Novel and less understood methods of food dehydration: understanding the effects of process conditions and the mechanisms of water loss

Dehydration is one of the oldest food processing operations. It continues to be extensively employed today, and all indications point to its continued use in the foreseeable future. There are several dehydration techniques available for use in domestic and commercial practice. The main objective of this thesis is to focus on novel and less studied variants of two specific dehydrating methods: 1. Osmotic dehydration and 2. Frying. Osmotic dehydration is a natural process where water loss occurs by osmosis when food is brought into contact with a concentrated salt or sugar solution. Although this process consumes lower energy than other drying methods - which predominantly involve supplying the necessary latent heat for water evaporation from the food - a key problem is the high salt/sugar uptake which potentially poses health issues. In this thesis, a novel variant of osmotic dehydration called post-dipping dehydration – has been developed and studied. The technique involves dipping, say, potato slices, briefly in an osmotic solution, withdrawing it and allowing the water released to evaporate and/or drain under ambient conditions. The aim is to promote water loss just as in osmotic dehydration, but at the same time, minimize the uptake of the osmotic solute. The effects of osmotic medium concentration and dipping time on the water loss were initially investigated. Dipping potatoes in a higher concentration of osmotic solution was found to enhance post-dripping water loss, but the dipping time had no significant effect. Post dipping dehydration was also conducted as a multi-stage operation by repeatedly dipping potato slices in the osmotic solution followed by ambient exposure for a time period. The resulting water loss was comparable to osmotic dehydration but the uptake of the osmotic solute was significantly lower (less than 50%). In the next part of the research, multi-stage dip dehydration was investigated as a pre-treatment prior to frying potato chips, in order to evaluate its effect on the product quality. Multistage dip dehydration was found to decrease the frying time considerably which also resulted in about 17% lesser oil in the product. Colour measurements showed that the pre-treated samples were brighter and suffered lesser browning than blanched samples. The final part of this thesis is dedicated to gaining insights into shallow frying of potato and chicken cubes – an extensively employed but less understood dehydration operation in relation to deep-fat frying. The experimental investigations involved determining transient temperature variations during the process, moisture loss, oil uptake and the development of product texture. A key outcome of this research was the observation that the significant amount of water released initially from the food tended to form an oil-in-water emulsion, which subsequently boiled off to result in a phase inversion (i.e. the formation of water-in-oil emulsion). The temperature clearly increased after the phase inversion when the crust and texture of the end-product were formed.