Fatty acid and tocopherol contents and oxidative stability of walnut oils

Abstract

Walnuts (Juglans regia L.) were collected during the 1997 harvest from 13 different cultivars of trees grown in a replicated trial in an experimental orchard at Lincoln University. Two U.S. commercial cultivars (Tehama and Vina), three European commercial cultivars (Esterhazy, G139, G120), and eight New Zealand selections (Rex, Dublin’s Glory, Meyric, Stanley, Mckinster, 150, 151, 153) were evaluated. Total lipids were analyzed for fatty acids by capillary gas chromatography, tocopherols by high‐performance liquid chromatography, and oxidation stability by Rancimat. The total oil content of the nuts ranged from 64.2 to 68.9% while the stability of the oil ranged from 3.9 to 7.8 h. The oleic acid content of the oils ranged from 12.7 to 20.4% of the total fatty acids, while 18:2 content ranged from 57.0 to 62.5% and the 18:3 contents ranged from 10.7 to 16.2%. Reduced stability of the oil as measured by the Rancimat method appears to be correlated to higher levels of 18:2 in the extracted oil. The total tocopherol contents of these nuts ranged from 268.5 to 436.0 µg/g oil. γ‐Tocopherol dominated the profile while α‐tocopherol was only 6% of the total content. Peroxide values of the fresh oil were measured spectrophotometrically to give an indication of the overall stability. The levels of total tocopherols when combined with the level of unsaturation in the oil in a multiple regression analysis had a significant relationship (R2=45.2%, P<0.001) with the peroxide value in the oil.