Abstract
Castor seed (Ricinus communis) is an oil-rich seed used in southeastern Nigeria for the production of fermented condiments such as ogiri. Because traditional fermentation is carried out under variable processing conditions, quantitative information on the combined effects of fermentation time, temperature and particle size is useful for process control. This study investigated the effects of fermentation time, temperature and particle size on nitrogen evolution during castor-seed fermentation and applied response surface methodology (RSM) to determine an optimum operating condition. Matured castor seeds obtained from Afor-Opi, Nsukka Local Government Area, Enugu State, Nigeria, were boiled, dehulled, oven-dried at 35°C, grated and sieved into particle sizes of 250–1200 µm. Fifty-gram portions were fermented at 25–45°C, with nitrogen evolution monitored at 24-h intervals for up to 120 h. A randomized three-factor design generated 20 experimental combinations. The fitted quadratic model was highly significant (F = 112.55, p < 0.0001) and explained 99.02% of the observed variation in nitrogen evolution (R² = 0.9902). The adjusted and predicted R² values were 0.9814 and 0.9212, respectively, and adequate precision was 41.206. Fermentation time, temperature, particle size, the time–temperature interaction and the quadratic time term were significant model terms. Numerical optimization selected 113.6 h, 43.67°C and 313.33 µm as the preferred combination, with a predicted nitrogen evolution of 0.984% and desirability of 1.000. The result demonstrates that the three operating variables can be jointly controlled to improve the predictability of castor-seed fermentation. Experimental confirmation of the predicted optimum is recommended before scale-up.