Lactic Acid Bacteria

Lactic Acid Bacteria (page 3)

(This chapter has 5 pages)

© Kenneth Todar, PhD

Fermentation of Foods by Lactic Acid Bacteria

Many human foods are plants or animal products which have been fermented by lactic acid bacteria, since these bacteria possess properties that can benefit food production or conversion. The acidic and organoleptic properties of fermented foods result from the metabolic activities of these microorganisms. Foods such as ripened cheeses, fermented sausages, sauerkraut and pickles have not only a greatly extended shelf life compared to the raw materials from which they are derived, but also aroma and flavor characteristics contributed directly or indirectly by the fermenting organisms.

Fermented dairy products have been made for thousands of years, but only within the last century have the microbiological bases of these fermentations been elucidated. Lactic acid bacteria are the principal organisms involved in fermenting dairy products. Prior to the availability of starter cultures, milk fermentations relied on the LAB naturally present in raw milk. The first commercial starter cultures were unknown mixes of microbes from raw milk that were prepared in Denmark around end of the 19th century. In the 1930s and 40s, the idea of pure single-strain starter cultures evolved.

Fermented dairy products are enjoying increased popularity as convenient, nutritious, stable, natural, and healthy foods. Lactic acid bacteria are the principal organisms involved in the manufacture of cheese, yogurt, buttermilk, cottage cheese, sour cream and cultured butter. In some fermented dairy products, additional bacteria, referred to as secondary microflora, are added to produce carbon dioxide, which influences the flavor and alters the texture of the final product.

Sausage is one of the oldest processed meat products. The writings of ancient Egyptians described the preservation of meat by salting and sun drying. The ancient Babylonians, Greeks, and Romans used sausage as a food source during times of war. Microorganisms were recognized as being important to the production of sausages about 1921. In the 1940s and 1950s, pure microbial starter cultures consisting of lactic acid bacteria became available but their use was not widespread until the early 1980s.

The fermentation of vegetables, a practice that originated in the Orient, has been used as a means of preserving food for more than 2,000 years. In the third century B.C., during the construction of the Great Wall of China, the Chinese produced fermented vegetables (cabbages, radishes, turnips, cucumbers, etc.) on a large scale. The most common fermented vegetables available in the United States are pickles, sauerkraut, and olives. Carrots, cauliflower, celery, okra, onions, and sweet and hot peppers also are sold as fermented vegetable products.

Generally LAB that are important in the fermentation of food products (dairy, meat, vegetables, fruits, and beverages), include only certain species of the genera Lactobacillus, Lactococcus, Streptococcus, Leuconostoc and Pediococcus. Some of these species are also members of normal flora of the mouth, intestine, and vagina of mammals.

Involvement of lactic acid bacteria in the manufacture of fermented dairy products

Product	Principal acid producers	Secondary microflora
Cheese
Colby, cheddar, cottage, cream	Lactococcus lactis subsp. cremoris	None
Colby, cheddar, cottage, cream	Lactococcus lactis subsp. lactis
Blue	Lactococcus lactis subsp. cremoris	Cit⁺ Lactococcus lactis subsp. lactis Penicillium roqueforti
Blue	Lactococcus lactis subsp. lactis
Mozzarella, provolone, Romano, parmesan	Streptococcus thermophilus	None
	Lactobacillus delbrueckii subsp. bulgaricus
	Lactobacillus helveticus
Swiss	Streptococcus thermophilus	Propionibacterium freudenreichii subsp. shermanii
	Lactobacillus delbrueckii subsp. bulgaricus
	Lactobacillus helveticus
Fermented milk
Yogurt	Streptococcus thermophilus	None
Yogurt	Lactobacillus delbrueckii subsp. bulgaricus
Buttermilk	Lactococcus lactis subsp. cremoris	Leuconostoc sp. Cit+ Lactococcus lactis subsp. lactis
Buttermilk	Lactococcus lactis subsp. lactis
Sour cream	Lactococcus lactis subsp. cremoris	None
Sour cream	Lactococcus lactis subsp. lactis

Lactobacilli

Lactobacillus is very heterogeneous genus, encompassing species with a large variety of phenotypic, biochemical, and physiological properties.
Most species of lactobacilli are homofermentative, but some are heterofermentative. The genus has been divided into three major subgroups and over 70 species are recognized. Group I lactobacilli are obligately homofermentative and produce lactic acid as a major end product (>85%) from glucose. They are represented by L. delbrueckii and L. acidophilus. They grow at 45^oC but not at 15^oC. Group II, also homofermentative, grow at 15^oC and show variable growth at 45^oC. Represented by L. casei and L. plantarum, they can produce more oxidized fermentations (e.g. acetate) if O₂ is present. Group III lactobacilli are heterofermentative. They produce lactic acid from glucose, along with CO₂ and ethanol. Aldolase is absent and phosphoketolase is present. Representative species include L. fermentum, L. brevis and L. keferi.

Lactobacilli are often found in dairy products, and some species are used in the preparation of fermented milk products. For example L. delbrueckii subsp bulgaricus are used in the preparation of yogurt; L. acidophilus is used in the preparation of acidophilus milk; L. helveticus, as well as L. delbrueckii subsp bulgaricus, are used to make Swiss, Mozzarella, provolone, Romano, and parmesan cheeses. Other species are used in the production of sauerkraut, silage and pickles. The lactobacilli are usually more resistant to acidic conditions than are other LAB, being able to grow at pH values as low as 4. This enables them to continue to grow during natural lactic fermentations when the pH has dropped too low for other LAB to grow, so they are often responsible for the final stages of many lactic acid fermentations.

Many sausage fermentations include a Lactobacillus species. L. plantarum is used in starter cultures for the manufacture of summer sausage, pepperoni and salami.

The natural fermentation of cabbage to make sauerkraut involves L. brevis and L. plantarum in the final succession of microbe. The starter cultures for manufacture of cucumber pickles includes L. brevis and L. plantarum. L. plantarum is the most essential species in pickle production, as it is for sauerkraut. Like cucumbers, olives are fermented under conditions similar to those of other vegetable products. The microbial population responsible for the fermentation of olives differs from that of sauerkraut and pickles mainly because the higher salt concentration of the brine prevents many salt-sensitive strains from growing and provides an advantage to salt-tolerant strains. LAB become prominent during the intermediate stage of fermentation. L. mesenteroides and P. cerevisiae are the first lactics to become predominant, followed by lactobacilli, with L. plantarum and L. brevis being the most important.

Streptococci and Lactococci

Note on Streptococcal Classification. Since 1985, members of the diverse genus Streptococcus have been reclassified into Lactococcus, Vagococcus, Enterococcus, and Streptococcus, based on biochemical characteristics, as well as ssRNA analysis. Historically, streptococci were segregated into serological groups based on the presence of specific carbohydrate antigens. Antigenic groups, or Lancefield groups (named for Rebecca Lancefield, a pioneer in Streptococcus taxonomy), are designated by letters A through O. Lancefield groups have proven to correlate well with the current taxonomic definitions. The beta-hemolytic streptococci found in humans contain the group A antigen, while "fecal streptococci" (enterococci) contain the group D antigen. Group B streptococci, usually found in animals, are a cause of mastitis in cows, and have been implicated in human infections. "Lactic streptococci" (streptococci and lactococci) contain the group N antigen and are nonpathogenic. However, Lactococcus has been defined as a genus separate from Streptococcus. As lactic acid bacteria, Lactococcus lactis and Streptococcus thermophilus are the cornerstones of cheese manufacture.

Streptococcus thermophilus

Streptococcus thermophilus is an alpha-hemolytic species of the viridans group. The bacterium is found in milk and milk products. It is not a probiotic (it does not survive the stomach) and generally is used in the production of yogurt and the manufacture of several types of cheese, especially Italian and Swiss cheeses. The organism is a moderate thermophile with an optimal growth rate at 45 °C. Although S. thermophilus is closely related to other pathogenic streptococci (such as S. pneumoniae and S. pyogenes), S. thermophilus is classified as a non-pathogenic, alpha-hemolytic species that is part of the viridan's group. It is closely related to S. salivarius in the oral cavity.

Lactococcus lactis

Lactococcus is a genus of of LAB with five major species formerly classified as Group N streptococci. The type species for the genus is L. lactis, which has two subspecies, lactis and cremoris. Lactococci differ from other lactic acid bacteria by their pH, salt and temperature tolerances for growth.

Lactococcus lactis is critical for manufacturing cheeses such as Cheddar, cottage, cream, Camembert, Roquefort and Brie, as well as other dairy products like cultured butter, buttermilk, sour cream and kefir. The bacterium can be used in single strain starter cultures, or in mixed strain cultures with other lactic acid bacteria such as Lactobacillus and Streptococcus.

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