Journal of Industrial Microbiology & Biotechnology (2000) 25, 104±108
D 2000 Society for Industrial Microbiology 1367-5435/00 $15.00
Acetaldehyde stimulation of the growth of Saccharomyces
cerevisiae in the presence of inhibitors found in
lignocellulose-to-ethanol fermentations
AR Barber, H Hansson and NB Pamment
Department of Chemical Engineering, University of Melbourne, Parkville, Victoria 3052, Australia
The addition of small quantities of acetaldehyde to fermentations containing inhibitory concentrations of furfural,
acetate and other compounds typically present in lignocellulosic hydrolyzates significantly reduced the lag phase of
yeast growth and stimulated ethanol production. Similar effects were observed when acetaldehyde (0.06g l À 1) was
added to fermentations of a birch wood hydrolyzate produced by steam/acid pretreatment. Acetaldehyde addition
appears to have potential as a low-cost alternative (or adjunct) to current procedures for medium detoxification in
lignocellulose-to-ethanol fermentations, particularly those in which high inhibitor concentrations are generated
through recycling of the culture broth. Journal of Industrial Microbiology & Biotechnology (2000) 25, 104±108.
Keywords: acetaldehyde; detoxification; lignocellulose hydrolyzates; Saccharomyces cerevisiae; inhibition; furfural
acetic acid diffuses across the plasma membrane, dissociating in the
Introduction
cytoplasm and reducing the intracellular pH [20].
The sugars present in lignocellulosic materials such as wood and
Lignocellulosic hydrolyzates can be partially detoxified by a
crop residues can be fermented by yeasts or ethanologenic bacteria
variety of methods, including treatment with lime, sodium
to produce ethanol for use as a motor fuel. Before fermentation,
hydroxide or enzyme preparations, adsorption onto activated
the substrate is either hydrolyzed completely with mineral acids
charcoal, ion exchange, solvent extraction, electrodialysis, and
[24] or given a milder pretreatment or prehydrolysis Ð often
filtration through molecular sieves [8,11,17]. The extent of
with steam and mild acid Ð to produce a hydrolyzate containing
detoxification achieved varies considerably, with the cost of
mainly the hemicellulose sugars and solubilised lignin;the
treatment contributing significantly to the overall cost of ethanol
residual insoluble cellulose is then hydrolyzed enzymatically
production. Overliming, one of the most widely used methods
[11]. Whichever route is chosen, lignocellulosic hydrolyzates
[11], raised the total production costs of a wood-to-ethanol
contain, in addition to sugars, a wide range of breakdown products
conversion process by more than 22%, sufficient to make the
inhibitory to microbial growth, so that some form of detoxification
overall process uneconomic [23].
of the medium is usually necessary to permit satisfactory rates of
An alternative to the above detoxification methods may be the
ethanol production.
addition to the pretreatment liquor of very small quantities of
The inhibitory substances in lignocellulose hydrolyzates may be
substances which stimulate growth and fermentation in the
broadly classified as furans, aromatic lignin degradation products,
presence of toxic materials. In previous works, we showed that
salts and acetic acid [2,10]. Furfural and other furans are formed
small quantities of acetaldehyde Ð a low-cost organic chemical
by the dehydration of pentose and hexose sugars [16,22].
Ð substantially relieved the toxic effects of ethanol on the growth
Lignocellulosic hydrolyzates may contain up to 5±7 g lÀ1 each
of Saccharomyces cerevisiae and Zymomonas mobilis in complex
of furfural and 5-hydroxymethylfurfural [21]. Furfural is a potent
media [5,18,19]. The lag phases of both organisms were reduced
inhibitor of yeast fermentations, particularly at the low cell
by 70±100% by the addition of less than 80 mg lÀ1 acetaldehyde,
concentrations prevailing at the time of inoculation [9,16] while
while the maximum specific growth rates increased by 15±60%.
the toxicity of 5-hydroxymethylfurfural is somewhat less [12,15].
To date, there have been no studies to indicate whether the toxic
Lignocellulosic hydrolyzates also contain many phenolic lignin
effects of chemicals other than ethanol can be relieved by
breakdown products including as much as 1 g lÀ1 hydroxybenzal-
acetaldehyde addition. Many components of lignocellulosic
dehyde [11], as well as hydroxybenzoic acid, syringaldehyde and
hydrolyzates (particularly the phenolic compounds) exert their
syringic acid [1,15]. These exert their inhibitory effect predomi-
primary inhibitory effect on the cell membrane, a mechanism
nantly by dissolving in the cell membrane and diminishing its
similar to that seen with ethanol inhibition. It thus appeared
integrity [4]. Acetic acid, derived from the acetyl substituents of
possible that acetaldehyde addition might also be effective in
hemicellulose sugars, is present in lignocellulosic hydrolyzates in
overcoming inhibition due to these compounds. In this paper, we
concentrations of the order of 3±14 g lÀ1 [13,15];undissociated
show for the first time the ability of acetaldehyde to alleviate
inhibition of yeast due to furans, lignin-derived aromatics and
acetic acid. In dilutions of a native hydrolyzate derived from steam/
acid pretreatment of birch wood, acetaldehyde reduced the lag
Correspondence: Dr NB Pamment, Department of Chemical Engineering, University
phase, stimulated ethanol production, and reduced the rate of cell
death.
of Melbourne, Parkville, Victoria 3052, Australia
Received 18 March 2000;accepted 2 June 2000