Microbial Production of Malic Acid from Biofuel-Related Coproducts and Biomass

Thomas West

doi:10.3390/fermentation3020014

Abstract

The dicarboxylic acid malic acid synthesized as part of the tricarboxylic acid cycle can be produced in excess by certain microorganisms. Although malic acid is produced industrially to a lesser extent than citric acid, malic acid has industrial applications in foods and pharmaceuticals as an acidulant among other uses. Only recently has the production of this organic acid from coproducts of industrial bioprocessing been investigated. It has been shown that malic acid can be synthesized by microbes from coproducts generated during biofuel production. More specifically, malic acid has been shown to be synthesized by species of the fungus Aspergillus on thin stillage, a coproduct from corn-based ethanol production, and on crude glycerol, a coproduct from biodiesel production. In addition, the fungus Ustilago trichophora has also been shown to produce malic acid from crude glycerol. With respect to bacteria, a strain of the thermophilic actinobacterium Thermobifida fusca has been shown to produce malic acid from cellulose and treated lignocellulosic biomass. An alternate method of producing malic acid is to use agricultural biomass converted to syngas or biooil as a substrate for fungal bioconversion. Production of poly(β-l-malic acid) by strains of Aureobasidium pullulans from agricultural biomass has been reported where the polymalic acid is subsequently hydrolyzed to malic acid. This review examines applications of malic acid, metabolic pathways that synthesize malic acid and microbial malic acid production from biofuel-related coproducts, lignocellulosic biomass and poly(β-l-malic acid).

Highlights

Malic acid is considered as a “building block” chemical for the production of biodegradable polymers [1,2]
Microbial production of poly(β-L-malic acid) (PMA) from agricultural biomass would seem to be as effective as direct microbial malic acid fermentation to synthesize this industrially-valuable organic acid
From a “green chemistry” perspective, it could help reduce the reliance on petroleum-based chemicals to synthesize malic acid by substituting biobased processes

Summary

Introduction

Malic acid is considered as a “building block” chemical for the production of biodegradable polymers [1,2]. The commercial production of malic acid involves the hydration of maleic anhydride (from the oxidation of benzene or butane) under higher pressure and temperature which synthesizes a racemic mix of D- and L-malic acid where the isomers require chiral resolution [1,2]. PMA may be useful in the production of biodegradable polymers for use in pharmaceuticals and agriculture since it is a water-soluble biopolymer [7,8,9,10]. Green chemistry approaches to produce malic acid from low value biomass or processing coproducts have begun being investigated. Such low value products could help to reduce the price of malic acid compared to its production using petroleum-based products. Possible production from biofuel-related coproducts, lignocellulosic biomass or polymalic acid is examined

Pathways of Malic Acid Biosynthesis

Malic acid Production from Biofuel-Related Coproducts

Lignocellulosic Biomass-Based Malic Acid Production

Findings

Conclusions