Quantitative analysis of drying behaviour of fruits and vegetables

In this study, the drying characteristics of seven vegetables (garlic, potato, bean, ginger, leek, onion, and carrot) and five fruits (avocado, banana, sultana, kiwi fruit and apple) were studied under idealised 'constant' controlled drying conditions using an automatic thermogravimetric analyser. Drying-rate curves were constructed and quantified in a systematic way using the least-squares method. This allowed the drying behaviour of each product to be expressed in terms of three variables: gradient of the 'constant rate' stage, gradient of the falling rate stage and critical moisture content (CMC). The drying curves of fruits and vegetables were found to vary greatly indicating the nature of foodstuffs to have a strong effect on the drying kinetics. The gradient of the constant rate period was not truly constant but had an average gradient of 3.1 x 10^-4 per second with bean having the lowest gradient (1.9 x 10^-4/s) and garlic having the highest gradient (5.3 x 10^-4/s). This was expected as bean had the highest moisture content (93% wet basis) and was able to feed the surface with sufficient water to maintain a near constant rate of evaporation. The falling-rate period ranged from 10 x 10^-4 per second with avocado and garlic having the lowest gradient while apple had the highest gradient of 30 x 10^-4 per second. The CMC for most of the fruits and vegetables studied was about 1 kg/kg dry mass. This means that, at the CMC, the mass of water was the same as the mass of dry matter but with a spread in values from 0.7 for a fruit like banana to 1.8 for avocado.