The ethanol fermentation potential of C6 sugar hydrolysates were performed using the industrial S. cerevisiae Thermosacc Dry .Three different yeast fermentation approaches were trialled to establish ethanol production and to investigate the effects of biomass degradation compounds generated during pretreatment and those naturally present in CGT. These compounds generally lead to an unfavourable environment for microbial growth resulting in low ethanol titres and productivities, and potentially require removal via an expensive detoxification step (Palmqvist and HahnHägerdal, 2000). In the first two approaches a separate enzymatic hydrolysis followed by fermentation was employed, where fermentation hydrolysates were produced from either recovered washed pretreated fibres (termed WF) or whole pretreated slurries (termed WS). The differences between the two hydrolysates being those from WS contain a complex mixture of sugars and compounds from CGT biomass and degradation reactions. In both approaches glucose was rapidly and completely metabolised by the yeast with the production of high ethanol titres (Table 6).
Based on glucose recovery data from optimised pretreatment
conditions, calculated ethanol fermentation yields for the three
processing options are 101, 149 and 142 L ethanol per metric tonne
of original CGT for WF, WS and SSF respectively. From an economic
and logistical perspective, the SSF approach would be more favourable
in an industrial setting. Although process efficiencies
described herein are similar to those reported by Jeoh and
Agblevor (2001), their estimated potential ethanol yield at 270 L/
t was greater because of the higher sugar content in their CGT biomass,
and the potential yields of ethanol from pentose sugars. On
the basis of this latter premise, conversion of released pentose sugars
(122 kg of xylose/t) would yield an additional 79 L/metric
tonne.