Microstructural Changes during Drying of Apple Slices

Abstract

In recent years some effort has been done trying to relate microstructural changes of dehydrated foods with macroscopical properties and quality factors of the processed product. This work assesses the microstructural changes of apple slices submitted to convective drying. Apple slices (25.7 mm diameter, 2.6 mm thickness) were dried in an oven at 70°C on a screen sample holder in order to allow mass transfer in any side of the sample. Microphotographs of the same sample were taken at determined time intervals using a stereomicroscope. The objects in these photographs were identified and classified in three size groups: (a) the cells group, containing objects with an area less than 0.03 mm²; (b) the mixed cells-intercellular spaces group, with an area between 0.03 and 0.06 mm², and (c) the intercellular spaces group, including objects with an area higher than 0.06 mm². Geometrical parameters of such objects, namely size (area, perimeter, equivalent diameter, and major and minor axis length) and shape parameters (compactness, elongation, and roundness) were evaluated. Shrinkage of cells and intercellular spaces was very clear during drying, observing a decrease of size with moisture content. Concerning shape factors, compactness remained constant, roundness slightly decreased during drying, and elongation increased in the final stage of the process. Macroscopical changes of the entire disc followed the same behavior as microscopical ones, suggesting that changes at the two scale levels are strongly related.

Keywords:

• Apple tissue
• Dehydration
• Image analysis
• Microstructure
• Shrinkage

Acknowledgments

M.A. Silva acknowledges CAPES (Brazilian Agency for Improvement of Graduate Personnel, Brazil, proc. n. BEX0956/02–3) for the financial support during her sabbatical stay at the University of Porto (UP), Portugal, as well as the facilities provided by Requimte/Department of Chemical Engineering at UP. L. Mayor acknowledges SFRH/BD/3414/2000 PhD grant to Fundação para a Ciência e a Tecnologia, Portugal.

Notes

All measurements in mm, except area in mm².