ARS | Publication request: New enzymes for hydrolysis and fermentation of citrus waste

Submitted to: Subtropical Technology Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: October 16, 2010
Publication Date: N/A

Technical Abstract: The expense involved to dry citrus processing waste into citrus pulp pellets (CPP) for use as a cattle feed continues to increase with rising fuel costs. While there have also been recent increases in the value of CPP, this value fluctuates considerably and does not always cover processing costs. Therefore there is still a great deal of interest to develop alternative co-products from citrus processing waste to increase crop value. Citrus processing waste contains an appreciable amount of fermentable sugars that can be converted to ethanol for use as a biofuel. The cost and amount of enzymes necessary to liquefy citrus processing waste and hydrolyze the polysaccharides to release complex sugars is a major factor limiting the use of this biomass to produce ethanol. The enzyme mixture needed for liquefaction consists of a combination of pectinase, cellulase and ß-glucosidase. Citrus waste contains very little lignin, so unlike woody biomass, pretreatments to remove lignin are not required. The newer generations of cellulase formulations now incorporate ß-glucosidase activity and these should lower enzyme requirements and cost. Addition of ß-glucosidase to supplement cellulase activity is necessary as cellulase enzymes by themselves only hydrolyze cellulose into the disaccharide cellobiose which can not be fermented by traditional fermentation organisms. ß-glucosidase cleaves cellobiose hydrolyzed from cellulose into two glucose molecules which can be fermented. This increases the amount of ethanol one is able to produce from citrus processing waste biomass for recovery as an alternative fuel. ß-glucosidase purchased as a separate enzyme is very expensive while the new cellulase formulations are reported to not be any more expensive than those previously available with insufficient ß-glucosidase activity. Accellerase 1000 and Accellerase 1500 enzyme products were tested for cellulase and ß-glucosidase activity as well as their performance and the products formed during simultaneous hydrolysis and fermentations of citrus processing waste materials. These enzymes had ß-glucosidase activity similar to that found in the separate ß-glucosidase enzyme formulation which was required to supplement earlier cellulases used. During simultaneous hydrolysis and fermentations, Accellerase 1000 and 1500 performed equally well and could be used to replace separate additions of ß-glucosidase with either Celluclast 1.5L or GC220 cellulase enzyme products. However, when enzyme loadings are optimized to minimize cost viscosity of the final product remains a problem. This needs to be solved in order to have ethanol production as part of an economical process for alternatives to utilize citrus processing waste.