Lactic acid bacteria (LAB) are industrious microbes working to transform ingredients such as milk into cheese and cabbage into sauerkraut through fermentation. These hardy bacteria also live in the human gastrointestinal (GI) tract, in symbiotic harmony with their host.
Lactic acid bacteria include Lactobacillus, Streptococcus, Lactococcus, Leuconostoc, and Pediococcus spp, a diverse group of Gram-positive bacteria. They're prized for their ability to convert sugars into lactic acid through carbohydrate fermentation.
This characteristic helps preserve food by lowering pH thus creating a more acidic environment, and inhibiting growth of spoilage organisms. Lactic acid contributes to the distinct flavors and textures in fermented foods such as yogurt, cheese, sauerkraut, and kimchi.
Lactic acid bacteria consume sugars found in fruits, vegetables, grains, and dairy products. They ferment these carbohydrates through the process of glycolysis.
As LAB ferment sugars, they excrete lactic acid as the main metabolic byproduct. In addition to lactic acid, LAB can produce other metabolites such as carbon dioxide, ethanol and flavor compounds, depending on the species and fermentation conditions.
Reproduction
Lactic acid bacteria primarily reproduce asexually through binary fission. During this process, a single bacterial cell divides into two identical daughter cells, promoting rapid population growth in favorable environments rich in carbohydrates.
Some LAB also form spores, which can withstand harsh conditions and enable survival until favorable conditions return. Under optimal conditions, LAB can multiply quickly; some can double population in just 20 to 30 minutes.
In fermentation, this rapid growth allows LAB to dominate and efficiently transform ingredients. In yogurt production, a few bacterial cells can grow into billions, resulting in rich, thick yogurt within hours.
Habitat
Lactic acid bacteria can be found in a variety of environments including:
Gastrointestinal tracts of humans and animals, where they contribute to digestive health.
Dairy products, like milk and cheese, enabling their transformation into fermented foods.
Surfaces of fruits and vegetables, aiding in natural fermentation processes. Yeast is also typically found on ripe fruit such as grapes and plums.
Marine environments, present in some seafood products.
Lactobacilli are part of the native intestinal microbiota of some fish. These include Atlantic cod, Atlantic salmon, rainbow trout, wolffish and Arctic char.
Fermentation Process of Lactic Acid Bacteria
The fermentation process of lactic acid bacteria begins with uptake of sugars from the environment. Through glycolysis, LAB break down glucose into pyruvate, which is then converted to lactic acid.
The process can be classified into two main types: homofermentative and heterofermentative fermentation.
Homofermentative Fermentation: This primarily produces lactic acid from glucose, with little to no other byproducts. LAB like Lactobacillus and Lactococcus are known for this type of fermentation, commonly used in yogurt and cheese production.
Heterofermentative Fermentation: In this process, LAB produce lactic acid, carbon dioxide, and ethanol from glucose. This is characteristic of LAB like Leuconostoc and some strains of Lactobacillus involved in sauerkraut and sourdough fermentation.
The fermentation process of lactic acid bacteria begins when they encounter available sugars. Through a series of enzymatic reactions, LAB begin converting carbohydrates into lactic acid.
An anaerobic process, it does not require oxygen. This immediately rules out competition from microbes who do. As lactic acid concentration increases, it lowers the pH of the environment and creates an acidic medium.
This acidic condition helps inhibit spoilage bacteria, promoting the desired fermentation. Products like sourdough bread see a significant change in flavor and texture due to the specific LAB used. The final taste varies widely, depending on the LAB strain and substrates.
The Special Relationship Between LAB and Yeast
Lactic acid bacteria often have a symbiotic relationship with yeast during food fermentation. They're found together in nature. While LAB ferment sugars into lactic acid, yeast such as Saccharomyces cerevisiae convert sugars into alcohol and carbon dioxide.
S. cerevisiae or baker's yeast cannot digest lactose, while LAB thrive on it; thus each microbial participant performs a certain job. This collaboration also enhances the complexity of flavors and textures in fermented products like bread, beer, and kefir.
In the sour beers often found in breweries today, Lactobacillus is the most widely used fermenter. It can produce ethanol, the alcohol in beer, wine, cider, mead and other drinks.
Yeast most enjoys a pH of 5.5, slightly acidic. Yeast provides LAB with metabolic byproducts to stimulate their growth. Both LAB and yeast can create biofilms, which protect the organisms living within or beneath, and help maintain optimal environmental conditions.
LAB and other microbes form biofilms in the human GI as self-protection from the highly toxic environment which includes hydrochloric acid. They may incorporate toxins which repulse or weaken pathogens.
Interesting Facts About Lactic Acid Bacteria
Health Benefits: Certain strains of LAB, particularly Lactobacillus and Bifidobacterium, are considered probiotics offering health benefits such as improved digestion, enhanced immune function, and prevention of digestive disorders.
Fermented Foods: LAB are integral to the production of yogurt, sauerkraut, kimchi, kefir, pickles and certain cheeses. The unique flavor profile of each food comes from the specific LAB involved in fermentation.
Industrial Use: LAB are common in the food industry, not only for fermentation but also as starter cultures. Their ability to produce lactic acid and other metabolites is used to improve food safety and shelf life.
Research Focus: Ongoing research into LAB focuses on their potential applications in biotechnology, such as bio-preservation, bioprocessing, and the production of natural preservatives.
Diversity: The LAB group encompasses over 100 species across different genera, each with unique traits and applications in food production.
Thermal Resistance: Some LAB can survive high cooking temperatures, remaining active and beneficial even in processed foods. For example, Lactococcus lactis is often present in both raw and cooked dairy products.
Lactic acid bacteria are an essential component of food microbiology and biotechnology, with significant roles in fermentation, health, and food preservation. Their specific traits and functions are applied in food production and preservation especially by artisan creators.
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