Metabolism of Residual Lactose and of Lactate and Citrate

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. Lactate may be racemized or metabolized during ripening via various pathways including to 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.