Changes in Cellulose Crystalline Structure and Enzymatic Digestibility After Alkaline and Liquid Ammonia Treatments

Cellulose is the principal carbohydrate in lignocellulosic biomass which can be enzymatically broken down (i.e., "hydrolysed"or "saccharified") to simple sugars that are necessary for biofuel-ethanol fermentation. In order for the cellulose to be effectively broken down by (cellulolytic or "cellulose") enzymes, it has to undergo a pretreatment step.  The purpose of pretreatment is usually  (1) to remove the tough lignin wrapping surrounding the cellulose molecules, and (2) to reduce cellulose-crystallinity. Removal of lignin from the cellulose molecules exposes the cellulose to more effective attack by cellulolytic enzymes. The reduction cellulose-crystalline structure into a more amorphous form, is also reported to facilitate enzymatic saccharification.

Treatments such as alkali or ammonia treatments are said to be effective in reducing cellulose crystallinity. Researchers from the National Renewable Energy Laboratory (NREL) (United States) investigated the effects of sodium hydroxide and liquid ammonia treatments on the cellulose-crystalline structure,and the ease of enzymatic digestibility of the treated cellulose for the production of sugars for ethanol fermentation. They found that alkali and ammonia treatments in combination with low or high temperatures, resulted in the formation of different forms of crystalline cellulose ("allomorphs"), and these forms have varied degrees of enzymatic digestibility. For example, treatment at a low temperature (25°C) resulted in a less crystalline product, whereas treatment at higher temperatures (130°C or 140°C) resulted in a more crystalline product. "Treatment of cellulose I with aqueous sodium hydroxide (16.5 percent by weight) resulted in formation of cellulose II, but also produced a much less crystalline cellulose". The chemical treatments were found to produce different allomorphs of cellulose, which in turn affected its crystallinity and enzymatic digestibility. The full paper is published in the open access journal, Biotechnology for Biofuels.