Microwave roasting of peanuts: Effects on oil characteristics and composition

Roasting of peanuts by microwave heating (MWH) for different periods was studied in comparison with conventional heating method (CHM). The oils extracted from microwave roasted peanuts (MWRP) showed gradual darkening by time of heating. Colour indices of the oil samples were calculated to show the effect of heating on the oil colour. Chemical characteristics and fatty acid composition of the extracted oils were determined. In addition, peroxide value, conjugated dienes and trienes were determined. It was found that very low amounts of epoxy and conjugated fatty acids as well as fatty acid peroxides were formed as detected by GLC, UV spectrophotometry and peroxide value determinations. Freshly extracted peanut oils subjected directly to MWH showed increase of formation of conjugated trienes. In addition, the MW treatment causes oil darkening and higher colour indices were obtained. It was generally concluded that even short period MWH accelerates the formation of some undesirable and harmful compounds (e.g. oxidation products, transformed pigments ... etc.) during peanut roasting.