Going beyond conventional osmotic dehydration for quality advantage and energy savings

Abstract

drying is a partial dehydration process, often considered more as a treatment, to give the product a quality improvement over the conventional drying process. The osmotic treatment involves soaking of a food in hypertonic solution of sugar and/or salt for specific times under controlled temperature condition. The process involves two counter-current mass transfers, a loss of water from the food to the solution and the simultaneous migration of solids from solution to the food. Such mass transfer phenomena are governed by pretreatment, osmotic solution, product and osmotic environment related factors. The method has two major advantages when combined or compared with other drying methods. The quality of osmotically dehydrated products is better and shrinkage is considerably lower as compared to products from conventional drying processes. Secondly, the technique helps to conserve the overall energy relative to other drying procedures. The first aspect has been widely studied while the energy aspects are addressed rather scarcely. The major objective of this paper is to discuss the advantage of osmotic dehydration in terms of energy reduction and its potential contribution to maximize profit by reducing the associated costs. The osmotic dehydration step can be done before, during or after the conventional drying process to enhance the mass transfer rate or to shorten the duration of drying time. After the osmotic treatment, the moisture content of fruits and vegetable are usually reduced by 30-50% (wet basis). The amount of residual moisture in the product determines the duration and the energy required to finish dry the product to achieve the desired product stability. This reduction in moisture has a significant impact in conservation of energy when the technique complements other conventional drying methods like convective, freeze, microwave and vacuum drying. Moisture removal by phase change (evaporation of water) is an energy intensive process due to high latent heat of vaporization of water. During osmotic dehydration, there is no phase transition and the process can be done with minimum supply of energy, which is the principal reason for the energy savings. Novel approaches in food drying are constantly being explored to minimize the energy demand and maximize profit. Keywords: Osmotic, dehydration, drying energy, efficiency