It is known that increasing frying time increases the peroxide value, free fatty acid value and panisidine value of the oil . According to one study, when refined soybean oil was used to fry tortilla chips at 190 °C for 60 hours, variations in colour readings became more visible after 30 hours of frying. Between 20 and 30 hours, foam and off-flavour formed. After 30 hours of frying, the convective heat transfer coefficient changed more quickly . C. this up with an experiment in which they heated oil to 180 °C without adding food. they monitor the refractive index of the oil, which they discovered increases with heating time . Daoud, J. I. and Mirghani, M. E. S. used multivariate analysis of variance (MANOVA) to show that there is a significant correlation between frying time and tocopherol content, iodine value, peroxide value, free fatty acids and polymer content of oil. They fry potato chips in palm oil, cottonseed oil and groundnut oil at 170 °C and found that peroxide value, free fatty acids and polymer content increase with heating time; while tocopherol content, and iodine value decrease . A linear correlation was reported between frying time and both total polar compounds and acid value in soybean and palm olein oil . Similarly, it was found mutagenic 4-hydroxy-2trans-nonenal formed in soybean oil and its concentration increased with increased heating time up to 6 hours. After 6 hours the concentration started to drop, possibly due to further oxidation of the compound . The frying temperature and time not only affect the oil's major component triglyceride but also change its minor components. For example, in a study, it was found that heating at 180 °C reduced the hydroxytyrosol derivatives of olive oil by 60% in 30 minutes; after 60 minutes only 10% of the original hydroxytyrosol derivatives were found to be left in the oil. Compared to this, tyrosol derivatives degrade only to a smaller extent while the lignans content of the oil remains unchanged. If the heating temperature is lowered to 100 °C, it was reported that only 20% deterioration of the phenolic compound was observed even after 2 hours of heating .