BACKGROUND: Continuous ethanol fermentation of concentrated food waste hydrolysates has been studied. The process was carried out in an immobilized cell reactor with beads of calcium-alginate containing immobilized Saccharomyces cerevisiae H058 at temperature 30 degrees C and pH 5.0. RESULTS: The total residual sugar decreased with increase of hydraulic retention time (HRT) under various reducing sugar concentrations. Ethanol production by immobilized cells increased with increase in HRT, regardless of the substrate concentrations employed. The highest ethanol concentration of 89.28 g L-1 was achieved at an HRT of 5.87 h and reducing sugar concentration of 200 g L-1. At an HRT of 1.47 h, the maximum volumetric ethanol productivity of 49.88 g L-1 h(-1) and the highest ethanol yield of 0.48 g g(-1) were achieved at reducing sugar concentration of 160 and 200 g L-1, respectively. The difference between the fresh and the 30-day Ca-alginate immobilized cell was also shown by scanning electronic micrographs of beads taken from their outer and inner surfaces. CONCLUSIONS: Continuous ethanol production from concentrated food waste hydrolysates using immobilized yeast cells is promising in view of the high ethanol productivity obtained at relatively high conversion and excellent reactor stability. (C) 2011 Society of Chemical Industry