Introduction

1. Fractal dimension of dried foods: A correlation between microstructure and porosity • Mohammad U. H. Joardder , Azharul Karim ,Chandan Kumar , Richard J. Brown • Faculty of Engineering and Science, Queensland University of Technology, Introduction The quality of dried food is affected by a number of factors including quality of raw material, initial microstructure, and drying conditions. The structure of the food materials goes through deformations due to the simultaneous effect of heat and mass transfer during the drying process. Shrinkage and changes in porosity, microstructure and appearance are some of the most remarkable features that directly influence overall product quality. Porosity and microstructure are the important material properties in relation to the quality attributes of dried foods. Fractal dimension (FD) is a quantitative approach of measuring surface, pore characteristics, and microstructural changes [1]. However, in the field of fractal analysis, there is a lack of research in developing relationship between porosity, shrinkage and microstructure of different solid food materials in different drying process and conditions [2-4]. Establishing a correlation between microstructure and porosity through fractal dimension during convective drying is the main objective of this work. Results and discussion Different food materials showed a wide range of three-dimensional FD in hot air drying and confirmed the dependence of variation of microstructure and porosity changes on original composition and structure of food materials. Microstructure ,tropograph and surface roughness can be expressed quantitatively by three dimensional FD. These structural properties of the three selective materials , as shown in figure 2-4, demonstrate clearly that structural characteristics significantly varies due to material composition and structure. An overall fractal dimension has been determined for these selected materials using enclosing boxes method as shown in the figure 5. A summery of the results for the selected materials in two drying ( microwave and convective ) in terms of porosity and fractal dimension is shown in the table 1. A greater fractal dimension was found for highly uneven disordered profile, which agrees with literatures . To illustrate this, the higher value of FD of the pore distribution manifests to a highly compact surface or volume .Value FD tends to 3 means compacted volume with filled pores. In addition of these, the result also express the 3 D fractal dimension allows the quantification of structural properties of dried food considering both microstructure and porosity. Whereas , 2D fractal dimension deals with microstructure and surface porosity . Therefore, determining 3D fractal dimension using priofilometer and its 3D image analysis software provides more comprehensive relationship among microstructure , porosity and fractal dimension. (c) (a) (b) Figure 2: (a) Microstructure, (b) topograph and (c) surface roughness of potato Materials and method Three selected food materials; namely carrot, apple and potato; were sliced in three different thickness 1.5,2.5 and 3.5 mm. All of the samples were dried in convective dryer at 700 C. Later on the samples were placed in microwave oven in order get same amount of moisture as remained after convective drying. After drying, the samples were analyzed in terms of microstructure , porosity and fractal dimension. An extensive experimental work has been conducted in this research. To study the microstructure of selected foodstuff after drying , a scanning electron microscopy (SEM ) was performed in a Hitachi Analytical Table Top Microscope TM3000 at 5 kV in high – vacuum mode. The images were then analysed using ImageJ 1.47. This programme evaluated the number of pores and their areas, in pixels, and transferred the measurements into SI units. In addition, a porfilometer A Nanovea ST400 Profiler has been used for observation of the topography and fractal dimension of the dried foodstuff. In general, it uses beams of light to read a surface. They shoot a beam out and measure the time it takes to return. The Nanovea 3D software is the acquisition software that is used with all Nanovea Profilers. The software allows to define the size of the area, or line, to be measured, as well as the fractal dimension measurement using the fractal box counting method. (c) (a) (b) Figure 3: (a)Microstructure, (b) topograph and (c) surface roughness of carrot (c) (a) (b) Figure 4: (a) Microstructure, (b) topograph and (c) surface roughness of apple Table 1: Ranges of porosity and fractal dimension Figure 5. 3D fractal dimension by enclosed boxes method Figure1. Process flow chart of this study Conclusion The preliminary results of this research provides enhanced understanding of microstructure and porosity from a quantitative point of view. Moreover, further study would be worthwhile for anticipating and maintaining the structural properties of dried food materials. 1.Sansiribhan, S., S. Devahastin, and S. Soponronnarit, Quantitative Evaluation of Microstructural Changes and their Relations with Some Physical Characteristics of Food during Drying. Journal of Food Science, 2010. 75(7): p. E453-E461. 2.Xu, P., A.S. Mujumdar, and B. Yu, Fractal theory on drying: A review. Drying Technology, 2008. 26(6): p. 640-650. 3.ShafiurRahman, M., Physical meaning and interpretation of fractal dimensions of fine particles measured by different methods. Journal of Food Engineering, 1997. 32(4): p. 447-456. 4.Sansiribhan, S., S. Devahastin, and S. Soponronnarit, Generalized microstructural change and structure-quality indicators of a food product undergoing different drying methods and conditions. Journal of Food Engineering, 2012. 109(1): p. 148-154. Corresponding author: m.joardder@qut.edu.au