Why food is cooked - to make it safe, palatable, digestible and varied - and how cooking and preparation affect the nutritional value of food, including the effect of heat, water and air on vitamins.

What this dot point is asking

AQA wants you to give clear reasons why we cook food and to explain how cooking and preparation can both improve and reduce nutritional value. You should link a preparation step to its effect on nutrients.

Why we cook food

Cooking also extends shelf life for some foods and develops appealing flavours through browning reactions such as the Maillard reaction and caramelisation.

How cooking affects nutrients

Cooking can have both positive and negative effects:

• Positive - heat makes some foods safer and easier to digest, and can make some nutrients more available.
• Negative - water-soluble vitamins (the B group and vitamin C) are easily destroyed by heat and dissolve into cooking water. The longer and hotter the cooking, the greater the loss.

Keeping nutrients in

To minimise nutrient loss when preparing and cooking:

• Steam rather than boil, or use only a little water and use it in a sauce.
• Cook for the shortest time at the right temperature.
• Prepare just before cooking and avoid cutting vegetables into very small pieces too early (more surface area means more oxidation).
• Serve promptly, as keeping food hot for a long time destroys more vitamins.

The science behind this is that vitamin C and the B vitamins are water-soluble, so they dissolve into cooking water, and they are also heat-sensitive and break down at high temperatures or over long cooking. They oxidise in air, so cutting vegetables small and early exposes more surface area and loses more vitamin C. By contrast, the fat-soluble vitamins (A, D, E and K) are far more stable and survive normal cooking. Knowing which nutrients are vulnerable lets you justify a preparation choice, for example steaming broccoli briefly rather than boiling it for a long time.

Cooking is not only about loss. Heat gelatinises starch and softens fibre and connective tissue, making energy and nutrients easier to digest and absorb, and it destroys some natural toxins and anti-nutrients (for example in raw kidney beans), making certain foods safe to eat. So the honest picture, and the one examiners reward, is that cooking both improves and reduces nutritional value depending on the food and the method.

Try this

Q1. Give two reasons why food is cooked. [2 marks]

• Cue. To make it safe, palatable, digestible or to add variety.

Q2. Explain why vegetables should be prepared just before cooking. [2 marks]

• Cue. To reduce vitamin C loss through oxidation in the air after cutting.