Nonsterile Open Fermentation for Producing Lignocellulolytic Enzymes Using a Streptomyces sp. and Untreated Agroindustrial Substrates
摘要
Lignocellulosic waste biomass can be used to produce microbial cellulolytic enzymes for releasing glucose from the same biomass in subsequent steps. Ideally, enzyme production should be carried out with untreated biomass in a nonsterile process as biomass pretreatment and sterilization are expensive. A newly isolated Streptomyces sp. proved to be an excellent producer of cellulolytic enzymes including xylanase, cellulase, and β-glucosidase. The bacterium was evaluated for enzyme production in batch cultures using several untreated lignocellulosic substrates (e.g., grain (wheat, rice, barley) straws and fruit peels), and mixtures of two substrates. Nonsterile open culture systems were used. Under the identified optimal conditions, a 96 h batch culture on a mixed substrate yielded a cell-free culture supernatant with the following enzyme activities: 18.2 ± 0.4 filter paper units mL−1 for cellulase; 43.5 ± 2.3 U mL−1 for β-glucosidase; and 225.3 ± 2.2 U mL−1 for xylanase. Subsequently, this cell-free crude enzyme cocktail was used to release reducing sugars from alkali-pretreated corn stover. The resulting sugar-containing crude hydrolysate was fermented in situ to ethanol with a yield that of > 91.5% of the theoretically possible yield. The use of untreated lignocellulosic biomass for obtaining bacterial enzymes via a nonsterile open fermentation proved to be possible.
Graphical abstract