Influence of Drying Techniques on Tomato Flavour

This thesis researched the best possible operating conditions of a test dryer and the mild drying technique for preserving fresh tomato flavour. Fresh flavour is the one with more green notes and less heat-induced notes. Three different type of test dryers, namely a convective dryer (Heat Pump Dehumidifier, HPD), a conduction and infra-red radiation dryer (Refractance Window Dryer, RWD), and a microwave radiation dryer (Microwave Vacuum Dryer, MWVD) were chosen as a representative of differing principles of heat transfer. Control dryers based on commercial tomato products were also chosen, the freeze dryer (FD), spray dryer (SPD) as a basis for comparison of the flavour quality. A thin layer model was developed to predict the drying behaviour of tomato pulp and energy modelling was used with the RWD. For the RWD, conduction and evaporation were proved to be the dominant mechanisms of heat gain and heat loss from the product, respectively. The radiation effect was found to be negligible, so disproving the concept of RWD. With the HPD, a preliminary lipoxygenase inactivation model was employed to understand the low temperature drying effect on the flavour producing enzyme. Volatile and non-volatile flavour measurements were carried out for all the test dried samples. FD and HPD- dried tomatoes of the same cultivar were not significantly different from each other. Generally, acid concentration increased while sugar concentration decreased with increased thermal processing. Microwave vacuum drying operated under high vacuum of 0.1 mbar (also called as microwave freeze drying) produced the best flavour quality, followed by HPD. An integrated approach of instrumental and sensory analysis was adopted to investigate the drying impact on the stability of flavour compounds and their associated sensory attributes. The volatile and sensory profiles of HPD- and FD-dried tomatoes were comparable, whereas loss of compounds contributing to fresh green aroma and presence of heat-ind