Figure 13.8 shows the concentration of ethanol produced from the fermentation experiments after various hydrolysis conditions on extracted seaweed. Hydrolysis at 0.1 M, 100% clearly produced the most ethanol (8.41% v/v), followed by hydrolysis 0.1 M, 30% (7.70% v/v); 0.4 M, 30% (4.73% v/v); and 0.4 M, 100% (3.36% v/v). Interestingly, the ethanol yield did not change with temperature. It was observed that ethanol yield was lower with 100%, 0.4 M acid hydrolysis (3.36% v/v) com­pared to 30%, 0.4 M acid hydrolysis (4.73% v/v). However, the ethanol yield showed a reverse trend with lower yield at 30%, 0.1 M acid hydrolysis (7.70% v/v) compared with 100%, 0.1 M acid hydrolysis (8.41% v/v).

The effect of temperature was not significant as compared to molarity change. Heating has been shown to increase the reducing sugar concentration (Fig. 13.7).

So, in theory, there would be more ethanol produced. The ethanol yield in the 100% acid hydrolysis was lower than in those hydrolysed at 30%, possibly due to the increased solubilisation of inhibitory compounds, which reduced the effi­ciency of the yeast (Adams et al. 2009). Larsson et al. (1999) suggested that the elevated temperature will produce poor fermentability hydrolysate. Sugars can be degraded to furfural which is formed from pentoses and 5-hydroxymethylfur — fural (5-HMF) which is formed from hexoses. 5-HMF can be further degraded, forming levulinic acid and formic acid. In addition, formic acid can be formed from furfural under acidic conditions at elevated temperatures. Acetate is liber­ated from hemicellulose during hydrolysis. S. cerevisiae is a non-pentose-utilis­ing yeast strain (Krishna et al. 1998). In this case, the fermentability of yeast will decrease as the hexose sugar for fermentation had reduced. Since there was no proof of the type of reducing sugars produced, hence it cannot be concluded here that this was the reason for the reduced ethanol production.

Increased in molarity also reduced the ethanol yield from 8.42% v/v to 3.36% v/v and 7.70% v/v to 4.73% v/v as shown in Fig. 13.8. An increase in molarity leads to a reduction in pH. The lower yields generated by the slurries with high molarity may be due to an increase in salt concentration in the slurry. The slurries initially contained low salt concentration from the seawater retained on the mac­roalgae and ions within the algae, but with the adjustment to 0.4 M and back to pH 7 with H2SO4 and NaOH, the salt concentration would have been increased further, potentially causing inhibition of the yeast and thus reducing ethanol production (Adams et al. 2009).

Higher molarity as acid hydrolysis was assumed to have been used to disrupt the cells, thus releasing the cellular contents. However, although there was an increase in reducing sugar yields in slurries with 0.4 M hydrolysis (Fig. 13.7), this increase was small, and the salt inhibition as seen in Fig. 13.8 fermentations overall proved this is a disadvantageous acid hydrolysis molarity (Adams et al. 2009).