3.3. Effect of frying cycles on kinematic viscosity
The kinematic viscosity of fresh RBO was 3.39 × 10−6 m2 s−1. It was found to increase from 3.81 × 10−6 m2 s−1 to
3 3.93 × 10−6 m2 s−1 and 4.11 × 10−6 m2 s−1 to 4.43 × 10−6 m2 s−1, respectively for heating and frying experiments for 1–6 consecutive cycles of frying (Fig. 2b). This increase in viscosity of frying oil was probably due to formation of higher molecular weight compounds involving carbon to carbon and carbon to oxygen to carbon bonds between oxidised fatty acids (Lalaset al., 2006). Teah (1988) indicated that polymeric materials are mainly responsible for the increase in viscosity. It was also found that because of high water content leaching from fried products, the polymerization of frying oil increased during the frying process. It is directly related to the formation of polymeric compounds due to exposure to oxygen at high temperature during frying (Kochhar and Gertz, 2004). It was
observed that formation of total polar material increased with frying cycle and correlating well with the change in oil
viscosity (R2 = 0.93).