Enzymatic Hydrolysis of Soybean Hull Without Pretreatment and Its Enhancement of Bioethanol Production Using Xylose-Fermenting Escherichia coli (FBR5).

Abstract

Background: Lignocellulosic materials, such as soybean hulls, possess a complex and recalcitrant structure that requires efficient pretreatment or enzymatic processing for effective conversion into valuable products. However, pretreatment processes often generate inhibitory byproducts (e.g., furfural, hydroxymethyl furfural (HMF), phenols, and lignin degradation products), which can impede enzymatic activity and increase overall production costs. This study explores soybean hulls, a byproduct of oil and meal production, as a potential high-carbohydrate biorefinery resource, assessing their chemical composition, fermentable sugar recovery, and bioethanol production potential.

Methods: Soybean hulls (5%, w/v dry basis) were subjected to enzymatic hydrolysis at 50 °C for 72 hours, utilizing a dual impeller mixing system at 250 rpm. An enzyme load of 45 mg enzyme protein per gram of solids was applied using a combination of commercial enzyme preparations, including Cellulase Blend and Multifect Pectinase. Conversion of cellulose, xylan, and arabinan into fermentable sugars was quantified. A moderate enzyme loading of 10 mg enzyme protein/g solids was also tested for comparison. Microbial fermentation was carried out using the xylose-fermenting Escherichia coli FBR5 strain to produce bioethanol.

Results: Hydrolysis of untreated soybean hulls resulted in conversion yields of 94.4% for glucan, 72.6% for xylan, and 69.3% for arabinan into glucose, xylose, and arabinose, respectively. In comparison, control experiments without cellulolytic enzymes showed significantly lower conversion yields (14.2%, 20.1%, and 15.5% for glucose, xylose, and arabinose, respectively). A moderate enzyme loading of 10 mg enzyme protein per gram of solids achieved a cellulose conversion of 90.6%, which was nearly equivalent to the conversion obtained with 45 mg enzyme protein/g solids. Microbial fermentation with E. coli FBR5 resulted in 94% theoretical ethanol yield, with a production rate of 0.33 g/L/h and a productivity of 0.48 g ethanol/g sugar.

Conclusions: The study demonstrates that enzymatic hydrolysis of soybean hulls, which are rich in cellulose and hemicellulose, can be effectively conducted without the need for pretreatment. The moderate enzyme load used in this study provides a promising platform for efficient sugar release and bioethanol production, presenting a cost-effective and viable approach for utilizing soybean hulls in biorefinery applications.