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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 percent.

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.

References:

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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