Lactic acid bacteria (LAB) have been used in fermentation and food processing for thousands of years. The two forms of LAB fermentation, homo and hetero, produce different products like lactic acid or CO2.
Homo-fermentation is a metabolic pathway in many species of lactic acid bacteria. It creates lactic acid as the primary product of glucose fermentation.
This process differentiates homo-fermenters from hetero-fermenters. The latter generate a variety of end products, including carbon dioxide, ethanol and acetic acid, along with lactic acid.
Hetero-fermentation creates a range of tastes and textures. In homo-fermentation the bacteria focus energy on a singular outcome, for the smooth, clean flavors in many fermented foods.
They may work together. Some LAB can even switch between processes as the need decrees. They may be found in the company of yeast, adding another depth to fermentation flavors.
LAB prosper in different environments, especially food rich in sugar. Lactobacillus delbrueckii converts over 90% of available lactose or milk sugar to lactic acid in dairy products and is favored in yogurt production.
Homo-Fermentation Process
The process begins with lactose being taken up by LAB bacteria. Lactose is converted to glucose and galactose. In glycolysis, in a series of enzyme-catalyzed reactions, glucose is decomposed to two molecules of pyruvate.
The pyruvate is then reduced to lactic acid by the enzyme lactate dehydrogenase (LDH). This process happens largely without oxygen. Glucose into lactic acid is represented by the metabolic equation:
C₆H₁₂O₆ → 2 CH₃CHOHCOOH
Also called the Embden-Meyerhof-Parnas (EMP) pathway, it's the common metabolic route for many organisms to break down glucose. In homo-fermentative LAB, glycolysis is the only pathway for glucose catabolism.
Examples of Homo-Fermentation
Yogurt: Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, popular in yogurt production, are mainly homofermentative. Their efficient production of lactic acid gives yogurt its characteristic tartness and thick texture.
Lactic acid lowers the pH of fermented products. This environment inhibits growth of spoilage organisms and pathogens. The pH of yogurt drops to around 4.0, significantly extending its shelf life.
Cheddar Cheese: While more complex microbial communities are involved in cheese maturation, homofermentative LAB like Lactococcus lactis are important to the initial stages of cheesemaking. They contribute to curd formation and acidification.
Sauerkraut: Sauerkraut fermentation involves heterofermentative bacteria but homofermentative strains can often be found adding depths of flavor and texture.
Common Homo-Fermentative Bacteria
1. Lactobacillus delbrueckii
Lactobacillus delbrueckii is widely studied in yogurt and fermented dairy food production. It efficiently ferments lactose into lactic acid, contributing to the tangy flavor and thick consistency characteristic of yogurt.
This species can thrive at temperatures ranging from 30-45°C (86-113°F). It's a versatile starter culture in the dairy industry.
2. Streptococcus thermophilus
Another popular example is Streptococcus thermophilus, often working with Lactobacillus delbrueckii in yogurt production. This bacterium is known for its rapid fermentation of sugars into lactic acid.
It's prolific at higher temperatures around 42°C (108°F). This speed enhances the texture and flavor of the final product and lets producers reduce fermentation times.
3. Lactobacillus plantarum
Lactobacillus plantarum is found in a range of fermented foods, such as sauerkraut and pickles. This species produces lactic acid and other metabolites to help preserve food and enhance its sensory qualities.
It can adapt to varying conditions. L. plantarum is found in the high salt concentrations of fermented vegetables to the acidic environment in yogurt.
Some LAB switch between homo-fermentative and hetero-fermentative pathways depending on conditions and available nutrients. Homo-fermentative LAB may also use different substrates for energy. For instance, L. plantarum can ferment fibers as well as sugars.
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