Lactose, or Milk-Sugar, is a member of the sugar group carbohydrates, and is closely allied to cane-sugar. It forms a crystalline solid soluble in water and possessing a sweet taste, though not so sweet as that of cane-sugar. It acts on polarised light, being dextro-rotatory. It does not ferment, yielding alcohol under the action of yeast; but if boiled with a dilute acid, or subjected to the influence of certain ferments, it yields a sugar galactose, which then yields alcohol as a product of the yeast fermentation. It, however, undergoes fermentation under the influence of an organism known as bacillus acidi lactici, present in sour milk or putrid cheese, and yields lactic acid (q.v.), and on further fermentation butyric acid. As lactose is itself present in milk, this fermentation is the cause of milk turning sour when exposed to air.