Abstract
A membrane bioreactor was developed to counteract the inhibition effect of furfural in ethanol production. Furfural, a major inhibitor in lignocellulosic hydrolyzates, is a highly toxic substance which is formed from pentose sugars released during the acidic degradation of lignocellulosic materials. Continuous cultivations with complete cell retention were performed at a high dilution rate of 0.5 h−1. Furfural was added directly into the bioreactor by pulse injection or by addition into the feed medium to obtain furfural concentrations ranging from 0.1 to 21.8 g L−1. At all pulse injections of furfural, the yeast was able to convert the furfural very rapidly by in situ detoxification. When injecting 21.8 g L−1 furfural to the cultivation, the yeast converted it by a specific conversion rate of 0.35 g g−1 h−1. At high cell density, Saccharomyces cerevisiae could tolerate very high furfural levels without major changes in the ethanol production. During the continuous cultures when up to 17.0 g L−1 furfural was added to the inlet medium, the yeast successfully produced ethanol, whereas an increase of furfural to 18.6 and 20.6 g L−1 resulted in a rapidly decreasing ethanol production and accumulation of sugars in the permeate. This study show that continuous ethanol fermentations by total cell retention in a membrane bioreactor has a high furfural tolerance and can conduct rapid in situ detoxification of medium containing high furfural concentrations.
Highlights
IntroductionLignocellulosic material can be converted to fermentable sugars by acid or enzymatic hydrolysis [1,2]
Lignocellulosic material can be converted to fermentable sugars by acid or enzymatic hydrolysis [1,2].Acid hydrolysis and pretreatments are efficient methods for opening up the crystalline structure of lignocellulose and hydrolyze both hemicelluloses and cellulose
The results of this work confirm that it is possible to use the inherent capacity of S. cerevisiae for in situ detoxification of feed medium containing furfural with up to 17.0 g L−1, and at the same time still produce ethanol rapidly at 23 g L−1 h−1
Summary
Lignocellulosic material can be converted to fermentable sugars by acid or enzymatic hydrolysis [1,2]. Acid hydrolysis and pretreatments are efficient methods for opening up the crystalline structure of lignocellulose and hydrolyze both hemicelluloses and cellulose. Several toxic substances are either released from the lignocellulosic structure or formed during the degradation process. These substances can severely inhibit the metabolism of the fermenting microorganism [3,4]. Three major groups of inhibitors are present in the hydrolyzate: aliphatic acids, phenolic compounds and furan aldehydes [5]. The furan aldehydes, furfural and 5-hydroxymethfural (HMF) are produced as primary degradation products from pentoses and hexoses during the hydrolysis when strong acids are present
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