The pore structure of a commercially available dialysis membrane was modified by the deposition of crystals of calcium octanoate in the membrane matrix. The modified structure resulted in a significant reduction in sucrose loss when dialysis was used to reduce the potassium concentration of a solution of sucrose and potassium chloride. The results of fermentation studies of artificial molasses substrates using a proprietary strain of Saccharomyces cerevisiae showed that the optimum potassium concentration for micro-organism activity was similar to 10.5 g l(-1). Subsequent fermentation studies on an authentic Australian molasses substrate with an initial potassium concentration (11.0g l(-1)) close to the optimum value showed time predicted decrease in micro-organism activity following potassium removal using the modified membrane. However, molasses from most Australian sources has a potassium concentration substantially higher than the optimum value when diluted for use as a substrate and in that case fermentation performance would be expected to be enhanced by potassium removal. Also, it was shown that the rate of fermentation of molasses could be increased by macromolecule removal using an ultrafiltration membrane with a nominal molecular weight cut-off of similar to 300 000 Da.