Finally, some other groups of bacteria, which have not been covered above, are nevertheless worth mentioning. The propionic acid bacteria constitute the essential secondary flora in Swiss-type cheese. After homofermentative lactic acid fermentation, they convert lactate into propionate, acetate, and carbon dioxide. The latter is responsible for the characteristic eye formation in these cheeses. Interest in the role of propionic acid bacteria in flavor formation has recently been renewed (59). Also for this group of bacteria, the raw milk environment appears to be a versatile and interesting source of strain variation (60).
A second group of bacteria, which play a major role in the maturation of surface-ripened cheese, comprises the brevibacteria and other coryneform bacteria. They are ob viously present in the smear of these cheeses and are strongly proteolytic, which results in high levels of sulfur-containing volatiles (45,61). They also show some lipolytic and es-terolytic activity, and produce distinct red-orange pigments (62). The variation in pro-teolytic activity, antimicrobial activity, and pigment biosynthesis in these bacteria may offer opportunities for the selection of appropriate variants for specific applications in the manufacture of smear-ripened cheeses or even other types of cheeses (63).
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