Alterations in fermentation conditions

The presence of lignocellulose inhibitors in fermenting medium affects the ethanol and biomass productivities as microorganism take more incubation times to convert into products [Chandel et al., 2007a; Nilvebrant et al., 2001; Zaldivar et al., 2001]. Usually the ethanol productivity is determined by cell-specific productivity and cell mass concentration, cumbersome by lignocellulose-derived inhibitors. To overcome by inhibitors, high cell-mass inocula are effective to tolerate the stress of inhibitory substances [Purwadi et al., 2007]. The ethanol productivity has been increased by maintaining the initial cell-mass at higher density [Brandberg et al., 2007]. By altering the initial cell density, the increased production of ethanol (0.44 g/ g) was reported at initial cell density (10 g/l dry weight) [Palmqvist et al., 1996]. The ethanol productivity in fed-batch fermentation was limited by the feed rate that in turn, was limited by the cell-mass concentration (Taherzadeh et al. 1999).

In continuous fermentation, the ethanol productivity also depends upon the rate of dilution. Since the microbial growth rate is known to decrease by the inhibitors, the productivity in continuous fermentation of lignocellulosic hydrolysates remains low [Lee et al., 1996; Palmqvist et al., 1998]. Purwadi et al. [2007] has achieved the ethanol yield of 0.42-0.46 g/ g sugar utilized from the crude hydrolysates of spruce wood as carbon source under continuous fermentations using the flocculating S. cerevisiae CCUG 53310. Cellular re­circulation strategy was employed in the fermentation of an enzymatic hydrolysate of spruce [Palmqvist et al., 1998], and in fermentation of bagasse hydrolysate [Ghose and Tyagi, 1979].

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