Effect of carboxymethyl cellulose-based active coatings containing savory and ginger essential oils on shelf life and some postharvest properties of fresh cucumber

Preparation of edible coatings is considered as one of the effective ways to increase the shelf life of fresh fruits (Hashemi et al 2017). Edible films or coatings contain a continuous matrix of protein, polysaccharide or lipid (Cagri et al 2004). The use of edible coatings has an effect on the product by changing the atmosphere inside the package, reducing the microbial load and delaying weight loss and respiratory reactions, as well as protection against physical damages. CMC, as a cellulose-derived compound, is a biopolymer that has the ability to form gel and shows outstanding film-forming properties. Also other properties such as water solubility, transparency, odorless, tasteless, high viscosity, non-toxicity and flexibility with medium strength and permeability to moisture and gases make it a suitable option for coating preparation (Fasihi et al 2017). Active packaging involves the addition of specific compounds to the film or packaging coating or inside packaging containers in order to increase the shelf life of the product. The most important benefit of adding active compounds to films and packaging coatings is the slow release of antimicrobial compounds from the packaging material, which results in high concentrations of the active compound at the product surface for a long time. This method is more effective than adding antimicrobial agent directly by spraying its solution on the surface of the product (Noshirvani et al 2017a). Spices and herbs are widely used in the food industry as flavorings. Recently, a large number of spices or essential oils have been identified as antimicrobial and antioxidant compounds. Many natural compounds, such as essential oils and their constituents, are recognized by the US Food and Drug Administration (GRAS) as healthy ingredients. Ginger and savory have outstanding antimicrobial and antioxidant properties and many studies have been done on these properties. The use of savory and ginger essential oils as strong antimicrobial compounds in the structure of active coatings can improve the quality properties of fresh cucumber.

The aim of this study was to prepare active coatings based on CMC containing ginger and savory essential oils in two concentrations of 250 and 1000 ppm in order to preserve fresh cucumber.

CMC based active coatings containing two levels (250 and 1000 ppm) of ginger and savory essential oils were prepared. In order to prepare coatings, CMC with two concentrations of 1 and 1.5% (w / v) was dissolved in distilled water and Tween 80 was added as emulsifier (25% w/w essential oil). Then different concentrations of savory and ginger essential oils (250 and 1000 ppm) were added after passing through the filtered syringe. Cucumbers were immersed in the solution for 3 minutes and then dried for 12 hours at 25°C. The samples were placed in polyethylene bags and stored at 10°C. To prepare the control sample, fresh cucumbers were placed in distilled water for one minute. The effect of coating on some properties of fresh cucumber such as weight loss, pH, acidity, total soluble solids, color, microbial count and sensory properties of coated cucumbers were examined during a 16-day storage period at 10°C. Fresh uncoated cucumbers were considered as control sample.

Based on the results, the use of active coatings containing ginger and savory essential oils significantly reduced weight loss and microbial growth in cucumber. The content of weight loss for the control sample was 13.6% after 16 days, while the samples containing the coating showed less weight loss. Weight loss is one of the most important changes after fruit harvest due to the migration of moisture from the surface of the fruit to the environment and its extent depends on the difference in water vapor pressure between the fruit tissue and the surrounding atmosphere (Sarker and Partners 2021). In addition, fruit respiration may lead to weight loss. Applying the coating on the fruit surface acts as a barrier against the passage of moisture from the fruit to the environment and reduces weight loss. Also, the reduction in fruit weight loss is probably related to the effects of the coating as a semi-permeable barrier against oxygen, carbon dioxide, moisture and solids, and thus will be associated with reduced respiration rate, weight loss and rate of oxidation reactions. The results indicated that in the treatments that used savory essential oil, the microbial spoilage of the fruit was reduced and the firmness of the fruit tissue was maintained. The treatment containing CMC coating was better controlled weight loss due to fruit moisture retention and increased the firmness of cucumber compared to the control sample at each stage of storage time. According to the results, the yellow color in all samples increased over time, which indicates a decrease in the green color of cucumber peel. However, the coated samples showed less yellow color than the control sample. Therefore, the presence of coating seems to reduce chlorophyll degradation by delaying fruit ripening and aging. The amount of total soluble solids, pH and acidity in the control sample changed more than the samples containing the coating due to the high rate of metabolic activities. The results of sensory evaluation indicated the lowest scores for control among the different samples, so that all scores obtained for this sample were below 3.5, which indicates that this sample is unacceptable in terms of the factors under study. The reason for the low score for the control sample is related to color changes (yellowing and browning), shrinkage due to high moisture loss and mold contamination. After the control sample, the samples coated with pure CMC in both concentrations used scored lower because of the microbial growth in these two samples after the storage period. Comparison of samples containing pure CMC coating and containing savory and ginger essential oils showed a higher score for samples containing essential oil. The presence of ginger and savory essential oils in the coating due to the antimicrobial effects of the essential oils used in the coating formulation prevents the activity of microorganisms, which leads to maintaining the quality of cucumber during storage. Comparison of samples with savory and ginger essential oils showed a higher score for savory compared to ginger essential oil, which is related to the stronger antimicrobial effects of savory essential oil than ginger.

Among the different coatings used in this study, the coating of CMC containing savory essential oil at both 250 and 1000 ppm concentrations showed a higher effectiveness in maintaining the quality of fresh cucumber. Due to the fact that CMC is a water-soluble biopolymer, so it is easily washed before consumption and no trace of coating will remain on the surface of the fruit. Based on the results, the use of CMC coating containing savory essential oil at both 250 and 1000 ppm concentrations indicated lower weight loss, desirable stiffness, better preservation of color, good microbial quality and sensory properties which is associated to strong antimicrobial properties of savory essential oil along with the presence of CMC coating.