We all enjoy cooked food and many of us cook every day but have you ever thought about the chemistry of cooking? Cooking makes food easier to digest and safer to eat as it kills micro organisms in the food. However it can also destroy some nutrients in food, such as vitamin C, so it's necessary to balance making food easier to digest and taste better without destroying the valuable nutrients that we need.

Food contains a variety of nutrients including vitamins, protein, carbohydrates, fats and minerals and we'll look at some of these and see how they are affected by the cooking processes. We can cook food in different ways using microwave radiation, dry heat (baking and grilling), moist heat (boiling) and heated fats (frying). Let's look at how the different nutrients in food are affected by these processes.
 

Proteins

Part of a protein showing amino acids bonded together

Proteins are found in eggs, meat, milk, cheese and other foods and are long chains of amino acids bonded together. They have a three dimensional structure that is held together by weak bonds between the chains.  When you boil an egg the white and the yolk contain proteins that are liquids. When these protein are heated the weak bonds are broken and the protein unravels. This is called denaturing and can also be brought about by the action of enzymes. The chains of amino acids now link together to form a network and the protein changes from a liquid state to a semi-solid state.

This is called coagulation and is what is happening as the egg white and yolk solidify. If you only boil the egg lightly the network of proteins traps water to form a soft gel that is easy to digest. If you continue boiling the egg it becomes rubbery and tough and not easy to digest at all! Coagulation is used to when you make souffles, custards and cakes. Coagulating milk protein using an enzyme called rennin results in cheese and coagulating soymilk using magnesium sulphate makes tofu.

The main protein in meat is collagen which is a tough building block found in connective tissue and needs to be softened before it is digestible. Long, moist cooking is best for this type of protein. Some meats however, such as steak, are low in collagen and are best cooked quickly with a dry heat.
 

Tenderising Meat

Meat can be tenderised in a variety of ways including:

1. Hanging - this stretches the muscle fibres so that they become loose.
2. Cooking - long moist cooking makes some meats more tender. using a pressure cooker can also help to make meats more tender.
3. Beating and grinding - hitting tough meats such as steak with a mallet or mincing can loosen connective and make the meat tenderer.
4. Use of enzymes - Some foods contain enzymes (called proteolytic enzymes) that will break down the protein chains. Pawpaw for instance contains papain which is often used to tenderise meat.
5. Salt - salt can be used to break down the connective tissues.
6. Alcohol - alcohol is often used in marinades and can denature the proteins in meat.

Starch

Part of a Starch Molecule

Starch is present in foods such as rice, potatoes and pasta. Starch is a natural polymer made from glucose molecules and is insoluble in water. When starch is boiled the grains begin to absorb water and swell. As the temperature rises above 70C the membranes of the starch grains weaken and the starch granules leak out. This is called gelatinisation and the mixture becomes jelly like and viscous.
 

Vitamin C

Vitamin C

This vitamin is present in fruits and vegetables and is essential for our health. It is water soluble so is not stored in the body so we need to eat foods containing vitamin C every day. A deficiency of this vitamin results in a disease called scurvy which was a big problem in the past, before people knew it was caused by the lack of Vitamin C. Vitamin C, or ascorbic acid, is easily oxidised to an inert form, the extent of oxidation being increased by heating, cutting or crushing the food. So if you chop up fruits or vegetables a long time before you eat them you'll be destroying the vitamin C they contain.

Oranges, lemons and limes are rich in Vitamin C

Cooking will also decrease the vitamin C content of the food so vegetables should always be cooked for the minimum time in a small quantity of water. Some cooks add a small quantity of sodium hydrogen carbonate (baking powder) to vegetables to improve the flavour but what will this do to the Vitamin C. Well as it is an acid and acids react with carbonates this will destroy the vitamin C in the vegetables! Not a good idea.
 

Raising Agents

Yeast

We've all used raising agents in cooking and baking but do you know the chemistry involved? There are two raising agents used in most recipes, yeast and baking powder. Yeast (Saccharomyces cerevisiae) is a micro-organism that contains the enzyme zymase that converts the sugars in dough into carbon dioxide and ethanol. The carbon dioxide is the raising agent.

Gluten in the dough is a fibrous compound that stretches as the bread rises and traps the carbon dioxide in an elastic framework. Yeast grows in a warm environment so the bread dough is kept warm until it rises. When it is placed in a hot oven the yeast increases production of carbon dioxide initially then dies as the temperature rises. The carbon dioxide trapped in the dough expands and the dough rises even more. Starch grains in the dough now absorb moisture and burst, the gluten becomes more rigid and sets forming a hard crust on the top of the bread. The flavour comes partly from the ethanol produced by the yeast.

Baking Powder

Baking powder is another raising agent used in baking and is sodium hydrogen carbonate. When this compound is heated carbon dioxide is produced which is the raising agent. In addition some recipes uses tartaric acid mixed with the sodium hydrogen carbonate. Acids react with carbonates to produce carbon dioxide so mixing tartaric acid with the baking powder increases the production of carbon dioxide and makes the food rise faster.

There's a lot more interesting chemistry of cooking to find out about! This is only a short introduction but I hope it has served to interest you in this fascinating aspect of chemistry in everyday life.