Application of postharvest bioactive edible coatings and natural antimicrobial as a green solution to alleviate shrivel and extend storage life of plums

In South Africa, brown rot has been exclusively attributed to M. laxa. M. laxa infects fruit, twigs and can remain latent during unfavourable conditions. Once fruit ripen or conditions improve, disease symptoms develop, resulting in brown decayed fruit. The nature of plums therefore greatly influences their shelf life. The commercial value of plums is therefore limited by their highly active metabolism and susceptibility to decay. The objectives of this study was to develop gum Arabic composite edible coatings incorporating thymol and salicylic acid; evaluate its effects on in vitro and in vivo activity of B.cinerea and M.laxa; and investigate their potential in delaying respiration and ripening and other quality attributes.

The effect of thymol and salicylic acid (SA) on mycelial growth of B. cinerea was evaluated using the method described by Karaca et al on 90mm plastic Petri dishes with PDA amended at 45-55°C with sterile aqueous solutions of bioactive. The concentrations tested for both bioactive agents were 0, 1.0, 1.5, 2.0 and 2.5mM. This was followed by a curative activity of the antifungal coatings. Quality attributes such as mass loss, peel and flesh colour, firmness, total soluble solids, titratable acidity, pH, ethylene production, respiration rate and shrivel and decay incidence were measured.

In experiment 1, the results confirmed that thymol (THY) and salicylic acid (SA) can be used to control the growth of B. cinerea and M. laxa in vitro. However, concentration optimizations of the bioactives in the coatings are necessary to achieve the full potential for in vivo activity. As observed in experiment 2, gum arabic (GA, 0.5%w/v) was the most effective treatment in maintaining fruit quality. GA had the lowest weight loss (5.51 %) compared to control (7.37 %). Fludioxonil (Flu) treated fruit had a high shrivel incidence of 25 % at the end of the 15 d storage period, while GA had a significantly lower shrivel incidence of 6.67 %. Fruit treated with Flu alone also exhibited 7.37 % similar to control GA- 1mM SA delayed fruit ripening rate as evidenced by a higher firmness value (2.54 N) compared with other treatments. All bioactives maintained significantly lower decay incidences at 15 d, when compared to the control (28.33%) and GA- 2 mM (8.33 %) and GA- 2mM (8.33 %) SA was the most effective.

Despite the benefits offered by thymol and salicylic acid, gum Arabic (GA) proved to be the most successful coating in maintaining quality attributes of the investigated fruit cultivar. Gum Arabic could effectively be used to reduce weight loss and maintain the colour of plums. It has also been shown to maintain firmness and reduce shrivel incidence. In terms of decay control, however, GA composites with SA at 2 mM and 4 mM exhibited the best performance. The results were comparable to the commercial fungicide, fludioxonil.

The treatments investigated during this study have shown to be useful in maintaining the quality attributes of plums. Optimizations of the bioactive concentrations in gum Arabic coatings should also be explored. The incorporation of these bioactive agents into different edible coatings such as methyl cellulose may also show promise in developing a green and effective antifungal product for the stone fruit industry. Future research should focus on investigation of other natural fungicides, including edible coatings and organic salts with or without edible coatings. The release of essential oil is also very important to investigate, as essential oils are active under certain temp and relative humidity.