Introduction

Modern dairy microbiology began with the study of the natural acidification process that occurs when milk, cheese whey, or buttermilk (from cultured butter manufacture) are held for a time. These acidified products had long been used as inocula to produce cheese, butter, and cultured milks, but the resulting fermentations were undependable and of uneven quality. Pasteur, in 1857, was the first to demonstrate that the lactic fermentation was of microbial origin; disputing the accepted theory of the time that chemical degradation of sugar to lactic acid resulted in spontaneous generation of microorganisms (Brock, 1961). It was not until 1878 that Lister isolated pure cultures of the lactic acid bacteria responsible for milk acidification (Brock, 1961). In the 1880s, Conn in the United States, Storch in Denmark, and Weigmann in Germany demonstrated the advantages of using selected lactic acid bacteria to culture cream for butter manufacture (Knud-son, 1931; Cogan, 1996). Commercial production and the use of starter cultures grew rapidly and was widespread at the beginning of the 20th century. The advantages of using starter cultures to initiate fermentation were convincing. Before the use of commercial starter cultures, Cheddar cheese took 6-7 h to produce, and much of the product was of too poor a quality to be sold (Conn, 1895). Slow fermentation was also a public health threat, because milk for cheese manufacture was not pasteurized. Currently, most cultured dairy products are produced using commercial starter cultures that have been selected for a variety of desirable properties in addition to rapid acid production. These may include flavor production, lack of associated off-flavors, bacteriophage tolerance, ability to produce flavor during cheese ripening, salt tolerance, polysaccharide production, bacterio-cin production, and heat sensitivity.

A starter culture is any active microbial preparation intentionally added during product manufacture to initiate desirable changes. These microbial preparations can consist of lactic acid bacteria, propionibacteria, surface-ripening bacteria, yeasts, and molds. Starter cultures have a multifunctional role in dairy fermentations. Their ability to produce acid rapidly aids in separation of curd from whey during cheese manufacture, modifies texture of cheeses and cultured milks, and enhances preservation. Production of low molecular weight compounds such as diacetyl contributes to flavor and aroma. Gas production can cause eye formation in cheese. Development of flavor and changes in texture during ripening of cheeses is associated with enzymes originating from bacterial and fungal cultures, depending on the cheese variety.

Lactic starter cultures may consist of single strains used alone or in combinations or undefined mixtures of strains (mixed-strain cultures). Cultures can also be either mesophilic (optimal growth at approximately 26°C) or thermophilic (optimal growth at approximately 42°C) (Cogan, 1996). Mesophilic mixed-strain starter cultures can be grouped by composition: O (or N) cultures consist of lacto-cocci that do not ferment citrate; B (or L) cultures contain Leuconostoc spp. and lactococci that do not ferment citrate; D cultures contain both citrate-fermenting and citrate-nonfermenting lactococci but no Leuconostoc spp; BD cultures contain Leuconostoc spp. as well as lactococci found in D cultures (Lodics and Steenson, 1993). The type of mixed strain culture used for a specific cheese variety depends primarily on the amount of gas production (if any) that is desired. Thermophilic starter cultures consist of a mixture of Streptococcus thermophilus and Lactobacillus sp., usually either Lb. helveticus, Lb. delbrueckii subsp. bulgar-icus, or Lb. delbrueckii subsp. lactis. These cultures are used to produce Italian and Swiss cheese varieties and yogurt.

This chapter discusses characteristics of lactic acid bacteria and other microorganisms found in dairy starter cultures; their interactions, preparation, and activity measurement; inhibitors of their activity; microbial inhibitors that they produce; and their genetic modifications.

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