Enzyme catalysis: green routes for high value-added products derived from cellulose and starch

A. VAZQUEZ; M.L. FORESTI; C. GOMEZ HOYOS; L. MALDONADO

Estrasburgo

Conferencia; III International conference on biodegradable and biobased – Biopol 2011; 2011

Enzymes are biological catalysts which have shown to efficiently catalyze a number of in vitro reactions. In comparison with chemical catalysts, enzymes show higher selectivity, work under milder conditions, and are environmentally friendlier. On the other hand, many processes mediated by those biological catalysts have shown limited scale up due to their still high costs compared with their chemical counterparts, and the fact that careful optimization of reaction conditions is needed for adequate performance. In the field of biopolymers enzymes find application in their synthesis, modification and degradation. In the current contribution a number of enzyme-catalyzed reactions with starch and cellulose as substrates that may lead to high value-added products will be briefly reviewed. The aim of the work is to exemplify the role of enzyme catalysis as a powerful tool for converting renewable abundant resources into high-value biodegradable products through sustainable routes. A well-known application of enzymes is their use in the production of starch hydrolysates. In the past decades, a shift from acid hydrolysis of starch to the use of starch-converting enzymes for the production of maltodextrins, glucose and fructose syrups has been evidenced. Currently, starch-converting enzymes (mainly those from the a-amylase family) comprise about 30% of the world’s enzyme production. In the last years, starch hydrolysis has received increased attention due to the potential use of the sugars produced as substrates for bioethanol production. For producing biotehanol starch is first hydrolyzed (with acids or enzymes) to sugars which are then fermented by yeast. An alternative to the use of energy crops as feedstock for ethanol production is the use of lignocellulosic biomass, which has the advantage that it is not directly linked to food production. However, the conversion of lignocellulosic material to ethanol requires more drastic hydrolysis steps due to the presence of various amounts of other sugars, such as xylose and arabinose. The breakdown of cellulose and hemicelluloses may be done by acids or by highly specific enzymes such as cellulases and xylanases, respectively. Enzymes can also play an interesting role in the modification of starch and cellulose when certain properties are required. Esterification of hydroxyls groups of polysaccharides decreases their hydrophilicity rendering them more adequate for certain applications such as reinforcement/filler of hydrophobic polymer matrixes, and as additives for food and pharmaceuticals. Acetylation (esterification using acetic as acyl donor) of cellulose and starch has traditionally been performed using strong acids, bases or pyridine as catalysts. Alternatively, lipases -a family of enzymes which selectively catalyze the formation of ester bonds-, may be considered as an eco-friendlier route which requires milder reaction conditions and avoids the use of concentrated acids or alkalis. Another interesting application of enzymes with starch and cellulose as substrates is the production of nanostarch and cellulose nanofibers. In the last decade, nanocellulose and starch nanoparticles have been produced by acid hydrolysis. Despite the biodegradability of the products and the renewable character of the substrates, the high acid concentrations currently used make the whole process definitely not sustainable. As an alternative, enzyme-catalyzed routes have recently been proposed (1,2), and further research on the topic is urgently needed.