A STUDY OF MECHANICAL AND STRUCTURAL PROPERTIES OF APPLE AND POTATO

Abstract

Among various fruits and vegetables, apple and potato are the most widely used products for consumption. Consumers select the food products which retain a natural texture, color and freshness. The overall quality and freshness of apple and potato depend on their mechanical properties (cell-wall tension, cell-wall stiffness, stretch ratios, turgor pressure, tissue strength etc.) and structural properties (moisture content, shrinkage, porosity, density, surface area etc.). From harvesting to consumption of apple and potato, mechanical and structural properties of these products are affected badly at several stages. Due to the application of external mechanical forces during post-harvest handling, several types of damage occur in these products. Mechanical damage is one of the major causes for the degradation of the quality of these products. On the other hand, during storage, drying and processing, water removal from apple and potato leads to change their structural properties. The first step in reduction of damage in apple and potato is the understanding of cell-wall mechanical properties. The mechanical properties of cell-wall can be calculated by developing constitutive relations/- mathematical models for cell-wall material during deformation. The dehydration process is frequently used to extend shelf-life of foods by increasing their stability during storage [95]. Previous research indicates that the water removal, from apple and potato during dehydration, caused the contraction in the cell material which results the formation of pores [11]. As drying process proceeds, moisture content in apple and potato decreases, consequently internal pressure in these products generated on pore surface. This pressure is responsible for change in porosity. Basically, the dependence of porosity on moisture content variation has been discussed in literature [81, 184]. The effect of initial i ii porosity and the stress, originated in cellular structure during water removal, have not been studied yet. Therefore, a suitable model is required to describe porosity variation with respect to pressure generated on the cell surface. It is evident from previous studies [82, 83, 184] that the mathematical models, developed by several researchers to predict structural properties of various food products, are the function of moisture content only. During dehydration, the effect of drying time on the structural properties has not been considered so far. For industries point of view, variation in structural properties with respect to time is important for allocation and optimization of resources. Therefore, more work is required for the development of mathematical models, so that, the moisture content variation and structural properties can be studied as function of drying time. Outline of the thesis The present thesis is compiled in 6 chapters, and the chapter wise description is given below. Chapter 1 is an introductory and contains some basic definitions related to thesis. It gives brief description of mechanical and structural properties, mathematical modeling and work done by various authors for apple and potato. In Chapter 2, we have proposed a strain energy function to study the mechanical properties (cell-wall tension and stretch ratios) of apple and potato by considering tissues as a lattice of hexagonal cellular structure under an external load. It has been considered that the tissues are isotropic, incompressible, homogeneous, thin, and shows hyperelastic behavior. The aptness of the proposed model is explored in the light of numerous experimental data [50, 87] of apple and potato tissues in isotropic condition. A Levenberg-Marquardt algorithm is used to estimate material constants of the model, which is based on the regression between predicted results and experimental data. A good fit of the proposed model is obtained with available experimental data. Comparisons are made between our results and previous studies. It is noticed that our results are better in respect of coefficient of determination (R2) and standard deviation (SD) than the previous results. It can be concluded that our model can describe those mechanical properties of apple and potato tissues for which different model iii were proposed earlier. In Chapter 3, a generalized form of the strain energy function, which has been developed in Chapter 2, is proposed. Turgor pressure plays an important role in stiffness of apple and potato cells and hence freshness of the whole product. To determine the quality of these food products, it is important to understand the effect of turgor pressure in the cells. The effects of turgor pressure can also be analyzed by using constitutive relations between turgor pressure and stretches in cell-wall material. Therefore, in this chapter, we have developed a relation between turgor pressure and stretch ratio by using strain energy function developed in Chapter 2. The developed relation is validated with the help of experimental data taken from literature [87]. A Levenberg-Marquardt algorithm is employed for regression analysis to calibrate the model constants by correlating predicted and experimental values of turgor pressure and stretch ratio for apple and potato tissues. A good fit of the developed relation to experimental data is obtained with the coefficients of determination of 98.02% for apple and 98.0% for potato. In Chapter 4, variation of porosity in apple and potato is discussed during dehydration. Porosity (volume fraction of pores) is one of the key parameter that affects the quality and other properties of foods (such as apple and potato). Therefore, to examine the porosity variation during dehydration in apple and potato, an arbitrary small cubic volume element is considered which contains pores (intracellular spaces) distributed in it. Further, it is assumed that each pore in the cubic volume element is spherical. A mathematical relation is developed between porosity and pressure generated (due to contraction of cells during water removal) in outward direction on the surface of spherical elements containing pore. The developed relation is tested with the help of experimental data [81, 83] for several drying methods for apple and potato and found satisfactory in respect of experimental observations. For the given pressure range, acquired porosity range is 0.1 to 0.92 for apple and 0.03 to 0.89 for potato which is matched with the existing experimental values. In Chapter 5, drying behavior and structural properties of apple and potato are investigated as a function of drying time. It is well known that mathematical modeling is a frequently used tool to study the drying kinetics of various fruits and vegetables. Therefore, in iv this chapter we have proposed a thin-layer drying (mathematical) model for investigating the drying characteristics of apple and potato. For validation of the proposed model, we have conducted experiments for one apple variety, namely, Fuji and three potato varieties, namely, Kufri Chipsona-1, Kufri Himsona and Kufri Bahar at temperature 60, 70 and 80oC. A nonlinear regression procedure is employed to fit the drying model to the experimental data. The model is compared with several existing thin-layer drying models according to chi-square (c2), coefficient of determination (R2) and root mean square error (RMSE). Results based on our model has close resemblance with experimental observations and are better in comparison of other models cited in literature. The effect of temperature on drying characteristics of apple and potato is determined. It is observed that the moisture content in all the samples decreases exponentially as the drying process progresses. On the successful validation of the drying model by experimental observations, drying model is used to calculate structural properties in terms of drying time. Our model is able to describe these properties suitably. Finally, Chapter 6 presents the summary and concluding remarks of this thesis and the possible directions of the future scope