Emulsion Techniques, for example, microencapsulation and nanoencapsulation, can be used in the formulation of food coatings. A variety of methods have been developed to obtain different emulsions, forming excellent biomaterials to be applied as edible films in combination with biopolymer matrices. The significance and widespread applications of edible films to obtain coatings to protect food are originated from their unique properties, such as biocompatibility, biodegradability, and no toxicity.
The current lifestyle of consumers has modified the fruit and vegetable consumption patterns. The request for natural, fresh, healthy, and minimally processed food is continuously increasing mainly due to the desire to consume beneficial health products driven by increased knowledge and awareness of the consumers related to the presence of healthy compounds in these products like antioxidant, antimicrobial or other micronutrients [1]. Minimally processed fruits and vegetables are products that have increased their consumption in a healthy diet. These types of foods have not been physically altered from their native state and remain in a fresh form [2], representing a very promising expansion sector for the food industry [3].
Nevertheless, the consumption of fruits and vegetables is directly related to the product’s freshness and quality when purchased by consumers. Undesirable characteristics, such as color changes, browning, or sweating, could cause rejection and should be avoided, resulting in economic losses, and ethical problems due to the production and accumulation of high concentrations of agro-industrial wastes. These changes related to the products freshness and quality can occur during processing operations, such as cutting, washing, or peeling, altering their integrity, modifying their surface properties, and damaging their tissue structure by the presence of undesirable effects, such as enzymatic reactions [2][4]. These degradation processes cause surface browning, undesirable flavor, water loss, and texture collapse, which should be avoided as much as possible. Besides, the presence of microorganisms on the vegetables’ surface could affect the products’ safety and the final quality [5].
Consequently, any solution to reduce these problems is essential to improve fresh food acceptance, reducing costs, and decreasing economic losses into food processing plants. In this context, the use of edible coatings in fruits and vegetables has been proposed as a valid methodology to improve their overall quality, extending shelf-life, perishability, and increasing profit for growers and marketers. These additives could act as a barrier against microbial agents or UV radiation, maintaining or improving the overall quality of foodstuff [6]. In contrast, the application of these thin layers of polymeric materials suitable for human consumption to protect food formulations from external agents can cause several changes in their organoleptic, mechanical, and textural properties [7].
Different approaches have been evaluated to increase food shelf-life and reduce the impact of external agents on their essential properties. In this sense, the combination of biopolymer matrices and emulsified structures based on the encapsulation of functional compounds by dispersion, forming micro- or nano-drops, shows high potential for the formulation of new edible coatings. Some of the research work related to edible bio-coatings focused on improving their properties by exploring the advantages of combining functional compounds, such as vitamins [8], essential oils (EOs) [9], and polyphenols [10][11] with natural polysaccharides like pectin [12], chitosan [13], cellulose [14], or starch [15]. Besides, encapsulation of active compounds resulting in efficient delivery systems requires a complete understanding of how these functional compounds can be encapsulated and their release mechanisms for their application in food formulations [6].
It is well known that different strategies can be used to encapsulate active chemicals, where emulsions show great potential for the food industry. This technique improves the bioavailability of functional compounds, protecting them from environmental storage conditions, and controlling their release into food products [16]. The development of microparticles and nanoparticles using emulsion methods has been widely studied in the last years. TableTable 1 1 lists some examples of emulsion systems used in the encapsulation of functional compounds to be further applied to extend shelf-life in vegetable foods.
Table 1. Some emulsion systems applied to vegetables.
Emulsion Technique * | Functional Compounds | Benefits | Food | Ref. | |
---|---|---|---|---|---|
Ionic gelation technique | Cuminum cyminum EO | Antimicrobial activity. | Mushroom | [17] | |
-- | Trans | -cinnamic acid | Antimicrobial and antibiofilm effects. | Lettuce | [18] |
Ultrasonication | Oregano EO | Antimicrobial activity. | Lettuce | [19] | |
HPH | Carvacrol | Antimicrobial activity. | Zucchini | [20] | |
HPH or ultrasonication | Carvacrol | Antimicrobial activity. | Shredded cabbages | [21] | |
HPH | Lemongrass EO Citrus extract Fermented dextrose Prolong 2 Concentrated cranberry juice |
Antibacterial activity. | Green peppers | [22] |
Vegetables are known for their nutritional benefits since they provide antigenotoxic, anti-inflammatory, antioxidant, anti-allergic, anticancer, and anti-diabetic functions. The new trends focusing on the need of a healthy diet have propelled the consumption of fresh-cut and ready-to-eat vegetables and fruits, changing the traditional consumption patterns of these food products.
It is well known that the processing operations (i.e., washing, cutting, or peeling) can produce alterations in food by modifying the vegetables' final appearance and decreasing their nutritional value. In some of these operations, the outer layers of vegetable cells are damaged and, on many occasions, they are the trigger of enzymatic reactions, which are most detrimental for food. Some authors described these modifications as changes in the surface by browning, unexpected flavor, water loss, and texture breakdown [23]. Besides, organoleptic degradation effects due to the growing of microorganisms onto the surface of the vegetables are observed and they must be eliminated or limited since they cause consumer´s rejection and represent a risk for health. In summary, these limitations in processed vegetables should be minimized. With this background, the use of coatings can be considered a valid alternative to incorporate active substances to protect the food structure and their resistance against aggressive external factors while increasing shelf life, decreasing production costs, and improving consumer acceptance [24].
One of the most explored options for the utilization of coatings in fresh-cut and ready-to-eat vegetables is focused on the development of the film layer directly onto the product surface by spraying or dipping the coating solution formed by an edible film containing the encapsulated active principles [4][25]. TableTable 3 2 summarizes the most relevant works published in the last decade, related to the use of polysaccharides to obtain edible coating films with functional properties to extend their shelf-life and retain the quality of vegetables.
Table 23. Active coatings based on polysaccharides from fresh and minimally processed vegetables and fresh-cut fruit.
Material Edible Coating | Functional Ingredient | Benefits | Food | Ref. | ||||||
---|---|---|---|---|---|---|---|---|---|---|
Chitosan | Carvacrol | Escherichia coli | reduction reaching >5 log UCF/g | Cucumber | [26] | |||||
Chitosan | Cinnamomum zeylanicum | EO | Inhibition of | Phytophthora drechsleri | , stored 7 days at 4 °C. Reduction of respiration rates, improving the microbiological quality, preserving the fruit weight. | Cucumber | [27] | |||
Chitosan | Limonene | Prolongation of post-harvest life maintaining weight loss, color, firmness, pH, and organoleptic properties. Reduction of fungal growth | Cucumber | [7] | ||||||
Quinoa protein and chitosan | Thymol | Cherry tomatoes inoculated with | Botrytis cinereal | . Considerable reduction in fungal growth after 7 days at 5 °C | Cherry tomatoes | [28] | ||||
Hydroxypropyl methylcellulose (HPMC), beeswax (BW) | Potassium carbonate, sodium propionate, ammonium carbonate, ammonium phosphate | Reduction of gray mold development on cherry tomatoes. Respiration rate, firmness, sensory flavor, color, off-flavor, and fruit appearance were not badly affected. | Cherry tomatoes | [29] | ||||||
Chitosan | Chitosan | A 5 days delay in ripening, enhancing the phenolic content, and maintaining a low respiration level. | Tomatoes | [30] | ||||||
Chitosan | Mandarin EO | Inhibition of | Listeria | species. No impact on firmness for 14 days and product color | Green beans | [31] | ||||
Chitosan | Mandarin EO | Control the growth of | Listeria innocua. | Reduction in the sample firmness and no color changes during storage. | Green beans | [32] | ||||
Chitosan | Bergamot, carvacrol, mandarin and lemon EOs | Inhibition during storage of | Escherichia coli | O157:H7 and | Salmonella Typhimurium | with carvacrol | Green beans | [33] | ||
Pectin | Sesame oil | Antioxidant activity, preservation of quality attributes, and control of microbial growth after 12 days. | Cut carrots | [34] | ||||||
Chitosan | Carvacrol | Control of microbial growth for 13 days at 5 °C. | Cut carrots | [35] | ||||||
Pectin | Biosecur F440D (citrus extract) and a mixture of four EOs | Increase the shelf-life by 2 days and control of | Listeria | species and | Penicillium chrysogenum | . | Cut carrots | [36] | ||
Maltodextrin and methylcellulose | Lactic acid, citrus extract, lemongrass EO | Inhibition against | Listeria monocytogenes and Escherichia coli. | Variations in the respiration rates with no major color modification. | Cauliflower florets | [37] | ||||
Starch and maltodextrin | Lactic acid, citrus extract, lemongrass EO | Inhibition against | Listeria monocytogenes. | No major changes in color, texture and respiration | Cauliflower florets | [38] | ||||
Chitosan | Lemongrass EO | Fungal growth was effectively controlled for 21 days at room temperature. Maintenance of the fruit quality: weight loss, firmness, color. | Bell pepper | [39] | ||||||
Hydroxypropyl methylcellulose | Oregano and bergamot EOs | Reduction in the respiration rate and ethylene production, total weight loss, no surface color change, and total cell count. | Plum | [40] | ||||||
Carboxymethyl cellulose | Potassium sorbate | Decrease in ripening and minimum changes in the green skin color with no loss of firmness. | Pears | [41] | ||||||
Alginate | Lemongrass EO | Complete inhibition of the natural microflora for 2 weeks and no significant influence on the quality parameters during storage. | Fuji apples | [42] | ||||||
Starch and carboxymethyl cellulose | Turmeric EO | Antioxidant activity and low weight loss, firmness loss, and moisture content | Fuji apples | [25] | ||||||
Chitosan and carboxymethyl cellulose | Citral | Good antimicrobial protection (up to a 5-log reduction), and significant extension of the shelf-life up to 13 days. | Melons | [43] | ||||||
Chitosan | Lemongrass EO | High inhibition of | Salmonella typhimurium | ; inhibition of yeasts, molds, and total mesophilic aerobes. Preservation of total soluble solid content, colour, and antioxidant activity during storage. | Grapes | [44] | ||||
Chitosan | Nisin, natamycin, pomegranate and grape seed extract | Antimicrobial effect against yeasts, molds, and mesophilic bacteria. | Strawberries | [45] | ||||||
Chitosan | Lemon EO | Antifungal activity and no effect on sensorial perception | Strawberries | [46] | ||||||
Alginate | Carvacrol Methyl cinnamate |
Antimicrobial effect. Maintenance of firmness, color retention, and weight loss reduction up to 13 days. Antioxidant effect. | Strawberries | [47] | ||||||
Alginate Pectin |
Eugenol Citral |
Antimicrobial and antioxidant effect. | Strawberries | [48] | ||||||
Chitosan | Bergamot, thyme, and tea tree EOs | Reduction of microbial growth and no changes in the food quality. | Oranges | [49] | ||||||
Chitosan | Trans | -cinnamaldehyde Cinnamaldehyde Carvacrol |
Reduction of the bacterial and yeasts/molds growth on the fruit. Antimicrobial effect. Shelf-life extension. | Blueberries | [50] | |||||
Alginate | Eugenol Citral |
Preservation of nutritional and sensory attributes and reduction of microbial spoilage. Antioxidant effect. | Arbutus unedo L | . fruit | [51] | |||||
Alginate | Lemongrass EO | Decrease in the firmness and sensory scores (taste, texture, and overall acceptability). Extension of the shelf-life up to 16 days. | Pineapple | [52] | ||||||
Alginate and pectin | Eugenol Citral |
Antimicrobial and antioxidant effect. | Raspberries | [53] | ||||||
Pectin | Cinnamon leaf EO | Increase the antioxidant activity, odor acceptability, and inhibition of | Escherichia coli | O157:H7, | Staphylococcus aureus, | and | Listeria monocytogenes. | Preservation of food quality. | Peach | [54] |
Pectin | Nisin Calcium chloride Citric acid |
Antibrowning effect and maintenance of the sensorial and microbiological quality for more than 9 days. | Persimmon | [55] | ||||||
Chitosan and pectin | trans | -cinnamaldehyde-CD inclusion complex | Antimicrobial effect. | Papaya | [56] | |||||
Basil seed gum | Oregano EO | Reduction of the microbial population and antioxidant activity. | Apricot | [57] | ||||||
Pullulan | Calcium chloride Lemon juice |
Antibrowning. Enhancement of the overall quality and extension of the shelf-life. | Bananas | [58] |
The encapsulation of functional substances, such as EO, by forming emulsions to maintain fresh-cut vegetables and fruits by forming coating systems in the food industry is owed to their particular and advantageous properties, such as:
-Antimicrobial activity: Vegetables coated with active emulsions have been proven to be less sensitive to microbial infection and proliferation than undamaged vegetables, resulting in increasing shelf-life and quality of minimally processed vegetables and fruits.
-Antioxidant performance to avoid or limit the lipid oxidation and quality parameters as appearance and color.
-Film-forming systems help to preserve cell wall integrity and the texture during storage to avoid enzymatic degradation and rejection by the consumers
Much research has been performed in the last few years and much more is currently underway to explore possible commercial applications of polysaccharide-based coatings with functional properties in minimally processed vegetables and fruits, but most of it has been carried out at the laboratory scale. However, the next step to get final commercial applications of these active coatings is still pending, and scale-up and industrial trials at a bulk volume should be carried out in the next future.