Values of QE related to original size particles were lower than those obtained with small size Finally, to assess the effectiveness of pretreatments in enhancing the SSF process, ethanol yields (YSSF and YE) were calculated. Their values are shown in Fig. 6. YSSF stands for the SSF yield, and was determined by Eq. (4). YOE is the overall ethanol yield, and represents the mass of ethanol produced at 72 h divided by the required mass of raw material to carry out the SSF process.
As expected, the highest YSSF data (99.8% and 98.0%) were obtained using E&DSA pretreated solids (Fig. 6A), since they were the substrates that had the lowest lignin content (13.3%). These results
indicated a complete conversion of pretreated glucans to ethanol and the absence of metabolic inhibitors in bioprocess. The ethanol yields derived from DSA and E&LHW pretreated solids were similar and ranged from 68.2 to 77.9% and from 70.0 to 84.3% respectively, while SSF performed with LHW pretreated solids resulted in YSSF of 47.0% for original size particles and 87.5% for fine particles. This latter value was an unexpectedly high result. It could be explained by the fact that LHW pretreated solids, derived from the grinding stage, had a specially-low particle size. SSF of original size DSA pretreated solids resulted in ethanol yields slightly higher to those obtained by Ballesteros et al. [18], who reported YSSF values of 59% working with acid steam-exploded olive stones at 210 C for 4 min using an enzyme loading of 15 FPU per gram of substrate and the yeast Kluyveromyces marxianus CECT 10895