Metabolism of residual lactose and of lactate and citrate

Abstract

This chapter discusses metabolism of residual lactose and of lactate and citrate. During the manufacture of cheese curd, lactose is converted to lactic acid (mainly the L-isomer) by the starter bacteria. In the case of Cheddar-type cheeses, most of the lactic acid is produced in the vat before salting and moulding whereas for most other varieties, acidification occurs mainly after the curds have been placed in moulds. For many common varieties, the pH of the curd reaches ∼5.0–5.3 within ∼12 h from the start of cheesemaking. Further, lactate is an important substrate for a series of reactions in cheese during ripening: in most cheeses, L-lactate is racemized to D-lactate by the non-starter lactic acid bacteria flora; lactate is catabolized in Swiss-type cheese by Propionibacteriurn freudenreichii subsp, shermanii, which is important for the development of characteristic eyes and flavor; lactate is catabolized to CO 2 and H 2 O by Penicillium camemberti in surface mould-ripened cheeses, such as Camembert and Brie, which is important for texture development; in the presence of O 2 . The relatively low concentration of citrate in milk belies the importance of its metabolism in many cheeses made using a mesophilic culture. Approximately 94% of the citrate in milk is soluble and most of it is lost in the whey; however, the concentration of citrate in the aqueous phase of cheese is ∼3 times that in whey, presumably reflecting the concentration of colloidal citrate; Cheddar cheese contains 0.2–0.5% citrate.