Human civilizations throughout history have placed great practical and economic value on methodologies to improve keeping qualities of foods. One of the most ancient of these practices involves fermentation by lactic acid bacteria (LAB) that are indigenous to raw milk, meat, vegetables, and cereal grains. The LAB are a diverse group of gram-positive (gram+) cocci, coccobacilli, and bacilli whose defining characteristics are that they (1) have a low (<55 mol%) G + C content; (2) are acid tolerant; (3) are nonsporing; (4) are nutritionally fastidious; (5) are aerotolerant but not aerobic; (6) are unable to synthesize porphyrins; and (7) have a strictly fermentative metabolism with lactic acid as the major metabolic endproduct.
The taxonomy of LAB is an active area of research, and several additions and refinements have been made in recent years. Among them are annexation of several new genera that satisfy the phylogenetic and physiological definition of a lactic acid bacterium (e.g., Aerococcus, Alloiococcus, Atopobium, Dolosigranu-lum, Eremococcus, Gemella, Globicatella, Lactosphaera, Melissococcus, and Vagococcus) (Axelsson, 1998; Collins et al., 1999; Vandamme et al., 1996), but which do not hold any important food fermentation species. The LAB that do have a significant role in food fermentation include Carnobacterium, Enterococ-cus (En.), Lactobacillus (Lb.), Lactococcus (Lc.), Leuconostoc (Leuc.), Oenococ-cus, Pediococcus, Streptococcus, Tetragenococcus, and Weissella. Discussions in this chapter will primarily address genetics of Lactobacillus, Lactococcus, Leuconostoc, and Streptococcus, because these genera include starter (and non-
Because LAB are common constituents of the raw milk microbiota, it is likely that fermented milk foods have been part of the human diet since milk was first collected in containers. Over the centuries, these inadvertent fermentations were slowly shaped into the more than 1000 unique cheeses, yogurts, and fermented milks that are available today. Because these products evolved well before the emergence of microbiological science, their manufacturing processes all relied upon spontaneous acidification of milk (caused, of course, by endogenous LAB). It was not until discovery of the lactic acid fermentation by Pasteur in 1857, and development of pure LAB dairy starter cultures later that century, that the door to industrialized milk fermentations was opened. Since that time, the economic value of fermented milk foods, and especially cheese, has experienced dramatic and sustained growth. Cheese production in the United States alone, for example, has increased more than 200% in the last quarter century, and total worldwide production now equals approximately 13 million tons per year (IDF, 1994,1999).
To sustain such a high level of productivity and diversity, the dairy industry has become a leader in starter microbiology and fermentation technology. Experience has proved that industrial production of uniform, high-quality fermented milk foods is facilitated by use of well-characterized starter bacteria. Thus, even though a number of traditional milk fermentations still rely on natural souring of raw milk, virtually all industrialized processes employ starter cultures. Because the economic vitality of this industry depends to a very large degree on starter cultures with known, predictable, and stable characteristics, great resources and efforts have been directed toward understanding the physiology and genetics of dairy LAB. The knowledge base that has been built from that work can and has been used genetically to effect precise refinements in metabolic attributes of dairy starter cultures. With literally hundreds of industrial and academic laboratories now devoting resources to LAB physiology and genetics research, it is clear that molecular-genetic strain improvement strategies will play an important role in tomorrow's dairy industry. Research during the last quarter century focused primarily on cellular biochemistry and development of genetics tools, with limited application in key areas such as bacteriophage resistance. Work in the coming decades should see widespread application of this knowledge in ways that will improve product quality and consistency, promote consumer health and well-being, reduce manufacturing losses and safety concerns, and further expand the diversity of fermented dairy products in the market place.
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