Cooking and its Effect on the Nutritional Value of Food
Some people believe that there is no need to cook food, they argue that animals manage
to survive on raw or uncooked food, so why should not humans also? Furthermore, in recent
years there have been attempts to treat some diseases by encouraging the eating of raw foods,
and this method has been quite successful. However, although it is quite pleasant, and safe, to eat
foods like fruit and some vegetables raw, rice, cocoyam or beans in their natural state would be
tasteless or nauseating to eat. On the other hand, badly cooked food can lead to the loss of
nutrients and to malnutrition, and it is part of the nurse's duty in nutrition counselling to make
people aware of this.
One of the main reasons for cooking food is to make it safe by destroying bacteria, their
toxins and the ova of parasites. Cooking also delays the decomposition of food so that it lasts
longer. Furthermore, it adds flavour to food and can enhance its nutritional value. For example,
by adding onions, tomatoes and salt to meat and frying them in oil, mineral salts, vitamins and
fats are added, which make the meat both more palatable and more nutritious. Finally, cooking
makes it easier for the body to digest and absorb the food. Some foods, like eggs, cannot be
absorbed when eaten raw.
Methods of cooking. There are many ways in which food can be cooked. Boiling,
stewing, steaming and pressure cooking make use of added moisture for cooking, while roasting,
baking, grilling, smoking and toasting do not require additional moisture. Frying, on the other
hand, uses oil. The method of cooking must be chosen carefully so as to achieve the most
appetising and nutritious results.
Generally speaking, most forms of carbohydrates, starches, sugars and cellulose require
cooking to make them more easily digested and absorbed by the body. For example, by boiling
yams or rice, the starch content is converted to a form which makes it possible for the enzyme
ptyalin to act on it during digestion. However, although cooking is necessary for the absorption
of starch, heat-unstable vitamins, like thiamine, may be lost. It is important to note that the
baking of bread causes a 5% loss of thiamine, and that toasting bread causes a further 15-30%
loss, depending on whether the slice is a thin or a thick one.
Similarly, cakes and biscuits lose an appreciable amount of thiamine during the cooking
process.
Sugars become more easily absorbed after being cooked, and cellulose is softened and made
more digestible. Unfortunately, in the case of the cellulose obtained from vegetables, much of
the thiamine and vitamin C content is lost into the water used for cooking; there may be as much
as a 70% loss of vitamin C. However, if vegetables are put directly into boiling water, the
vegetable enzymes which destroy vitamin C during cooking are themselves destroyed. In these
circumstances vitamin C loss is greatly reduced.
Meat and fish. Coagulation of proteins occurs during cooking. This causes meat to shrink;
the colour change which also occurs is due to decomposition of the blood in meat. Cooking has
little effect on the nutritional value of fats, proteins and the vitamin A content of meat. However,
its mineral salts pass into the water used during cooking. Also, about 30% thiamine, 10%
riboflavin and 10% niacin are lost. The cooking of fish results in 40% loss of mineral salts. The
water in which the food is cooked should be used for soups and sauces to retain some of these
losses.
Eggs are most easily digested when lightly cooked; hard boiled eggs are less digestible,
and raw egg cannot be absorbed. However, 5-15% thiamine and 5-15% riboflavin are lost after
boiling, scrambling and frying. The iron content of the egg yolk causes the colour of the yolk to
darken after the egg has been hard boiled.
Milk is first pasteurised to make it safe for human consumption. This causes a 10% loss
of thiamine and a 20% vitamin C loss. On the other hand, sterilisation of milk causes a 50% loss
of vitamin C and a 30% loss of thiamine. Evaporated milk is sterilised at a temperature of 45°C
after the water has been removed at a low temperature, resulting in 60% vitamin C and 40-90%
thiamine loss.
Decomposition of fats takes place at a very high temperature and causes irritating odours
and fumes. Fats in this state are indigestible. Some communities in Nigeria believe in
overheating palm oil before using it so that it resembles groundnut oil in colour. Suchpractices
should therefore be discouraged to prevent decomposition of the fats.
When cooked in 'hard' water, which contains calcium, the calcium content of food can be
doubled. Calcium is otherwise not usually affected by cooking. Sodium is lost if food is boiled
but this is not important as sodium chloride can be added to the food just before it is eaten. Iron
is more readily absorbed when it is cooked. When meat is cooked, the iron content escapes into
the meat water and will be lost if the water is thrown away. However, the use of iron cooking
pots can increase the iron content of food to an alarming and dangerous degree. Potassium and
magnesium are lost in moist cooking.
Vitamin C and thiamine are the most unstable vitamins, and are lost during food
preparation and cooking. The best way to preserve vitamins in vegetables is to put the vegetables
in boiling, salted water. In this way the enzymes which destroy the vitamins are rendered
ineffective by the heat of the water. Foods that are exposed to too much light lose their riboflavin
content.
Generally speaking, long periods of cooking and the rewarming of food should be
discouraged, as this makes the food less nutritious. It should be remembered that nutrients that
are lost by cooking can be made up by eating other food items which contain them, but which do
not require cooking, such as fruit and salad vegetables.