How cheese turns blue

Few flavours are as distinctive as that of blue cheese. You could attempt to describe it as spicy, savoury, or punchy, but it may be most accurate to opt simply for “blue.” According to our Technical Director, Bronwen Percival, "we often use the catch-all term ‘blue’ to describe the piquant, peppery, sometimes-slightly-soapy flavour that blue mould produces within a cheese.”  

Where does that unique flavour come from? When mould inside the cheese is exposed to oxygen, it activates: changing texture, adding flavour, and, of course, turning the white paste blue. 

Cheese can become blue intentionally or accidentally. When producing Stilton, for example, cheesemakers pierce the cheese, allowing oxygen to activate the Penicillium roqueforti that has been added to the milk. But the maturation process can introduce bluing in cheeses like Cheddar, Cheshire, Lancashire, and Wensleydale, too.  

How is blue cheese made? 

To become blue, cheese requires the presence of blue mould and exposure to air. Take Colston Bassett Stilton, for example. “It’s already inoculated with Penicillium [roqueforti] in the vat,” says cheesemonger Orla Jackson. “You pierce it and let the air in and that’s what makes it go blue.” 

The blue mould cultures added to the milk during the cheesemaking process remain dormant until the cheese is pierced. Piercing is a process where long, thin metal needles are inserted into the cheese. Once exposed to the air, the mould inside the cheese becomes active. The outcome is blue streaks in between the open textured curd which matures the cheese from the inside out; turning a young, acidic, dry cheese into a softer, mellow, more broken-down cheese as it ages.   

When and how much you pierce a cheese will alter its taste and appearance. When piercing a Stilton cheese, most producers would wait until the cheese is at least four weeks old. Any earlier and the cheese will be too fragile to withstand the physical nature of the process — not to mention bland. Commercial Stilton producers often pierce cheeses up to three times during the cheeses fourth and fifth weeks, which encourages a quick and heavy bluing. This allows the cheeses to be sold quickly, delivering a speedier return on the milk and increasing the volume of cheese produced in a year. In consequence, the blue taste is prevalent, and customers will taste a bright, acidic cheese with a dominant blue flavour.    

The cheese makers we work with prioritise taste over expediency. For example, Billy Kevan, who makes Colston Bassett Stilton, allows his cheeses to mature over six to seven weeks. This allows the cheese to develop its own flavour characteristics before piercing. Billy only pierces our Colston Bassetts twice, which reduces the number of holes in the rind of the wheels and reduces bluing. All of these considerations result in a cheese with the blue flavour in the background, rather than dominating the palate. 

How does hard cheese become blue? 

Bluing isn't just for blue cheese. We often see vibrant streaks of blue shooting through typically non-blue cheeses, like Montgomery’s Cheddar or Appleby’s Cheshire. These accidentally blue cheeses are completely natural and edible. They're simply the result of air inadvertently reaching the interior of the cheese and making contact with mould spores naturally present there.  

How? We don’t know for certain. The simplest explanation is that wheels of hard cheese are turned frequently throughout their maturation. If an affineur flips the wheel and bumps it on a shelf, that can introduce microscopic fissures – and potentially, eventually, bluing. The green mould which happily lives in the air of humid, dark environments (like a cave, or a maturation room) gets into the interior of the cheese and proliferates.  

This happens more often to cheeses with a slightly brittle, crumbly texture. British territorials like Cheshire, Wensleydale, Lancashire, and Cheddars are particularly prone to bluing.   

Accidental mould in cheeses can be delicious and is sometimes sought after by customers. Rarities like bluing in non-blue cheeses is one of the things that make handmade cheese so special. Each batch, and even each wheel, will be unique. Some cheeses will differ so much through accidental bluing that they become entirely different cheeses. For example, a blue-veined Appleby’s Cheshire is known as “Green Fade” and is sometimes sold as a different product. 

If you have any questions about – accidentally or intentionally – blue cheese, please feel free to contact us. We would be happy to help you choose a delicious batch and take care of it once it arrives.