Читать книгу Poly(lactic acid) - Группа авторов - Страница 27

Sucrose and starch have in common that they are used for food and nowadays, with oil wells drying out and prices rising, also for biofuels. A decrease in the availability of fossil fuels is envisaged for the future, and with increasing population, more food is needed at reasonable prices. Therefore, the ideal raw material for biofuels and bioplastics is carbohydrates that are not edible. Such material is abundantly available around the globe as lignocellulose, like in corn stover or wheat straw. Lignocellulose consists of the glucose homopolymer cellulose, the heteropolymer hemicellulose, and lignin. Hemicellulose consists of hexoses and pentoses. In all, lignocellulose contains roughly 80% fermentable sugars, but this largely depends on the source [54]. The remainder, lignin, is a phenolic polymer that is difficult to degrade and is not directly usable for lactic acid production. It may be used for energy production though, which can be returned to the lactic acid plant.

A purer source of cellulose without lignin is waste paper that can be used for lactic acid production at lab scale [56]. Thus, even this book can eventually be converted into PLA!

Complete utilization of cellulose and hemicellulose requires selection or genetic modification of an organism that is able to ferment pentoses. To obtain monosaccharides from the raw material, several pretreatments and/or separations are required. First, the lignocellulosic material is mechanically treated and then delignified (pulped) by strong alkali or acid treatment. The (hemi)cellulose part becomes more accessible for enzymes at the same time. Subsequent enzymatic treatment mainly yields glucose and xylose and some arabinose. The enzymatic treatment and subsequent fermentation can be done in separate reactors or in one fermenter, in an SSF concept similar to starch SSF [57].