Kitchen lab: Why does custard get lumpy, and why bother cooking at all?

What has cooking got to do with science?

Well, if you think about it, much of chemistry, the branch of science that's all about the composition, properties and transformations of matter, is just a very sophisticated form of cooking. We observe chemical phenomena everyday in the kitchen. However, they are so commonplace that we've come to consider them unremarkable. And chemistry isn't the only science at work in the kitchen: consider the importance of heat, pressure and microwaves in your recipes - that's physics. Then there's spoilage, bacteria and yeast - that's biology.

The activities in this article aim to help you understand, and to try out for yourself, just a little of the science involved in food preparation.

COOKING - WHY BOTHER?
If you’re not careful, many of the nutrients in food are destroyed before they reach the plate – boiling, baking, frying and freezing all take their toll, so why do we bother? Well, cooking food kills off bacteria, it can deactivate the natural toxic chemicals in plants such as beans, and it tenderises animal tissues and solubilises starch in cereals to make them easier to digest. It’s an activity that distinguishes us from other species, whose diets are, as a result, more limited.

But cooking’s also a little magical: mix together a selection of rather unappetising ingredients, cook them, and hey presto, you have a delicious meal. The variety of foods you can create by cooking is almost limitless. For example, take humble batter. It tastes awful uncooked, but fry it and you’ve got pancakes; bake it and you get fluffy Yorkshire puddings. By altering the ingredients and the method of heating you can design any foodstuff you can dream of – you just need the scientific know-how.

PHYSICAL CHANGES
Boiling water, freezing foods and dissolving sugar in your tea are all examples of a change in physical state (gas, liquid or solid) and are easily reversed. All matter is made up of tiny particles which are too small to see. In a solid, the particles are held tightly together in fixed positions.

The arrangement of particles in the solid state

On heating, the particles start to move more vigorously about their fixed positions until eventually, on melting, they break away and flow over each other to give a liquid – but even as a liquid they’re still held closely together.

The arrangement of particles in the liquid state

If we heat the liquid to its boiling temperature, the particles gain enough energy to pull completely away from each other, and so move independently – the gaseous state.

The arrangement of particles in the gaseous state

CHEMICAL CHANGES
A chemical reaction occurs when the particles of one substance combine with the particles of another to form a new compound. Unlike physical changes, many chemical changes are not easily reversed.

ACTIVITY 1
Take a small glass of vinegar (ethanoic acid) and add a teaspoonful of baking soda (also known as sodium hydrogen- carbonate, or sodium bicarbonate). The mixture immediately fizzes as bubbles of carbon dioxide gas are given off. It’s this ability of baking soda to react to give another compound, carbon dioxide, which makes it useful as a raising agent. The ‘airiness’ created when a batter or dough rises is actually caused by bubbles of carbon dioxide.

You can get baking soda to give off carbon dioxide just by heating it, but the soda (sodium carbonate) residue produced would affect the taste and colour of the product. You may have tasted this with soda bread, which contains baking soda and soured milk.

To make them more effective, most ready-made baking powders are actually a mixture of baking soda and an acid (together with starch or flour). Both the baking soda and acid are solid, so they don’t start to react to form carbon dioxide until water is added, and even more gas is released when the mixture is heated. Any soda residues don’t affect the flavour of the baked product since they are neutralised by the acid. The flour keeps the mixture dry by absorbing any moisture.

ACTIVITY 2
Try making some baking powders of different strengths for yourself using the recipes below. The ingredients you'll need can be found in pharmacies and supermarkets.

You can test the raising power of your home-made baking powders in Activity 3. Compare your products with supermarket brands that contain different ingredients (see the label).

RECIPES FOR BAKING POWDER
FAST ACTING
48g cream of tartar
28g baking soda
21g cornflour
3g tartaric acid
SLOW ACTING
32g acid calcium phosphate
28g baking soda
40g cornflour

ACTIVITY 3

Take the white of one egg and mix it thoroughly with 200 cubic centimetres (1 cubic centimetre is the same as 1 millilitre) of water. Allow the mixture to stand for 15 minutes, then pour off the upper, clear liquid into a container, and discard the lower, cloudy residue (it doesn’t matter if the upper layer is still slightly cloudy). Place 2 teaspoons. of each of your baking powders into separate tall glasses or jars.

Add 25 cubic centimetres (about 2 tablespoons) of the egg-white mixture to each glass, and stir briefly, allowing the mixture to foam. The height of the foam that you obtain in each case is a measure of the activity of the baking powder.

You could make this activity more interesting by using different baking powders with a basic sponge recipe, and comparing your results. Of course, the only real way to assess the outcome is to taste it!

LUMPY CUSTARD
To understand why custard sometimes goes lumpy, you need to know why it thickens. The traditional thickening method is to add an egg. The albumin proteins in eggs consist of long molecular chains that are folded into globules.

On heating, these chains unfold and bind together into a three-dimensional network. It’s the formation of this network that produces the thickening.

However, if you overdo the heat the coagulation of the chains becomes so extensive that they squeeze out the water, forming lumps.

Fortunately, help is at hand. Even Delia Smith confesses to using a teaspoon of cornflour in her ‘proper custard sauce’ to reduce the risk of it going lumpy. Cornflour is predominantly maize starch, which consists of two types of long-chain molecule (sugars). On heating, these form gels with water, which hold the sauce together. The sugars overlap with the albumin protein to form a gel rather than aggregate, and this prevents the lumps forming.

Find a recipe for custard, and have a go at making it both with and without cornflour.

So who first came up with the idea of a cornflour-based custard powder, and who invented baking powder? It was a chemist, Alfred Bird, who did it because his wife’s digestive disorder meant that she couldn’t eat eggs or yeast. It seems science and scientists are not so far removed from cookery after all.

WEBSITES

Fine Cooking Food Science
Science of Cooking

READING

Coultate T.P., Food: The chemistry of its components, Royal Society of Chemistry paperbacks, 2nd edn., 1989 ISBN 0 8518 6433 3

Kilgour O.F.G., Complete Catering Science, Heinemann, 4th edn., 1986 ISBN 0 4349 1051 1

McGee H., On Food and Cooking: the science and lore of the kitchen, Fireside, 1997 ISBN 0 6848 4328 5

Pollock S. and Marshall J., Help Your Child With Science, BBC Books, 1992 ISBN 0 5633 6215 4

Selinger B., Chemistry in the Marketplace, 5th edn., Harcourt Brace, 1998 ISBN 0 7295 3300 X

FURTHER GENERAL READING

Here are some books and articles that you may want to try and get hold of:

Barrow J. D., The Artful Universe, Oxford University Press, 1995 ISBN 0 1985 3996 7.
A quite remarkable book that will change the way you view the world. Extremely accessible.

Burton et al., Chemical Storylines, G. Heinemann Educational Publishers, 1994 ISBN 0 435 63106 3.
Part of the Salters Advanced Chemistry course, which explores the frontiers of research and the applications of contemporary chemistry. For A level and other science courses aimed at 16 to 19-year olds.

Fraser A. and Gilchrist I., Starting Science (Book 1), Oxford University Press, 1998 ISBN 0 19 914235 1.
Part of an integrated science course for the National Curriculum Key Stage 3 and Scottish Environmental Studies (science) for S1 and S2.

Northedge A. et al., The Sciences Good Study Guide, The Open University, 1997 ISBN 0 7492 3411 3.
Indispensable for students of science, technology, mathematics and engineering. Packed with practical exercises and activities, all aimed at making studying more enjoyable and rewarding. Lots of hints and tips for those returning to study.

Selinger B., Chemistry in the Marketplace, 5th edn., Harcourt Brace, 1998 ISBN 0 7295 3300 X.
An excellent and informative reference source for all kinds of real-life applications of chemistry. Explores the world of chemistry that surrounds us in our daily lives, explained in terms that everyone can understand. ‘Makes chemistry come alive.’

PS547 Chemistry for Science Teachers course materials, The Open University, 1992
A course designed for use by science teachers from a wide variety of backgrounds, with varying experience of teaching science. A familiarity with some basic science (perhaps physics or biology) is assumed, but little understanding of chemistry is required. The mathematical understanding needed for the course is not great.