Effect of Roasting Temperature and Duration on the Quality of Oil from Walnut (Plukenetia conophorum) Using Response Surface Methodology

Abstract

This study investigated the effect of heat treatment and duration on yield and qualities of Walnut (Plukenetia conophorum) oil. The experimental design was carried out using the central composite rotatable design (CCRD) of response surface methodology with a total of thirteen experimental runs. The process variables and their ranges were roasting temperature 87.57ºC to 172.43ºC and roasting duration 10 min to 46.21 min. Oil from each experimental run was extracted using Piteba screw oil expeller. The expressed oil yield was determined and the results revealed that the oil yield ranged from 33.87% to 30.67% for the maximum and minimum values which were achieved at temperatures and duration of 172.43% at 25 min and 87.57ºC at 25 min respectively. Optimum process condition was achieved at roasting temperature and duration of 160ºC and 40 min with two possible solutions of 0.82 desirability which gave oil yield (33.50%); peroxide value (7.8 MeqO2/g); colour (13.69 mg/L); free fatty acid (1.46%) and iodine value (136.82 mmol/100 g). The low free fatty acid and peroxide values in walnut oil is an indication of its overall quality thus its suitability as an alternative oil to supplement existing edible oils.