BACKGROUND: Cheese whey powder (CWP) is a concentrated source of lactose and other essential nutrients for ethanol fermentation. CWP solution containing different concentrations of total sugar was fermented to ethanol in an up-flow packed-column bioreactor (PCBR) at a constant hydraulic residence time (HRT) of 50 h. Total sugar concentration in the feed was varied between 50 and 200 g L-1 and a pure culture of Kluyveromyces marxianus was used for ethanol fermentation of lactose. Variations of ethanol and sugar concentrations with the height of the column and with the feed sugar concentration were determined. RESULTS: Ethanol concentration increased and total sugar decreased with the column height for all feed sugar contents. The highest effluent ethanol concentration (22.5 g L-1) and ethanol formation rate were obtained with feed sugar content of 100 g L-1. Percentage sugar utilization decreased with increasing feed sugar content above 100 g L-1 yielding lower ethanol contents in the effluent. The highest ethanol yield coefficient (0.52 gE g(-1) S) was obtained with a feed sugar content of 50 g L-1. Biomass concentration also decreased with column height, yielding low ethanol formation in the upper section of the column. CONCLUSION: The packed column bioreactor was found to be effective for ethanol fermentation from CWP solution. The optimum feed sugar content maximizing the effluent ethanol and the specific rate of ethanol formation was found to be 100 g L-1. High sugar content above 100 g L-1 resulted in low ethanol productivities due to high maintenance requirements. (C) 2008 Society of Chemical Industry