Calculations of bioprocess parameters
Production rates were calculated in terms of mlproduct/Lreactor·day.
Yield (of lactic acid or ethanol) on a substrate is defined as the amount of the metabolite obtained per unit of PPW used and given in Eq. (1):
Y = (C – C0 ) · V / M · 100% (Equation 1)
where Y is the yield (ml/g), C is the actual concentration (ml/L), C0 is the initial concentration (ml/L), V is the bioreactor working volume (L), and M is the initial mass of PPW in the bioreactor (g).
Sugar recovery was defined as the efficiency of the microorganism to convert the starch available in the PPW to glucose. It is calculated as the ratio between the mass of glucose in the hydrolysate to the total mass of the starch in the PPW and expressed as a percentage (%).
A particularly useful parameter for characterizing ethanol and lactic acid production is substrate conversion efficiency, which is a measure of how much of the substrate has been utilized for ethanol and lactic acid production rather than growth, cell maintenance, or alternative biosynthesis. It was determined as the ratio of the mass of ethanol and lactic acid that has actually been produced per mass of ethanol and lactic acid expected through stoichiometric conversion of a substrate according to the hypothetical reactions of lactic acid and ethanol fermentations and expressed as a percentage (%).