Effect of hot water extraction and liquid hot water pretreatment on the fungal degradation of biomass feedstocks

Abstract

Exhaustive hot water extraction (HWE) and liquid hot water (LHW) pretreatment were evaluated for their effects on degradation of biomass feedstocks (i.e., corn stover, wheat straw, and soybean straw) by Ceriporiopsis subvermispora. HWE (85 °C for 10 min) partially removed water soluble extractives and subsequently improved fungal degradation on wheat straw while it had little or no effect on the fungal degradation of corn stover and soybean straw. In contrast, LHW pretreatment at 170 °C for 3 min improved the fungal degradation of soybean straw; thus, lignin removal of 36.70% and glucose yield of 64.25% were obtained from the combined LHW and fungal pretreatment. However, corn stover, which was effectively degraded by fungal pretreatment alone, was less affected by this combined pretreatment. Our results indicated that a HWE or LHW pretreatment conducted under mild conditions worked synergistically with fungal degradation for some recalcitrant feedstocks.

Introduction

It has been reported that microbial pretreatment with white rot fungi under solid state fermentation effectively improved enzymatic hydrolysis of various biomass feedstocks (e.g., hardwood, corn stover, and switchgrass) (Wan and Li, 2011) .The advantages of microbial pretreatment include low energy input, little or no use of chemicals, and no waste stream output. However, microbial pretreatment has inherent problems, such as long pretreatment time and simultaneous degradation of carbohydrates and lignin. Our previous study showed that some biomass feedstocks, such as soybean straw and wheat straw, were strongly resistant to fungal degradation, even with the addition of nutrients and enzyme inducers (Wan and Li, 2011). Combined pretreatment can potentially overcome the problems associated with physical and chemical pretreatments alone, such as intensive energy use, severe pretreatment conditions, high chemical loading, and toxicity of the waste stream. These benefits have been demonstrated for biopulping technologies, where fungal pretreatment with white rot fungi is widely applied along with mechanical or chemical pulping (Akhtar et al., 1998, Kang et al., 2003). Most importantly, the combined pretreatment process could result in a synergistic effect, improving the yields of end products.

Several studies have investigated a combination of fungal pretreatment with one of the other types of pretreatment methods for biomass. The results showed that combined pretreatments improved enzymatic hydrolysis and biofuel production when compared to a single pretreatment process. Fungal pretreatment with Ceriporiopsis subvermispora followed by ethanolysis was found to substantially increase ethanol yield of woody biomass over ethanolysis alone (Baba et al., 2011, Itoh et al., 2003). Similarly, fungal pretreatment followed by steam explosion further increased saccharification of beech wood meal by about 10% (Sawada et al., 1995). Yu et al. (2008) tested fungal pretreatment of ultrasonic- or H₂O₂-pretreated rice hulls and found that the combined pretreatment resulted in increased lignin degradation and enzymatic hydrolysis yield. Kadimaliev et al. (2003) found that modification of birch and pine sawdust by ultrasound accelerated lignin degradation by Panus tigrinus. Combined fungal pretreatment with mild alkaline pretreatment of corn stover was also shown to enhance delignification and xylan removal as well as glucose yield (Yu et al., 2010). The ethanol yield of water hyacinth pretreated with combined fungal and dilute acid pretreatment was 1–2 folds higher than that obtained from dilute acid pretreatment alone (Ma et al., 2010).

Liquid hot water (LHW) pretreatment has been one of the leading pretreatment methods for improving cellulose digestibility of lignocellulosic materials (Mosier et al., 2005b). This method, generally conducted at elevated temperatures (120–260 °C), is regarded as an environmentally-friendly pretreatment process because no chemicals are used (Kim et al., 2009, Weil et al., 1998). Water and acetyl groups within hemicelluloses, which act as acids at around 200 °C, are believed to catalyze extensive hydrolysis of hemicellulose to its component sugars, primarily xylose. The effectiveness of LHW pretreatment on cellulose digestibility is strongly related to pretreatment severity. Severe pretreatment conditions, which result in accumulation of organic acids and, subsequently, an acidic environment, cause degradation of monomeric sugars present in the liquid fraction to compounds inhibitory to ethanol fermentation (e.g., hydroxymethyl furfural (HMF), furfural, formic acid, levulinic acid) (Weil et al., 1998). The pH controlled LHW pretreatment can prevent degradation of fermentable sugars but involves the use of a base (Mosier et al., 2005b). To eliminate the use of chemicals while minimizing the degradation of fermentable sugars, LHW pretreatment conducted at less severe conditions can be followed by other pretreatment methods. Inoue et al. (2008) reported that ball milling combined with hot-compressed water treatment of Eucalyptus reduced energy usage during pretreatment and enzyme loading for enzymatic hydrolysis. The combination of LHW pretreatment and fungal pretreatment has not been reported.

It was the goal of this study to investigate the synergistic effects of hydrothermal pretreatments and fungal pretreatment on degradation of biomass. Two uncatalyzed hydrothermal methods, HWE and LHW pretreatments, were compared. Their effects on the modification of the chemical compositions of plant cell walls and improvement on glucose yields were also investigated.