"These advantages include their amylolytic characteristics, low nutrient requirements, low-cost downstream process for separation of biomass, valuable fungal biomass which can be used in biosorption processes for purification of contaminated effluents, for fungal chitiosan production and as additive in animal feeds," wrote lead author Zhan Ying Zhang from the University of South Australia.
Lactic acid is used extensively by the food industry as a flavour agent, preservative, and acidity adjuster in foods. A review published in 1995 (FEMS Microbiol. Rev. Vol. 16, pp. 221-231) stated that 85 per cent of lactic acid was used in food and food-related applications in the USA.
The review is a timely reminder of the potential of the fungi to produce lactic acid more cost-effectively than other sources, notably bacteria. Prices for lactic acid have been rising in recent years with producers like Purac and Galactic announcing hikes.
Energy costs for example have doubled in the last 12 to 24 month and the costs for carbohydrates have gone up by 40 to 60 per cent.
The surging cost for carbohydrates is explained by the restructuring of the sugar regime in the European Union and the strongly increasing demand for carbohydrates to produce bio-ethanol.
An advantage offered over other methods of lactic acid production, suggest the authors, is that other, less expensive carbohydrate sources, including refined sugars, molasses, raw starch materials and even lignocellulose using fungal strains of Rhizopus genus.
The review, published in the Biochemical Engineering Journal, also points out that the Rhizopus strains only generate the l-lactic acid isomer, which is the preferred form for the food industry since elevated levels of the d-isomer form are reported to be harmful to humans.
Zhang and co-reviewers state that studies on cost-estimation of the process of using the fungi are few and far between, but what studies have been performed indicated that fed-batch culture was the optimal operation mode for the industrial production of l-lactic acid, based on carbon source, steam and waste treatment costs. Indeed, this method was 12 per cent cheaper than the batch culture method, while the continuous culture method was 40 per cent more expensive than batch cultures.
According to the authors, the main limitation associated with the Rhizopus species is the low lactic acid production and productivity. However, "optimization of process parameters, such as pH and oxygen supply, and control of morphology result in improving performance of Rhizopus species in lactic acid production," they said.
"Furthermore, studies on the enzymetic and metabolic pathways of Rhizopus species provide insight into possibilities for enhancement of lactic acid production.
"Molecular genetics and bioinformatics appear to be promising research tools to get a better understanding of enzymatic pathways and metabolism, and of genetic regulation involved in the simultaneous saccharification and fermentation process using Rhizopus species for lactic acid production, and consequently for enhancing production yield and productivity," concluded the reviewers.
Source: Biochemical Engineering Journal (Elsevier)
1 August 2007, Volume 35, Issue 3, Pages 251-263
"Production of lactic acid from renewable materials by Rhizopus fungi"
Authors: Zhan Ying Zhang, Bo Jin, and Joan M. Kelly