To thrive, every organism need to extract energy from an energy source. Bacteria and yeasts use fermentation process as their energy source. Fermentation is a way of getting energy, just like respiration process which is used by plants and animals. Lactic acid fermentation is a type in which lactic acid is formed as a result of the fermentation process by lactic acid bacteria.
- What is Lactic Acid Fermentation
Lactic acid bacteria perform an essential role in the preservation and production of many varieties of foods. It has got various applications in food, pharma, and allied industries due to its peculiar flavour and aroma. It is also inexpensive, and needs little or no heat in the application, making them fuel-efficient as well.
The production of lactic acid from hexoses ( 6 Carbon sugar) is a peculiar metabolic activity, also named ‘fermentation’, of LAB (Lactic Acid Bacteria) such as Lactococcus, Lactobacillus, Enterococcus, and Streptococcus spp. For these reasons, fermentative bacteria are commonly employed in the food industry as starter cultures for the industrial processing of fermented dairy, meat, cereal, and vegetable products.
On the other hand, lactic acid can be also used as a food additive in the industry of edible products without the presence of LAB fermentation. This option can be extremely useful in various ambits. Both the homofermentative and the heterofermentative lactic acid bacteria are generally fastidious on artificial media but they grow readily in most food substrates and lower the pH rapidly to a point where other competing organisms can not survive.
- Bio-preservation of Foods using Lactic Acid Fermentation
Bio-preservation refers to extended storage life and enhanced safety of foods using their natural or controlled microflora and (or) their antibacterial products.
It may consist of:
- adding bacterial strains that grow rapidly and (or) produce antagonistic substances.
- adding purified antagonistic substances
- adding the fermentation liquor or concentrate from an antagonistic organism
- adding mesophilic LAB as a ‘fail-safe’ protection against temperature abuse.
LAB produces lactic acid or lactic and acetic acids, and they may produce other inhibitory substances such as diacetyl, hydrogen peroxide, reuterin (b-hydroxypropionaldehyde), and bacteriocins.
Lactic acid bacteria have a major potential for use in biopreservation because they are safe to consume and during storage, they naturally dominate the microflora of many foods. In milk, brined vegetables, many cereal products and meats with added carbohydrates, the growth of lactic acid bacteria produces a new food product. In raw meats and fish that are chill stored under vacuum or in an environment with elevated carbon dioxide concentration, the lactic acid bacteria become the dominant population and preserve the meat with a ‘hidden’ fermentation. The same applies to processed meats provided that the lactic acid bacteria survive the heat treatment or are inoculated onto the product after heat treatment.
- Industrial Production of Lactic Acid
Lactic acid also has a prime position due to its versatile industrial applications in food, pharmaceutical, textile, leather, and other chemical industries. Lactic acid is widely used in food-related applications but recently it has gained many other industrial applications like biodegradable plastic production. Food and food-related applications account for approximately 85% of the demand for lactic acid, whereas non-food industrial applications account for only 15% of the demand.
Lactic acid was first isolated from sour milk by Carl Wilhelm Scheele in 1780 and was first commercially produced in 1881 by CE Avery in Littleton, MA, USA. Pasteur, Lister, and Delbrueck identified lactic acid as a microbial metabolite. The production demand for lactic acid has been increased over years due to due to its high potential of application in a wide range of fields.
Lactic acid has been mainly used for food and food-related applications. It is due to the mild acidic taste of lactic acid. In addition, lactic acid is non-volatile, odourless, and classified as GRAS (generally recognized as safe) for use as a general-purpose food additive. Therefore, many industries choose lactic acid as a safe flavour and preservative in food. Lactic acid also has been utilized in the cosmetic industry such as in the manufacture of hygiene and aesthetic products due to its moisturizing, antimicrobial, and rejuvenating effects on the skin, as well as of oral hygiene products.
The other promising application of lactic acid lies in its polymer, the poly-lactic acid (PLA). It offers tremendous advantages like biodegradability, thermos-plasticity, high strength, etc. PLA is considered as an environment-friendly alternative to substitute plastics derived from petrochemicals. PLA can be applied in medical applications for filling the gaps in bones, producing sutures (stitching material), and joining membranes or thin skins in humans.
- Lactic Acid Fermentation Process
Lactic acid can be produced by the fermentation of sugars or sugar-containing hydrolyzates or the single-step conversion of starchy or cellulosic wastes by direct conversion using amylolytic lactic acid-producing microorganisms or by the simultaneous hydrolysis and fermentation with concomitant addition of saccharifying enzymes and inoculum together. There are different processes for the biotechnological production of lactic acid. Generally, hydrolyzate is used instead of refined sugars which can be utilized for submerged fermentation or solid-state fermentation.
- Future of Lactic Acid Fermentation
Biodegradable plastic i.e., polylactic acid, can replace synthetic polymers to avoid environmental pollution. So lactic acid production must be economic and environmental friendly with the utilization of renewable biomass. The production of lactic acid from fossil fuels is now widely accepted as unsustainable due to depleting resources and the accumulation of environmentally hazardous chemicals. Even though fermentation can replace the chemical synthesis, the cost of production must reduce for the bulk production of lactic acid for the biodegradable plastic. High energy consumption and cost in raw material pre-treatment can be reduced by simultaneous saccharification and fermentation. This process helps to increase the yield of lactic acid and increase productivity.
The simultaneous saccharification and fermentation of lignocellulosic and starchy materials have its advantages over separate hydrolysis and fermentation. Consumable sugars like glucose released by cellulase or amylolytic enzyme are simultaneously converted to the end product by the microorganism. Glucose inhibition on the enzyme is therefore minimized. Many of the lactic acid bacteria are mesophilic and fermentation can carry out at the optimum temperature of their growth. Simultaneous saccharification and fermentation offer the controlled release of sugar at the optimum growth temperature. The operating temperature of the simultaneous saccharification and fermentation can thus be brought to the level close to the optimum of the cellulase enzyme by using thermotolerant organisms for the efficiency of the whole process.