During rapid ethanol fermentation (2-3 h) of sugar-cane blackstrap molasses, a significant increase in the ethanol yield was frequently observed as fermentation proceeded, eventually leading to yields higher than the theoretical value when the end of the process was approached. In order to explain the above facts, three assumptions were examined: 1. temporary ethanol accumulation within the yeast cells; 2. variation of the dry matter content and/or of the microorganism density during the fermentation; 3. transformation of sugars into undetectable extra-cellular fermentable compounds at the initial stages of the process. Based on the experimental results presented here, the third of the above assumptions seems to explain the observed increase in the ethanol yield.

Abstract - The gas balance technique applied to batch cultures of the aerobic microorganism Aspergillus awamori NRRL3112 results in respiratory coefficient,RQ, values as high as 1.4 for glucose concentrations around 50g/L and no nutrients or oxygen concentration limitation. RQ decreases during the culture to values around 1.0 for glucose concentrations lower than 10g/L. Besides of an indication of drastic metabolic changes, these data suggest the need for some care when utilizing the RQ value for process control purposes. This RQ variability also has influence on the determination mode of the specific oxygen uptake rate, qO2 (mmol/gh). If the RQ value is constant, qO2 can be determined through a carbon dioxide (CO2) gas balance. This determination mode gives more reliable results than the one based on the oxygen gas balance since CO2 determination via a spectrophotometer of infrared gives accurate results and is a fast response relative to fluctuations in the CO2. In the case of this process however, reliable values of specific oxygen uptake rate, qO2, can be obtained only through the oxygen gas balance