ReviewAlternatives for detoxification of diluted-acid lignocellulosic hydrolyzates for use in fermentative processes: a review
Introduction
Forestry and agricultural residues are abundant, renewable, and inexpensive energy sources. When hydrolyzed, these lignocellulosic materials release sugars (d-glucose, d-galactose, d-manose, d-xylose, and l-arabinose) and several compounds derived from sugar and lignin degradation (furfural, hydroxymethylfurfural, acetic acid, syringic acid, p-hydroxybenzoic acid, vanillin, and other compounds). The lignocellulose hydrolyzates can be used as fermentation media to obtain xylitol, ethanol, and other useful products. However, the by-products of sugar and lignin degradation negatively affect fermentation efficiency, because some of them are toxic to fermentative microorganisms and inhibit their metabolism. Therefore, knowing about these inhibitors and how to minimize their effects is very important. This review summarizes the inhibition mechanisms of several toxic compounds found in lignocellulosic hydrolyzates, as well as the most recent detoxification methods used to partially or completely remove these inhibitors, consequently improving the efficiency of fermentation processes.
Section snippets
Lignocellulosic hydrolyzates
Lignocellulose is mainly composed of cellulose, hemicellulose, and lignin; the contents vary according to plant species. The complex structure of lignocellulose in plants forms a protective barrier to cell destruction by bacteria and fungi. To make this structure suitable for conversion in fermentative processes, cellulose and hemicellulose must be hydrolyzed into their corresponding monomers (sugars) for utilization by microorganisms (Iranmahboob et al., 2002).
Although several methods for
Inhibitors of lignocellulosic hydrolyzates
The kind of toxic compounds and their concentration in lignocellulose hydrolyzates depend on both the raw material and the operational conditions employed for hydrolysis. Moreover, fermentation variables––such as cell physiological conditions, dissolved oxygen concentration, and pH of the medium––are also associated with the toxicity of these compounds, in several cases accentuating their toxic effect (Taherzadeh et al., 2000). Toxic compounds can stress fermentative organisms to a point beyond
Hydrolyzate detoxification methods
When compared with the fermentation of commercial sugars or detoxified hydrolyzates, the fermentation of non-detoxified hemicellulosic hydrolyzates is characterized by slow kinetics, with limited yield and productivity. This is due to the presence of a variety of compounds that act as potent inhibitors of microbial metabolism. Therefore, the lignocellulosic substrates need to be pretreated and neutralized to attain the fermentation pH, thereby becoming more suitable for microorganism metabolism
Activated charcoal treatment
To purify or recover chemicals, activated charcoal is widely used to remove compounds from the liquid phase by adsorption. Nevertheless, the effectiveness of activated charcoal treatment depends on the following variables used for the adsorption process: pH, temperature, contact time, and activated charcoal concentration. The effects of these variables are described in the next sections.
Conclusions
Because several inhibitory compounds are formed during hydrolysis of the raw material, the hydrolytic process has to be optimized so that inhibitor formation can be minimized. When low concentrations of inhibitory compounds are present in the hydrolyzate, detoxification is easier and fermentation is cheaper. The choice of a detoxification method has to be based on the degree of microbial inhibition caused by the compounds. As each detoxification method is specific to certain types of compounds,
Acknowledgements
The authors acknowledge financial support from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Conselho Nacional de Pesquisa e Desenvolvimento (CNPq), Brazil. The authors also wish to thank Maria Eunice M. Coelho for revising this manuscript.
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