3.2. Effect of frying cycles on the colour of oil
The change in the colour of the oil subjected to 6 cycles of heating experiments and frying experiments are shown in
Fig. 2a. After 6 cycles, the red colour of the oil increased from 2 units to 3 and 4.5 units during heating and frying, respectively. Whereas, the yellow colour was increased from 15 units to 20 and 25 units during heating and frying, respectively. The oil was found to increase after frying due to presence of unsaturated carbonyl compounds, formation of degraded compounds by oxidation, non-polar compounds of foodstuff leached in frying oil (Augustin et al., 1987; Gutierrez et al.,1988). The colour of oil samples was found to change considerably after frying in comparison to heating. The increase in colour in oil after frying may be due to the Maillard reaction (carbohydrate/aldehydes react with aminocompounds) took place during frying. It occurs in poori and the carbon material is leached out from the poori to the oil, which darkens the colour of the fried oil. However, Burton (1989) pointed out that the alpha and beta unsaturated carbonyl compounds derived from sugars are first formed intermediates that react with substances containing alpha amino
groups to give carbonylnitrogen compounds which conjugate to form brown products.