Abstract
Refined vegetable oil (Kings brand), was purchased from Mile 3 Market, Port Harcourt, Nigeria and was repetitively used in frying Akara balls for a period of 24 hours, at intervals of four hours per day, for six days. The effect of heat and repetitive use for frying on quality of the oil was monitored by the measurement of the physico-chemial parameters of the oil after each frying period. The density, % free fatty acid, viscosity, peroxide value and iodine value of the fresh and used oil were determined to investigate the level of deterioration. The results obtained show that the values of the parameters increased with frying time except that of iodine value which decreased with frying time. The free fatty acid value increased from 0.06 -.56%, density increased from 0.900-0.910 g/ml, viscosity increased from 23.33-28.31mm<sup>2</sup>/s, peroxide value increased from1.988-4.879meqO<sub>2</sub>/Kg and iodine value decreased from 43.35 – 29.83wijis. These changes are attributed to the destruction of the chemical structures of the triacylglycerols and formation of new products in the oil by heat. The increase in the density and viscosity shows that as frying progressed, denser and higher molecular weight compounds are formed thereby rendering the used oil more viscous. The increase in peroxide values may be due the formation of peroxides as the primary products of oxidation. The formation of peroxides leads to loss of unsaturation in the oil which is seen as a reduction in iodine value of the oil. The sweet aroma of the oil was also lost as frying progressed as a result of de-esterification of the oil and formation of secondary oxidation products that impart off-flavours to the oil. The findings show that repeated use of vegetable oil for frying leads to deterioration/degration of the oil.
Highlights
Vegetable oils are composed of triglycerides which are extracted from seeds or nuts of vegetables
In 1991, investigation of the changes in deep-fat frying was carried out and the researchers found out that the presence of air and water accelerated the rate of deterioration of the frying oil and density, viscosity, % free fatty acid, peroxide value and iodine value in other to assess the changes in physicochemical parameters and degree of deterioration
Peroxide value: The results from the analysis show that the peroxide values of the oil samples increased with increase in frying time from 1.54 meqO2/kg oil to 5.99 meqO2/kg oil for the heated sample
Summary
Vegetable oils are composed of triglycerides which are extracted from seeds or nuts of vegetables. In addition to the triacylglycerols, contain beta-carotene and some phenolics that enable the oil to have antioxidant potentials. The exposure of oils to light, heat or air leads to the alteration of these parameters and an attendant loss in quality due to formation of offflavours, colour defects as well as loss in nutritional quality [2] This is because the essential fatty acids and vitamin present in the oil are oxidizable. Some common foods prepared by frying include: fish, meat, and repeated frying on the quality of vegetable oil with time so that the use of oil that has gone bad can be discouraged in other to prevent possible health consequences. Oil used for the study is the ‘’kings brand’’ vegetable oil with smoke point value of 270°C, justifying the oil’s suitability for frying purposes
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