Physical Properties of Food Oils and Factors Affecting Bubble Dynamics During Frying

Shreya N Sahasrabudhe, Purdue University

Abstract

Deep fat frying is one of the most complex transport problems in the food industry. According to the US Department of Agriculture, world vegetable oil consumption has increased from 151.68 to 177.16 million metric tons from 2011/12 to 2015/16. Out of the total production, 73.4 % of the oil is used for food purposes; 20 % of which is for shallow and deep fat frying. Frying oil degrades via exposure to heat, oxygen and water, forming volatile and non-volatile products which reduce oil interfacial tension (IFT). The rate of heat transfer, and oil uptake are affected, in part, by physical properties of oil such as density, viscosity and interfacial tension. There is a significant gap in knowledge on the effect of physical properties of oil on heat and mass transfer, especially at high temperatures relevant to the frying process. Hence, the aim of the present research is to understand and mathematically model the effect of temperature and oil quality on oil’s physical properties and develop hypotheses to describe the impact on heat and mass transfer during frying. Oil physical properties including density, viscosity, surface tension and contact angle were measured, and mathematical models were developed to understand the effect of temperature on the physical properties of vegetable oils before and after degradation due to frying. Density and surface tension decreased linearly with temperature; viscosity decreased exponentially. There was no effect of oil quality on viscosity at frying temperatures. Surface tension also remained unaffected by change in oil quality. Contact angle decreased as oil quality degraded with frying indicating increased wettability. When compared with their corresponding experimental values, predicted density values had < 2.0 % error, while predicted surface tension values had < 10.1 % error, and predicted viscosity values had < 12 % error. The error was within the equipment’s accuracy range. Thus, the mathematical models presented in this work can be used as a tool to predict the behavior of oils at high temperatures. This will help to gain a better understanding of oil absorption, as the properties affecting heat and mass transfer rates during frying may be accurately predicted at frying temperatures. Heat is transferred by convection from the hot oil to the food surface and by conduction from the surface of food to the core. The food material acts as a vapor generating matrix resulting in the formation of bubbles in hot oil at the food’s surface. The heat transfer coefficient during frying shows a bell-shaped curve; reaching its peak value at a high rate of moisture loss when bubbling is observed. Thus, the rate of heat transfer is dependent on bubbling characteristics. Hence, a customized assembly was built to understand how oil physical properties as well as process parameters such as oil temperature affect bubble formation during frying with an aim of understanding the effect of oil quality on heat transfer. A separate assembly was built to understand oil absorption during the immersion frying stage by understanding the effect of liquid properties and liquid-solid interaction on meniscus formation between consecutive bubbles when the orifice is submerged in the fluid.

Degree

Ph.D.

Advisors

Corvalan, Purdue University.

Subject Area

Agronomy|Fluid mechanics|Food Science|Mathematics|Mechanics|Thermodynamics

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