Protective role of onion and garlic on physicochemical alterations and toxicity of heated soybean oil

Abstract

Fresh and soybean oil heated with or without onion or garlic were analyzed for their physicochemical and toxicological properties. Darkened appearance, off flavors, rancid taste and significant reduction of iodine value was obtained for the heated oil. Acid value, peroxide value, viscosity and concentration of malondialdehyde of the thermooxidized soybean oil were also significantly elevated (P<0.05) with considerable loss of ascorbic acid and tocopherols. Heating with onion and garlic considerably preserved the quality of the oil as indicated in the significant reduction in levels of lipid peroxidation
indicators as well as reduction in loss of ascorbic acid and tocopherol. Rats fed with diet containing heated oil for a period of four weeks showed significant elevation (P<0.05) in the serum levels of alanine
aminotransferase (ALT) and aspartate aminotransferase (AST) starting from the second week to the end of the experimental period. This might be as a result of cellular damage caused by peroxides and other
lipid oxidation products which allowed the enzymes to leak out from the tissues. The levels of these enzymes were maintained close to the control in rats fed with diets containing heated oil with garlic and
onion indicating their protective role in lipid oxidation.