Enzymatic conversion of first- and second-generation sugars

Processes for the hydrolytic conversion of polysaccharides to fermentable sugars as feedstocks for biofuels and commodity chemicals are discussed. The production of first-generation biofuels, for example, bioethanol, involves the conversion of sucrose or starch; the latter requires initial enzymatic hydrolysis of the starch to glucose in a two-step process catalyzed by a-amylase and glucoamylase. These methods are established industrial processes that are conducted on an enormous scale. Although the enzymes involved are relatively inexpensive, they are used on a single-use, throw-away basis, and substantial cost savings can be achieved by immobilization of the enzymes to enable their recycling. In particular, immobilization of the enzymes as magnetic cross-linked enzyme aggregates (mCLEAs), in combination with magnetic separation using commercially available equipment, offers possibilities for achieving substantial cost reductions. The production of second-generation biofuels involves, in the long term, more sustainable conversion of waste lignocellulose to fermentable sugars, a much more complicated process requiring multiple enzymes. The hydrolytic step is preceded by a pretreatment step that opens the structure of the recalcitrant lignocellulose to make it accessible for the hydrolytic enzymes. This step is usually conducted in water, in which the lignocellulose is insoluble, but there is currently much interest in the use of ionic liquids or deep eutectic solvents in combination with water. Subsequent hydrolysis of the cellulose and hemicellulose to fermentable sugars involves a complex cocktail of enzymes referred to as "cellulase." In this case the percentage cost contribution of the enzymes to the biofuel is even higher than with first-generation biofuels. Consequently, it is even more important to reduce the costs of enzyme usage by immobilization, and magnetic separation of magnetic immobilized enzymes, such as magnetic CLEAs, is a potentially attractive way to achieve this.