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New two-step method improves bioethanol production efficiency

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An international group of researchers has developed a two-step method to more efficiently break down carbohydrates into single sugar components, an essential process in the production of green fuel.

Led by scientists at Tokyo University of Agriculture and Technology (TUAT) in Japan, the researchers published their results in April in the American Chemical Society Journey, Industrial & Engineering Chemical Research.

The process, known as saccharification, results in the production of single sugar components – monosaccharides – which can then be fermented into bioethanol or biobutanol.

"For a long time, considerable attention has been focused on the utilisation of homogenous acids and enzymes for saccharification," said Eika W. Qian, paper author and professor in the Graduate School of Bio-Applications and Systems Engineering at TUAT. "Enzymatic saccharification is seen to be a reasonable prospect since it offers the potential for higher yields, lower energy costs, and it's more environmentally friendly."

The use of enzymes to break down carbohydrates, however, can be hindered – particularly in biomass materials such as rice straw.

Rice straw comprises three carbohydrates: starch, hemicellulose and cellulose; the latter two carbohydrates cannot be broken down by enzymes due to their cell wall structure and surface area, so must be pre-treated, which can be expensive.

Another option is to use solid acid catalysts, according to the researchers, which can be recovered after saccharification and reused.

However, as Qian explains, it’s not as simple as switching the enzymes for acids, as the carbohydrates are not uniform.

Hemicellulose and starch, for example, degrade at 180 degrees Celsius and below, whereas cellulose only degrades at temperatures of 200 degrees Celsius and above.

Consequently, to maximise the resulting yield of sugar from the rice straw, the researchers have developed a two-step process: one for the hemicellulose and a second for the cellulose.

The first step requires a gentle solid acid at low temperatures (150 degrees Celsius and below), while the second requires a stronger solid acid and higher temperatures (210 degrees Celsius and above).

This new process has proved effective in testing, producing approximately 30% more sugars than traditional one-step processes.

"We are now looking for a partner to evaluate the feasibility of our two-step saccharification process in rice straw and other various materials such as wheat straw and corn stoke etc. in a pilot unit," Qian said. "Our ultimate goal is to commercialise our process to manufacture monosaccharides from this type of material in the future."