Abstract
Lignocellulosic biomass comprises the most abundant biopolymers on earth, cellulose, hemicelluloses, pectin and lignin. The total annual production of lignocellulosic biomass is estimated at about 10-50 billion metric tonnes of which approximately 4 billion tonnes consist of annual crop residues, the by-product of crop production. While the basic constituents of cellulose and hemicelluloses, the hexose and pentose sugars, are key nutrients in human and animal nutrition, they are locked up in a plant lignohemicellulose-cellulose matrix that is largely resistant to hydrolysis by mammalian enzymes. Mammals can partially utilize lignocellulosic biomass through microorganisms hosted in their fore-stomach (ruminants) or hindgut (monogastrics) that secrete enzymes that degrade cellulose, pectin and hemicelluloses, thereby releasing fermentable sugars. Since the early twentieth century, the abundance of lignocellulosic biomass and the potential nutritive quality of its basic sugar constituents has attracted animal nutritionists who searched for physical and chemical treatments to make those sugars more accessible. The work on second-generation biofuels (biofuels derived from lignocellulosic biomass) was motivated by reasons very similar to those of the early animal nutritionists: the abundance of lignocellulosic biomass and its content of polymerized sugars. This work has attracted US dollar multi-billion investment during the last two decades. It may be feasible to utilize spin-offs from second-generation biofuel technologies to upgrade lignocellulosic biomass for animal feeding, particularly combinations of pretreatment approaches that render the hemicellulose, pectins and celluloses more accessible to enzymes, and enzymes applications. There are numerous mechanical and chemical approaches that have been applied in second-generation biofuel technologies. Among these, fibre expansion approaches using moderate temperature and pressure in an alkaline environment (ammonia) that generate only solid substrates are promising. One such pretreatment, the Ammonia Fiber Expansion (AFEXTM; AFEXTM is a trademark of MBI, International, Lansing, Michigan, USA.) pretreatment increased mean cell wall digestibilities by rumen microorganisms on average by 80%. Steam explosion is another promising pretreatment potentially effective without pH interventions if partially hydrolysed hemicelluloses are recovered. Applications of tailor-made enzyme mixes resulted in pentose and hexose recovery from lignocellulosic biomass of more than 90%. In other words almost all sugars in cellulose and hemicelluloses can potentially be made directly accessible for mammalian digestion and absorption. However, more research is needed to understand optimal combinations of pretreatments and enzymatic digestions and to determine the economical viability of such approaches for accessing sugars in lignocellulosic biomass for ruminant and monogastric livestock and even building blocks for new food ingredients for direct human consumption. The paper draws from reviews, revisiting and recalculating published data sets and use of as yet unpublished data material.