Core Concepts
In this article you will be able to understand the biochemical pathway that generates lactic acid and its importance in the food industry. After reading this article you will know the basics of lactic acid fermentation.
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Lactic Acid
Lactic acid is a carboxylic acid existing in two isomeric forms: L-lactic acid (levorotary) and D-lactic acid (dextrorotary). The majority of lactic acid found in biochemical systems is L-lactic acid, which plays a pivotal role in various metabolic pathways, including glycolysis. Lactic acid production occurs when the enzyme lactate dehydrogenase reduces pyruvate, a glycolysis byproduct, into lactic acid. This anaerobic process is often observed in low-oxygen environments as an alternative to aerobic respiration.
Lactic Acid Fermentation
Glycolysis
Lactic acid fermentation is a metabolic pathway converting sugars, primarily glucose, into lactic acid. This process has a profound impact on food preservation. The resulting low pH inhibits spoilage by slowing microorganism growth. Further, it imparts unique flavors and textures to fermented foods and beverages!
First, the process commences with glycolysis, which breaks down glucose into two pyruvate molecules. Glycolysis is the metabolic pathway that converts glucose, a six-carbon sugar, into two three-carbon compounds called pyruvate. This process occurs in the cytoplasm of cells and is the first stage of cellular respiration, a series of biochemical reactions that generate adenosine triphosphate (ATP), the primary energy currency of cells. At the end of glycolysis, one molecule of glucose is converted into two molecules of pyruvate, and a net gain of two molecules of ATP and two molecules of NADH is achieved. Pyruvate can then enter the mitochondria for further processing in aerobic conditions, or it can undergo fermentation in anaerobic conditions, depending on the availability of oxygen.
Conversion of Pyruvate to Lactate
Pyruvate is converted to lactic acid (lactate) in a reaction catalyzed by the enzyme lactate dehydrogenase. This reaction involves the transfer of electrons from NADH to pyruvate, producing NAD+ and lactate. The primary purpose of lactic acid fermentation is to regenerate NAD+ from the NADH produced in glycolysis. NAD+ is essential for the continuation of glycolysis since it is required as a cofactor for the glyceraldehyde-3-phosphate dehydrogenase reaction in glycolysis.
The key enzyme is lactate dehydrogenase, which catalyzes the conversion of pyruvate to lactate. The fermentation takes place in the cytoplasm of the cell. Unlike aerobic respiration, which occurs in the mitochondria, lactic acid fermentation is an anaerobic process and doesn’t require oxygen.
While lactic acid fermentation helps regenerate NAD+ and allows glycolysis to continue, it is not an efficient way to produce ATP. The net ATP gain from glycolysis itself is relatively small (two ATP molecules), and lactic acid fermentation does not contribute additional ATP. In the absence of oxygen, this process provides a temporary solution for cells to generate some ATP and continue essential glycolytic reactions.
Lactic Acid Bacteria
Lactic acid bacteria are microorganisms that have mastered the art of lactic acid fermentation. Moreover, they are responsible for converting sugars and organic compounds into lactic acid, contributing significantly to food processes such as yogurt and cheese production. Additionally, lactic acid bacteria inhabit diverse environmental niches, including the human gut, soil, and fermented food products.
The lactic acid bacteria group encompasses numerous species, including Lactobacillus, Streptococcus, and Pediococcus. Additionally, each species exhibits its own unique adaptations and fermentation characteristics.
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Lactic Acid Fermentation Applications
The applications of lactic acid fermentation are vast, impacting diverse industries and aspects of daily life. Prominent applications include:
- Food Production: Lactic acid fermentation is fundamental to food preservation. These fermented products boast extended shelf lives and unique flavors and textures due to lactic acid production.
- Beverage Production: Numerous fermented beverages, including kefir, kombucha, and select types of beer, derive their distinctive tastes and health benefits from lactic acid fermentation, though these beverages can often involve other fermentation processes beyond lactic acid fermentation.
- Probiotics: Lactic acid bacteria are utilized in producing probiotic supplements and fermented foods that support gut health. Probiotics maintain a balanced gut microbial community and offer various health benefits.
- Biomedical Applications: Lactic acid-based polymers like polylactic acid (PLA) are gaining popularity in biodegradable plastics. Additionally, lactic acid finds applications in the pharmaceutical industry, used to synthesize drugs and as an excipient in oral and topical formulations.
- Chemical Industry: Lactic acid serves as a precursor in the chemical industry for synthesizing other chemicals, including lactate esters and lactide. These have applications in producing biodegradable plastics and green solvents.
Further Reading
If the topic interested you, we recommend you to read this article!