Citrate metabolism is important among L. lactis subsp. lactis (biovar diacetylactis) and Lc. mesenteroides subsp. cremoris strains used in the dairy industry, as it results in excess pyruvate in the cell. The pyruvate may be converted via a-acetolactate to diacetyl, an important flavour and aroma component of butter and some other fermented milk products (Cogan and Hill, 1993) (Fig. 2). Strategies designed to increase the carbon metabolic flux towards diacetyl production have resulted in mutants which produce large amounts of this compound (Hugenholtz, 1993; de Vos, 1996; Swindell et al., growth in milk when mutants are deficient in PrtP 1996).