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In this article we will discuss about:- 1. Discovery of Lactic Acid 2. Fermentation of Lactic Acid 3. Medium and Manufacturing Process 4. Recovery 5. Uses.
Discovery of Lactic Acid:
Lactic acid was first discovered by Scheele (1789) from sour milk. Later on, Pasteur (1857) identified the microorganism involved in lactic acid production. In the year 1881, first commercial production was started by M/S Clinton Processing Company, Clinton, Iowa (USA). This was based on the fermentation process.
Lactic acid production by using chemical process was not economical and recovery and purification were also not upto the mark, hence continuous efforts were made to improve the process. Moreover, the requirement in plastic industry was of very high purity.
Fermentation of Lactic Acid:
Lactic acid is produced by several microorganisms which differ in their ability to produce either D (-) lactic acid, L (+) lactic acid or the racemic mixture. The particular acid, formed seems to be characteristic of the individual microorganism. The racemic mixture is formed due to the production of an enzyme called ‘recemase’. The lactic acid recovered is optically active but becomes inactive due to the action of the enzyme.
Various microorganisms are involved in the production of lactic acid. Rhizopus oryzae produces only L (+) lactic acid. However, the production is quite slow and yield is also low.
There are mainly two important processes based on end product formation:
(a) Homo-fermentative Process:
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This process involves certain bacteria namely, Lactobacillus delbruckii, L. bulgaricus, L. pentosus, L leichmanii, L. casei, Streptococcus lactis, etc. These bacteria utilize the EMP pathway to produce pyruvic acid which is then reduced by the lactate dehydrogenase to lactic acid. All the microbes are considered to be anaerobic, although they can withstand some oxygen. The end product is lactic acid with traces of others.
(b) Heterofermentative Process:
This process involves the action of Leuconostoc mesenteroides which produces lactic acid, carbon dioxide, ethanol, acetic acid, water and few other products.
Medium and Manufacturing Process of Lactic Acid:
The culture medium contains semi-refined sugar, (molasses or whey contains semi-refined sugar), molasses or whey starch, maltose, lactose, sucrose, calcium carbonate with ammonium hydrogen phosphate,. The malt sprouts are mixed and pH is kept between 5.5 and 6.5.
Lactic acid is quite corrosive, hence metals are avoided, and consequently wooden fermenters are used. The thermophilic Clostridia results in the production of some butanol and butyric acid which are the major contaminants in lactic acid production.
The colonies of L. delbruckii are transferred into large culture vessel kept at 45-55°C. Each stage of culture building requires 16-18/!. A slight excess of calcium carbonate is present in each stage.
The inoculum volume is usually 5% and the fermentation is carried out for 5-10 days. The sugar be reduced to 0.11% or less during the fermentation because residual sugar makes the recovery of better quality of lactic acid difficult. Aeration and agitation are required (Fig. 20.11).
Recovery of Lactic Acid:
To the fermentation medium, CaCO3 is added; pH adjusted to 10, broth is heated and filtered. Lactic acid is converted to calcium lactate. It decomposes residual sugar which kills bacteria. The H2SO4 is added to remove Ca as CaSO4. Lactic acid is recrystallized as calcium lactate. The activated charcoal is added to remove impurities and lactic acid is recovered.
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In the solvent extraction procedure free lactic acid is extracted with isopropyl ether directly. It is washed with cold water in a centrifuge to get lactic acid. In another process, methyl ester of free lactic acid is prepared.
The fermentation broth is distilled to get ester. It is boiled with water (hydrolysis) which allows the ester to decompose. The Fig. 20.11: Lactic acid production, product is distilled to get aqueous solution of lactic acid and distilled product is methanol.
The lactic acid can also be recovered as tertiary alkaline salt. Hence in the fermentation broth, organic solvent is added to get salts of lactic acid which is decomposed and free acid is released.
Uses of Lactic Acid:
Since it is a weak acid with good solvent properties, it polymerises readily for the production of polymers. It provides acidity in foods and beverages and served as a preservative in food stuff. The de-lining of hides in leather industries is also carried out by its utilization.
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Certain other industries such as textile and laundry use lactic acid in fabric treatment. Calcium lactate is employed in baking powder and as a source of calcium in pharmaceutical industries. The pure form of lactic acid is used in plastic industry.
