The effect of repetitive frying on physicochemical properties of refined, bleached and deodorized Malaysian tenera palm olein during deep-fat frying

Abstract

Refined, bleached and deodorized tenera palm olein is widely used for domestic and commercial food frying. However, repeated heating and frying might lead to oil spoilage. This study focuses on the effect of deep-frying on physicochemical properties of repeatedly heated palm olein. Recycled palm olein was prepared by frying potato strips up to 5 cycles with potato-to-oil (g/ml) ratio of 3:20 prior to colour, density, viscosity, moisture content, contact angle, peroxide value and iodine value analyses. The fluctuation in the temperature of the oil was insignificant for every cycle (p > 0.05). The increase in red colour of the oil was significant (p < 0.05) after two cycles of frying from 3.6 to 5.0 Lovibond unit. After 5 frying cycles, there was no change in the density of RBDPO-2 and RBDPO-3. The viscosity of the oil after 5 frying cycles was higher as compared to after 5 heating cycles (67.7 ± 0.6 and 58.2 ± 0.9 cP respectively). Besides, the viscosity of RBDPO-3 was lower than RBDPO-2 only. The moisture content of the oil increased from 3.7 ± 0.1% to 30.4% (w/w) for RBDPO-2, whereas for RBDPO-3, the viscosity increased from 3.7 ± 0.1% to 30.4 ± 0.1% (w/w) due to the presence of the moisture from the potato. The change in the contact angle of the RBDPO-1 (control) was significant (p < 0.05) between the 1st cycle and the 5th cycle from 19.16 ± 0.45° to 12.03 ± 0.45° respectively. Besides, there was a significant change (p < 0.05) of contact angle for RBDPO-2 between the 1st cycle and the 5th cycle from 32.53 ± 0.5° to 18.06 ± 0.15° respectively. The peroxide value of the fresh oil was 2.8 ± 0.2 meq/kg and the value after the 1st cycle for all RBDPO(s) treatment increased, indicating that primary oxidation had occurred after the 1st cycle of heating and frying. A decreasing trend was observed from the 2nd until the 5th frying cycle because the instability of peroxide at a high temperature, causes rapid degradation and produced a dimer and volatile compound. Frying also decreased the unsaturated value of the oil as indicated by a decrease in the iodine value. Based on Pearson’s correlation test, there was a positive correlation between the colour, density and viscosity of the oil. The higher the peroxide value, the higher the density of the oil. There was also a positive correlation between the contact angle, moisture content and peroxide value of the oil. Whereas, a negative correlation was found between the iodine values, colour, density and viscosity of the oil. In conclusion, the potato repetitive deep-frying darkened the colour of the oil, increased its density, viscosity, contact angle, and peroxide value and decreased the degree of unsaturation of the oil as it shows a prominent sign of oxidative rancidity.