Fungal Cell Factories - Food and Industrial Bioproducts and Bioprocessing - page 205

Figure 8.1 Light micrograph showing the A. niger pelleted morphology important for high citric acid
production (Image: Sue A. Karagiosis, Pacific Northwest National Laboratory).
and Vorlop, 2001 ; Okabe et al ., 2009). This organic acid has an annual global production
of 70 000 tons. Unlike citric and gluconic acid, itaconic acid is used exclusively in non-
food applications, such as making synthetic fibers, coatings, adhesives, thickeners and
binders.
The zygomycete fungus Rhizopus oryzae is a significant industrial source of
enantiomerically pure L(+)-lactic acid. Lactic acid production has an annual global market
of over 275 000 tons, with forecasted increasing demands due to expanding use in cosmetics
and biodegradable packaging. Lactic acid is used to generate poly lactic acid and ethyl
lactate, a biodegradable polymer and solvent, respectively. Additionally, this mild organic
acid is utilized as a food preservative and flavor enhancer. Unlike Lactobacillus
species, which produce mixed isomers, Rhizopus generates the preferred L(+)-lactic acid.
The principal limitation of the R. oryzae fermentation process is the diversion of carbon
away from lactic acid production to ethanol and fumaric acid fermentation. Fungal
fermentation of glucose yields lactic acid at 60-80% efficiency (Skory, 2004), and
productivity rates and yields are lower than those obtained for commercial Lactobacillus
fermentations.
A clearer understanding of R. oryzae 's fermentative pathways and lactic acid biosynthesis
may provide novel insight into more efficient and cost-effective methods of lactic acid
synthesis. Targeted molecular genetic approaches, such as increasing lactate dehydrogenase
activity to redirect pyruvate away from ethanol fermentation and increase the overall yield
of lactic acid, are promising (Skory 2000, 2004). However, the current paucity of molecular
tools for genetic manipulation of Rhizopus and an incomplete understanding of the
recombination and replication mechanisms that affect the fate of introduced DNA have
challenged this process (Skory, 2005).