Effect of particle size
In order to investigate how the bioprocess was affected by PPW particle
size in the nutrient media, grounded PPW was separated into six
fractions in size (<0.125, 0.125- 0.250, 0.250-0.500,
0.500-1.000, 1.000-2.000, and 2.000-4.000, all in mm) were tested.
Nutrient media were prepared by using these PPW as the main substrate at
a loading rate of 2%. In media prepared with a PPW particle size
smaller than 0.125, some gelling was observed which resulted in a more
viscous media. No such effect was observed for other PPW size ranges.
The fermentations were carried out and DP4+, glucose, lactic acid, and
ethanol concentrations were determined throughout the runs (Figure 1).
Based on these data, sugar recovery, substrate conversion efficiency,
produced amounts, average production rates, and yields of both ethanol
and lactic acid were calculated and tabulated in Table 1.
R. oryzae utilized PPW as a substrate in all cases studied to
produce glucose, DP4+, ethanol, and lactic acid.
It can be clearly seen in Figure 1 that the fungi show a distinct
pattern of substrate utilization and consume back the metabolites they
produced; all concentrations peak at the early stages of the
fermentation and then decrease steadily. The consumption rates can be
used as an indicator of the substrate preference order of the
microorganism. Figure 1B shows that R. oryzae consumed the
initial glucose available in the nutrient media within the first 10
hours, and after this is depleted, it started to degrade the starch
available in the PPW, as suggested by an increase in glucose and DP4+
concentrations (Fig. 1A and B), followed by lactic acid and ethanol
fermentations (Fig. 1C and D). After the depletion of glucose, lactic
acid, and ethanol consumption started and lasted until the end of the
runs. Ethanol production was much higher compared to lactic acid
production, for all cases.
Regression analysis did not show any meaningful correlation between PPW
particle size and the produced amounts, average production rates, and
yields of both ethanol and lactic acids (R2< 0.4 for all). However, some significant findings were
obtained for the extreme cases. The lag time of substrate utilization
for particle size <0.125 (Fig. 1) was the longest, which was
probably caused by the gelling observed for this particle size range,
which created mass transfer limitations. On the other end of the
spectrum tested, for particle size 2-4mm, DP4+ production was
significantly higher (Fig. 1A) compared to the other size ranges. The
maximum glucose produced (Fig. 1B) was also the lowest for this particle
size. This is expected since starch hydrolysis due to amylase activity
should yield less glucose and more high molecular weight products (DP4+)
as the particle size increases. Sugar recovery percentages given in
Table 1 support this claim, they decrease from 72% to 42% as the
particle size increases.