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Why we drool over tasty snacks

Saliva is a remarkable substance. It might be 99 per cent water, but it is far more than that. Without it, you would find it almost impossible to chew and swallow a meal and your teeth would quickly decay. You would also be prone to nasty infections like oral thrush, ulcers and gum disease.

Saliva is 99 per cent water but also contains hormones including testosterone, cortisol and melatonin as well as minerals such as calcium, electrolytes and antibacterial compounds. It carries with it human cells shed from the lining of the mouth, which is why a swab saliva test can analyse DNA, as well as molecules responsible for expression of genes known as RNA.

It also contains long-chain protein molecules, which looks like strings under the microscope, which give it a unique elasticity and stickiness combined which allows it to form a protective barrier over the mucosal layer (the membrane lining of the mouth).  It protects underlying tissues from mechanical damage.

There are three pairs of salivary glands under the tongue, in the cheeks and jaw, which send saliva through ducts into the mouth. Each produce a slightly different formula. ‘The parotid glands produce watery saliva which helps to moisten food that is chewed in the mouth. These are the glands that work overtime when you are chewing,’ says Professor Proctor.

Saliva from the submandibular and sublingual glands under the jaw and the tongue is much stickier and this is the special ‘protective layer’ saliva which coats the inside of the mouth when you are not eating food.

The average person produces between 1-2 litres of saliva each day, about the same amount of daily urine output. Saliva flow changes throughout the day and night, depending on what we are doing and how we are feeling.

Drooling at the mouth? You’ve probably just caught a whiff of a particularly delicious smelling treat.

But simply looking at food in a magazine won’t make you salivate. This can be demonstrated in all animals except human beings. We don’t appear to salivate when we see a picture of food, but we do salivate when we see and smell food. The brain sends signals to the salivary glands which start to produce greater volumes of saliva. Sucking a lemon sweet is also likely to increase your production of saliva because acid tastes are a strong salivary gland stimulant. At night, we produce much less saliva which is why we wake up feeling like we need a sip of water.

Saliva production responds to chewing because mechanoreceptors, sense cells which register touch or sound, in the mucosal layer of the mouth and in the gums register the pressure of chewing and passes signals through to the medulla area of the brain. This, in turn, signals to the parotid glands to start producing more saliva. Hypersalivation, when someone produces too much saliva, is a real condition and can be caused by gastric reflux, liver disease or sometimes pregnancy.  Some medications can trigger hypersalivation including clozapine, used to treat schizophrenia and ketamine which is used as an anaesthetic and pain relieving drug.

Professor Gordon Proctor
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