Edible Coatings as Carriers of Active Ingredients for Fresh-cut fruits

M Alejandra Rojas-Graü, Robert Soliva-Fortuny and Olga Martín-Belloso

Abstract

Minimal processing operations alter the integrity of fruits bringing about negative effects on product quality such as browning, off-flavor development and texture breakdown. The search for methods that aim to retard these negative effects is of great interest to all the sectors involved in production and preservation of fresh-cut fruits. In this way, edible coatings are potential systems for the improvement of quality and shelf-life of fresh-cut commodities. The incorporation of additional nutrients, enhancement of sensory characteristics, and/or inclusion of antimicrobials are among the list of potential benefits of an edible coating. The development of edible films and coatings as carriers of active ingredients is considered as a promising packaging system to maintain freshness of fresh-cut fruits and vegetables.

Introduction

Edible films and coatings are generally defined as continuous matrices that can be prepared from edible materials such as polysaccharides, proteins and lipids. They can be used as thin wraps or pouches, or formed as coatings on food or between food components. In fact, edible coatings have been used for centuries to prevent moisture migration, improve food appearance and increase shelf-life. Potential properties and applications of edible coatings have been extensively reviewed. However, in the last years edible coatings have been recognised for more innovative uses. One major advantage of coatings is that several active ingredients can be incorporated into the polymer matrix and consumed with the food, thus enhancing safety or even nutritional and sensory attributes.

Potential active ingredients to be carried by edible coatings One of the distinctive functions of edible coatings is their ability of incorporating active ingredients into the matrix to enhance its functionality (Figure 1). As a result, quality, shelf-life stability and safety of fresh-cut fruits can be significantly improved with the incorporation of antioxidants, antimicrobials or functional ingredients.

Figure 1. Edible coatings as carriers of active ingredients on fresh-cut fruits

Antibrowning agents

Fresh-cut fruit processing operations can induce undesirable changes in colour and appearance of these products during storage. Most strategies to control browning have focused on theoretical approaches to modulate PPO enzyme activities. Application of antioxidant dips after peeling and/or cutting is the most common way to control browning of fresh-cut fruits. However, antioxidants can also be added into the coating matrix to protect the cut surface against enzymic browning. Ascorbic acid is the most extensively used antioxidant to avoid enzymic browning of fresh-cut fruits. However, this treatment is not completely effective to control enzymic browning, since once the ascorbic acid is completely oxidised to dehydroascorbic acid, o-quinones are no longer reduced and darkening may occur due to formation of melamines (Nicolas et al. 1994). The incorporation of other antibrowning agents into edible coatings applied on fresh-cut fruits has been studied by various authors, who have achieved excellent results using several thiol-containing compounds such as cysteine, N-acetylcysteine, and reduced glutathione as an alternative to ascorbic acid.

Antimicrobial agents

The use of edible coatings as carriers of antimicrobial compounds is another potential alternative to enhance the safety of fresh-cut produce. Antimicrobial edible coatings may provide increased inhibitory effects against spoilage and pathogenic bacteria by maintaining effective concentrations of the active compounds on the food surfaces. There are several categories of antimicrobials that can be potentially incorporated into edible coatings, including organic acids (acetic, benzoic, lactic, propionic, sorbic), fatty acid esters (glyceryl monolaurate), polypeptides (lysozyme, peroxidase, lactoferrin, nisin), plant essential oils (cinnamon, oregano, lemongrass), nitrites and sulphites, among others (Franssen and Krochta 2003). Although several types of antimicrobials incorporated into edible coatings have been used for extending shelf-life of fresh commodities, their use in fresh-cut fruits is yet limited. Currently, organic acids and plant essential oils are the main antimicrobial agents incorporated into edible coatings for fresh-cut fruits. Despite the good results achieved so far with the incorporation of essential oils into edible coatings, the major drawback is their strong flavour which could change the original taste of foods.

Texture enhancers

Processing operations may result in a dramatic loss of firmness in fruit tissues due to the action of pectic enzymes. The most common way of controlling softening phenomena in fresh-cut fruits is the use of treatments with calcium salts. Texture enhancers can be incorporated into the formulation of edible coatings to minimise softening phenomena of fresh-cut produce. In fact, some polysaccharide coatings such as alginate, gellan and pectin have the capacity to bind a number of divalent ions, for instance calcium, thus undergoing conformational changes and giving rise to the well known “egg box” model of gelation. Rojas-Graü et al. (2008) and Oms-Oliu et al. (2008) observed that apple or melon wedges coated with alginate, gellan or pectin edible coatings crosslinked with calcium salts outstandingly maintained their initial firmness during refrigerated storage.

Nutraceuticals

Edible coatings are also an excellent vehicle to enhance the nutritional value of fruits and vegetables by carrying basic nutrients that lack or are present in low amounts in fruits and vegetables. However, only a few studies have suggested their integration into edible coatings. Chien et al. (2007) maintained the vitamin C content of sliced dragon fruit coated with low molecular weight chitosan. Tapia et al. (2008) reported that the addition of ascorbic acid to the alginate edible coating helped to preserve the natural vitamin C content in fresh-cut papaya. Hernández-Muñoz et al. (2006) indicated that chitosan-coated strawberries retained more calcium gluconate (3079 g/kg dry matter) than strawberries dipped into calcium solutions (2340 g/kg). Tapia et al. (2007) maintained counts of Bifidobacterium lactis Bb-12 above 10⁶ cfu/g on papaya and apple pieces coated with alginate or gellan solutions containing the probiotic microorganism during 10 days of refrigerated storage.

Sensory Implications

Despite the good results achieved so far with the incorporation of active compounds into edible films and coatings, the use of certain ingredients into formulations may have detrimental consequences on the flavour of the coated product. For instance, the use of some antibrowning agents in edible coatings can yield an unpleasant odour, particularly when high concentrations of sulfur-containing compounds such as N-acetylcysteine and glutathione are used. However, the incorporation of antimicrobial agents, especially essential oils, could impart undesirable sensory modifications in the fresh-cut fruits. Finally, many nutraceutical compounds have natural bitter, astringent, or other off-flavours that could lead to rejection of the product by consumers. For this reason, more studies are required in order to develop edible coatings with high sensory performance.

References

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Hernández-Muñoz, P, Almenar, E, Ocio, MJ and Gavara, R (2006) Effect of calcium dips and chitosan coatings on postharvest life of strawberries (Fragaria×ananassa). Postharv. Biol. Technol. 39: 247–253. Franssen, LR and Krochta, JM (2003) Edible coatings containing natural antimicrobials for processed foods. In Roller, S (ed.), Natural Antimicrobials for Minimal Processing of Foods. Boca Raton: CRC Press; 250-262.

Nicolas, JJ, Richard-Forget, FC, Goupy, PM, Amiot, MJ and Aubert, SY (1994) Enzymatic browning reactions in apple and products. Crit. Rev. Food Sci. Nutr. 34: 109-157.

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Rojas-Graü, MA, Tapia, MS and Martin-Belloso, O (2008) Using polysaccharide-based edible coatings to maintain quality of fresh-cut Fuji apples. LWT-Food Sci. Technol. 41: 139-147.

Tapia, M.S, Rojas-Grau, MA, Carmona, A, Rodriguez, FJ, Soliva-Fortuny, R and Martin-Belloso, O (2008) Use of alginate and gellan-based coatings for improving barrier, texture and nutritional properties of fresh-cut papaya. Food Hydrocoll. 22: 1493-1503.

Tapia, MS, Rojas-Graü, MA, Rodríguez, FJ, Ramírez, J, Carmona, A and Martin-Belloso, O (2007) Alginate- and gellan-based edible films for probiotic coatings on fresh-cut fruits. J. Food Sci. 72: E190-E196.

Dr M Alejandra Rojas-Graü is a Postdoctoral Researcher, Dr Robert Soliva-Fortunyis an Associate Professor and Dr Olga Martín-Belloso is a Professor in the Department of Food Technology, UTPV-XaRTA, University of Lleida, Lleida, Spain; E-mail: omartin@tecal.udl.es