4. Discussion
Enzymatic hydrolysate of soybean stalk was investigated for
lactic acid production effectiveness. The lactic acid conversion
from enzymatic hydrolysate was only 48% for L. sake and 56%
for L. casei, respectively. In order to find the cause of the lower
lactic acid conversion from enzymatic hydrolysate, the effect of
sugar composition of enzymatic hydrolysate and different types
of lactobacillae strains were investigated. The hydrolysate from
cellulose and hemicellulose contains glucose, xylose and
cellobiose. During fermentation it was found that the content of
glucose is very close to zero at the end of fermentation as shown
in Fig. 3(a) and (b). However, L. sake and L. casei was not able
to utilize xylose and cellobiose completely. This explained why
L. sake and L. casei could not utilized these two-types sugars
completely. Consequently, xylose and cellobiose may be the
reason for the decreased lactic acid conversion.
The production of lactic acid from the MRS model medium
with glucose, xylose and cellobiose is summarized in Fig. 5(a)
and (b). L. casei showed higher conversion on glucose and
cellobiose than L. sake. However, L. sake showed higher
conversion on xylose than L. casei. From the substrate uptake
patters of L. casei and L. sake it was obvious that the potential of
the medium could be better exploited if the two cultures were
co-fermenting. By co-inoculation of both strains, better
substrate utilization led to an increase in lactic acid production
rate, resulting in a final conversion of 71%. Combination of the
two strains had especially advantages since incomplete sugar
utilization could lead to increased cost for purification of the
lactic acid.