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Drying Technology, 25: 135–146, 2007 Copyright # 2007 Taylor & Francis Group, LLC ISSN: 0737-3937 print/1532-2300 online DOI: 10.1080/07373930601160973

Fractal Characterization of Some Physical Properties of a Food Product under Various Drying Conditions
Soraya Kerdpiboon and Sakamon Devahastin
Department of Food Engineering, King Mongkut’s University of Technology Thonburi, Bangkok,Thailand

The present work was aimed at monitoring and studying the relationship between microstructural changes of a food product and its physical property changes during two different types of drying; i.e., conventional hot air drying and low-pressure superheated steam drying, using fractal analysis. The external changes of a model food product, viz. carrot, which were represented in terms of thepercentage of shrinkage and the rehydration behavior, correlated well with its microstructural changes, which were represented by the rate of change of the fractal dimension of the microstructural images. The changes of physical properties and microstructure of carrot could be divided into two periods, which are periods of uniform and nonuniform deformation. The microstructural changes of the samplesundergoing different drying techniques were quite different, however. Fractal dimension of carrot undergoing drying increased with drying time for both hot air drying and low-pressure superheated steam drying cases. Fractal dimension was found to be a good indicator of the microstructural changes of a product undergoing different drying techniques and conditions. Keywords Carrot; Fractalanalysis; Hot air dryer; Low-pressure superheated steam dryer; Microstructure; Shrinkage; Structure-quality relationship

INTRODUCTION During drying, physical properties of food change mainly because of the loss of its moisture. Attempts have been made to characterize these physical property changes in terms of such parameters as the changes in volume, area and shape.[1–5] However, these externalchanges are caused by internal changes within the sample, which are directly related to the structure of the drying material. Among many methods that could be used to describe the structural and physical changes of materials are illustrating and evaluating their microstructural changes using such optical instruments as light scanning microscope (LSM) and scanning electron microscope (SEM).[6] TheseCorrespondence: Sakamon Devahastin, Department of Food Engineering, King Mongkut’s University of Technology Thonburi, 126 Pracha u-tid Road, Bangkok 10140, Thailand; E-mail: sakamon.dev@kmutt.ac.th

microstructural images cannot be easily quantified without the use of other appropriate evaluation techniques, however. For example, Leonard et al.[7] applied X-ray microtomography in combination withimage analysis to evaluate the shrinkage and crack formation of a sample (wastewater sludge) during convective drying. They found a good relationship between the drying kinetics and crack development, which was caused by internal diffusion limitations leading to moisture gradients and mechanical stresses within the sample. Due to the usefulness of image analysis, some researchers have applied thistechnique in combination with the technique called fractals (or fractal analysis) to quantify property changes of different samples. Fractals have been used successfully to quantify, among others, the shapes of brain cells, gold colloids, sponge iron, etc.[8,9] Some researchers have used fractals to describe the microstructural changes and mechanical property changes of foods (via the descriptionof the changes of images representing food surfaces) such as viscoelastic property changes of rice starch suspensions during gelatinization; stress cracks in grain kernel endosperm (corn kernel), which were caused by rapid drying at high temperatures; ruggedness of instant coffee after agglomeration; as well as ruggedness of restructured potato during deep fat frying, among others.[10–13]...