Excessive acidification occurs frequently in food waste (FW) anaerobic digestion (AD) due to the high carbon-to-nitrogen ratio of FW. In this study, zero-valent iron (ZVI) was applied to prevent the excessive acidification. All of the control groups, without ZVI addition (pH similar to 5.3), produced little methane (CH4) and had high volatile fatty acids/bicarbonate alkalinity (VFA/ALK). By contrast, at OLR of 42.32 g VS/L-reactor, the pH of effluent from the reactors with 0.4 g/g VSFWadded of ZVI increased to 7.8-8.2, VFA/ALK decreased to <0.1, and the final CH4 yield was similar to 380 mL/g VSFWadded, suggesting inhibition of excessive acidification. After adding powdered or scrap metal ZVI to the acidogenic reactors, the fractional content of butyric acid changed from 30-40% to 0%, while, that of acetic acid increased. These results indicate that adding ZVI to FW digestion at high OLRs could eliminate excessive acidification by promoting butyric acid conversion and enhancing methanogen activity. (C) 2016 Elsevier Ltd. All rights reserved.