(iii) from lactose

Any country with a large dairy industry has a problem with wastes. Cow manure can be turned into methane and, later, soil conditioner.

What happens to the wastes from the large dairy factories?
The liquid waste, whey, contains lactose (milk sugar). Bacteria can use this carbohydrate as an energy source. Pouring whey over pasture produces an unpleasant smell (lactic acid). Pouring whey into a stream pollutes the water.

Lactose is a disaccharide. Why not ferment it?
Ordinary yeast, Saccharomyces cerevisiae, does not produce an enzyme to break the bond between the two monosaccharide units.

A different yeast, Kluveromyces fragilis, does produce an enzyme which catalyses the hydrolysis of lactose into galactose and glucose. The monosaccharides are easily fermented to produce alcohol.