Osmotic dehydration as a pre-treatment before combined microwave-hot-air drying of mushrooms

Combined microwave hot air drying can greatly improve the structure and bul k volume of dried mushrooms. However, the geometry and dielectric propertie s of mushrooms are such that overheating of the centre hampers the applicat ion of this technology. Osmotic dehydration can be used as an effective met hod to remove water from vegetable tissues while simultaneously introducing solutes in the product. The residues of the solute inside the product not only influence the taste and flavour of the product, but as well the dielec tric properties. In this paper halved mushrooms are osmotically pre-treated with a sodium chloride solution and subsequently dried by application of c ombined microwave/hot air heating. The increased salt concentration has a s trong effect upon the loss factor. The results demonstrate that the mushroo ms are more homogeneously heated due to the reduced centre heating, have a slightly shorter drying time, show improved rehydration properties, reduced shrinkage and a higher open-pore porosity. (C) 2001 Elsevier Science Ltd. All rights reserved.