For example, no more than 2 g l−1 and 5 g l−1 are controlled for the residual reducing sugar and total sugar, respectively, in the ethanol production from starch materials.
Any ethanol fermentation research which is expected to be practical needs to bear these criteria.
During ethanol fermentations, yeast cells suffer from various stresses. Some are environmental such as nutrient deficiency, high temperature and contamination, while the others are from the yeast cell metabolism such as ethanol accumulation and its corresponding inhibition on yeast cell growth and ethanol production, especially under very high gravity (VHG) conditions that will be discussed later.
Fig. 3 summarizes some of these stresses.
Many of them are synergistic, affecting yeast cells more severely than any single one, leading to reduced yeast viability and vigor as well as lower ethanol yield.