Tracy P. Houghton,1 David N. Thompson,*1
J. Richard Hess,1 Jeffrey A. Lacey,1
Michael P. Wolcott,2 Anke Schirp,2 Karl Englund,2
David Dostal,2 and Frank Loge2

11dahn Natinnal Engineecing and Envicnnmental Labncatncy,
Idahn Falls, ID 83415-2203,

E-mail: thnmdn@inel. gnv; and
2Washingtnn State Univecsity,

Pullman, WA 99164-1806

Abstract

Combining biologic pretreatment with storage is an innovative approach for improving feedstock characteristics and cost, but the magnitude of responses of such systems to upsets is unknown. Unsterile wheat straw stems were upgraded for 12 wk with Pleurotus ostreatus at constant temperature to estimate the variation in final compositions with variations in initial mois­ture and inoculum. Degradation rates and conversions increased with both moisture and inoculum. A regression analysis indicated that system perfor­mance was quite stable with respect to inoculum and moisture content after 6 wk of treatment. Scale-up by 150x indicated that system stability and final straw composition are sensitive to inoculum source, history, and inoculation method. Comparative testing of straw-thermoplastic composites produced from upgraded stems is under way.

Index Entries: Fungal upgrading; engineered storage; biological prepro­cessing; Pleurotus ostreatus; straw composite.

Introduction

Agricultural crop residues are a valuable renewable biomass resource. In 1999, American farmers harvested 53,909,000 acres of wheat (1). The straw from this acreage of wheat represents >50 million t annually. Cur­rently, some of the straw is harvested (baled) for use as livestock bedding or low-grade animal feed. However, these low-value uses provide only a

*Author to whom all correspondence and reprint requests should be addressed. Applied Binchemistcy and Bintechnnlngy 71 Vnl. 113-116, 2004

minimal return. Nationally, only about 3.2% of the economic return on wheat is from straw (1). Producers have long recognized the potential eco­nomic and environmental benefits of producing forage, bioenergy, and bioproducts from excess wheat straw residue. However, because of the low bulk density of straw and the loss of fermentable sugars to microbial activ­ity during storage, there are harvest, transportation, storage, and prepro­cessing methods and logistics issues that must be worked out before the excess straw can be economically utilized on a national scale.

The U. S. Department of Energy and U. S. Department of Agriculture recently began a concentrated national effort under the Biomass Research and Development Act of 2000 to develop and demonstrate working biorefineries in the near term. The "vision" and "roadmap" documents for near-term utilization of agricultural residues to produce fuels, chemicals, and bioproducts have recently been completed and focus primarily on corn stover and cereal straws as the feedstocks (2,3). Objectives and research pathways identified in the roadmap document for stover and straw pre­processing and storage issues include the following (3):

1. Cost-Effective Pre-Delivery Treatment Processes—The develop­ment and testing of cost-effective pre-conversion treatment pro­cesses to increase energy — and chemical-density of raw materials at the point of harvest.

2. Best Practices for Harvesting and Storage—The biomass/agricul — tural communities must identify, develop, test, and implement best practices for cost-effective and environmentally sound pre-treat­ment, collection, storage, and transport of plant and animal resi­due-based feedstocks. This should lead to improved plant and animal residue recovery, more effective separation, improved han­dling and storage technologies/procedures, and reduced environ­mental impacts.

Thus, several issues related to preprocessing and storage have been identified as important research and development priorities for the near term. An innovative and potentially useful approach to addressing these issues would be to combine preprocessing and storage into a single sys­tem. In this way, energy use and infrastructure could be reduced by modifying the feedstock while it is waiting to be utilized. These modifi­cations could be biological or chemical in nature. In the case of biological treatments, for such a system to be workable, it would be necessary for microbes carrying out the desired modifications to outcompete indig­enous microorganisms vying for the same resources.

Straw utilization for composites is limited by poor resin and polymer penetration, and excessive resin consumption owing to the straw cuticle, fines, and the lignin-hemicellulose matrix (4). Some white-rot fungi, in­cluding Pleurotus ostreatus, degrade the cuticle and selectively degrade lig­nin and hemicellulose, leaving behind relatively more cellulose (4). Thus, treatments by these fungi could potentially be used to improve resin pen­etration and resin binding without the use of physical or chemical pretreat­ments. Although long treatment times and large footprints limit the use of fungal treatments on a large scale, distributed fungal pretreatments could alleviate land requirements.

In a previous study (4), we presented the results of a preliminary inves­tigation to determine whether P. ostreatus could be competitive with indig­enous organisms in unsterilized wheat straw stems. A detailed description of the potential benefits of preparing straw-thermoplastic composites from wheat straw stems upgraded by selective degradation by a white-rot fun­gus was provided in that study (4). In general, the potential benefits focus primarily on the reduction of fines (reduced external surface area) via a selective harvest method (5), and removal of amorphous matrix compo­nents by P. ostreatus to increase internal surface area and allow better pen­etration of composite formulation components into the lignocellulose matrix (4). Our previous study was conducted with the aim of moving toward the development of a passive, potentially distributed fungal upgrading system to improve feedstock characteristics for production of straw-thermoplastic composites (4). As envisioned, the system would be constructed and oper­ated similarly to passive composting systems and could be operated for 12 wk or longer in a distributed or centralized manner, depending on land use requirements. Such a system fits within the frameworks of both engineered storage systems and pre-conversion processing.

In the preliminary study it was found that above about 11 mg of P. ostreatus/g of stems and 0.77 g of H2O/g of stems, the inoculated P. ostreatus was generally competitive with indigenous microbes (4), which is consistent with a previous report showing good competitiveness of Pleurotus sp. with soil microorganisms (6). In the present article, we describe completed laboratory studies conducted at the Idaho National Engineering and Environmental Laboratory (INEEL) that were tasked with determin­ing acceptable moisture and inoculum ranges for pilot-scale fungal upgrad­ing tests. Inoculated P. ostreatus was found to more completely dominate degradation of the straw stems as inoculum size and moisture content increased, but to be less selective with respect to polysaccharide degrada­tion. Inoculum and moisture levels of 40 mg of P. ostreatus /g of stems and

1.6 g of H2O/g of stems, respectively, allowed successful competition of the inoculum with indigenous organisms and gave acceptable amounts of deg­radation of xylan and glucan (on a total degradation basis). Statistical analy­sis of the data was conducted to predict the variability of final compositions in response to ±30% variations in initial moisture and inoculum levels. Mini­mal variations in final composition would be desirable to ensure consistent product composition in outdoor systems having few environmental con­trols. In addition, we present the experimental design for the composite formulation/extrusion testing, as well as initial results from several extru­sion tests conducted at the Wood Materials & Engineering Laboratory at Washington State University (WSU). In the near term, these data will be used to devise and test a pilot-scale fungal upgrading windrow system at WSU for demonstration of larger-scale operation and extrusion.