Reduction of postharvest decay in organic citrus fruit by a short hot water brushing treatment

Abstract

Abstract The marketing of organic citrus fruit has markedly increased during the last few years. However, these fruits are not treated with chemical fungicides and suffer from relatively high rates of decay. In this study, we examined the possible use of a new hot water brushing (HWB) treatment, to disinfect the fruits and reduce decay development during postharvest storage. Preliminary observations have shown that a minimum exposure period of 20 s at 56°C was required to inhibit Penicillium digitatum (Pers.: Fr.) Sacc spore germination in vitro. In vivo studies, carried out by rinsing and brushing the fruit 24 h after artificial inoculation with a P. digitatum spore suspension, indicated that HWB at 56, 59 and 62°C for 20 s, reduced decay development in the infected wounds to only 20, 5 and less than 1%, respectively, of that in untreated control fruits or fruits treated with tap water. The effects of HWB at 56, 59 and 62°C for 20 s on epiphytic microflora, was further confirmed by the reduction of the microbial counts (CFU) on the fruit surface to only 24, 12, and less than 1%, respectively, of those on fruit that had been rinsed and brushed with tap water. Scanning electron microscopy showed that HWB at 56°C for 20 s had smoothed the fruit epicuticular waxes and thus covered and sealed stomata and cracks on the fruit surface, which could have served as potential pathogen invasion sites. Postharvest storage experiments using various organically grown citrus cultivars such as ‘Minneola’ tangerines, ‘Shamouti’ oranges and ‘Star Ruby’ red grapefruit, showed that HWB at 56°C for 20 s reduced decay development by 45–55%. The HWB treatment at 56°C did not cause surface damage, and did not influence fruit weight loss or internal quality parameters.