Development of t(50) and its application to evaluate very-high-gravity ethanol fermentation.

A three-parameter logistic growth model was modified to monitor the glucose uptake profile of yeast during very-high-gravity (VHG) ethanol fermentation. The modified model was used to define t(50) as a quantifier to differentiate among various fermentation conditions. There are two types of t(50); t(50)(g) is the time required to convert 50% of the initial glucose, and t(50)(e) is the time required to produce half of the final ethanol. A 2(4) factorial experimental design was implemented to illustrate the applicability of using t(50) to isolate active ingredients in VHG growth media. The analytical results obtained from the experimental design and from a modified model were compared, which demonstrated that t(50) could serve the proposed objectives. A shorter t(50) implies a faster fermentation. A tailing of the ethanol profile after t(50)(e) indicates that there is an inhibitory effect imposed on yeast, i.e., the stronger the tailing in the ethanol profile, the stronger the inhibitory effect. When t(50) is equal to or near to the halftime of the total course of the fermentation, a bell-shaped curve was seen for the glucose uptake rate or for the ethanol production rate, indicating that the inhibitory effect exerted on yeast was evenly distributed.