High temperatures, either before or after harvest, can result in injuries to several tropical and subtropical fruits. This injury can be induced by exposure to direct sunlight or excessive high temperatures, such as during the use of heat (hot air or hot water) treatments. Preharvest exposure to the sun can result in sunburn (such as in fi gs) and sunscald (such as in pomegranates). Transpiration in growing plants maintains temperatures in the optimal range, but once removed from the plant, fruits lack the protective effects of transpiration, and direct sources of heat such as full sunlight can rapidly heat tissues to above the thermal death point of their cells. Heat injury is usually expressed as bleaching, scalding, uneven ripening, excessive softening, and desiccation. Tropical and subtropical fruits vary signifi cantly in their tolerance to heat injury (Paull, 1994). Temperatures above 25°C cause uneven softening, skin discoloration, fl esh darkening, and off-fl avors development in some fruits, such as avocados. Impaired ripening of other fruits occurs at temperatures above 35°C (reduced ethylene production and action). Heat treatments (hot water or hot air) have been commercially applied to some tolerant fruits such as mango, papaya and citrus, especially for the control of decay and insects (Lurie, 1998)
3.6.2 Relative humidity (RH)
As discussed in section 3.5.5, water loss from fruits is affected by both commodity factors (e.g. morphological and anatomical characteristics) and external or environmental factors. Focussing on the environmental factors, the rate of water loss depends on the vapor pressure defi cit (VPD) between the fruit and the surrounding ambient air, which is infl uenced by temperature and relative humidity (RH). At a given temperature and rate of air movement, the rate of water loss from the fruit depends on the RH. At a given relative humidity, water loss increases with the increase in temperature (Kader, 2002c). The optimum RH during storage ranges from 85 to 95% for subtropical and tropical fruits. Lower RH (55 to 65%) is recommended for tree nuts and dried fruits. Very high RH (above 95%) and water condensation on the fruits can accelerate pathogen attack and decay development.