Effects of postharvest storage and processing techniques on the main fatty acids in the profile of oil extracted from African Walnut (Tetracarpidium conophorum Mull. Arg.)

Chikere G. Nkwonta, M. Carmen Alamar, Sandra Landahl, Leon A. Terry

Plant Science Laboratory, Cranfield University MK43 0ALUK.

Abstract

African Walnut (Tetracarpidium conophorum Mull. Arg.) is a perennial climber which grows in the western and central regions of Africa. The nuts are processed by boiling and roasting and are sold within 1 – 5 days to consumers through the open market system. During processing, storage and distribution, the nuts are typically exposed to high temperatures raising concerns over nutrient quality and safety. Although African walnut, like several other nuts, contains high amount of oil, there is no study reporting on how the common processing methods (boiling and roasting) affect the fatty acid profile. Nut samples (n = 702) at both early and late maturity were harvested and stored at 5°C. Randomized sampling was done (0, 10 and 20 days) and nuts grouped according to treatments (boiling, roasting, unprocessed). Nuts were then held for 3 and 7 days at either 25°C or 37°C to simulate normal retail practices. Oil was extracted and analysed as fatty acid methyl esters using gas chromatography flame ionization detection and gas chromatography coupled mass spectrometry. Retention times were compared with known standards. Results indicated the presence of C16:0, C18:0, C18:1 cis -9, C18:2, cis - 9, 12, C20:0, C18:3 with C18:3 being the most abundant (1.1 – 8.2 mg g^-1 dry matter). In general, postharvest storage at 25°C or 37°C for 3 or 7 days after boiling and roasting significantly increased concentrations of the fatty acids (> 50%) in nuts stored for 10 days compared to unprocessed. Current processing methods and retail storage practices improved concentrations of the fatty acids in African walnut stored up to 10 days at 5°C.

Keywords: Food analysis; Food composition; African walnut; Gas chromatography; Tetracarpidium conophorum (Mull. Arg.); Postharvest storage; Methyl esters; Nut processing.