The Rheological Properties Of Rheology

Rheology is ‘the science of deformation and flow of matter. It is used in food science to define the consistency which is described by two factors i.e. viscosity and elasticity. And so, rheology stands for viscosity measurements, characterization of flow behavior and determination of material structure. Basic knowledge of these subjects is essential in end product quality evaluation. The rheological properties displayed by fat and oil system depend strongly on the physical state of the material. The rheological properties of fats and oils are the result of a combined effect of the SFC and the microstructure of the fat and oil crystal networks, including the shape, size, and spatial distribution pattern of the fat and oil crystals. Rheology

Viscosity measurements are used to monitor changes in shortening rheology due to crystallization. However, Bell et al. (2007) and Jirasubkunakorn et al. (2007) utilized a controlled stress rheometer to study fat rheology. The temperature is one of the most influential parameters effecting on rheological behaviors and consequently viscosity. According to Alonso et al. (1990) viscosity is the fundamental factor in the rheological study of liquid foods, which characterize the fluid texture. Viscosity of major fats and oil is summarized in Table 6. Fat and oils show the flow behavior of true Newtonian liquids. Near the melting point non-Newtonian behavior may occur due to the presence of fat and oil crystals. Viscosity increases with molecular weight but decreases with increasing unsaturation and temperature. The viscosity of an oil decreases by approximately 30% for each 10oC rise in temperature (Timms, 1985). According to Valdes and Garcia (2006) research of the two original vegetable oils studied, the viscosity of sunflower oil was reported to be more sensitive to temperature, it may increase

It determines a fat’s spread capability. It is major characteristic to consider when selecting a fat to be used in the preparation of cookies and other baked products. Although, most fats seem solid at room temperature, but actually comprised of liquid oil with a network of solid fat crystals keeping it in place. This combination of solid and liquid proportion allows the fat to be molded into different shapes. Temperature influences plasticity, hard fats such as butter becoming soft and more spreadable when warmed while chilled butter has very little plasticity. Unsaturation of fat increases its plasticity. Harvey (1937) stated that increasing the plasticity of a fat by certain ways, such as mechanical manipulation or the addition of edible oil, increased its shortening power also. Maximum of the aeration of dough is due to air trapped in the solid fraction of fat. Thus, a wide plastic range is essential for appropriate