Abstract
Heat-treatments have been evaluated and utilized commercially to reduce postharvest decay, chilling sensitivity, and maintain quality of perishable horticultural products. Recent studies exposing grapefruit (Citrus paradisi Macf.) to short-duration, high-temperature water [e.g., 133 to 144 F (56 to 62 C) for 20 seconds] have shown promise at reducing subsequent development of mold (Penicillium) and increasing resistance to chilling injury (CI). Among the most prevalent citrus decay organisms in Florida are the stem-end rots (Diplodia natalensis and Phomopsis citri). Whereas Penicillium species invade citrus tissue through wounds, the stem-end rot organisms develop latent infections within the button tissue that are more protected from physical and chemical treatments. Here we report on efforts to identify heat-treatments that do not result in visible grapefruit peel injury, while reducing subsequent postharvest decay from natural infections. 'Marsh' or 'Ruby Red' grapefruit were exposed to liquid or vapor water at temperatures between 122 and 149 F (50 to 65 C) for 0 to 120 seconds. Fruit tolerance to heat injury followed a time × temperature relationship that usually shifted slightly between experiments. However, washing and waxing the fruit immediately after the heat-treatment greatly reduced the development of visible heat injury. Grapefruit could usually tolerate a 10-second exposure to 138 F (59 C) water, but extending exposure time to 120 seconds required lowering the temperatures to 127 F (53 C) to prevent peel injury. While some time × temperature combinations significantly reduced stem-end rot (SER), only one did not result in significant peel injury. In one experiment, hot water treatment of 138 F (59 C) for 10 seconds was non-injurious and resulted in about 90% reduction in SER incidence (to 3.5%). Injurious treatments were associated with elevated fruit respiration. Simulated commercial degreening conditions before or after the heat-treatments had no affect on the development of peel injury.
