POLYSACCHARIDES

A variety of polysaccharides, like cellulose, starch, pectin, carrageenan, alginates, pullulan, chitosan, and their derivatives, are used in the edible coatings formulations. Generally, these polymers are derived from the agricultural, animals and marine sources. They work by blocking the oxygen efficiently as their structure contains the network of hydrogen-bonds that lead to the desired modified atmosphere, thus extending the shelf life of fruits without creating the severe anaerobic conditions (Khan et al., 2013). However, they are poor in preventing the moisture transfer due to their hydrophilic and crystalline nature (Olivas et al., 2008), but may be used to reduce the moisture loss of some fruits on the basis of short-term storage. Polysaccharides impart the compactness, crispiness, hardness, adhesiveness, thickening quality, and viscosity to the edible coatings. Such coatings furnish an oil-free appearance, negligible caloric content, and are tough, colorless, and flexible that minimizes the dehydration, oxidative rancidity, and surface-darkening. The coatings from complex polysaccharides, like alginate and gellan exhibited the good colloidal characteristics when combined with calcium ions producing the strong gels (Rojas-Grau et al., 2008). The behavior of fungi and bacteria produced complex polysaccharides, like xanthan, curdlan, and are not much exploited that may offer great potential as edible coatings in the coming years.

7.4.1.1 STARCH BASED CO A TINGS

Starch, the reserve polysaccharide of plants, is an inexhaustible and extensively available material ideal for many industrial applications due to its comparative less cost, biodegradable nature, good mechanical properties, and variety of functionality showing the potential to replace the plastic polymers. It is a homopolymer composed of two kinds of glucose polymers: a straight chain glucose polymer, known as amylose (1 -4 linkage) that is water soluble, and a branched chain glucose polymer called as amylopectin (1-6 linkage) that is water insoluble (Fig. 7.2) (Rodriguez et al., 2006). Starch coatings are odorless, clear, tasteless, good gas barrier, and poor water vapor barrier because of their hydrophilic nature (Pascall and Lin, 2013). Most edible coatings are produced from the high-amylose starch, like com starch, containing 25% of amylose and 75% of the amylopectin. The modified cultivars containing 85% amylose have also been developed by the scientists. The coatings derived from the high amylose com starch (71%) were an excellent oxygen barrier at the relative humidity even less than 100% (Lin and Zhao, 2007).

Chemical structure of (a) amylose and (b) amylopectin

FIGURE 7.2 Chemical structure of (a) amylose and (b) amylopectin.

Corn starch-derived coatings are colorless, tasteless, odorless, semiper- rneable to oxygen and CO„ biologically absorbent, and nontoxic exhibiting the physical properties similar to the plastic based coatings. On the other side, the amylopectin due to its branched structure produce the coatings that have reduced elongation and tensile strength (Hassan et al., 2017). Its freeze-thaw stability and solution clarity can be modified by replacing the hydroxyl (-OH) groups that reduce the hydrogen-bonding ability. Starch must be treated with additives like plasticizers, mixed with other biopolymers, modified chemically or genetically or combined with all in order to formulate the coatings of good mechanical characteristics demonstrating the excellent elongation, flexural, and tensile strength. The incorporation of biodegradable plasticizers may balance the starch brittleness that is a great challenge in the coatings of high starch content. The extensively used plasticizers in starch-based coatings formulations are polyether, glycerol, urea, and lower-molecular-weight polyhydroxy-molecules that restrict the multiplication of micro-organisms by decreasing the water activity. The coatings from starch derivatives, like dextrins, showed better water bander properties as compared with the starch coatings (Rodriguez et al., 2006). Interestingly, the starch coatings are the best for fruits with high respiration because of its good oxygen barrier characteristics, thus lowering the oxidation and respiration of coated fruits.

7.4.7.2 CELLULOSE AND ITS DERIVATIVES

Cellulose is the most abundant natural polymer in the earth and found as a structural material of the plant cell walls. It is a linear polymer comprising of the repeating unit of D-glucose units containing the 0-1,4 glycosidic linkage together with the hydroxyl-methyl groups placed above and below the backbone of polymer chain (Fig. 7.3) (Han and Gennodios, 2005). The molecular weight of cellulose determines the mechanical barrier characteristics of edible coatings and higher the molecular weight, the cellulose possesses the excellent barrier properties (Bourtoom, 2008).

The cellulose-based coatings are typically colorless, odorless, tasteless, hydrophilic, low or zero calories, bendy, transparent, water-soluble, resistant to oil and fat, and moderately moisture, and gas permeable. The cellulose derived edible coatings that are commercially viable include the methyl cellulose (MC), carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), and hydroxypropyl methyl cellulose (HPMC) (Bourtoom, 2008). Such substances are suitable with the salt, surfactants, and other water-soluble polysaccharides and are nonionic and may be dissolved in any aqueous or aqueous-ethanol solutions to synthesize the water-soluble and fats and oils resistant coatings (Lin and Zhao, 2007). As a whole, the cellulose-based coatings are not good in the water and gas barrier properties. MC being highly water-resistant and least hydrophilic in nature exhibits the better moisture barrier characteristics than others. The food researchers have even examined the composite coatings comprising of MC and HPMC or containing the beeswaxes and fatty acids (Bourtoom, 2008). CMC coatings had protected the flavor compounds, retained the fresh-like crispness and firmness of Indian blueberry (Gol et al., 2015). More efforts are needed in the efficient production of cellulose derivatives-based coatings with improved functionality in a cost-effective way.

Chemical structure of cellulose

FIGURE 7.3 Chemical structure of cellulose.

7.4.1.3 PECTIN

Generally, pectin is a fruit derived polysaccharide comprising of a-1, 4-linked D-galacturonic acid units in which the uronic acid carboxyls are either partially (low methoxy pectin) or fully [high methoxy pectin (HMP)] methyl esterified (Fig. 7.4) (Caffall and Molmen, 2009). Pectin coatings have lower water barrier properties thus suitable for low moisture fruits only. HMP usually produce excellent coatings. The citrus pectin, when combined with high amylose starch, furnishes the flexible coatings that are stable even at high temperature (180°C) (Fang and Hanna, 2000). The coatings prepared from pectin in combination with other substances were used to prolong the storage life of fresh-cut cantaloupe (Martinon et al., 2014). Likewise, pectin (2%, w/v) derived coatings comprising the glutathione (0.75%, w/v) and N-acetylcysteine (0.75%, w/v) were employed to improve the microbiological stability of fresh-cut pears demonstrating the maintenance of aesthetic attributes of pear chunks up to 2 weeks (Oms-Oliu et al., 2008).

Chemical structure of pectin

FIGURE 7.4 Chemical structure of pectin.

7.4.1.4 CHITIN AND CHITOSAN

After cellulose, chitin is the widely present, nontoxic, allergen-free, safe, and naturally-existent biopolymer that can be obtained from the exoskeleton of crustaceans, including the crab and shrimp shells, cell walls of fungus, and other organic matter. Chitin consists of the N-acetyl-D-glucos- amine residues resulting in a structure similar to the cellulose. The major difference in the chemical structure of cellulose and chitin is presence of acetamide group on the second carbon atom of hexose repeating unit in chitosan rather than the hydroxyl group (-OH) On deacetylation of chitin in the presence of concentrated alkali like NaOH, the chitosan is formed (Fig. 7.5) (Dutta et al., 2004).

Chemical structure of (a) chitin and (b) chitosan

FIGURE 7.5 Chemical structure of (a) chitin and (b) chitosan.

Semi-permeable coatings may be created using the chitosan that improves the interior conditions of fruit by retarding the respiration rate thus postponing the ripening. The chitosan-based coatings are transparent, tough, pliable, bendy, smooth, excellent antimicrobial agent, good viscosity like gums, good oxygen and carbon dioxide barriers, cohesive, well-suited to other materials, like minerals, vitamins, and antibrowning agents, without influencing the antifungal and moisture barrier properties and contain the excellent mechanical properties providing better strength and resistance (Park and Zhao, 2004; Chien et ah, 2007). Methylation of polymer chain modifies the permeability of carbon dioxide. Chitosan coatings have the potential to absorb the heavy metal ions that are beneficial to retard the oxidation reactions catalyzed by the free metals (Shiekh et ah,

  • 2013). These coatings are applied on a variety of fruits, including the strawberries as an antimicrobial coating and on the plums, peaches, apples, and pears as the good gas barrier (Devlieghere et ah, 2004; Gol et ah, 2013). The chitosan, when coated on the fruit surface, induces the generation of chitinase—the plant defense enzyme that deteriorates the fungal cell walls. The derivatized chitosan-based composite coating known as Nutri-Save is widely employed to prolong the shelf-life of fruits like pears, apples, pomegranates, etc. The positive impact the chitosan coatings, either alone or in combinations, on the freshly-peeled litchi and peeled prickly pear has been examined by Dong et ah (2004). The chitosan coating containing the oleic acid preserved the strawberries where oleic acid refines the water vapor barrier properties in addition to boosting the antimicrobial action of chitosan (Vargas et ah, 2006). Instead of a huge application of chitosan coatings, selecting the ideal chitosan concentration is still a great concern in the industry so that the desired functionality can be achieved without altering the aesthetic properties.
  • 7.4.1.5 PULLULAN

The pullulan is a neutral polymer consisting of a-(l —► 6)-linked malto- triose units that are in turn made up of three glucose molecules attached to each other by a- (1 —»■ 4) glycosidic bond (Fig. 7.6) (Singh et ah, 2008). It is an extracellular, edible, and biodegradable microbial polysaccharide derived from starch. Pullulan coatings are crystal clear, tasteless, odorless, good oxygen, and moisture barrier, and used to perverse the kiwifruits and strawberries (Diab et ah, 2001). The pullulan coatings along with chito-oligosaccharides and glutathione improve the shelf life of a variety of fruits (Wu and Chen, 2013). Very few works have been conducted till date focusing on the effect of pullulan coatings containing the antibacterial and antibrowning agents on the fruits for the shelf-life extension.

Chemical structure of pullulan

FIGURE 7.6 Chemical structure of pullulan.

7.4.1.6 ALGINATES

Alginate, the brown seaweed-derived linear polysaccharide, is an important component of an edible coating as it has unique colloidal properties, including gel formation, thickening, emulsification, and stabilizing (Rojas- Grau et al., 2007). This structural polymer consists of p-D-mannuronic acid (M) and a-L-guluronic acid (G) in the different molecular weight, arrangements, and levels. Two polymer chains containing the guluronic acid units when combined with the divalent or multivalent ions, like calcium, ferrum, or magnesium, produce the three-dimensional network, that is, alginate (Cha and Chinnan, 2004). The chemical make-up of alginate gels, such as the amount of guluronic (G), to mannuronic (M) acid units and their order, G-block length and overall molecular weight decides their physical characteristics. Alginate coatings are the transparent, water-soluble, oil and fat resistant, and poor water barrier (Acevedo et al., 2012). The alginate coatings lose the moisture without dehydrating the fruits thus work as a sacrificing agent that improves the batter adhesion to fruit surface. Coatings sourced from calcium alginate lower the shrinkage, moisture transfer, oxidative rancidity, and oil absorption and improve the sensory properties of fruits prolonging their shelf-life (Olivas et al., 2007).

7.4./.7 OTHERS

The red seaweeds-based polysaccharide carrageenan is a complex mixture of several polysaccharides and another potential coating material for fruits.

Coatings from carrageenan suppress the moisture loss, oxidation, and degradation of fresh apples. After combination with the antibrowning substances, like ascorbic acid, these coatings improve the appearance and color and reduce the microbial load on apple slices (Lee et al., 2003). Interestingly, the antimicrobial compounds, like nisin, grapefruit seed extract, ethyl- enediaminetetraacetic acid (EDTA), and lysozyme, are also present in the «•-carrageenan based coatings (Choi et ah, 2001).

In addition, a variety of gums including the fermentation gums, such as xanthan gum, and exudate gums, like acacia gum, gum karaya, and gum arabic, are the good coating materials for fruits. Xanthan gum imparts better adhesive properties to the coatings while gum arabic improves the sensoiy characteristics of the fruit surface. Besides these, gums from several sources like aloe vera, psyllium husk, locust bean, basil seed, and flaxseed can be employed for the edible coating formulation (Rojas-Argudo et ah, 2009; Benitez et ah, 2013). The coatings from psyllium seed gum were found better than the chitosan coating as they effectively inhibited the enzymatic browning along with retaining the color of apple slices (Banasaz et ah, 2013). The edible coatings can be formulated from aloe gel and shellac that lower the oxidizing enzymes action and rate of respiration and ethylene synthesis along with minimum changes in the firmness of apple slices (Chauhan et ah 2011). The aloe vera coatings also improved the appearance, moisture barrier, and other sensoiy properties of kiwifruit slices during storage (Benitez et ah, 2013). It is known to have antifungal activity (Jasso de Rodrnguez et ah, 2005). Another gum called locust bean gum can also be successfully applied to coat the “Fortune” mandarins for enhancing their shelf-life (Rojas-Argudo et ah, 2009).

 
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