylalanine ammonia lyase and pectin methylesterase activities, superoxide free radicals, malondialdehyde content   Ascorbic acid                                                                                                           [@B50]
  Sweet cherry   Malondialdehyde content and superoxide dismutase, catalase, ascorbate peroxidase, polyphenol oxidase, guaiacol peroxidase lipoxygenase activities                                                                                                       --                                                                                                                      [@B60]
  Strawberry     Over 5000 differently expressed genes                                                                                                                                                                                                                   --                                                                                                                      [@B46]
                 18 defence genes                                                                                                                                                                                                                                        --                                                                                                                      [@B47]
                 Fruit color                                                                                                                                                                                                                                             --                                                                                                                      [@B26]
                 Fruit firmness, anthocyanin and total phenol content                                                                                                                                                                                                    --                                                                                                                      [@B70]
                 Weight loss, titratable acidity, pH, total soluble solids, total phenols, anthocyanin and ascorbic acid content, activity of polygalacturonase, pectin methyl esterase, β-galactosidase and cellulose                                                   Carboxymethyl cellulose, hydroxypropylmethyl cellulose                                                                  [@B32]
                 Weight loss                                                                                                                                                                                                                                             Lavander and thyme essential oil                                                                                        [@B71]
                 Titratable acidity, soluble solids content                                                                                                                                                                                                              --                                                                                                                      [@B10]
                 pH and soluble solids content                                                                                                                                                                                                                           Natamycin, nisin, pomegranate, grape seed extract                                                                       [@B21]
                 Weight loss, ascorbic acid                                                                                                                                                                                                                              Poeny extract                                                                                                           [@B58]
                 Weight loss, respiration rate, skin and flesh color, firmness, pH, titratable acidity, soluble solids content, reducing sugars content                                                                                                                  Beeswax                                                                                                                 [@B78]
                 Weight loss, firmness, color and total soluble solids content                                                                                                                                                                                           Sodium benzoate, potassium sorbate                                                                                      [@B76]
                 Weight losses, total soluble solids and titratable acidity                                                                                                                                                                                              Olive waste extract                                                                                                     [@B41]
                 Allergen-related gene                                                                                                                                                                                                                                   --                                                                                                                      [@B62]
  Table grapes   Phenylalanine ammonia lyase, chitinase, and β-1, 3-glucanase, phenolic compounds, respiration rate, weight loss, total soluble solids, titratable acidity                                                                                               Salicylic acid                                                                                                          [@B74]
                 Total phenols, flavonoids and ascorbic acid content, activities of peroxidase, polyphenoloxidase, polygalacturonase, and xylanase, fruit firmness                                                                                                       --                                                                                                                      [@B2]
                 Fruit color                                                                                                                                                                                                                                             --                                                                                                                      [@B36]
                 Weight loss, titratable acidity, pH and soluble solids content, resveratrol content                                                                                                                                                                     Ultraviolet-C                                                                                                           [@B29]
                 Weight loss, soluble solids content and titratable acidity                                                                                                                                                                                              Salvia officinalis essential oil                                                                                        [@B39]
                 Firmness, titratable acidity, soluble solids, color, weight loss                                                                                                                                                                                        Menta essential oil                                                                                                     [@B33]
                 Total soluble solids, ascorbic acid content, titratable acidity, weight loss, respiration rate, activities of peroxidase and superoxide dismutase                                                                                                       Glucose complex                                                                                                         [@B30]
                 Titratable acidity, soluble solids, color, firmness                                                                                                                                                                                                                                                                                                                             [@B16]
                 Chitinase activity, quercetin, myricetin, and resveratrol content                                                                                                                                                                                       --                                                                                                                      [@B28]
  Citrus         Chitinase and phenylalanine ammonia lyase                                                                                                                                                                                                               --                                                                                                                      [@B52]
                 640 differentially expressed genes, many involved in secondary metabolism and hormone metabolism pathways                                                                                                                                               --                                                                                                                      [@B14]
                 Fruit firmness, weight loss, total soluble solids                                                                                                                                                                                                       Carboxymethyl cellulose                                                                                                 [@B4]
                 Peroxidase and phenylalanine ammonia-lyase                                                                                                                                                                                                              Cyclic lipopeptide antibiotics from *Bacillus subtilis*                                                                 [@B79]
                 Contents of chlorophylls and total carotenoids                                                                                                                                                                                                                                                                                                                                  
                 Phenylalanine ammonia-lyase, β-1,3-glucanase, chitinase                                                                                                                                                                                                                                                                                                                         
  Jujube         Fruit firmness, cellulase, pectinase                                                                                                                                                                                                                    --                                                                                                                      [@B34]
  Pear           Total phenolic and flavonoid contents, superoxide dismutase, peroxidase and catalase activities, total antioxidant activity                                                                                                                             Calcium chloride                                                                                                        [@B43]
                 Malic acid-metabolising enzymes and related genes expression                                                                                                                                                                                            Calcium chloride                                                                                                        [@B44]
  Mango          Peroxidase (POD) and polyphenol oxidase (PPO) gene expression                                                                                                                                                                                           --                                                                                                                      [@B35]
  Kiwifruit      Induced gene expression and increased enzymatic activity of catalase, superoxide dismutase and ascorbate peroxidase                                                                                                                                     --                                                                                                                      [@B84]
                                                                                                                                                                                                                                                                                                                                                                                                 

Chitosan Antimicrobial Activity
===============================

Numerous studies on chitosan inhibitory activities toward numerous microrganisms have been carried out since the first report of almost half a century ago ([@B1]). The antimicrobial activities of chitosan against a wide range of plant pathogens have been confirmed by any of *in-vitro* and *in-vivo* studies. The antimicrobial activity of chitosan is one of its main properties, and this depends on the concentration at which it is applied. In the control of postharvest decay of fresh fruit, the antimicrobial activity can account for 35--45% of its effectiveness, as an antifungal barrier on a fruit inhibits the germination of fungal spores and slows down the rate of decay-causing fungi of already infected fruit, both latently and actively (Figure [2](#F2){ref-type="fig"}). A standard application rate of chitosan to provide a significant control of postharvest decay of fruit and vegetables can be considered 1%, except for the control of *Penicillia*, where higher concentrations may be needed to provide a good effectiveness. The degree of deacetylation and the molecular weight of chitosan characterize its properties, such as the number of positively charges of amino groups and therefore, its electrostatic interactions with different substrate and organisms at different pH. Chitosan with a higher degree of deacetylation, which has greater numbers of positive charges, would also be expected to have stronger antibacterial activities. On the other hand, numerous studies have generated different results relating to correlations between the chitosan bactericidal activities and its molecular weight ([@B63]). In addition, there are many differences between the chitosan antifungal and antibacterial activities and several mechanisms relating to these remain still unclear and further researches are needed ([@B63]).

Chitosan Film-Forming Properties
================================

Once applied to a plant surface by dipping or spraying, chitosan can form an edible coating, the properties of which (e.g., thickness, viscosity, gas, and water permeability) greatly depend on the acid in which the biopolymer is dissolved. The film-forming properties of chitosan account for 20--30% of the chitosan effectiveness in the control of postharvest decay of fruit and vegetables (Figure [2](#F2){ref-type="fig"}). Coating produces a barrier for gas exchanges and reduced respiration, and slows down fruit ripening. Of note, a less ripe fruit is less sensitive to postharvest decay.

Toward Large-Scale Commercial Applications
==========================================

When first used in experimental trials, chitosan needed to be dissolved in an acid (e.g., hydrochloric acid, acetic acid, which were among the most effective ones; see [@B66]), and then taken to the optimal pH (∼5.6) This approach can even take 1--2 days, and it is impractical for use by growers. More recently, several commercial chitosan formulations that can be dissolved in water have become available on the market to be used as a biopesticides (Table [5](#T5){ref-type="table"}). Some of these are formulated as powders, and then the cost of shipping is lower (although still higher compared to most of the commercially available synthetic fungicides), although the chitosan needs to be dissolved in water, in some cases a few hours before its application. This makes chitosan more difficult to use, as the grower wants to use an alternative to synthetic fungicides in the same way as a commercial compound, such that it should have the same effectiveness. This objective can be achieved with liquid formulations, which have concentrations of 2--15%. In this case, the cost of shipping is higher, as the volumes are larger due to the amounts of water that travel with the chitosan. In tests of three different commercial products, even when used at the same concentration, differential effectiveness was seen ([@B27]). The higher cost of chitosan treatment compared to standard applications might also induce companies toward the use of low doses (e.g., even well below 0.1%), Based on data in literature, the optimal dose is around 1%, while decreasing the concentration, the effectiveness declines. Furthermore, when the concentration of chitosan is decreased, its effectiveness also declines. However, applications to the plant canopy also need to take in account possible phytotoxic effects, mainly if repeated applications occur. This has been shown for grapevines ([@B67]), such that for these purposes a good concentration might be 0.5%. However, under some particular conditions, even low concentrations of chitosan (e.g., 0.02%) in a commercial formulation can be beneficial, such as for the improved storage of litchi ([@B37]).

###### 

Some chitosan-based commercial products that are available for control of postharvest diseases of fruit and vegetables.

  Product trade name   Company (Country)                                                     Formulation                 Active ingredient (%)
  -------------------- --------------------------------------------------------------------- --------------------------- -----------------------
  Chito plant          ChiPro GmbH (Bremen, Germany)                                         Powder                      99.9
  Chito plant          ChiPro GmbH (Bremen, Germany)                                         Liquid                      2.5
  OII-YS               Venture Innovations (Lafayette, LA, United States)                    Liquid                      5.8
  KaitoSol             Advanced Green Nanotechnologies Sdn Bhd (Cambridge, United Kingdom)   Liquid                      12.5
  Armour-Zen           Botry-Zen Limited (Dunedin, New Zealand)                              Liquid                      14.4
  Biorend              Bioagro S.A. (Chile)                                                  Liquid                      1.25
  Kiforce              Alba Milagro (Milan, Italy)                                           Liquid                      6
  FreshSeal            BASF Corporation (Mount Olive, NJ, United States)                     Liquid                      2.5
  ChitoClear           Primex ehf (Siglufjordur, Iceland)                                    Powder                      100
  Bioshield            Seafresh (Bangkok, Thailand)                                          Powder                      100
  Biochikol 020 PC     Gumitex (Lowics, Poland)                                              Liquid                      2
  Kadozan              Lytone Enterprise, Inc. (Shanghai Branch, China)                      Liquid                      2
  Kendal cops          Valagro (Atessa, Italy)                                               Liquid                      4
  Chitosan 87%         Korea Chengcheng Chemical Company (China)                             TC (Technical material)     87
  Chitosan 2%          Korea Chengcheng Chemical Company (China)                             SLX (Soluble concentrate)   2
                                                                                                                         

Concluding Remarks
==================

The effectiveness of chitosan application arises from the integrated combination of its three mechanisms of action. There are increasing consumer requests for fruit and vegetables to be free from residues of synthetic pesticides, such that the rules defined by the public administration have become more limiting in terms of the active ingredients allowed and the maximum residue limits. Also, large stores compete with each other to further reduce these limits, compared to the legal thresholds ([@B67]). These trends make the concept of the application of alternatives to synthetic fungicides more popular, and among these the main one that is already used in human medicine is chitosan, which is particularly welcomed by public opinion. These aspects have promoted further studies based on the multiple actions of chitosan on fruit and vegetables. Therefore, further increases in our knowledge are expected following the widespread practical application of chitosan due to the regulation of its use in agriculture and the interest of companies to promote chitosan-based products, with potential benefits for the growers, the consumers and the environment.

Author Contributions
====================

GR proposed the review, collected data on chitosan popularity over time and on commercial products, coordinated the authors, and wrote the article. EF collected papers on effectiveness of chitosan on temperate fruit and on the mechanisms of action in the tables, and helped with the writing. DS collected papers on effectiveness of chitosan on tropical fruit and on the mechanisms of action in the tables, and helped with the writing.

Conflict of Interest Statement
==============================

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

[^1]: Edited by: Boqiang Li, Institute of Botany (CAS), China

[^2]: Reviewed by: Xianghong Meng, Ocean University of China, China; Hongbing Deng, Wuhan University, China

[^3]: This article was submitted to Food Microbiology, a section of the journal Frontiers in Microbiology
