Application of conjugated diene value as a measure of walnut oxidation progress

Lipids and oils are susceptible to oxidation. Analytical protocols are requiredfor measuring their oxidation progress. Spectrophotometric, iodometric, andcolorimetric methods may apply for measuring the early stages of lipid oxidation.In this study, walnut samples were selected and were kept shelled or unshelled at62, 72 and 82˚С to evaluate the relationship between peroxide value andconjugated diene value. A number of chemical and physical characteristics ofwalnut kernel oil were determined. Measuring walnut oil conjugated dienes wascarried out spectrophotometrically by reading UV absorption at 233 nm. Peroxidevalue was measured by conventional titration method. The results showed thatperoxide value, conjugated diene value and acidity (by oleic acid) of fresh walnutwere 0.3 meqO2/kg, 4.354 μmol/g and 0.064 %, respectively. First order linearequations were found to correlate peroxide and conjugated diene values inextracted oil from whole walnut and in walnut kernel, during storage at differenttemperatures (62, 72 and 82˚С), with a regression coefficient of 0.98 and 0.99. Ageneralized equation was used for expressing the relationship between peroxideand conjugated diene values in walnut, because there was no significant differencebetween the equations slopes and intercepts for three temperature conditions andtwo walnut forms.