Going beyond conventional osmotic dehydration for quality advantage and energy savings

Abstract

Osmotic drying is a partial dehydration process, often considered more as a treatment, togive the product a quality improvement over the conventional drying process. Theosmotic treatment involves soaking of a food in hypertonic solution of sugar and/or saltfor specific times under controlled temperature condition. The process involves twocounter-current mass transfers, a loss of water from the food to the solution and thesimultaneous migration of solids from solution to the food. Such mass transferphenomena are governed by pretreatment, osmotic solution, product and osmoticenvironment related factors. The method has two major advantages when combined orcompared with other drying methods. The quality of osmotically dehydrated products isbetter and shrinkage is considerably lower as compared to products from conventionaldrying processes. Secondly, the technique helps to conserve the overall energy relative toother drying procedures. The first aspect has been widely studied while the energyaspects are addressed rather scarcely. The major objective of this paper is to discuss theadvantage of osmotic dehydration in terms of energy reduction and its potentialcontribution to maximize profit by reducing the associated costs. The osmoticdehydration step can be done before, during or after the conventional drying process toenhance the mass transfer rate or to shorten the duration of drying time. After the osmotictreatment, 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 andthe energy required to finish dry the product to achieve the desired product stability. Thisreduction in moisture has a significant impact in conservation of energy when thetechnique complements other conventional drying methods like convective, freeze,microwave and vacuum drying. Moisture removal by phase change (evaporation ofwater) 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 donewith 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 energydemand and maximize profit.