Enzymatic saccharification of hot-water pretreated corn fiber for production of monosaccharides
Introduction
The U.S. annual production of ethanol is 3.4 billion gallons of ethanol (Renewable Fuel Association, 2005), which consumes 1.4 billion bushels of corn (National Corn Growers Association, 2005). Ethanol is manufactured from corn by either the dry grind or wet mill process. Corn fiber is a residue of wet milling that is currently incorporated into corn gluten feed, a low-protein animal feed product. Redirecting this fiber for conversion to ethanol would simultaneously increase the ethanol yield from wet milled corn (by as much as 10%) and increase the value of the corn gluten feed by reducing its fiber content. Enough corn fiber is produced each year in the U.S. to generate an additional 385 million gallons of ethanol per year [1].
Corn fiber contains approximately 70% carbohydrates including cellulose, xylan, and residual starch. The first step for converting corn fiber to ethanol is saccharifying these carbohydrates to fermentable monosaccharide sugars. Most research on corn fiber conversion to ethanol has focused on pretreating corn fiber with dilute sulfuric acid. Typically this involves lowering the pH to 1.0–1.3 with sulfuric acid and treating at a temperature of 150–180 °C for anywhere from 2 to 20 min [2], [3], [4], [5]. The sugar yields from these studies are generally very high [4], [5], [6]. However, from a processing view-point, dilute-acid pretreatment has several shortcomings, including the need for expensive reactors capable of withstanding the combination of low-pH and high-temperature, formation of pretreatment-associated side-products that often stall the subsequent fermentation, and generation of acid-related waste streams (e.g., gypsum) that adds to the cost of waste treatment and complicates downstream processing.
Hot-water pretreatment is an effective alternative to dilute-acid for pretreating herbaceous biomass [7], [8], [9], [10], including corn fiber [11], [12], [13]. The major advantages of hot-water pretreatment compared to dilute-acid are avoiding the use of mineral acid (with its myriad of disadvantages) and reducing sugar degradation products. The technology is also conveniently integrated into a wet milling operation as demonstrated by Ladisch and co-workers [14]. However, corn fiber treated with hot-water requires further processing than that treated with dilute-acid because the former is not severe enough to saccharify xylan sugars, as needed for their fermentation. Fermentation of xylan is critical because it represents approximately 50 wt% of the corn fiber carbohydrates [5]. Recovering corn fiber xylan as monosaccharide sugars is difficult because its xylan is exceedingly complex in structure and resilient to treatment with most commercially available xylanases.
This paper examines the feasibility of using custom hydrolytic enzyme blends for recovering carbohydrates from hot-water treated corn fiber as monosaccharide sugars. Enzymes were prepared by growing Trichoderma reesei Rut C30 and Aspergillus niger NRRL 2001 on hot-water treated corn fiber. The recovered cultures were profiled for carbohydrase activities and evaluated on hot-water treated corn fiber for release of monosaccharide sugars. The effect of adding additional enzyme activities and modifying the pretreatment conditions was also examined for the possibility of increasing final sugar yields.
Section snippets
Materials
Microbial strains were obtained from the ARS Culture Collection (NCAUR, Peoria, IL). Corn fiber was received from Aventine Renewable Fuels (Perkin, IL) and stored at −20 °C. Feruloyl esterase is a recombinant purified enzyme originating from Clostridium thermocellum that has been over expressed in E. coli [15]. A purified xylosidase/arabinosidase, isolated from Selenomonas ruminantium, was expressed in and purified from E. coli [16]. The following commercial enzymes were used: glucoamylase
Pretreating destarched corn fiber with hot-water
The corn fiber used in this study was partially destarched to minimize background glucose originating from the starch; corn fiber contains up to 20% (w/w) residual starch. The corn fiber was destarched by treating with a purified glucoamylase (Megazyme) and repeatedly washing with distilled water. The partially destarched corn fiber (DSCF) was determined to have the following carbohydrate content (per gram DSCF, db): 145 mg arabinose, 286 mg xylose and galactose, and 276 mg glucose of which 76 mg
Discussion
Hot-water treatment of corn fiber is an efficient method for dissolving intact xylan. However, these dissolved carbohydrates still need to be saccharified to monosaccharides before they are fermentable by most ethanol producing microorganisms. This is problematic because commercially available xylanases that have been evaluated are largely ineffective for saccharifying corn fiber (pericarp). For example, Hespell et al. [17] was only able to release 18% of the sugars from ammonia fiber explosion
References (26)
- et al.
A bioethanol process development unit: Initial operating experiences and results with a corn fiber feedstock
Bioresour Technol
(2004) - et al.
Saccharification of corn fibre by combined treatment with dilute sulphuric acid and enzymes
Process Biochem
(1997) - et al.
Fractionation of lignocellulosics solubilization of corn stalk hemicelluloses by autohydrolysis in aqueous medium
Biomass Bioenergy
(1998) - et al.
Cell wall polysaccharide interactions in maize bran
Carbohydr Polym
(1995) - et al.
Isolation and structural determination of two 5-5’-diferuloyl oligosaccharides indicate that maize heteroxylans are covalently cross-linked by oxidatively coupled ferulates
Carbohydr Res
(1999) - et al.
Ferulate cross-linking in cell walls isolated from maize cell suspensions
Phytochemistry
(1995) - et al.
Release of ferulic acid from agroindustrial by-products by the cell wall-degrading enzymes produced by Aspergillus niger I-1472
Enzyme Microb Technol
(2002) - et al.
The US corn ethanol industry: an overview of current technology and future prospects
Int Sugar J
(2002) - et al.
SO2-catalyzed steam explosion of corn fiber for ethanol production
Appl Biochem Biotechnol Part A Enzyme Eng Biotechnol
(2002) - et al.
Conversion of corn fiber to ethanol by recombinant E. coli strain FBR3
J Ind Microbiol Biotechnol
(1999)