Enzymic saccharification of alfalfa fibre after liquid hot water pretreatment

Liquid hot water (LHW) at a high temperature has been advocated as a pretreatment for herbaceous and lignocellulosic materials prior to enzymic saccharification. The focus of our research was the suitability of LHW pretreatment of alfalfa (Medicago saliva) fibre in the presence and absence of mild acid for optimum saccharification using enzymes. Enzymic saccharification was optimised in terms of substrate and enzyme concentrations. The main components of the enzymic 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 predominantly cellulose) of alfalfa were recovered; the yield was 48 and 41%, respectively. Enzymic saccharification released more reducing sugars from pretreated fibre than from untreated fibre. 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-pretreated 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% sulphuric acid to the LHW pretreatment facilitated hemicellulose solubilisation; 9.9 g/l reducing sugars were released from the alfalfa extract by enzymic saccharification. However, pretreatment with sulphuric acid reduced sugar release and decreased fibre degradation of the raffinate. (C) 1999 Elsevier Science Ltd. All rights reserved.