Maltose Transport by Brewer's Yeasts in Brewer's Wort

Abstract

The kinetics of maltose transport by two industrial yeasts were studied. The ale and lager strain each showed both high and low affinity transport. For the lager strain, maltose transport was only weakly inhibited by maltotriose, sucrose and trehalose, suggesting that its dominant maltose transporter is the maltose-specific type coded by MALx1 genes. For the ale strain, maltose transport was strongly inhibited by maltotriose, sucrose and trehalose, suggesting that its dominant maltose transporter may be the AGT1-encoded type that also carries these sugars. Also glucose inhibited transport by the ale strain more than that by the lager strain. Instantaneous inhibition by ethanol at concentrations met in brewery fermentations was moderate (about 25% at 50 g ethanol · L−1). The apparent Vmax for high affinity transport increased about 100-fold between 0 and 30°C, whereas the Km (3 ± 1 mM) was constant. Standard activities of maltose transport and maltase were followed through pilot fermentations of 11–24°P worts. Rapid (20 s) measurements of the zero-trans-rate of maltose uptake were also made with each day's yeast (rapidly harvested and washed) in reaction mixtures containing the same day's wort labelled with tracer 14C-maltose. Results suggested that maltose uptake is the dominant factor controlling the rate of maltose utilization in these wort fermentations.