3.3. Effect of urea nitrogen on the co-fermentation performance
Yao et al. (2011) reported that soy skim with 11% solids produced
from EAEP provided adequate nitrogenous nutrients for yeast
growth. We hypothesized that soy skim not only could replace the
water used in corn fermentation but also could provide the necessary
nitrogen when the 100% replacement was used. Fig. 4 shows
that fermentations with or without urea gave the same rates when
the 100% skim replacement was used; however, the 20% skim
replacement without urea gave significantly lower rate than the corresponding
treatment with urea, which indicated that the 20% skim
replacement hadinadequate nitrogenous nutrients for yeast growth.
This agrees with our earlier observation that the yeast growth rate
decreased andless cell biomasswasproducedwhensoy skimwasdiluted
because the diluted skim did not contain enough utilizable
amino nitrogen to support normal yeast growth (Yao et al., 2011).
The control without soy skim did not give measurableCO2whenurea
was not added. No significant difference in total ethanol yield was
found between the treatments with or without urea.