Bleu cheese is a cow's, sheep's, or goat's milk cheese that is ripened with mold cultures to produce a cheese with flavor that ranges from mild and earthy to sharp and peppery.
A mold called Penicillium roqueforti is added during the cheesemaking process. After the cheese is formed into wheels and left to age, it is spiked with steel rods that introduce oxygen into the middle of the wheel. This allows the mold to grow, resulting in the iconic blue veins. Creamy or soft bleu cheese is usually a younger variety with higher moisture content, or a cheese where cream has been added to the curd. Crumbly bleu cheeses have less butterfat and have been aged longer, lowering the moisture content in the cheese to create a harder, denser curd.