A Possible Industrial Solution to Ferment Lignocellulosic Hydrolyzate to Ethanol

It is known that fermentable sugars can be obtained from lignocellulosic biomass through enzymatic or chemical hydrolysis processes. Although there are great interests in the application of enzymatic hydrolysis due to its specific products and high yield, in a short term, the time-consuming process and rather expensive enzyme cost make enzymatic hydrolysis still uncertain for use in large-scale industry.

One of the disadvantages of chemical hydrolysis process such as dilute-acid hydrolysis is the formation of by-products (furan derivatives, phenolic compounds, and aliphatic acids), which inhibit cultivation of yeast in lignocellulosic hydrolyzates. Hence on one hand, a great effort has to be made to reduce the inhibitor formation; on the other hand, detoxification step such as membrane separation is needed in the process. In addition, since the fermenting microorganisms are able to detoxify the hydrolyzates in situ, any process that can provide low concentration of the inhibitors and high cell density may be successful in cultivation of these toxic hydrolyzates. To some extent, continuous cultivation can be one of the approaches to provide low concentration of the inhibitors in the reactors and the high cell density can be provided by using cell recycling, immobilization or encapsulation.

According to s new research Paper, "a toxic dilute-acid hydrolyzate was continuously cultivated using a highcell-density flocculating yeast in a single and serial bioreactor which was equipped with a settler to recycle the cells back to the bioreactors. No prior detoxification was necessary to cultivate the hydrolyzates, as the flocks were able to detoxify it in situ. The experiments were successfully carried out at dilution rates up to 0.52 h-1. The cell concentration inside the bioreactors was between 23 and 35 g-DW/L, while the concentration in the effluent of the settlers was 0.32 ± 0.05 g-DW/L. An ethanol yield of 0.42-0.46 g/g-consumed sugar was achieved, and the residual sugar concentration was less than 6% of the initial fermentable sugar (glucose, galactose and mannose) of 35.2 g/L".