Abstract
Continuous growth on a substrate consisting of a mixture of glucose and fructose was studied. Growth below the critical dilution rate was measured as purely respiratory and both sugars were utilized equally and completely. At dilution rates greater than the critical, growth by fermentation and respiration was observed. Incomplete utilization of glucose and fructose was measured at dilution rates greater than approximately 0.40h-1.
Batch studies using fructose, glucose and fructose, and sucrose with total initial concentrations of 20g/l, showed no significant differences compared to those carried out with initial sugar concentrations of 10g/l. However, in chemostat culture, washout of the sugars occurred at high dilution rates when the feed was 20g/l (of total sugar) but did not occur when the feed was 10g/l (of total sugar). Apparently this was indicative of the saturation of the sugar uptake mechanism rather than substrate repression. High inlet sugar concentration may cause less than complete utilization of sugars encountered industrially.
There were two types of sugar uptake profile in batch cultures: uptake of glucose in preference to fructose, and equal uptake of the two sugars. The ability of the yeast to convert from one sugar uptake profile to the other during the course of a batch experiment over a small time period implies a pseudo-instantaneous switch in transport mechanisms and not new protein synthesis.
