Ultrasound synergistic slightly acidic electrolyzed water treatment of grapes: Impacts on microbial loads, wettability, and postharvest storage quality

Abstract

Microbial contamination is the principal factor in the deterioration of postharvest storage quality in grapes. To mitigate this issue, we explored a synergistic treatment which combines ultrasound (US) and slightly acidic electrolyzed water (SAEW), and rigorously compared with conventional water cleaning (CW), exclusive US treatment, and standalone SAEW treatment. The US + SAEW treatment proved to be markedly superior in reducing total bacterial, mold & yeast counts on grapes. Specifically, it achieved reductions of 2.23 log CFU/g and 2.76 log CFU/g, respectively, exceeding the efficiencies of SAEW (0.78, 0.75), US (0.58, 0.65), and CW (0.24, 0.46). The efficacy of this synergistic treatment is attributed to the ultrasound removal of the wax layer on grape skins, which transitions the skin from hydrophobic to hydrophilic. This alteration increases the contact area between the grape surface and SAEW, thereby enhancing the antimicrobial efficacy of SAEW. From a physicochemical quality standpoint, the US + SAEW treatment exhibited multiple advantages. It not only minimized weight loss, color deviations, polyphenol oxidase activity and malondialdehyde synthesis in comparison to CW-treated samples but also preserved firmness, sugar-acid ratio and the activities of key enzymes including phenylalanine ammonia-lyase, superoxide dismutase and catalase, and thus maintaining high levels of total phenolics, total ascorbic acid, total anthocyanins, and antioxidants. Consequently, US + SAEW treatment put off the times of decay onset in grapes by 12 days, outperforming both SAEW (8) and US (4) in comparison to CW. These results highlight the potential of US + SAEW as an effective strategy for maintaining grape quality during their postharvest storage period.

Keywords: Grapes; Postharvest Storage Quality; Slightly Acidic Electrolyzed Water; Sterilization; Ultrasound Treatment.

Fig. 1

Illustrative diagram of the production of SAEW, the treatment and analysis of grapes. CW: Conventional water washing. US: Ultrasound treatment. SAEW: Slightly acidic electrolyzed water treatment. US + SAEW: Ultrasound synergistic slightly acidic electrolyzed water treatment.

Fig. 2

Changes in microbial counts and decay rate of grapes treated with CW, US, SAEW, and US + SAEW during treatment and storage. A: Total bacterial counts. B: Mold & yeast counts. C: Decay rate.

Fig. 3

Appearance (A), SEM images (B), and contact angle (C) of untreated grapes and grapes treated with CW, US, SAEW, and US + SAEW. The appearance of grapes was photographed when they were submerged in water or SAEW.

Fig. 4

Illustrative diagram of the antimicrobial mechanism where ultrasound treatment enhances the efficacy of slightly acidic electrolyzed water.

Fig. 5

Physical photographs of grapes treated with CW, US, SAEW, and US + SAEW during storage. The area marked by the red circle indicate the visible site of decay. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6

Changes in physical quality of grapes treated with CW and US + SAEW during storage. A: Weight loss rate. B: Hardness. C: Sugar-acid ratio. D: Color change (ΔE). Error bars indicate standard errors from three replicates, with significance levels calculated by Student’s t-test (ns, p > 0.05; *, p < 0.05; **, p < 0.01; ***, p < 0.001), the same as below (Fig. 7 and Fig. 8).

Fig. 7

Changes in bioactive components and antioxidant activities of grapes treated with CW and US + SAEW during storage. A: Total phenolic content. B: Total ascorbic acid content. C: Total anthocyanin content. D: DPPH radical scavenging rate. E: ABTS radical scavenging rate.

Fig. 8

Changes in enzyme activities and MDA content of grapes treated with CW and US + SAEW during storage. A: PAL activity. B: PPO activity. C: SOD activity. D: CAT activity. E: MDA content.