Lactic Acid Bacteria

Traditionally, fermentation in dairy foods was achieved by the use of wild-type lactic acid bacteria that were naturally present in raw milk.

Advances in Dairy Products, First Edition.

Edited by Francesco Conto, Matteo A. Del Nobile, Michele Faccia, Angelo V. Zambrini, and Amalia Conte. © 2018 John Wiley & Sons Ltd. Published 2018 by John Wiley & Sons Ltd.

Lactic acid bacteria (LAB) are a group of Gram-positive bacteria that are catalasenegative, nonmotile, non-sporeformers, oxygen-tolerant cocci or rods, able to produce lactic acid as the major end-product during the fermentation of carbohydrates. The core group of LAB is represented by the genus Lactobacillus, Leuconostoc, Pediococcus, Lactococcus and Streptococcus. Taxonomic revisions of these genera have suggested that LAB may also comprise Aerococcus, Alloicoccus, Carnobacterium, Dolosigranulum, Enterococcus, Globicatella, Lactosphaera, Oenococcus, Tetragenococcus, Vagococcus, and Weissella. The genus Bifidobacteria, once considered in the same context and even included in the genus Lactobacillus, is philogenetically unrelated to lactic acid bacteria and has unique mode of sugar fermentation.

There are two main sugar fermentation pathways of LAB:

  • Glycolysis (the Embden-Meyerhof pathway): It is used for sugar metabolism by bacteria whose end-product under standard conditions is almost 100% lactic acid. This metabolism is named homolactic fermentation and the bacteria homofermenters.
  • • The 6-phosphogluconate/phosphoketolase pathway: It results in significant amounts of other end products such as ethanol, acetate, and CO2, together with lactic acid. This metabolism is known as heterolactic fermentation and the bacteria heterofermenters. The end-product formation may be significantely altered due to variation in the growth conditions (Maarth and Steel, 2001; Fox et al., 2004).

The physiology of lactic acid bacteria was always recognized for its role in the acidification of food and feed products. LAB are generally associated with habitats rich in nutrients, like many food products: milk, meat, beverages, vegetables; but they are also present as autochthonous microflora of the mouth, gastrointestinal tract, vagina, of mammals (humans and animals).

A summary of the differentiation of the LAB genera is shown in Table 1.2.2.1 and Table 1.2.2.2.

 
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