Mass Transfer Kinetics During Osmotic Dehydration of Oha (Pterocarpus Soyauxii) Leaves




1.1     Background to the study

          Vegetables are important protective foods, which are highly beneficial for the maintenance of good health and prevention of disease. They contain valuable food nutrients, which can be successfully utilized to build up and repair the body. They are rich sources of carotene, ascorbic acid, riboflavin, folic acid and minerals like calcium, iron and phosporus (Sheela et al., 2004: Nnamani et al., 2010).

          Leafy vegetables represent a veritable natural pharmacy of minerals, vitamins and phyto chemicals (George 2005). For example, the potassium content of leafy vegetable is good in the control of hypertensive complication because it lowers arterial blood pressure. The fiber content of vegetables contribute to the feeling of satisfaction and prevents constipation (Noonan and Savage, 1999), while the protein in vegetables are superior to those found in fruits, although inferior to those found in grains and legumes (George, 2003).

          Dehydration is one of the most common natural and reliable methods of food preservation. Although reaction rates are generally reduced y dehydration, undesirable changes due to reactions such as enzymatic browning may result in quality changes (Charles, 2010) in order to reduce the rate of reaction, salt and sugar were introduced to help the drying of fruits and vegetables. How ever, sugar was used to preserve the quality of the dried product while salt helps to remove the moisture content. In other to achieve a presentable product, osmotic dehydration was introduced.

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          Osmotic dehydration is the process of water removal by immersion of the material (fruits and vegetables) in a concentrated aqueous solution (Charles, 2010). The fundamental purpose of food dehydration is to lower the water content in order to minimize rates of chemical reactions and to facilitate distribution and storage, while osmotic dehydration is the immersion of food material in a saline or sugar solution. This results in three types of counter mass transfer phenomenon, first, water outflow from the food tissue to the osmotic solution, second, a solute transfer from the osmotic solution to the food tissue, third, a leaching out of the food tissue’s own solutes (sugar, organic acids, minerals, vitamins) into the osmotic solution (Charles, 2010).

          All these mass exchanges between the osmotic solution and food materials may have an effect on the final dehydrated product. Complex internal structure and possible damage 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 food materials will decrease as well as the water activity.

          According to Yetenayet and Hosahlli (2010). They reported that up to 50% reduction in the fresh weight of fruits or vegetables can be achieved by osmotic dehydration.


          Some researchers have tried to increase the rate of osmotic mass transfer to reduce the processing time (Mundad et al, 2011; Bchir et al., 2012). However, some researches concern to minimize the uptake of osmotic solids, as it can severely alter organoleptic and nutritional characteristics such as the loss vitamin and mineral salt of the products (Shi and Xue, 2009; Jalaee et al., 2010 Phisut, 2012). During osmotic dehydration, a high osmotic rate would make the process more efficient and practical.

          The use of osmotic dehydration in vegetable  can help to extend the shelf-life thereby making it available all year round.

1.2     Justification

 The osmotic dehydration of vegetable will help to extend the shelf-life making it available all year round.

1.3     Objectives of the Study

          The objectives of the study are to

  1. To dehydrate orha (Pterocarpus soyauxii) leaves using osmotic method.
  2. To evaluate mass transfer kinetics during the dehydration process
  3. To evaluate the sensory quality of the dehydrated orha (Pterocarpus soyauxii) leaves.




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