NATURAL ANTIMICROBIALS IN EDIBLE COATINGS
Edible coatings spread on the surface of fruits and vegetables form an additional barrier to mass transfer and thus potentially reduce moisture loss or gas diffusion. At the same time, edible coatings enriched with antimicrobials might inhibit microbial proliferation. Edible coatings are basically composed of biopolymers, such as polysaccharides, cellulose, starch and derivatives, chitin and chitosan, alginates, and carrageenans or proteins, such as soy proteins, corn proteins, gelatin, casein, keratin, collagen, and whey proteins. All components should be prepared with materials generally regarded as safe. Among the raw fruits that are marketed now with edible coatings, we can find apple, kiimow, grapefruit, passion fruit, avocado, orange, lime, peach, lemon, fresh-cut apple, fresh-cut pear, and fresh-cut peach (Valdes et al., 2017).
In recent years additional research has been conducted in order to supplement these edible coatings with powerful natural bactericidal agents, such as EOs, organic acids, and natural phytochemicals from plants with demonstrated antimicrobial potential (Dhall, 2013; Muranyi, 2012; Valdes et al., 2017). The development of edible coatings to achieve effective entrapment of natural antimicrobials has been proposed as an alternative in order to keep raw or minimally processed fruit surfaces in contact with antimicrobial agents dosed at desired stable concentrations to keep microbial growth below critical levels during long periods of storage (Quiros-Sauceda et al., 2014).
Some of the most celebrated examples have to do with the use of chitin and EOs in edible coatings of fruits to inhibit microbial proliferation (Valdes et al., 2017). Chitin, an abundant constituent of crustacean shells and fungi, has been used to obtain chitosan, one of the most versatile compounds included in the preparation of edible coatings. Nowadays, chitosan is considered as a biocompatible, nonantigenic, nontoxic, and biofunctional food additive (EFSA, 2011). Concentrations up to 3% of chitosan applied as edible coatings have been effective in reducing native mesophilic bacteria and populations of inoculated E. coli and S. cerevisioe, and in inhibiting the growth of naturally occurring microorganisms on coated fresh-cut cantaloupe, pineapple, lychee, papaya, and mango. The use of N, О carboxyl methyl chitosan as an edible coating on apple, pear, and pomegranate guarantees prolongation of the shelf life of these raw fruits by inhibiting microbial proliferation (Farber et al., 2003).
Alginate and pectin coatings supplemented with citral and eugenol have also proved to be effective against aerobic mesophilic microorganisms, yeast, and molds. Mucilage coatings supplemented with oregano EO revealed a wide spectrum of antimicrobial potential, being effective against L. monocytogenes, Salmonella typhimurium, Bacillus cereus, Yersinia enterocolitica, P. aeruginosa, S. aureus, E. coli, and E. coli 0157:H7. The proliferation of microorganisms, such as E. coli 0157:H7, S. aureus, P. aeruginosa, and Lactobacillus plantarum has also been inhibited in edible soy-protein coatings supplemented with oregano and thyme EOs (Guerreiro et al., 2016; Youki et al., 2014; Emiroglu et al., 2010). The combination of natural EOs from plants (lemongrass, oregano oil, and vanillin) with alginate and gellan edible coatings has also been used to prolong the shelf life of fresh-cut apples. A reduction of 4 logI0 cycles in the inoculated population of Listeria innocua in the fresh-cut apple was achieved owing to the effect of lemongrass or oregano oils incorporated into the alginate edible coating
(Rojas-Grau et al., 2007). Organic acids (sodium metabisulfite, malic acid, and glutathione) have been incorporated in alginate edible coatings to control microbial proliferation in raw apples, melon, and strawberry, among other fruits (Hauser et al., 2016).
Future trends in the development of antimicrobial edible coatings for fruit preservation will probably be in the research line of micro- and nanoencapsulation of active compounds, helping to control their release from intelligent packaging and edible coatings under specific conditions (Majid et al., 2016).
NATURAL ANTIMICROBIALS IN POWDERS: VALORIZATION OF AGRO-INDUSTRIAL BY-PRODUCTS
More than a million tons of horticultural by-products are produced every year in the European Union (Stojceska et al., 2008). Agri-food by-products represent a great volume of residues for the food industry, and they are also becoming environmental and economic problems. These fruit and vegetable by-products mainly consist of peel, pulp, fruit stones, stalks, leaves, and by-products derived from feimentation or manufacture of juice, wine, or jam. Valorization of agro-industrial by-products is now one of the principal priorities guiding sustainable development in the European Union, according to the Horizon 2020 research program. There is an increasing number of published articles reporting the great value of vegetable and fruit by-products in bioactive compounds (e.g., hydroxycinnamic acids, flavo- noids, stilbenes) with associated antioxidant, anticarcinogenic, and antimicrobial properties (Volden et al., 2009; Teixeira et al., 2014; Sanz-Puig et al., 2015a). Accordingly, the need to find alternative methods of final disposal or valorization of these nutritious and potent functional by-products is gaining importance for the agro-food chain.
According to studies conducted by Sanz-Puig et al. (2015b), powdered cauliflower by-product has shown great potential as an antimicrobial agent against S. typhimurium, achieving levels of bactericidal capability close to 6 log10 cycles underexposure at refrigeration temperature (Sanz-Puig et al., 2015a, 2015b). Also, 2.5 log10 cycles were achieved in the reduction of a
B. cereus microbial population by use of cauliflower by-product powder at a concentration of 5% (w/v) at 37°C. Similarly, powdered mandarin by-product (5% (w/v)) has demonstrated a great antimicrobial potential against both S. Typhimurium (8 log10 cycles) and E. coli 0157:H7 (1.6 log10
cycles), probably associated with the high polyphenolic content and EO richness present in this fruit by-product (Sanz-Puig et al., 2016).
These findings are extremely interesting for the future decontamination of fruits and vegetables in primary postharvest steps involving the development of novel liquid (for spraying in the field) or powdered extracts based on the specific profile of bioactive phytochemicals in these fruit and vegetable by-products.
FUTURE TRENDS ANALYSIS
The tendency toward the production of high-convenience foods is spreading to all food sectors, including fruits and vegetables. Ready-to-eat products and “take and go” concepts are a reality in dairy (go-yogurts for children), meat products, and prepared dishes (such as mix and go rice dishes). As a result of the evident establishment of new lifestyles, living habits, and work patterns in the population worldwide, there is little time to dedicate to cooking and preparing food. However, consumers do not want to renounce healthy, savory meals so there is increasing demand for fresh, natural fruit and vegetable products, subjected to minimum processes to guarantee microbiological safety, but at the same time preserving all the flavor and nutrients inherent in these raw products. Consequently, the presence of freshly prepared fruits and vegetables, such as fresh-cut products, has increased in supermarkets in recent years. In addition to the convenience and health aspects related to the naturalness of fresh vegetables and fruits, reduced volume in prepackaged products that only contain the edible part of these foodstuffs is another valid reason to be taken into account in the selection criteria for the consumption of these products.
Novel technologies for decontamination of fruits have been assessed during the last 20 years as possible answers to the food industry’s demand for effective and environmentally friendly strategies for reducing microbial contamination in food. They can be used alone, or combined showing synergistic effects in microbial inactivation, and preserving at the same time the nutritional value and original sensory properties of food products. Washing, chemical or thermal decontamination of fresh vegetables or processed fruits could be replaced or complemented with the application of these novel methods. However, the selection of the best-suited decontamination technology should be oriented according to the main criteria that are the control of microbial risks and the minimization of losses of nutritional values. The performance of presented technics concerning these aspects were discussed in previous chapters and is related to a number of boundary conditions (character of microbial risk, location of contaminations in the food matrix, additional hurdles in the preservation concepts, the sensitivity of specific bioactive to heat, etc.).
Other criteria are processing costs, industrial readiness, availability and particle know-how in the use of equipment, energy consumption, control of technical parameters, and automation, packaging concepts, or consumer acceptance of the technic. For illustration, packaged products could be exposed to pulsed light if packed in transparent, UV-light permeable plastic bags. By shadowing in the bulk of fruit pieces, the efficiency is seriously reduced. An alternative might be the ignition of a plasma in the packaging. For both technologies, a certain efficiency was demonstrated, but the industrial readiness of technical systems is not given. Another alternative could be irradiation, which has to be labeled, but is related to rejection by consumers.
HHP is an effective consolidated process to be applied on fruits decontamination, in which the balanced good results between microbial inactivation levels achieved and the commercial versatility/industrial implementation of the processes are favoring its success worldwide. On the other hand, further studies are required to assess the application of cold plasma to these food matrices, optimizing microbiological criteria, maximizing the nutritional value of the processed food, and avoiding changes in quality/sensory characteristics that could take place under certain processing conditions (depending on time and exposure factors). The nature of the cold plasma technology and the operating conditions should be defined for each of the fruit products under study, so optimization and scale-up of the technology to commercial treatment levels requires a more complete understanding of the interaction of plasma with food. On the other hand, HHP is presented as a clearly consolidated process to successfully treat fresh-cut fruit pieces, ready-to-serve products freshly prepackaged, achieving rates of microbial inactivation close to 5 log 10 cycles in combination with additional antimicrobial strategies (e.g., ascorbic acid addition).
Novel perspectives should be explored by food scientist and processors to progressively reduce the need for chemical preservatives in these high valued fruit and fruit-derived products.