Cheddar cheese contains approximately 108 viable lactic acid bacteria (LAB)/g curd throughout the ripening period. The LAB present may have been added deliberated by the cheesemaker either as the primary starter culture or as a secondary adjunct culture during manufacture or alternatively may have entered the product adventitiously from the milk and other sources during the manufacturing process. Unlike the starter and adjunct cultures that are carefully selected, the cheesemaker currently is unable to control the diversity and development of the adventitious nonstarter population. The identity and characteristics of these distinctive lactic acid bacterial populations will be considered in the following sections.
Starter cultures containing lactic acid bacteria have a definitive role in the manufacture of cheese and fermented dairy products (21). Starter cultures used in dairy fermentations are selected for rapid lactic acid formation and their ability to contribute to the development of the aroma, flavor and texture characteristics of the product, through a diverse range of metabolic attributes. For large-scale Cheddar manufacture in commercial plants, consistency of acid production throughout the cheesemaking process is essential, and the strains used are selected rigorously for bacteriophage resistance, salt sensitivity, acidification capabilities, and contribution to flavor formation. Two types of starter systems, bulk starter and direct vat inoculation, are used widely in Cheddar manufacture with the majority of production utilizing a multiple-strain starter comprising a mixture of at least two or three strains. The LAB used in type O starter cultures for Cheddar cheese manufacture are homofermentative mesophilic strains of Lactococcus lactis that are unable to ferment citrate and exhibit optimal growth at temperatures in the range 26-30°C (21-23). These O-type starter cultures are mixtures of strains of only Lactococcus lactis subsp. lactis and Lc. lactis subsp. cremoris. Mesophilic mixed-strain starters for other fermented dairy products comprise different combinations of citrate-utilizing and citrate-negative strains of biovars of the two Lc. lactis subspecies and Leuconostoc spp.
Many cheeses contain, in addition to the starter population, a secondary microflora that may have been added deliberately as defined adjunct cultures or may have developed as a consequence of adventitious contamination.
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