Explain how cooking affects the nutrients in food and describe ways to conserve nutrients during preparation and cooking

What this dot point is asking

The syllabus wants you to explain how cooking changes the nutrients in food - sometimes for the better, often reducing vitamins - and to describe practical ways to keep more of them. The central idea is that cooking is a trade-off: it makes food safe and digestible but can destroy or wash out fragile nutrients, so a careful cook minimises the loss.

The answer

Effects on vitamins

Vitamins are the nutrients most affected by cooking, and the water-soluble ones (vitamin C and the B group) are the most vulnerable:

• Leaching: water-soluble vitamins dissolve out of food into the cooking water, which is often thrown away.
• Heat destruction: vitamin C in particular is destroyed by prolonged heat and by exposure to air.
• The longer, hotter and wetter the cooking, the greater the loss. Fat-soluble vitamins (A, D, E, K) are much more stable.

Effects on protein

Heat denatures protein (changes its shape) and then coagulates it (sets it). This is a useful change: it sets an egg, firms meat and fish, and makes some protein more digestible. But overcooking can make protein tough, shrunken and harder to digest, as in overcooked meat that turns chewy.

Effects on carbohydrates

Heat gelatinises starch, swelling and softening it so it is digestible and able to thicken sauces. Dry heat can caramelise sugar (browning and flavour) and brown starchy foods. These are mostly positive changes that improve texture and palatability, though burning is undesirable.

Effects on fat

Heat melts solid fat to liquid and helps brown and crisp fried food. At very high temperatures fat can break down, smoke and develop an unpleasant flavour. Frying also adds fat (and energy) to food, while grilling and roasting let some fat drain away.

Effects on minerals

Minerals are fairly stable, but some, like water-soluble vitamins, can leach into cooking water. Using that water in a dish recovers them.

Conserving nutrients

To keep the most nutrients, especially vitamins:

• Prepare vegetables just before cooking; do not cut them too small or soak them.
• Cook in little water, or steam or stir-fry rather than boil.
• Cook for the shortest time and avoid keeping food hot for long.
• Use the cooking water in soups and sauces.
• Eat suitable foods, such as fruit and salad, raw.

Examples in context

Example 1. Steamed versus boiled broccoli. Broccoli steamed for a few minutes keeps most of its vitamin C and stays bright, while broccoli boiled hard for ten minutes loses much of its vitamin C into the discarded water and turns dull and soft. The same vegetable, cooked two ways, shows the impact of method on nutrients.

Example 2. Stir-fried mixed vegetables. A quick wok stir-fry of mixed vegetables uses high heat for a short time with little liquid, so it preserves vitamin C well while improving flavour. This is why stir-frying is both a tasty and a nutrient-friendly choice for everyday Singaporean cooking.

Try this

• Cue. Explain two ways cooking lowers the vitamin C in vegetables. Recall leaching into the cooking water and destruction by prolonged heat and air.
• Cue. Describe how cooking changes protein, with an example. Recall denaturation and coagulation that set the food, such as an egg white turning firm.
• Cue. List four ways a cook can conserve nutrients. Recall little water, short cooking, last-minute preparation, and reusing the cooking water.