Why Does Cheese Have a Skin? What Happens on the Outside

A rind forms at the boundary between the cheese and the air, and it serves several functions at once. It slows moisture loss, since an uncovered surface would dry the whole cheese out and a completely sealed one would leave it wet. It admits oxygen at a controlled rate, which matters because the organisms responsible for ripening are mostly aerobic and work inward from the surface. It excludes unwanted organisms by being colonised deliberately by wanted ones, which occupy the surface and leave no room. And it contributes flavour directly, since the enzymes produced at the surface break down fat and protein there and the products diffuse inward, which is why many cheeses ripen from the outside in and taste different near the rind.

Rinds differ according to what the maker encourages on the surface:

• •Natural rinds, allowed to dry and develop whatever moulds arrive, brushed or turned to control them
• •Bloomy rinds, sprayed with a white mould that forms a soft coat and ripens the cheese inward
• •Washed rinds, wiped repeatedly with brine or alcohol, which favours bacteria producing the strongest smells
• •Waxed and clothbound, where a barrier controls moisture loss mechanically
• •Rindless, made in sealed plastic, which ripens throughout rather than from outside
• •Ash-coated, where a layer of ash reduces acidity at the surface and lets particular moulds establish

The strongest smelling cheeses get that way by a specific and deliberate route. Wiping the surface repeatedly with brine removes the acid-loving moulds that would otherwise dominate and raises the surface pH, which allows particular bacteria to establish, including one that is closely related to the organism responsible for the smell of human feet. Those bacteria produce sulphur compounds and short-chain fatty acids that account for the aroma, and they also break down protein aggressively, which softens the paste beneath and produces the texture such cheeses have. The orange colour comes from pigments the bacteria produce. The process requires attention, since the surface must be wiped on a schedule for weeks, and the resulting cheese smells far stronger than it tastes.

Ripening from the outside inward produces a gradient that is visible when a cheese is cut. In a bloomy-rind cheese the paste immediately beneath the rind softens first as enzymes from the surface mould work inward, so a young one has a chalky core surrounded by a running layer, and a fully ripe one is soft throughout. That gradient is why such cheeses are cut into wedges from the centre outward, so each portion carries the same proportion of rind and centre, and why cutting the point off is regarded as bad manners rather than arbitrarily. Hard cheeses ripen more evenly because the rind admits less and the interior is drier. Blue cheeses ripen from the inside instead, since the mould grows along channels pierced into the paste.

The practical question has a straightforward answer that depends on which kind it is. Bloomy and washed rinds are part of the cheese and are intended to be eaten, and removing them discards much of what the ripening produced. Natural rinds on hard cheeses are edible and are frequently gritty or tough, so it is a matter of preference rather than safety, and the rind of a long-aged hard cheese is worth keeping to flavour cooking. Wax, cloth and plastic are packaging and are not food. Rinds treated with an antifungal coating, which is common on industrial hard cheeses, should be cut away and the treatment is required to be declared. Mould growing on a cut surface of hard cheese can be trimmed away, while mould in a soft cheese means discarding it.

The rind controls moisture loss, admits oxygen at a measured rate, excludes unwanted organisms by hosting wanted ones and contributes flavour that diffuses inward. Bloomy, washed, natural, waxed and rindless are the main approaches. Washed rinds are wiped with brine to raise surface pH, which lets particular bacteria establish and produce both the smell and the soft texture.