The ABE and acids production profile for the simultaneous process of pretreated OPEFB is presented in Fig. 4-b.
Maximum acid production was detected at 48 h equivalent to 7.78 g/l, consisting of 2.66 g/l of butyric acid and 5.12 g/l of acetic acid. Butyric acid production here was found to be lower compared to those obtained in the two-step process. However, this fermentation still experiences some inhibition phenomenon possibly due to the low sugar concentration in the system. The synthesis of all products ceased after 96 h although some amount of sugar (4.6 g/l) was still available in the system. This occurred when low sugar concentration was supplemented in the ABE fermentation system, usually below 40 g/l (data to be published elsewhere). At low sugar level, the cells were not able to convert the remaining sugar into ABE because it has been used for acids production. These acids should be reassimilated with the remaining sugar for ABE formation. However, due to low sugar concentration, reassimilation of acids ceased and thus inhibited cell metabolism to produce ABE. This situation was observed by Linggang et al. [11] when more acids were accumulated in the fermentation using 30 g/l compared to 50 g/l of sago pith residue hydrolyzate.