Saliva fulfils several roles: keeping mouth and teeth healthy, helping with speaking and last but not least enabling eating, chewing and swallowing.
Saliva consists of 97-99 percent water, so it is tempting to assume that it should be easy to replace saliva by water. Quite wrong, the small amounts of proteins and ions making up the remaining 1-3 percent make saliva a rather special, high-tech viscoelastic solution (for the specialists: this is called a non-Newtonian fluid). The viscosity of saliva depends on the shear force working on it when we chew or move food around in the mouth, or try to swallow. It actually has very special properties (tomato ketchup has some such, though simpler, properties: it gets slightly more liquid when you shake the bottle vigorously).
In addition the process of swallowing is a truly complicated process. It is governed by a chain of commands, obtained from multiple sensors all over your mouth and throat, signalling when it is safe to swallow, and how (liquid or solid foods). Only when this feedback is positive and returns to brain and nerves the message that the lump of food in your mouth is ready for swallowing will this be a smooth and pleasant eating experience. Saliva produces the right kind of consistency of lumps of food in the mouth (called a bolus) to trigger a positive swallowing signal. Copious amounts of water will help to get food down but without the effects of saliva eating will feel like hard work: water will not properly lubricate most foods to a consistency that the swallowing sensors will accept as ‘okay’.
It is essential that saliva is such a versatile high-tech material because it needs to be ready, on the spot, for whatever we put in our mouth – from brittle biscuits to soft purees to very nearly liquid soups, not to mention chemically vastly different foods, ranging from starchy potatoes to fibrous protein-rich meat or fish. The feedback & saliva system must be able to cope with all these circumstances almost instantaneously. Normally this works perfectly well but if one or several of the components in this complicated system do not function properly (for example, insufficient saliva production) we will have problems with swallowing and eating.
There are different modes of saliva production and different kinds of saliva, depending on what is needed at any given time and circumstance.
We have three pairs (left/right) of major salivary glands as well as many small glands all over the mouth (the minor salivary glands). When we rest only a little saliva is produced mostly by the submandibular and sublingual glands, just enough to keep the soft tissue (mucosa) in your mouth moist and to protect your teeth. Chewing and the tastes of foods (and their smell) signal the need to produce more saliva. About 60 percent of the saliva under these working conditions is produced by the parotid glands (which are much less active at rest) and the composition of this on-demand saliva is different from that produced at rest.
The slightly more viscous saliva produced at rest by the submandibular and sublingual glands contains large protein molecules, called mucins. These proteins are an important part of the food lubrication at work, and they give the saliva its viscosity at rest. Under working/eating conditions the additionally on-demand produced saliva from the parotid glands is less viscous and does not contain mucins. Instead, the on-demand saliva contains an enzyme called amylase. Enzymes from the amylase family split very large molecules (such as starch) into smaller units (such as sugar molecules; you may have noticed that after chewing starchy food for a very long time it starts tasting slightly sweet). The role of amylase is not entirely clear beyond some such form of pre-digestion. Amylase may contribute to exposing food particles to taste buds and may also play a role with clearing food from the mouth (amylase enzymes typically are a component of dishwasher tablets).
Another important component in the on-demand saliva are molecules called statherins. These are powerful emulsifiers, they help to combine watery and fatty foods into smooth and homogeneous lumps, ready for swallowing.
If you suffer from dry mouth in order to help with eating you will likely want to try some artificial salivas or take advantage of the lubricating powers of some foods.
Next section: Saliva and eating