Biohydrogen production using palm oil mill effluent (POME) as source of nutrients was evaluated via dark fermentation under mesophilic conditions. The effect of adding a plant enzyme preparation (PEP) extracted from dormant castor bean seeds (Ricinus communis L.) to improve the availability of nutrients from POME was also evaluated and carried out in two different approaches. The addition of PEP in a one step process, a simultaneous hydrolysis and fermentation, resulted in a significant reduction of adaptive phase by approximately 50%. Furthermore, hydrogen yield increased by 14% and hydrogen productivity by 48%. The two step process, which consists in a pre-hydrolysis followed by dark fermentation, promoted a hydrogen yield of 2.58 mmol H-2/g(DQO), reduced the adaptive phase by 75% and increased hydrogen productivity by 102% when compared to the original conditions of raw POME fermentation. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.