For most dairy fermentations, the role of starter culture bacteria is quite simple— they ferment lactose and produce lactic acid. As a result, the pH is reduced, and the ensuing low pH serves to preserve the product. In addition, lactic acid and low pH also are responsible for enhancing syneresis in cheese manufacture and for causing caseins to coagulate in yogurt, sour cream, and other cultured dairy products. However, lactic acid bacteria used as dairy starter cultures perform a number of other important functions in fermented milk products. They produce or generate several flavor compounds or flavor precursors, and they produce enzymes and other products that have profound effects on texture and body characteristics of cheese and cultured milk products. Not surprisingly, many functions performed by starter culture organisms are directly related to metabolic and physiological characteristics of those organisms. In this chapter, the specific means by which carbohydrates and proteins are metabolized and how endpro-ducts are produced by lactic acid bacteria will be reviewed. The pathways for flavor production, not only by lactic acid bacteria but also by non-lactic acid bacteria used as culture adjuncts and by fungi will also be described.

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