Milk

1Carbohydrate breakdown(souring,fermentation).Lactose to lactic acid+alcohols.Casein coagulates at low pH-Curdling leaving whey.Pasteurized milk spoils differently

2Protein breakdown(proteolysis)-1st amino acids,unpleasant smelling gas(H2S),"putrid"

3Fat breakdown(lipolysis)production of fatty acids+glycerol catalysed by lipases(soapy,rancid)

Beneficial changes-cheese manufacture-Acid production(by rennet),curd is obtained&heated&separated from whey,pressed into blocks&allowed to maturate