Combined osmotic and microwave-vacuum dehydration of apples and strawberries

The combination of osmotic pre-treatment - mainly in sucrose solutions - and microwave-vacuum dehydration of strawberries and apples has been studied. Water removal during osmotic treatment is accompanied by solute uptake from the osmotic solution. The resulting changes in composition bring about better properties of the final product in terms of structure and volume. Compared to the solely microwave-vacuum dried samples, osmotic pre-treatment improved volume retention from 20% to 50% (strawberries) and from approximately 20% to 60% (apples) based on the fresh volume. SEM pictures revealed that the cellular structure is also preserved better, when osmotic pre-treatment is used. Gel formation between pectins, sucrose and in some cases, calcium ions is believed to be the main cause of structure build-up. Vitamin C retention was around 60% with the microwave procedure applied here. (C) 2001 Elsevier Science Ltd. All rights reserved.