Major losses in quality and quantity of fresh fruits occur between harvest and consumption. Savings obtained through reduction of postharvest fruit losses is regarded as ‘a hidden harvest’. Through a better understanding of the respiration process of fresh fruits, several techniques have been developed that are successful in extending shelf life. Consumers around the world demand for food of high-quality, without chemical preservatives and an extended shelf life. Many storage techniques have been developed to extend the marketing distances and holding periods for commodities after harvest. Emerging research shows polysaccharides, bacteriocins, essential oils, enzymes, proteins and lipids are all natural coatings that have unrealized potential in food preservation. Among various methods to extend the post-harvest life of fruits, the use of the edible coatings has gained a good momentum.
The application of edible coatings is increasingly demonstrating to be a relatively new and simple technology effective in preventing the appearance and textural deterioration of several products. Edible coatings based on cellulose gums effectively delay ripening in some climacteric fruits like mangoes, papayas and bananas and significantly reduce enzymatic browning on sliced mushrooms
(Nisperos-carriedo et al., 1991). Several polysaccharides (chitosan, alginate, methylcellulose or pectin) and proteins (casein, collagen, gelatin, phaseolin, zein, soy or whey proteins), or mixtures of them, were shown to give rise to edible films effective as water vapour and gas barriers for a wide range of food products and as carriers for antimicrobials. In particular, antimicrobial containing films are recently gaining potential interest in reducing the deleterious effects caused by minimal cut processing of fresh fruits and vegetables
(Conte et al., 2009). Some of the most commonly used antimicrobials include benzoic acid, sorbic acid, lysozyme, bacteriocins and plant-derived secondary metabolites, such as essential oils and phytoalexins.
The mechanism by which edible coatings preserve fruits and vegetables is the establishment of a modified atmosphere around the product, which serves as a partial barrier to O
2 and CO
2, water vapour and aroma compounds, decreasing the respiration rate of the fruit and water loss and preserving texture and flavor
(Olivas et al., 2008). In addition to the use of edible coatings, food additives can be added to plant foods to control detrimental reactions
(Rojas-Grau et al., 2009). An edible coating is a thin layer that is deposited on the surface of a fruit and is co - consumed. It is used to improve handling properties, prevent moisture loss, to increase the shelf life and to reduce the need of packaging material during transport
(Rojas-Grau et al., 2009). Edible coatings act as moisture and oil barrier at low to intermediate RH because the polymers make effective hydrogen bonds. Ideal edible coatings have good eating properties, acceptable colour, odour, taste, flavor and texture and also act as barrier for microbes. The use of coating has gained importance in reducing the moisture loss and maintaining firmness
(Farrooqhi et al., 1988 and
Chouhan et al., 2005).
Effect of chitosan
Chitosan is modified, natural biopolymer derived from chitin, is obtained from waste products of the shellfish exoskeletons or the cell walls of some microorganisms and fungi and it has shown great aptitude for their application in food preservation. It is a cationic polysaccharide with a high molecular weight and a linear polymer which is composed of
β-1, 4-linked glucosamine (GlcN) with various quantities of N-acetylated GlcN residues. Chitosan has antibacterial and antifungal properties and is used for food protection. Antimicrobial films can be made from chitosan blended with essential oils and clays. Chitosan films with different combinations can increase the shelf life of food.
Recently, chitosan has received increased attention for its commercial applications in the biomedical, food and chemical industries. Use of chitosan in food industry is readily seen due to its several distinctive biological activities and functional properties. The antimicrobial activity and film-forming property of chitosan make it a potential source of food preservative or coating material of natural origin
(No et al., 2007). It has the property to create a semipermeable barrier that controls gas exchange and reduce water loss, thereby maintaining tissue firmness and reducing microbial decay of harvested fruit and vegetables
(Alvarez et al., 2013).
Jiang and Li (2001) studied the effects of chitosan coating in extending postharvest life of longan fruits as well as investigated the maintenance of their quality. Their study revealed that the application of chitosan coating reduced respiration rate and weight loss, delayed the increase in PPO activity and the changes in colour and eating quality and partially inhibited decay of fruit during storage. Furthermore, increasing the concentration of chitosan coating enhanced the beneficial effects of chitosan on postharvest life and quality of the fruit.
The application of chitosan coating delayed discoloration associated with reduced activities of PAL, PPO and POD as well as lower total phenolic content and slowed down the loss in eating quality associated with higher contents of total soluble solids, titratable acidity and ascorbic acid of fresh-cut CWC. Disease development of the fresh-cut CWC-treated chitosan coating was also inhibited compared to the control. Increasing the concentration of chitosan coating markedly enhanced the beneficial effects
(Pen and Jiang, 2003).
Dong et al. (2004) reported that application of chitosan coating retarded weight loss and the decline in sensory quality, with higher contents of total soluble solids, titratable acid and ascorbic acid and suppressed the increase in activities of PPO and POD and effectively maintained quality attributes and extended shelf life of the peeled fruits of litchi.
Application of chitosan coating delayed the decrease in anthocyanin content, increase in PPO activity and changes in colour index as well as reduction in eating quality decrease in concentrations of total soluble solids and titratable acidity and partially inhibited decay. In addition, treatment with chitosan coating was potential for shelf life extension at ambient temperature when litchi fruits were removed from cold storage
(Jiang et al., 2005).
Munoz et al. (2008) revealed that in strawberry fruits, the chitosan coating reduced respiration activity, thus delaying ripening and the progress of fruit decay due to senescence.
Chien et al., (2013) reported that chitosan coating had the ability to maintain the lightness of the sliced papayas. The chitosan coating on the sliced papaya effectively retarded water loss and inhibited the growth of microorganisms. The results reveal that applying a chitosan coating effectively maintained the quality attributes and prolonged the shelf life of the sliced papayas.
Effect of Aloe vera Gel
Aloe vera is a well known plant for its marvellous medicinal properties.
Aloe vera gel has been proven one of the best edible and biologically safe preservative coatings for different types of foods because of its film forming properties, antimicrobial actions and biodegradability and biochemical properties.
Aloe vera gel forms a protective layer against the oxygen and moisture of the air and inhibiting the action of micro-organisms that cause food borne illnesses through its various antibacterial and antifungal compounds
(Serrano et al., 2006). Aloe vera gel based edible coatings have been shown to prevent loss of moisture and firmness, control respiratory rate and maturation development, delay oxidative browning and reduce microorganism proliferation in fruits.
Serrano et al. (2006) coated grapes fruits with
Aloe vera gel and their study revealed that uncoated clusters showed a rapid loss of functional compounds, such as total phenolics and ascorbic acid. These changes were accompanied by reduction of the total antioxidant activity (TAA) and increases in total anthocyanins, showing an accelerated ripening process. On the contrary, table grapes coated with
Aloe vera gel significantly delayed the above changes, such as the retention of ascorbic acid during cold storage or SL. Consequently,
Aloe vera gel coating, a simple and non-contaminating treatment, maintained the functional properties during postharvest storage of table grapes.
Romero et al. (2006) studied the effect of
aloe vera gel coatings on postharvest treatment of sweet cherry and their study revealed that fruits treated with
Aloe vera gel decreased respiration rate, slowed down weight loss and colour changes, reduced softening and ripening, stem browning and decreased microbial populations significantly as well extended storage period.
Singh et al. (2011b) studied the effect of different concentrations of
Aloe vera gel coatings on refrigerated strawberry quality and shelf-life and their study revealed that strawberries treated with
Aloe vera gel (1: 3 ratio) significantly reduced weight loss, maintained colour, firmness, quality characteristics (TSS, acidity and ascorbic acid) and ultimately extended storability up to 16 days. The sensory analysis for taste, aroma and flavour further confirmed their findings.
Padmaja and Bosco (2014) reported that
Aloe vera gel of 1:3 ratios with a dipping period of 5 minutes effectively inhibited the undesirable physicochemical and physiological changes during storage of Jujube packaged in LDPE film, extending its shelf life to 45 days under refrigerated conditions. This edible coating along with surface cleaning of fruit with potassium meta bisulphate was able to reduce the initial microbial counts for both bacteria and fungi, which significantly increased the shelf life of edible coated jujube fruits to 45 days over storage. Jujube fruits with above treatment scored appreciable sensory scores as compared to control.
Nasution et al. (2015) studied the effect of additives in
Aloe vera gel (AV) coating on fresh-cut guava stored at 5
oC and 75-80% relative humidity. The additives used were 1.5% ascorbic acid (AA), 2% calcium chloride (CaCl
2) and 0.2% potassium sorbate (PS). The coated samples had less change in the color lightness and yellowness compared to the uncoated sample. AV + AA +PS-coated guava was the most acceptable sample with respect to sensory acceptance. Moreover, it gave the highest ascorbic acid content.
Sharmin et al. (2015) studied the effects of
aloe vera gel coating on storage behaviour of papaya at room temperature (29
oC-31
oC). Among the physico-chemical parameters, colour, physical changes, total weight loss and TSS contents increased significantly, whereas moisture content, vitamin C and titratable acidity decreased during storage. The overall results showed the superiority of 1.5%
aloe vera gel coating in extending the shelf-life of papaya up to 15 days compared to that of 0.5%, 1%
aloe vera gel coating and control papaya.
Ali et al. (2016) studied the effect of
Aloe vera coatings on shelf life of grapes. Their results revealed that 20 per cent
Aloe vera coating gave the best visual and physicochemical results and most effective and appropriate for the extension of shelf life of grapes. It was also found that the use of low temperature storage in combination with edible coating and packaging extends marketability by reducing moisture loss.
Effect of paraffin wax
Paraffin wax is a white or colourless soft solid derivable from petroleum, coal or oil shale that consists of a mixture of hydrocarbon molecules containing between twenty and forty carbon atoms. It is solid at room temperature and begins to melt above approximately 37
oC (99
oF); its boiling point is >370
oC (698
oF).
Sindhu et al. (2009) studied the effect of waxing on fruits of pear cv. Punjab Beauty. Their study revealed minimum PLW, shrivelling and spoilage percentage in the waxed fruits packed in the CFB cartons and maximum in non-waxed fruits packed in wooden boxes. No core browning was observed up to 60 days of cold storage in waxed fruits packed in CFB cartons. Waxed fruits could be cold stored for 75 days in CFB cartons with minimum weight loss, spoilage, excellent appearance and better fruit quality. CFB packaging proved better than wooden boxes.
Singh et al. (2011a) studied the effect of waxing on weight loss, decay loss and the chemical changes associated with ripening of purple passion fruit. The shelf life of paraffin waxed fruits was 25 days with maximum juice density and maximum sugars of the fruits.
Effect of CMC (Carboxy Methyl Cellulose)
CMC or cellulose gum is a cellulose derivative with carboxy methyl groups (-CH
2-COOH) bound to some of the hydroxyl groups of the glucopyranose monomers that make up the cellulose backbone. It is often used as its sodium salt, sodium carboxy methyl cellulose.
Togrul and Arslan (2004) treated peach and pear fruits with different compositions of emulsions to extend shelf-life of fruits. The coating of peach and pear surfaces with emulsions containing CMC extended the shelf-life of peach and pear to 12 and 16 days, respectively.
Hussain et al. (2010) used Carboxymethyl cellulose (CMC) coatings alone and in combination with gamma irradiation for maintaining the storage quality and extending shelf life of pear. CMC coating in combination of 1% w/v CMC and 1.5 kGy irradiation proved significantly (
P≤0.05) effective in maintaining the storage quality and delaying the decaying of pear.
There is increasing public interest in development of edible natural biodegradable coatings to replace the currently used commercial synthetic waxes for maintaining postharvest quality of citrus fruits.
Arnon et al. (2014) studied the efficacy of carboxy methyl cellulose (CMC) and chitosan in preserving postharvest quality of various citrus fruits, including ‘Or’ and ‘Mor’ mandarins, ‘Navel’ oranges and ‘Star Ruby’ grapefruit after simulated storage and marketing. In all citrus species, it was found that the CMC/chitosan bilayer coating was equally effective as the commercial polyethylene wax in enhancing fruit gloss. Furthermore, the CMC/chitosan bilayer coating slightly increased fruit firmness, especially of oranges and grapefruit, but was mostly not effective in preventing post-storage weight loss.
