Metabolism of Residual Lactose, Lactate and Citrate

Abstract

Most cheeses contain a very low level of lactose after ripening; most is lost in the whey and the small amount of residual lactose trapped in curd is metabolized by starter or nonstarter lactic acid bacteria during the early stages of ripening. Metabolism of lactose, lactate and citrate impact on cheese quality (flavor compounds formation, gas production, and crystals formation). Lactate may be racemized to D-lactate and the formation of crystals can be an associated defect. Lactate can also be metabolized during ripening via various pathways including propionic acid, acetic acid, CO2 and H2O in cheeses with a propionic acid fermentation, oxidatively by the molds in mold-ripened varieties or anaerobically by Clostridium to butyrate, CO2 and H2 which leads to the serious defect known as late gas blowing. Metabolism of citrate by citrate-positive strains of lactococci or Leuconostoc spp. is of significance in Dutch-type cheeses where it leads to the formation of flavor compounds such as diacetyl and small amounts of gas.