Blue cheeses are all made from cow’s milk except for the famous French Roquefort cheese, which comes from ewes’ milk.
The blue in Blue Cheeses is essentially blue mould. But it is extremely good for you. What happens is that an organism similar to penicillin is added to the milk or curd used to make ordinary white cheese. The mould, during three to six months of ripening, grows either in small, irregular, natural openings in the cheese or in machine-made perforations, depending on the type of cheese.
The small amounts of mould reproduce or spread to give the typical blue streaks in the white cheese and once ripened, the distinctive flavor. Blue cheeses are usually heavily salted to add to the flavor.