Effects of hot water dipping on apple heat transfer and post-harvest fruit quality

Post-harvest sanitation of fruit and vegetables is one major concern of the food producing industry. In particular, amount and type of microorganisms (MOs) on fresh commodities determine how long a produce can be stored and sold. Therefore, great efforts are put into the development of effective decontamination methods, which reduce the consumers' health risk and extend shelf life of the produce. One relevant method is hot water dipping. Sanitizing fruits with hot water treatment (HWT) has to manage the conflicting priorities of efficient deactivation of MOs and avoiding compromising fruit quality. Comprehensive knowledge of heat transfer properties within the fruit and of tissue physiological responses to heat treatment are prerequisites for an effective optimization of sanitation conditions. Therefore, objectives of this study were to determine the optimal temperature for a limited heating of the fruit surface and to evaluate potential microscopic alterations of the apple peel by heat. Short-term HWTs at 55 degrees C for 0.5 min exclusively warm the outermost apple layer to 30.1 degrees C without any microscopic changes of the fruit surface. At apple calyx and stem regions MOs were concentrated 13 - 200-fold compared to the equator area.