Why does halloumi cheese “squeak” against your teeth when you bite into it? I don’t know of any other food that does this, including other cheeses.
David Muir Edinburgh, UK
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Cheese curds, sometimes eaten as a snack, have a springy, rubbery texture rather like halloumi. They also have a tendency to squeak when bitten, so in places where they are eaten, they are often called squeaky cheese. The fresher the curd, the better the squeak. Undercooked green beans are another squeaky-bite food.
The explanation for the squeak is the alternate “stick and give” of the food’s elastic surface against the inflexible enamel of teeth. This phenomenon is called stick-slip and is responsible for the squeal of car brakes and the screech of a braking car’s tyres on the road.
You can feel stick-slip by dragging your finger along an inflated balloon or a piece of polystyrene. You can enjoy stick-slip by listening to a bow being drawn across the strings of a cello.
Adrian Bowyer Foxham, Wiltshire, UK
Human food has a vast range of textures, but elastic, rubbery foods are quite rare. Halloumi is one of them, and because of its texture, it squeaks when it rubs on our teeth. Other cheeses tend to be crumbly or hard (in other words, not elastic) or soft (and therefore more viscous).
The squeak is a friction phenomenon called stick-slip: the cheese sticks to your teeth, slips a bit, sticks again and so on. When cheese and teeth are stuck, the friction between them is high, and the cheese stretches, increasing the spring force in the cheese. Eventually, this spring force exceeds the friction force and the cheese starts to slip, so the motion continues as the spring in the cheese relaxes.
Eventually, the cheese’s spring force drops to the point that the friction force overcomes it and the cheese sticks again. Then the whole process repeats, producing an oscillation – the squeak.
So why doesn’t Turkish delight, another rubbery food, squeak? The reason is its high sugar content, which forms a film of syrup against your teeth. This viscous layer negates the stick-slip effect, just as the presence of viscous oil stops sliding metals squeaking.
“The squeak when biting halloumi cheese is caused by stick-slip, the same thing that causes the squeal of car brakes”
Decades ago, I spent some years researching stick-slip, with the result that my daughter introduces me thus: “This is Dr Bowyer. He has a PhD in squeaks.”
Photis Papademas Cyprus University of Technology, Limassol
Squeaking against the teeth is a characteristic of halloumi’s texture. The reason why lies in the fact that it contains a large amount of the milk protein casein in its intact form, which creates a dense network. This is because, unlike many cheeses, halloumi has a high pH, as no starter cultures are used in its production.
It seems that when this dense casein network “rubs” against the enamel of the teeth, it produces a squeak. Other cheeses usually have a lower pH, which tends to make the casein more fragile.
Halloumi is made by coagulating sheep or goat milk, then leaving the curds to drain in moulds (the collected whey is used to make a cheese called anari). The curds are then cooked at a high temperature, around 90°C. Finally, the cheese is folded while still hot and salt is then added to the surface.
Sue Thompson Bury St Edmunds, Suffolk, UK
Our (home-grown, not overcooked) French beans squeak, to the extent that our children called them squeaky beans.
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