Kitchen Chemicals

A kitchen is just like a science laboratory, don't think the only chemicals in your kitchen are those under the sink. All the ingredients you cook with are themselves made up of chemical compounds - some complex and some quite simple.

Salt
Salt is the original food additive. Essential for life, salt (Sodium Chloride) controls many body functions. Salt is one of the five taste sensations we can detect with the sensors (or taste buds) on our tongues. In the kitchen we use salt as a preservative, to help strengthen doughs, etc. and to emphasise flavours. Salt preserves food by taking out moisture and thus preventing bacterial growth.

The charge on the sodium and chloride ions in salt in doughs and meringues can help bind charged protein molecules and thus make them "stronger". Our tongues are particularly attuned to the taste of salt - presumably since we cannot make salt and need to get all our needs from our diet. Although salt was once one of the most precious of commodities (e.g. Roman soldiers were paid a "salarium" to buy their salt), today salt is common place and added to nearly all processed foods both as a flavour enhancer and as a preservative.

Sugars
Sugars are made of molecules that consist of 6 carbon atoms joined together in a ring with associated hydrogen and oxygen atoms. The common sugar we buy from the supermarket is sucrose and consists of two such sugar rings joined together. Sugar provides energy. Sweet foods have always been important in our diet. In prehistoric times man had to get energy to chase the wildebeest to feed the family, so we developed a real sweet tooth. When refined sugar became available it was a very expensive commodity, but now that sugar is cheap and readily available we often eat too much - which is not good for us. In the kitchen sugar has many uses. Apart from simply making food taste sweet we use sugar in many dishes. Sugar molecules can link proteins together - making it much easier to beat egg whites into a meringue or helping the egg proteins thicken a custard.

Oils and Fats
Fats and oils give foods a rich, creamy, feel in the mouth so they are used in "comfort" food products such as chocolates and ice creams. In the kitchen we also use fats to fry - the high boiling points allows us to cook foods (e.g. chips and meats) at temperatures above 100°C where chemical reactions occur that develop interesting "browned" flavours. As with sugars our liking for fatty foods probably developed in pre-historic times. Fats are an excellent means of storing food energy - people who could lay down fat deposits when food was plentiful could then store the food energy for later lean times. Thus evolution would have favoured those of our ancestors who could put on weight easier. These days, with food readily available, it is a disadvantage to be able so easily to convert fats in food to fat on the body (something I know only too well!).

To a scientist oils and fats are members of the same group of molecules; they consist of three chains of carbon atoms with two hydrogen atoms attached to most carbons. The longer the chains the higher the melting point - so short chains give us the liquid oils and long chains the solid fats. In saturated fats all the carbon atoms are joined by single and form straight chains. In mono-unsaturated two of the carbons and are joined to each by a double bond. This double bond introduces a kink in the chains and makes it difficult to pack the fat molecules in a crystal - so they have lower melting points than saturated fats. Poly-unsaturated fats have several such kinks in them giving them even lower melting points. It is the high melting point of saturated fats that makes them particularly dangerous to us - if solid fat deposits form in blood vessels they can stop the flow of blood leading to heart disease, etc.

Proteins
Proteins are long molecules made by joining small building blocks (amino acids) together. Most of the biochemistry of our bodies is controlled by proteins - the sequence of amino acids determines the shape of the protein and its shape helps the protein perform its own function. For example, haemoglobin has a special shape that allows it to carry oxygen molecules around in the blood stream . When a muscle needs some oxygen it sends a chemical signal and the haemoglobin changes shape so the oxygen pops out (the change of shape also causes a colour change from red to purple).

We need a good deal of protein in our diets - our bodies break down the proteins we eat into their constituent amino acids which we then recycle to make the proteins our own bodies need. Proteins are amongst the most important molecules we use in the kitchen as they change their properties when heated or beaten and react together chemically at high temperatures. Eggs are mostly proteins dissolved in water. If we whisk eggs the proteins change shape (denature) and can form stable foams. If we cook eggs the proteins react to form a solid network - as in a hard-boiled egg. We use these changes when we make cakes and other baked goods - they are held together by the "glue" formed by the reacting proteins.

Starches
The other main food group is made up from starches. Starches are large molecules made by joining many sugar rings together. Scientists often classify starches and sugars together - starches don't taste sweet since the long molecules are too large to reach the sensitive parts of the taste buds on our tongues. There are two main types of starch molecule; amylose in which the sugar rings are joined to make long strings and amylopectin in which the sugar rings are joined in a branched structure like a Christmas tree.

In the kitchen the major sources of starch are from root vegetables such as potatoes and from cereals - usually in the form of flour. We use starches to provide bulk and texture in baked goods - imagine a cake with no flour - it would just be a soufflé with no substance. Starch is formed by many plants in small granules - a typical granule may be a few thousandths of a millimetre across. Of course, the granules are not purely amylopectin and amylose, the plants also incorporate some proteins as they make the granules. Starch granules with a high protein content will absorb a lot of moisture at room temperature, while those with low protein contents absorb but little water. Starch granules can absorb astonishing amounts of water (potato starch granules can easily absorb 100 times their own volume of water) so they make excellent thickeners.

Peter Barham