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Nutrient retention in foods
Posted by Carol
Nutrients in foods, especially fruits and vegetables, vary greatly in their
stability. Some nutrients are not affected to any great extent by ordinary
handling. Other nutrients, such as vitamin C, vitamin A, thiamin, folate,
and potassium are readily lost from some foods, although stable in others.
The amount of nutrients left in cooked vegetables, in some cases such as the
water soluble vitamins, depends upon how much water they are cooked in and
how long they are cooked. That is why it is best to properly store all
vegetables, eat some raw ones. Cooked vegetables retain more nutrients if
cooked in only about 1/4 cup or less liquid, covered with a tight fitting
lid, and cooked ONLY until they are "tender crip." However, the lid should
be left off the cabbage family vegetables for 5 to 10 minutes to allow the
sulfur to escape before covering them.
Sometimes there are more nutrients left in canned or frozen vegetables than
in fresh ones if the fresh ones were not stored properly or if kept too long
before in the store OR AT HOME before using. Commercially, the vegetables
are harvested at their peak and processed immediately (some begins even in
the fields now); they have the maximum level of nutrients at this time and
few nutrients are lost in processing, although frozen ones are slightly
better in nutrient retention if held at 0 degree F. and if not held too
long. All begin to lose nutrient value after being harvested, but those
left too long before using have fewer nutrients left.
Vitamin C is soluble in water; air and heat hasten its loss. Conservation
of vitamin C is often used as an index to the retention of other nutrients,
because vitamin C is more easily destroyed than other food nutrients.
Measures that protect vitamin C, such as an acid medium in food preparation,
usually protect other nutrients. Important nutrients that vegetables can
contribute include vitamins and minerals. The vitamins include: vitamin C
(ascorbic acid), thiamin, riboflavin, niacin, pantothenic acid, viamin B-6,
folate, viamin A, and vitamin B-12. Mineral elements include calcium, iron,
magnesium, phosphorus, potassium, zinc, copper and manganese. They also
contain some other compounds which may play a role in prevention of cancer
and heart disease.
In general, freshly harvested vegetables have a higher vitamin content than
those held in storage. However, proper storage of fresh vegetables helps to
conserve most of their original nutrient values. The length of time fresh
vegetables are stored, as well as storage temperature and humidity, affects
retention of thier nutrients.
Vegetables such as broccoli, turnip greens, and salad greens, need to be
refrigerated in the vegetable crisper or in moisture-proof bags. They keep
thier nutrients best at near-freezing temperatures and slightly higher humidity.
Frozen vegetables stored at zero degrees lose from 1/3 to 3/4 of their
vitamin C content if stored for a year. Canned vegetables lose only 10
percent if stored a year at 65 degrees. When the temperature is 80 degrees
F., losses may reach 25 percent per year.
Carotene, a precursor of vitamin A, is well retained in canned vegetables.
Losses average only about 10 percent in a year when cans are stored at room
temperature. Canned tomato juice, a particularly stable, year around source
of carotene, shows no loss of this nutrient.
Thiamin in canned vegetables is well retained when stored for 1 year at room
temp. When stored for one year at higher temps, losses may increase to 25
Vitamin C, thiamine vitamin B-6 and pantothenic acid are found equally in
solid and liquid of canned peas. After one year of storage, canned peas,
liquid and solids, retained about 80 percent vitamin C and thiamiln, while
retention for for other vitamins rang from 90 to 100 percent.
If the amount of water used in cooking cabbage equals about 1/3 the amount
of cabbage, 90 percent of the vitamin C will be retained. When a large
amouint of water is used, such as 4 times as much water as cabbage, the
retention of vitamin C drops to less than 50 percent.
Minerals in vegetables cooked in the microwave oven are 80 to 100 percent
retained; minerals in vegetables cooked in a minimum amount of water are 90
to 100 percent retained. Potassium retention in most vegeetables ranges
from 90 to 100 percent except for microwave-cooked green leafy vegetables,
which have a potassium retention of 80 percent.
Most cooked vegetables, including microwave-cooked vegetables, have vitamin
retentions of 80 to 100 percent. Green leafy vegetables, such as spinach,
have 60 percent vitamin C retention when cooked in water and 25 percent
vitamin C when microwave-cooked. Roots, bulbs, and vegetables of high
starch or sugar content, including carrots, green peas, lima beans, and
squash, retain 70 percent vitamin C when boiled in water and retain 80
percent vitamin C when cooked in a microwave oven. Other vegetables, such as
asparagus, broccoli, cabbage, and snap beans, retain 80 peracent vitamin C
with cooking. Baked, boiled, or stewed tomatoes have highly acid pH, which
protects the vitamin C at 95 percent retention.
Potatoes and sweetpotatoes retaim 80 percent of their vitamin C with boiling
and baking. Frozen potatoes which are heated, such as baked stuffed
potatoes and french fried potatoes, also have 80 percent retention. Vitamin
C retention in microwave-cooked sweetpotatoes is 100 percent.
Folate retention for green leafy vegetables when cooked in water is 60
percent and 100 percent when microwave-cooked. Rotes, bulbs, and
vegetables of high sarch content retained 70 percent folate when cooked
inwater and 90 percent folate when cooked in a microwave-oven. Other cooked
vegetables, including asparagus and tomatoes, retained 70 percent folate.
(New research shows that folate may play an important role in helping to
prevent heart disease.)
Potatoes and sweetpotatoes, whether baked or boiled in the skin, retained 85
percent folate. Boiled without skin and fried potatoes regtained 785
percent folate, while hash-browns only retained 60 percent of thier folate.
The folate retention of microwave-baked sweetpotatoes is 100 percent.
Vitamin A retention of vegetables ranges from 80 to 100 percent with
cooking. Green leafy vegetables, cooked in water and microwave-cooked, have
about 100 percent vitamin A retention. Root/bulbs have a 90 peracent vitamn
A retention when cooked in water and an 80 percent vitamin A retention when
microwave-cooked. Other vegetables retain 90 percent vitamin A when cooked
in water. Sweetpotatoes retain 90 percent to 95 percent vitamin A whether
they were baked or boiled in thier skins or cooked in the microwave-oven.
I don't know of any vegetables except the oil of corn and soybeans,
that would contain essential fatty acids or EFA's
(linoleic acid, arachidonic acid, and linolenic acid), in them.
The primary source is the meat, poultry, fish, eggs and nuts food group and
the fat food group. Nearly all diets supply enough EFA to meet the
required amount. Deficiencies are usually seen only in infants fed a
formula that lacks EFA and in hospitalized patients who have been fed
through a vein for a long time with a formula lacking it. Even in an
otherwise totally fat-free diet, only 2 teaspoons of corn oil (which is 50
percent EFA) would be adequate to meet the amount of EFA for an adult.
However, such a low amount of fat is not recommended. U.S.D.A. recommends
that our diets contain 25-30 percent fat.
More than 100 body compounds, beside linoleic acid, plus other oils,
vitamins, minerals and hormones are needed to ensure health. No single food
has magical, miraculous, or curative powers, but a well balanced diet and
sufficient exercise help most people to stay healthy.
McCarthy, M.A. and Matthews, R.H., Conserving Nutrients in Foods. United
States Dept. of Agriculture, Human Nutrition Information Service, Nutrition
Monitoring Division, Administrative Report No. 38l4, 1988.
E.N. Whiteney and E. Hamilton (1987) UNDERSTANDING NUTRITION. West Publ.
Co., New York, NY.
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The information presented is intended to be used for educational purposes only. The statements made have not been evaluated by the Food and Drug Administration (U.S.). This product is not intended to diagnose, treat, cure or prevent any condition or disease. Please consult with your own physician or health care practitioner regarding any suggestions and recommendations made.