Helen Money, Nutritionist
Preparing and cooking food alters nutritional composition. Cooking food does generally lower vitamin content but to varying extents.
Vitamins are categorised into two groups: 
fat soluble and water soluble. Fat soluble vitamins (A, D and E) show reasonable stability, with the largest losses occurring when frying at high temperatures for long durations of time. Tests show that retention of vitamin A when exposed to heat varies from 65–100%; retention of vitamin D, 60–100%; whilst retention of vitamin E is much lower at 30–50% (1).
Water soluble vitamins (B complex and C) are less stable. Water soluble vitamins ‘leak’ from foods when they come into contact with water. This is clearly visible when boiling beetroot or green vegetables. Any contact with water will reduce the water soluble vitamin density of a food, including when washing them, but losses become significant when heat is added. Any moist method of cooking will destroy vitamins, including steaming, but to a greater extent when boiling.
The table below shows the loss of vitamins C, B1, B2 and B6 using various cooking methods (2).
|
Cooking method |
Percentage loss of vitamin C, B1, B2 & B6 |
|
Boiling |
35 – 60% |
|
Roasting |
10 – 47% |
|
Steaming |
10 – 25% |
|
Microwave |
5 – 25% |
|
Stewing |
10 – 12% |
|
Frying |
7 – 10% |
|
Pressure cooker |
5 – 10% |
Whilst cooking food in most cases reduces micronutrient density there are some exceptions. For example, the nutritional value of carrots actually increases when cooked. Additionally cooking has many important functions in the diet – making food more palatable, easier to digest and increasing the range of foods eaten.
Both fat soluble and water soluble vitamins are sensitive to light, water soluble vitamins more so. Chopping, juicing and pureeing all increase the surface area of a food leading to higher levels of light exposure and therefore vitamin loss.
Ways to reduce vitamin loss from food include preparing food only when it is about to be eaten, where possible eating raw, using cooking methods that reduce cooking time duration, leaving the skin on when cooking and, when preparing food cut into large chunks rather than small.
References
1. Hrncirik K. Stability of fat-soluble vitamins and PUFA in simulated shallow-frying. Lipid Technology; 22(5):107-109
