A fundamental mechanical analysis of the texture of french fries

Texture is a major index of food quality and it is a major index of quality in food processing plants. Texture is one of the most important quality attributes of the french fry and is important for quality control in the potato processing industry. Indentation tests are frequently used in the food industry as a simple means of texture evaluation. However, indentation tests generally provide empirical results for product texture rather than fundamental results. This thesis attempted to onderstand how mechanical parameters characterizhg the textural properties of french frics could be derived from indentation tests. The focus of the first part of this study was to obtain a fundamental mechanical parameter, the elastic modulus, of a model food system (an agar gel), from the indentation test. Cylindrical indenters with varying diameters were used as indentation probes on large slabs of a 3% agar gel. A 3% agar gel was chosen because its mechanical properties are similar to those of cold french fry interior. Compression testing, using lubricated tenon platens and a travelling microscope, was used to independently measure the gel's elastic modulus (52 kNnf2) and Poisson's ratio (0.32). Indentation measurements of reaction force (P) and deformation (d) were analyzed with the theoret+ relationship for fiictionless flat cylinder indentation. A good linear fit (2 = 0.991) was obtained for Pld versus indenter radius, indicating the vaiidity of the analysis. However, the value of the elastic modulus derived from indentation was 79 The indentation derived elastic modulus overestimated the compressive elastic modulus by 52%. Such a large overestirnate suggests that additional factors to those considered in the theoretical model influence the indentation process. The presence of pores in the gel, frictional forces during indentation, and elastic mismatch between indenter and gel al1 may affect the indentation process. Nevertheless, similarity in results for the elastic modulus determined from indentation compared to compression, indicates that a simple testing method cm yield a more comprehensive evaluation of food textural quality. The focus of the second part of this study was to use a fundamental engineering approach to quantify the mechanical properties of the fried potato crust. The effect of fqing time on the mechanical propereies of fried potato crust was investigated and values for the fracture stress, fracture strain, elastic modulus, and fracture toughness of the fried potato crust were quantified. It was found that frying time had an effect on the mechanical properties of the fned potato crust. The fracture stress and elastic modulus values increased as fry time increased while the fracture strain and fracture toughness values decreased as fry time increased. The fracture stress and elastic modulus values were in the range of 100 and I MN^^, respectively. Fracture strain was in the range of 1-10 % strain. Fracture toughness was of the magnitude of 10 ~ r n ~ . With respect to fracture toughness, as length of fv time increased the crust material changed from a ductile or pliant material to a more brittle material. A value of the Poisson's ratio was unable to be quantitatively measured, but microscopy was employed to deduce that the value was 0.5. Therefore, it was determined that the tensile test commonly used to measure fundamental mechanical parameters of engineering materials cm also be employed to characterize the texture of fried potato crusts. Thus, an accurate means of obtaining indices of french fry quality may be possible through c ru t property measuremen ts. The focus of the third part of this study was to apply a full mechanics approach to the indentation analysis of a biological composite material. A 3% agar gel was modeled as the core of the composite materiai and fried potato cmsts of various thicknesses were modeled as the facesheets of the composite material to simulate the structure of a french fry with the aim of characterizing the indentation process. Using a theory which incorporated srna11 plate defonnation theory and plastic deformation theory, and membrane deformation theory, the load-deformation response of this biological composite material was characterized. The theoretical predictions of Ioad-deformation response for indentation of a biological composite were in reasonable agreement with the experimental results. At indentation deformations less than half the cmst thickness, the indentation process was characterized by the linear-small plate deformation theory. At indentation deformations greater than half the cmst thickness but less than the total cmst thickness the indentation process was characterized by non-linear plastic deformation theory. At indentation deformations greater than the total cmst thickness, the indentation process was defined by the membrane theory.