Induction of resistance to Penicillium digitatum and chilling injury in 'Star Ruby' grapefruit by a short hot-water rinse and brushing treatment

Postharvest heat treatments have been used for many years as alternatives to chemical control of fungal diseases and insect infestation of fruits and vegetables. In this study, the effects of a new hat-water brushing (HWB) treatment on the resistance of red grapefruit (C. paradisi cv. Star Ruby) to green mould decay caused by Penicillium digitatum (Pers.: Fr.) Sacc. and on the development of chilling injury (CI) symptoms during cold storage were examined. The HWB treatment comprises rinsing hot water on the fruits as they move along a belt of brush rollers. A twenty second HWB treatment at 59 or 62 degrees C reduced decay, after artificial inoculation of wounded fruit, by 52 or 70%, respectively, compared with control unwashed fruit, whereas rinsing and brushing the fruit with tap water (similar to 20 degrees C) or with hot water at 53 or 56 degrees C. were ineffective. HWB treatments applied 1-3 d prior to inoculation were most effective in enhancing the disease resistance of fruit, but were much less effective when the fruit were inoculated on the same day or 7 d later. HWB treatments at 59 or 62 degrees C for 20 s also significantly reduced the CI index and the percentage of fruit displaying CI symptoms by 42 and 58%, respectively, after six weeks' storage at 2 degrees C and an additional week at 20 degrees C. Furthermore, HWB treatments cleaned the fruit and improved its general appearance without causing any surface damage, and did not influence fruit weight loss, percentage of total soluble solids (TSS) in the juice, juice acidity or fruit colour.