3.3. Final effects and kinetics of fermentation of waste bread using GSHE as affected by pretreatment method
It was observed that application of all pretreatment methods used in present study increased the final ethanol yield in comparison to pretreated waste bread fermentation (Table 2). For enzymatic-ally and microwave pretreated waste bread fermentation with GSHE and SHF process the yield of the process was the highest, in the case of other variants lower ethanol yields were achieved, but still they were very high (above 80% of theoretical). The highest values of ethanol productivity were achieved in all studied samples within the first day of fermentations. Between the second and third day of fermentation the changes in the yields and productivity for all studied variants of experiment did not change significantly what suggested that these processes could be shortened to 48 h.
Previous studies dealing with utilization of waste bread into ethanol reported similar or even lower ethanol yields. Ebrahimi et al. studied the separate hydrolysis and fermentation of waste wheat bread at a raw material loading of 350 g kgÀ1. They obtained overall ethanol yield of 350 g per kilogram of bread dry matter and the alcohol concentration in the fermentation media about 100 g LÀ1. Kawa-Rygielska et al. reported that prehydrolysis of waste wheat-rye bread with Ceremix 6X MG preparation increase the ethanol yield from mashes at 320 g kgÀ1 waste wheat-rye bread loading from 352.4 to 366.0 g kgÀ1 in comparison to control. In present study the ethanol yield from substrate (sugars in raw material) unit was also very high without high-temper-ature liquefaction and separate saccharification, however the raw
material loadings were relatively low so the impact of substrate and product inhibition on yeast cells was minor. In industrial application higher raw material loadings would be more profitable because if the higher ethanol concentration is achieved the less energy is needed to distill it.