|عنوان فارسی مقاله:||فراتر رفتن از آبزدایی تراوشی معمول با هدف مزیت کیفیتی و صرفه جویی در انرژی|
|عنوان انگلیسی مقاله:||Going beyond conventional osmotic dehydration for quality advantage and energy savings|
|رشته های مرتبط:||صنایع غذایی، فراوری مواد غذایی، زیست شناسی و بیوشیمی|
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Osmotic 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.
Osmotic dehydration (OD) is a dehydration process of foods that involves soaking a food in hypertonic sugar and/or salt solution to reduce the moisture content of foods before actual drying process. OD is performed to reduce the moisture content of food products in minimal processing under ambient or modified environment conditions (Escriche et al., 2000). During the process two simultaneous counter-current flows may occur; waterflow out of the food into the solution, the simultaneous transfer of water soluble solutes from the solution into the food, and migration of natural solutes (sugars, organic acids, vitamins, reducing sugars, some flavor compounds, volatiles, minerals, etc.,) from the food into the solution (Le Maguer and Biswall, 1988). The process can be represented as shown in Fig 1. Since the hypertonic solution has higher osmotic pressure with reduced water activity, it serves as a driving force for water withdrawal from the cells solution to the osmo-active solution. The removal of water during osmotic process is mainly by diffusion and capillary flow, whereas solute uptake or leaching is only by diffusion (Rahman, 2007). All these mass exchanges between the osmotic solution and foodstuff may have an effect on the overall yield and quality of the dehydrated product.In perfectly semi-permeable membrane, the solution is unable to transfer through the membrane into cells, but it is hardly to obtain such type of membrane in food materials due to its complex internal structure and possible damages during processing (Shi, 2008). Hence, such conditions are important in osmotic dehydration processes to allow counter flow of solutes and water. In plants due to semi-permeable nature of plant tissue and low molecular size of water molecules the flux of water coming out of the food is much larger than solute gain from osmo-active substance. This result in a decrease of water content of the product with time till equilibrium condition is established. Therefore, the weight of the foodstuff will decrease, as well the water activity. According to some works, it is reported that up to 50% reduction in the fresh weight of fruits or vegetables can be achieved by osmotic dehydration (Rastogi and Raghavararo, 1997).