This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
The losses that occur in cooking fruits and vegetables are of three types: first, the losses due to volatile substances; second, the losses due to the solubility of some of the substances of the fruit or the vegetable in water during preparation and cooking, and the discarding of this water; third, the losses due to the destruction of some substances by heating.
The volatile loss is composed largely of water but includes some acids and substances that give flavor and odor. The greatest part of the volatile loss can be replaced by the addition of water. The other volatile substances affect the flavor more than the food value and will be considered later.
Water in which fruit is soaked and cooked is seldom discarded, so that this type of loss pertains more to the cooking of vegetables. Vegetables contain the following food nutrients: protein, fat, carbohydrates, minerals, and vitamins. Decrease of these nutrients during cooking is due largely to their loss. Vegetables contain little fat, thus fat loss in their cookery is relatively unimportant. The starch from vegetables is insoluble in water. The loss of starch from vegetables occurs when the cell walls are broken by cutting in preparation, disintegration through over-cooking or from abrasion, which may occur in violent boiling and cause the sloughing off of parts of the vegetable near the surface.
Nitrogen. The nitrogenous or nitrogen-containing part of fruits and vegetables may be classified for convenience as composed of proteins and non-proteins. The nitrogenous content of vegetables, with the exception of the legumes, is low. The albumins are soluble in water and dilute salt solutions, the globulins in dilute neutral salt solutions. They are both coagulable by heat. Their greatest loss would occur when the vegetable is soaked before cooking, or when cold water is added to the vegetable to start the cooking process. In salted water some of the albumins and globulins would be dissolved.
McKee and Smith in their investigations of the protein of the cauliflower found that 68 per cent of the nitrogen of the edible part belongs to constituents soluble in water or dilute salt solution; 12 per cent to compounds soluble in 0.3 per cent sodium hydroxide, but not soluble in water; and 16 per cent to substances insoluble in either water or dilute alkali. Their results show a large proportion of the nitrogen compounds soluble in water and dilute salt solution, indicating that it is preferable to start cooking in boiling water to coagulate the albumins and globulins.
Some of the nitrogenous substances are soluble at all temperatures. The foaming of the cooking water is partially due to these substances, and partially to tannins and saponins. All these substances lower the surface tension of the cooking water and favor foaming. Foaming occurs in the cooking of some vegetables, such as peas and asparagus, more than in others.
The important changes in the cookery of vegetables come from the loss of sugars, vitamins, and minerals. The sugars are very soluble in cold and hot water. Their loss is lessened if the vegetable is left in large pieces, which give less surface for contact with the water than small pieces.
Vitamins. The loss of vitamins in cooking may come from destruction by heat or by oxidation as well as loss in the discarded cooking water.
Mineral salts. The principal minerals in foods that may be lost during cooking are the salts of sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), phosphorus (P), sulfur (S), iron (Fe), chlorine (CL), and iodine (I). Sodium, potassium, magnesium, and calcium are a valuable part of the mineral content of fruits and vegetables, as an excess of this group over the phosphorus, sulfur, and chlorine group produces an alkaline ash reaction in the body after the food nutrients have been absorbed from the intestinal tract. Potassium particularly is found in high percentage in most vegetables. Most of its salts as well as those of sodium are quite soluble in water so that their loss in discarded cooking liquor may be high. As a rule, the calcium salts are not so soluble as the other salts found in vegetables; in some vegetables they seem to be in an insoluble form, but in others the calcium loss is about one-fourth of the total calcium of the food. Magnesium salts are more soluble than those of calcium. Vegetables, with the exception of the legumes, contain small percentages of phosphorus when compared with some other foods. The phosphorus salts are easily soluble, and the phosphorus loss in discarded water may amount to 50 per cent or more of the total phosphorus of the vegetable.
The solubility of most of the salts found in vegetables increases with increasing temperature of the water, so that at 100°C. they are more soluble than at ordinary temperatures.
Peterson and Hoppert, using medium and large quantities of water for cooking vegetables, report the loss of calcium to be practically nothing in spinach. In other vegetables the loss is about 30 per cent of the total calcium. They report that the magnesium loss varies from 20 to 45 per cent, the phosphorus from 20 to 50 per cent. Berry reports as high as 60 per cent for magnesium and 52 per cent for phosphorus.
The following losses for cooking vegetables in water are reported in U. of Minn., Agri. Expt. Sta. Bull. 54, and in U. S. Dept. Agri. Farmers' Bull. 73.
Table 13 Losses in Cooking Vegetables Various Ways in Water
Vegetable and method
Loss of total nitrogenous matter, per cent
Loss of total mineral, per cent
Loss of total sugar, per cent
Potatoes peeled and soaked several hours before cook-ing......................
Potatoes started to cook in cold water...............
Potatoes, cooking started in boiling water.............
Potatoes cooked in skin......
Carrots cut in small pieces. . .
Carrots cut in medium pieces
less than 42
less than 47
Carrots cut in large pieces. . .
Cabbage, started in hot water
Cabbage, started in cold water
Iron. The iron of foods is very important nutritionally, because of the fact that many dietaries do not contain the minimum daily amount recommended by Sherman. The loss of iron in cooking should be avoided. Iron salts are quite soluble. Blunt and Otis report the following losses of iron when the vegetables were cooked in water: spinach 43 per cent; string beans 39 per cent; navy beans 32 per cent; peas 36 per cent; and potatoes 15 per cent. Peterson and Hoppert report from 20 to 30 per cent iron loss for various vegetables. Morgan reports 46 to 50 per cent loss for canned peas.
Table 14 Iron Loss in Salted and Unsalted Water (Steinbarger)
Unsalted water, per cent
Salted water, per cent
Potatoes cut in halves.................
Steinbarger reports the loss of iron as greater in salted than in unsalted water. Table 14 is from her results.
The vegetables given in Table 14 were cooked the following lengths of time: potatoes and onions 20 minutes; green beans 90 minutes; and the others 30 minutes. The results suggest some connection between iron salts and the globulins. The iron salts of potatoes have been reported as more concentrated near the surface, which may account for their high iron loss.
Losses due to method of cooking and preparation. The losses through cooking of vegetables depend upon several factors, which include: (1) the method of cooking, (2) the nature of the vegetable, (3) its preparation, (4) the length of time of cooking, (5) the temperature of cooking, (6) the quantity of cooking water, (7) whether started in hot or cold water, and (8) the reaction of the cooking medium. Some of these have been considered under nitrogen and vitamin losses.
Baking results in small nutritive losses.
All investigators report small losses in steaming; this seems to be an ideal way of cooking vegetables whose color and flavor are not altered by this method.
Masters and Garbutt report practically no loss in cooking vegetables in water, if the amount of water added is small and is evaporated just to dryness at the close of the cooking period. They designate this as the conservative method of cooking. If it is not practicable to evaporate the water to dryness, the use of as small a quantity of water as possible seems preferable to using a very large quantity.
Leafy vegetables like spinach offer a larger surface area for loss than the compact ones like carrots. Soaking in water before cooking may increase the losses, and vegetables like carrots and potatoes, when cooked whole, in their skins, or left in large pieces lose less than if cut in small pieces.
Loughlin reports that the amount of the sugars, the vitamins, and mineral salts dissolved in the cooking water is increased if a large amount of liquid is used, if much cut surface is exposed, and if the cooking time is prolonged.
Thompson reports that the total loss of solids, ash, and iron is greater in vegetables that are over-cooked than in those just sufficiently cooked.