THESE UNIQUE El Hassane Kéhien-Piho TOU - Nutridev

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E.H. Tou et al. / International Journal of Food Microbiology 106 (2006) 52–60
7
SOAKING
6
5
4
pH
SETTLING
6
5
4
3
0 2 4 6 8 10 12 14 16 18 20 22 24 26
Time (h)
TPU1 TPU2 TPU3 TPU4
TPU5 TPU6 TPU7 TPU8
Fig. 2. Changes in pH during soaking and settling of the pearl millet grains in eight (1–8) traditional production units (TPU).
3.3.1. Kinetics of substrate and product formation during the
soaking step
To characterize what could be the main substrates responsible
for fermentation during the soaking step, changes in
sugars (glucose, fructose, maltose, sucrose, melibiose, raffinose
and stachyose) in the grains and in the soaking water
(supernatant) were investigated in pilot scale experiments
(PSE) (Fig. 3). The main sugars in the grains were sucrose
(24.9 mmol/100 g DM), glucose (12.5 mmol/100 g DM),
fructose (12.1 mmol/100 g DM) and raffinose (7.6 mmol/100 g
DM). Their concentration decreased during soaking with the
biggest changes in sucrose, fructose and glucose (Fig. 3).
Glucose, fructose and maltose were the major mono- and
disaccharides which appeared in the supernatant transiently
between 2 and 12 h.
Ethanol, acetic and lactic acids were identified as the
fermentation products in the supernatant during the soaking
step (Fig. 4). Surprisingly, ethanol was the main end-product
and the same observation was made in samples taken from five
different TPUs (Fig. 5). During the soaking step in both TPUs
and PSEs, ethanol concentration increased to reach mean final
values of 41 and 21 mM, respectively. From these results it can
thus be inferred that the soaking step is dominated by alcoholic
fermentation. The relatively limited production of lactic and
acetic acids might account for the limited decrease in pH
observed in poor buffering conditions. Furthermore, the first
phase of the limited decrease in pH observed in TPU
conditions (Fig. 2) may correspond to the period in which
organic acid production is hardly detectable (up to 8 h) (Fig. 4).
3.3.2. Kinetics of substrate and product formation during the
settling step
After the onset of the fermentation, only traces of maltose
and a-galactosides were detected and the sugars initially
present in the supernatant during the settling step (Fig. 6),
mainly glucose (8.0 mM) and fructose (3.6 mM), decreased to
reach mean final values of 0.43 and 0.02 mM, respectively.
As a result of sugar fermentation, ethanol, acetic and lactic
acids were formed during the settling step (Fig. 4). Lactic acid
was the main fermentation product in both TPUs and PSEs
(Fig. 7) and its concentration increased to reach mean final
values of 35.4 and 31.0 mM, respectively. Furthermore, lactic
acid production increased up to the end of fermentation (24 h)
whereas ethanol production lasted only 8 h (Fig. 4).