Figure 4-5. Manufacture of cultured buttermilk.

producing lactic acid, as well as small amounts of acetic acid, ethanol, and carbon dioxide. These metabolic end-products contribute to the flavor and, in the case of the carbon dioxide, to the mouth feel of the product. Importantly, these bacteria also have the metabolic capacity to ferment citric acid and to produce diacetyl, a compound that has major impact on the flavor of cultured buttermilk. Diacetyl has a buttery aroma and flavor, and, according to buttermilk afficiona-dos, imparts a delicate and characteristic flavor. The ability of the starter culture to perform the citrate fermentation, therefore, is a critical trait. How lactic acid bacteria convert citrate into diacetyl has been a subject of considerable interest, not only because of the practical importance of the pathway in fermented foods, but also because it posed a particular challenge to biochemists. For many years, two pathways, one enzymatic and the other, non-enzymatic, were thought possible.

In both pathways, the early steps are the same (Figure 4-6; also see Chapter 2). Citrate is first transported by a citrate permease (CitP) that is pH-dependent, with an optimum activity between pH 5.0 and 6.0. The intracellular citrate is then hydrolyzed by citrate lyase to form acetate and oxaloacetate. The oxaloac-etate is decarboxylated by oxaloacetate decarboxylase to give pyruvate and carbon dioxide, and the acetate is released into the medium. Normally, lactic acid bacteria reduce pyruvate to lactate, which is then excreted. However, pyruvate reduction requires NADH, which is ordinarily made during glycolysis, but which is not made during citrate fermentation. In the absence of reduced NADH, pyruvate would accumulate inside the cell, eventually reaching toxic or inhibitory levels.The excess pyruvate is instead oxidatively decarboxylated by thi-amine pyrophosphate (TPP)-dependent pyruvate decarboxylase and acetaldehyde-TPP is formed (see Chapter 2 for details). The latter compound condenses with another pyruvate molecule to form a-acetolactate, a reaction out citrate

Figure 4-6. Citrate fermentation pathway in lactic acid bacteria. Adapted from Hutkins, 2001.

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