The Production and Uses of Lactic Acid

The industrial production of lactic acid by fermentation involves the use of microorganisms, typically lactic acid bacteria, to convert carbohydrates into lactic acid. Lactic acid is a naturally occurring organic acid used in various industries, including food and beverage, pharmaceuticals, and cosmetics (Prescott & Dunn, 1959; Buchta, 1983).

Overview of the process

Microorganism Selection

The first step is to select a suitable microorganism that has the ability to produce lactic acid through fermentation. Lactic acid bacteria, such as Lactobacillus spp. or Lactococcus spp., are commonly used due to their natural ability to ferment carbohydrates and produce lactic acid as a metabolic byproduct. A number of fermentations use Lactobacillus plantarum, L. annylophilus & L. delbrückii.

Other microorganisms which have been investigated include Rhizopus oryzae.

Fermentation Medium Design

The fermentation medium is prepared, providing the necessary nutrients and carbon sources for the lactic acid bacteria’s growth and lactic acid production. Carbon sources such as sugars (e.g., glucose, sucrose, lactose) or starch-containing raw materials (e.g., corn, cassava, or sugarcane) are commonly used for industrial scale production. Most Lactobacilli though do not metabolise starch so it is better if it is hydrolysed using enzymes to produce reducing sugars. We shall see that it is the availability of the reducing sugars which helps create a suitable medium for fermentation.

The batch and continuous fermentation of L. delbrückii in a glucose-yeast extract was explored with high level pH control (Hanson & Tsao, 1972). In a continuous stirred tank bioreactor, a constant ratio of two parts glucose to one part yeast extract plus added mineral salts appeared ideal. 

One fermentation used enzyme-hydrolyzed tapioca flour, tapioca starch and soluble starch (Shamala & Sreekantiah, 1988) and another used molasses (Aksu & Kutsal, 1986).Enzyme hydrolysis yielded 77, 95 and 99% of reducing sugars from tapioca flour, soluble starch and tapioca
starch, respectively in the former case. In the latter case, invertase was added to the fermentation medium to encourage further hydrolysis of the starch in the molasses. 

Viable lactobacilli are also grown as biomass for use in medical applications and as probiotics. 

Inoculation

The selected lactic acid bacteria are inoculated into the fermentation medium, typically in large-scale fermentation tanks (bioreactors). The fermentation process is carried out under controlled conditions, including temperature, pH, oxygen supply, and agitation.

Lactic Acid Production

As the fermentation progresses, the lactic acid bacteria consume the carbohydrate substrate and convert it into lactic acid through anaerobic fermentation. The chemical reaction involves the reduction of pyruvate to lactic acid.

Monitoring and Optimization

Throughout the fermentation process, various parameters are monitored, such as cell density, lactic acid concentration, and pH levels. If necessary, adjustments to the fermentation conditions may be made to optimize lactic acid production.

The amount of lactic acid produced varies between 16g/100g reducing sugar used and 34g/100g when tapioca starch was used (Shamala & Sreekantiah, 1988).

pH Control

Lactic acid production results in a decrease in pH due to the formation of lactic acid. To maintain optimal conditions for bacterial growth and lactic acid production, pH control is crucial (Hanson & Tsao, 1972). Calcium carbonate or other buffering agents may be added to the fermentation medium to regulate pH.

Harvesting and Recovery

Once the fermentation process is complete, the lactic acid-rich broth is harvested from the fermentation tank. To obtain pure lactic acid, the broth is subjected to downstream processing steps, which may include filtration, centrifugation, and purification using ion-exchange or solvent extraction methods.

Lactic Acid Purification

The extracted lactic acid is further purified to remove impurities and unwanted components, resulting in a high-purity lactic acid product.

Product Formulation

After purification, the lactic acid is formulated into the desired product forms, such as liquid or powder, suitable for its intended applications.

The industrial production of lactic acid by fermentation offers a sustainable and cost-effective method to meet the demand for this important organic acid. Lactic acid finds application in various industries, including food and beverage as a food additive and preservative, in pharmaceutical formulations, and as a component of cosmetic products.

References

Aksu, Z., & Kutsal, T. (1986). Lactic acid production from molasses utilizing Lactobacillus delbrueckii and invertase together. Biotechnology letters8, pp. 157-160

Buchta, K., (1983) Lactic acid. In: Biotechnology, eds H. J. Rehm & G. Reed. Verlag Chemie, Vol. 3

Hang, Y. D., (1989) Direct fermentation of corn to L(+)-lactic acid by Rhizopus oryzaeBiotechnol. Lett.11  pp. 299– 300

Hanson, T. P., & Tsao, G. T. (1972). Kinetic studies of the lactic acid fermentation in batch and continuous cultures. Biotechnology and Bioengineering14(2), pp. 233-252 (Article)

Nakamura, L. K. & Crowell, C. D., (1979) Lactobacillus anylophilus, a new starch hydrolyzing species from swine waste-corn fermentationDev. Ind. Microbiol.20  pp. 531– 40

Prescott, S.C. and Dunn., C.G. (1959). The production of lactic
acid by fermentation. In: Industrial Microbiology, pp. 304
331, McGraw-Hill Book Co. Inc., New York

Shamala, T. R., & Sreekantiah, K. R. (1988). Fermentation of starch hydrolysates by Lactobacillus plantarum. Journal of Industrial Microbiology and Biotechnology3(3), pp. 175-178

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