The extracts of Citrus limetta (sweet lime) peelings were evaluated as a fermentable substrate for hydrogen (H(2)) production by dark-fermentation (acidogenic) using both anaerobic mixed consortia and selectively enriched acidogenic mixed consortia. Extraction was carried by pretreating sweet lime peelings at 121 degrees C (1 bar pressure) at variable pH (6 and 7) and digestion time (20 and 40 min). Maximum organic matter extraction was observed at pH 7.0 (40 min). Fermentation was performed at different organic loading conditions [OL1, 1.17 kg COD/m(3); OL2, 2.35 kg COD/m(3); OL3, 4.69 kg COD/m(3)] under acidophilic microenvironment. H(2) production was found to depend on the concentration of the substrate and composition. Increase in organic load showed consistent improvement in H(2) production. Operation at OL3 employing selectively enriched inoculum documented higher cumulative H(2) production (10.07 mmol) and H(2) production rate (0.345 mmol/h) (pH 7; 40 min). Substrate degradation was also found to increase with increase in organic loading. Maximum substrate degradation (SD) was registered at pH 6 (40 min) with anaerobic culture (2.80 kg COD(R)/m(3); xi(COD) 31.82%) and at pH 7 (40 min) with selectively enriched acidogenic culture (3.20 kg COD(R)/m(3); xi(COD) 36.36%). Concentration of volatile fatty acids (VFAs) also improved with increase in organic load. Maximum VFA concentration (1098 mg/l) was observed with OL3 (pH 7; 40 min) by using selectively enriched culture. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.