Ziaiifar, Aman-Mohammad (2008) Oil absorption during deep-fat frying: mechanisms and important factors. PhD thesis Génie des Procédés Alimentaires, Département Génie Industriel Alimentaire, AgroParistech 2008AGPT0084 p.173.

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Abstract

This research was performed to better understand the oil absorption during deep-frying. High oil content in fried products is one of the major issues making them unsuitable for daily consumption. In spite of all research efforts, fried products still contain significant amounts of oil. Experimental studies were performed in order to show by which mechanisms, where and when the oil absorption takes place during frying process. At first, the effect of pore development on the oil uptake was studied. The physical properties of adhered oil at the surface of product on this uptake were then studied. Finally, the effective thermal conductivity of crust and core regions and their effect on heat transfer were investigated. Potatoes were cut into rectangular shapes and fried at different oil temperatures (140, 155, 170, and 185°C). An improved Lees apparatus was successfully used to determine the effective thermal conductivity of fried samples. Results showed that the oil uptake increases as the oil bath temperature decreased from 185 to 140°C. During the frying period, the porosity increases due to forceful water evaporation and pore formation. However, during the cooling period, it starts to decrease as a result of the absorbed oil implanted in the pore spaces and collapse phenomenon. During the cooling period, when the surface oil temperature tends to decrease, the adhered oil interfacial tension and viscosity increase, resulting in more oil absorption. The different regions of product (core and crust) showed different thermal conductivity behaviours. The physico-chemical changes of product which occur during frying influence the thermal conductivity at these regions. In the core region, the starch gelatinization, taking place during the first minutes of frying (3 min), causes an increase in core thermal conductivity; while the moisture loss which starts after 3 min of frying decreases thermal conductivity. In the crust region, the thermal conductivity decreases with frying time due to moisture loss and formation of a porous structure.

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