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ARS Home » Midwest Area » Madison, Wisconsin » U.S. Dairy Forage Research Center » Research » Publications at this Location » Publication #101343
Title: ENZYMATIC SACCHARIFICATION OF ALFALFA FIBER AFTER LIQUID HOT WATER PRETREATMENT

Author
item SREENATH, HASSAN - UDSA FOREST SERVICE
item Koegel, Richard
item MOLDES, ANA - UNIV WISCONSIN MADISON
item JEFFRIES, THOMAS - UNIV WISCONSIN MADISON
item STRAUB, RICHARD - UNIV WISCONSIN MADISON

Submitted to: Process Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: Fibrous plant material consists largely of chains of sugars. If these chains can be broken down into single sugar molecules by enzymes, the sugars can be fermented by microorganisms into useful substances such as alcohol or organic acids. A model here is conversion of alfalfa fiber to lactic acid which is a needed ingredient in biodegradable plastic. This research deals with the first step, conversion of alfalfa fiber to sugars by selected enzymes with or without a pretreatment for two minutes in a water bath heated to 430 degrees F and held under high pressure to maintain it as a liquid. This pretreatment is proposed to allow the enzymes to work more rapidly, yielding more sugar. The liquid hot water (LHW) treatment divided the fiber into a liquid fraction (extract) and a fibrous fraction (raffinate). Enzyme treatment of both fractions yielded 27g and 25g of sugars for a total of 52g from 100g of original alfalfa fiber. Without the LHW pretreatment 51g of sugars were produced from 100g of alfalfa fiber. While there was no difference in sugar yields, the untreated fiber and the extract required two types of enzymes (cellulase and pectinase) while the pretreated fiber required only cellulase. Since the pretreatment cost must be weighed against any cost savings for enzymes, the economics of the pretreatment has not yet been determined, but a significant advantage appears questionable. Economically viable conversion of plant fiber (including crop residues) to liquid fuels or chemical feedstocks would create a tremendous value-added income for the agricultural sector while displacing non-renewable, imported petroleum. In addition, use of renewable feedstocks avoid addition of "greenhouse" gases to the atmosphere.

Technical Abstract: Liquid hot water (LHW) at high temperature (220 degrees C) has been advocated as a pretreatment for herbaceous and lignocellulosic materials prior to enzymatic saccharification. The focus of our research was the suitability of LHW pretreatment of alfalfa (Medicago sativa)fiber in the presence and absence of mild acid for optimum saccharification using enzymes. Enzymatic saccharification was optimized in terms of substrate an enzyme concentrations. The main components of the enzymatic hydrolyzates were sucrose, glucose, xylose, arabinose, and low amounts of uronic acid. After LHW pretreatment, both the soluble hemicellulose-rich "extract" fraction and the insoluble residue "raffinate" fraction (which contains predominately cellulose) of alfalfa were recovered; yield was 48% and 41%, respectively. Enzymatic saccharification released more reducing sugars from pretreated fiber than from untreated fiber. For untreated alfalfa, a commercial pectinase and cellulase mixture caused maximum release of reducing sugars; using 2% and 4% (w/v) enzyme, a maximum of 51 g/L reducing sugars was released from 100 g/L untreated alfalfa. The soluble extract, LHW-pretrated alfalfa, was clarified and saccharified with the same cellulase and pectinase mixture releasing 8.4 g/L reducing sugars from 15.5 g/L solids. Release of reducing sugars from the insoluble raffinate ranged from 59 to 65 g/L from 100 g/L substrate, using 2% and 4% (w/v) cellulase. Addition of 0.07% sulfuric acid to the LHW pretreatment facilitated hemicellulose solubilization; 9.9 g/L reducing sugars were released from the alfalfa extract by enzymatic saccharification. However, pretreatment with sulfuric acid reduced sugar release and decreased fiber degradation of the raffinate.