Effects of Cooking and Preservation on Food Values

New Zealand Journal of Agriculture, Volume 81, Issue 1, 15 July 1950, Page 93

Effects of Cooking and Preservation on Food Values

By

EVELYN E. MOORE,

Rural Sociologist, Department of Agriculture, Palmerston North.

THAT the preparation and cooking of food are two of the housewife’s major tasks and, many would aver, the most interesting is born out by the fact that cookery books, according to most booksellers, are always among the best sellers. It is , not sufficient only to be a good cook, however, for to provide a family with a healthful diet some knowledge of the most nutritious foods and the best way to cook them to preserve their value is also necessary. In fact, the proverbial remark that half the digestive ills of the world are caused by good cooks and the other half by bad cooks serves to emphasise the three major influences of cooking on foodits pronounced effects on palatability, digestibility, and nutritive value. IN recent years a realisation of the importance of good nutrition in family health has resulted in the effect of cooking and preservation on the nutritional value of foods being much in the news, and has even produced a serious-minded cult of people who consider eating most of their food raw essential to health. That, of course, is unnecessary and, if carried to extremes, even harmful, for though poor cooking methods undoubtedly detract from the nutritive value of foods, good cooking often enhances it if factors other than a slight inevitable loss of some vitamins are considered. Therefore it is desirable to know which methods of preparing and cooking foods best conserve the food value, and also, as the use of food preserved by bottling, canning, drying, or freezing has become common, whether their effects on nutritive value are sufficiently great to make it advisable to allow for losses by serving more of other foods which are good sources of the lost nutrients. • Most housewives have heard of the deleterious effects of peeling, soaking, and other methods of preparation on the nutritive value of vegetables and fruits, losses in the preparation of other foods usually being insignificant. The statement that “All the goodness lies in or just under the skin or in the outside leaves of green vegetables” is made very frequently, but results of the work of experienced investigators show that this . statement requires considerable modification.

Peeling or Discarding Outside Leaves It has been shown that the outside leaves of most green vegetables and the peel of root vegetables and fruits do not contain any nutrients not present in the flesh or heart (excepting green outer leaves .of cabbage and lettuce, which are much better sources of vitamin A than the colourless hearts) and may even contain less of some nutrients, and that where the peel is a richer source of nutrients than the inside flesh, the flesh usually contains three-quarters or more of the nutrients concerned. < Therefore, when practical considerations are against using outside leaves or unpeeled vegetables—for example, when potatoes suspected of being infected with blight must be used irreplaceable nutrients are being lost. Potatoes boiled in their skins have a higher vitamin C content only if the skin remains intact, so preventing the vitamin from leaking out during cooking. When they are cut up, or when the skin has burst, they do not retain significantly more of this vitamin, and presumably of other nutrients, than potatoes peeled before being boiled. Potatoes cooked in other ways with or without being peeled, have about the same nutritive value per unit of weight. Avoidance of Waste However, most dietitians would advise that in the interests of economy and the preservation of food value there be as little waste as possible in the preparation of vegetables and fruits. Potatoes and other root vegetables should be scrubbed, cooked, and, if desired, peeled after cookery, and green vegetables and fruits should be used in season and as fresh as possible when there is least possibility of waste. Out-of-season vegetables and

fruits, as well as being more expensive, are often less nutritious and more wasteful to prepare than the same foods bought or grown to be ready during the normal period. Soaking, Grating, and Mincing At one time preliminary soaking of vegetables was thought to cause a very great loss of the minerals and vitamins, but it has been shown that, unless root vegetables are left soaking for a long time or are cut up finely or shredded, the losses are comparatively small. If necessary, the housewife may peel potatoes in the morning for dinner and leave them soaking in salt water to prevent the surfaces going brown, but green vegetables should be washed and kept fresh in a damp cloth. Minced, grated, or very finely shredded vegetables should be prepared as close to the cooking or meal time as possible, for enzymes—stances set free when plant cell walls are damaged—destroy the vitamin C, rapidly in green vegetables and slowly but steadily in root vegetables. Grated vegetables used in soups should be cooked immediately in boiling water or stock, for this destroys the enzyme and cooking losses are then comparatively small, though the amounts of vitamins B ( and C dissolved in the cooking water are considerably greater. Therefore, if the water is to be discarded, vegetables should not be cut too finely; for example, one group of investigators found that 12 per cent, of the vitamin C of runner beans cut in lin. lengths was dissolved in the cooking water and 36 per cent, was lost from beans sliced finely in a bean slicer. Effects of Cookery on Nutrients Many foods are acknowledged to be quite unfit for human consumption in their natural state and therefore must be cooked, or processed before being eaten. Whether this habit of cookery has been extended to include many

foods which would be better eaten raw has been a subject for controversy, and a great many experiments have been carried out to test the claims of those who favour the use of more uncooked foods, especially after some vitamins were found to be partially lost in cooking processes. Many of those experiments showed the beneficial effect of cookery on the digestibility and absorption of other nutrients, so that a consideration of all the nutrients is necessary in any impartial discussion of the effects of cookery on food. Proteins As the main solid constituents of muscles and other body tissues, proteins are essential for health. Consequently the effect of cookery on them is just as important as the effect on vitamins. Though most proteins are solidified or coagulated by heat (egg white, for example), that does not affect their nutritive value and frequently improves their digestibility (boiled milk and softly boiled eggs are more digestible than the raw foods) unless cooking is carried near to charring point, as in the formation of a brown, crusty layer on roasted meat or crinkled edges on eggs fried at too high a temperature.

- It was once thought that meat for roasting should be seared well at a high temperature to keep the juices in and then cooked at a lower temperature, but this method causes more shrinkage and loss of weight and lower digestibility of the seared outer -crust than roasting at a constant temperature of 300 to 350 degrees F. The opinion that placing meat to be stewed or boiled in boiling water would seal the surface and retain more mineral salts and extractives in the meat also has been proved wrong, losses being about equal whether the cooking is started in cold or boiling water. Fats No changes take place in the nutritive value of fats when they are cooked, whether by deep- or shallowfat frying or by roasting, unless they are overheated considerably, when an irritating substance, acrolein, is formed. However, fried foods have gained a bad reputation as a cause of indigestion, and their nutritive value also is often regarded as poor, probably because few cooks realise that the' temperature of fat for frying is as important as, for example, the oven temperature for cooking sponges and meringues and that the wrong temperature can produce just as disastrous results. Fried foods properly

cooked and drained do not contain an undue amount of fat and are readily digested by most people, nor does fried food have a significantly lower nutrient content than the same foods baked, roasted, or grilled. > As the temperature in deep fat in a saucepan is much easier to control than that of shallow fat, many authorities recommend using deep fat always, for it is no more extravagant and, apart from the slightly longer time j required to heat the fat, it is as quick as using a frying pan. Starches and Carbohydrates The effect of cookery on starches and carbohydrates in the cell walls of vegetables is altogether favourable. In baked foods some of the raw starch is changed into easily digestible dextrins. The remainder, and the starch in steamed or boiled foods, is changed into the much more easily digested soluble starch. The cell walls of plants are also softened, making their contents available to the body. Some investigators have found that, when carrots are used as a source of vitamin A, smaller amounts of cooked ones than raw ones are needed by animals for satisfactory growth, probably because the vitamin A in cooked carrots is more easily digested and used.

Sugars Among the most easily digested and absorbed of all food nutrients, sugars probably are also the least affected by cookery. Cooking of acid fruit changes some of the cane sugar into glucose and fructose, or fruit sugar, which require no digestion at all, but as cane sugar itself is very easily digested this improvement normally is not of practical importance. Minerals Mineral salts are not destroyed by any of the ordinary cooking processes. In boiling or stewing some of the more soluble ones are dissolved out of the food into the cooking water, but these losses are unimportant compared with losses of vitamins .in the same cooking processes (the minerals dissolved out in greatest amounts fortunately are least likely to be lacking in most diets), so methods of cookery which conserve vitamins certainly retain the maximum amount of minerals. Vitamins Though they are only one of five groups of essential nutrients, vitamins assume great importance in cookery because they are most likely to be discarded or destroyed in poor cooking practices, especially with vegetables. The losses to guard against are, chiefly, those caused by the solubility of vitamin C and the B group of vitamins in water, those caused by the destruction of these vitamins by heat and the oxygen in air, and those caused by the action of soda and other alkaline substances. Vegetables and fruits are the only useful sources of vitamin C in the diet and, when cooked to conserve their vitamins, probably provide a quarter of the daily vitamin B t requirement, assuming that the amounts recommended for health are eaten. Particular attention to methods of cooking these foods considerably improves the nutritive value of the diet, for poor and careless cookery may destroy vitamin C. Therefore a method which reduces losses to a minimum, especially in the cookery of green vegetables, has been evolved. In it the following points are emphasised as most important for retaining food value: Garden-fresh vegetables should be ■ used when possible, cut up (but not too finely) so

that they will cook quickly, placed in a minimum of boiling salted water without soda, cooked rapidly, and served immediately; the cooking water should be used in soups, gravies, or sauces where practicable. This avoids as far as possible conditions which cause the destruction of vitamins B x and C. Cooking in a minimum of water reduces the amounts of vitamins dissolved in the cooking water. For example, one experiment showed that when green vegetables were boiled covered completely with water, half covered, and a quarter covered the losses of vitamin C into the cooking water - were respectively 80, 60, and 40 per cent. Rapid boiling produces a thick blanket of steam which excludes air. The addition of soda is not advocated because, though it reduces cooking times, it increases the destruction of vitamins B and C. In one experiment cabbage cooked with soda and kept hot for 10 minutes lost about 40 per cent, of its vitamin C, compared with 15 per cent, lost from cabbage cooked without soda during the time after cooking. Because even a 15 per cent, loss is worth avoiding if possible, it is recommended that vegetables be served immediately, especially if they have been mashed, for the loss is then much greater. In addition, cold cooked vegetables lose vitamin C during storage, . one investigator reporting an additional

40 per cent, loss from cooked potatoes stored for 24 hours. Vegetable dishes which make use of twice-cooked vegetables (especially if they have been stored overnight), such as scalloped vegetables, vegetable pies, stuffed baked potatoes, potato scones, and fried cooked vegetables (bubble and squeak), cannot be regarded as good sources of vitamin C, though they may be good sources of calories, minerals, protein, and some other vitamins. The effect of pressure cookery on the vitamin C value of vegetables has been investigated carefully since this method has become so popular. In no case has it been found to affect the nutritive value more adversely than the recommended method, and in one of the most recent experiments reported, using 10 different vegetables, the results showed that on the average more vitamin C was retained in vegetables cooked in a pressure cooker. Baking, steaming, and roasting are all satisfactory methods of cooking root vegetables, for though they may destroy slightly more of vitamins Bi and C than does boiling, there is no loss through solution in the cooking water. Consequently the amounts retained in the cooked root vegetables compare favourably with those in boiled or pressure-cooked root vegetables. However, because exposure to air is an important factor in the cookery of green vegetables, • these methods are not satisfactory for them, as a greater surface is exposed to the oxygen in the air, resulting in greater destruction of vitamin C.

The acidity of fruits partially protects their vitamins from destruction. In addition, fruit syrup is always used, so that losses of vitamins in fruit cookery are less important than those in vegetable cookery. Some experiments have shown losses in pies and baked fruit tarts to be considerable, however, and it seems probable that in flummeries and other desserts for which fruit is sieved or pulped and mixed with air some of the vitamin C is lost. The effect of cooking on vitamins in other foods is of less importance because they are not sources of vitamin C. Vitamins A and D are not destroyed readily, except by prolonged heat treatment, nor are they dissolved in cooking water. The vitamins of the B group with the exception of B x are also practically unaffected in the cookery of meat, cereals, and other foods. Because fruit and vegetables are the only useful sources of vitamin C in the diet, the effect of cooking on them has merited special attention. For the same reason, cereals, as the chief sources of vitamin Bp also deserve prime consideration. If high-extraction flour and wholemeals are used, they provide half or more of the daily requirement as flour, oatmeal, and similar products. Consequently, if losses in baking bread, scones, or cakes and cooking porridge were considerable, modifications in the menu would be necessary to ensure an adequate human intake of vitamin Bp Fortunately, experiments have shown the loss of vitamin B x in the cookery of bread, porridge, and milk puddings to be small —seldom more than 20 per cent, in yeast bread and 5 per cent, or less in porridge. On the other hand, baking powder, because it contains baking soda, causes appreciable destruction of vitamin Bp so that baking-powder breads, scones, and cakes, even when made with wholemeal flour, should not be allowed to replace too large a proportion of yeast breads in the diet. Another advantage of yeast breads is the destruction during fermentation of some phytic acid (a substance present in wheat which makes calcium unavailable to the body), no comparable destruction occurring in bakingpowder breads or " scones. Toasting bread or scones also causes losses of 12 to 20 per cent, of the vitamin Bp so the commonly held opinion that toast is more digestible and nutritious than bread requires modification. In meat cookery losses of vitamin B, tend to be fairly uniform whatever the method used —about half. Improvement in palatability and digestibility more than offsets this loss, for there are plenty of alternative sources of this vitamin, and cooked pork still provides a third and cooked bacon a sixth of the daily requirement per serving. Simple and efficient cookery has no very serious effects on the nutritive value of food. Some unnecessary destruction of nutrients may be caused in more elaborate recipes for desserts, party savouries, rusks, biscuits, hard-baked toast, and foods cooked for needlessly

long times, but when these predominantly energy foods are not allowed to replace the vegetables, milk, eggs, meat, fish, sandwiches, salads, and other constituents of a well-balanced diet they have a useful place in the menu. Other Methods of Processing Food Drying, bottling, canning, freezing, and preserving with salt, vinegar, or sugar are other present-day methods of processing foodstuffs. Dried Foods Any foods may be dried, and many, especially cereals and legumes, are in fact naturally dried seeds, but the deleterious effect of drying on the flavour of most foods limits their popularity. On the whole, nutrients are well retained, but vitamin C is destroyed almost completely in commercial or home-dried fruits and vegetables. When they are sulphured to improve the colour some vitamin C is preserved, but at the expense of vitamin B. Consequently dried fruits cannot be regarded as substitutes for fresh fruits except for many culinary purposes. They are useful sources of calories or energy, and by adding flavour to desserts they provide a welcome addition to the winter menu. The nutritive value of dried milk and eggs when they are reconstituted is comparable with that of the fresh foods, for, apart from a slight loss of vitamin B x , their -nutrients are well retained. Indeed, dried milk may be of greater value'than fresh milk for infant feeding as it forms much finer and softer curds in the stomach and therefore is more digestible. Frozen Foods As a method of food preservation freezing has probably the greatest possibilities of expansion, for, provided it is carried out properly, it preserves food with a flavour more closely approximating that of the fresh food than any other method yet developed. As a , means of transporting meat, butter, and other perishable foods refrigeration has been of vital importance to New Zealand’s economy, and the high quality of such foods is acknowledged widely. Their nutritive value, too, is usually indistinguishable from that of the fresh food. Frozen fruits and vegetables are comparative novelties to many New Zealanders, though if the American lead is followed, many more people ■ are likely to obtain home freezers and the variety and quantity of frozen foods on the market will increase steadily. Consequently, such foods may become important factors influencing the. nutritive value of diets, though their present cost is such that they are luxury foods. Their nutritive value has yet to be investigated thoroughly, but from preliminary reports it seems possible that, like foods preserved in other ways, they lose some vitamins during preparation and storage. For that reason their vitamin C content . usually is slightly inferior to that of high-quality, freshly cooked vegetables, though they are equally good sources of other nutrients.

For maximum retention of vitamin C frozen vegetables should be defrosted and cooked immediately, for losses of this vitamin occur rapidly after thawing; thawed peas, for example, have been found to lose 27 per cent, of their vitamin C in an hour. Vitamin C may also be lost during the storage of frozen vegetables if they have not been blanched before being frozen and if the storage temperature is allowed to rise above zero F., losses of up to 90 per cent, having been reported. Therefore great. care is necessary in their preparation, storage, and cookery if their nutritive value is to be comparable to that of freshly cooked food. Canned and Bottled Foods One of the best methods for the preservation of foodstuffs is still canning or bottling. The. carbohydrate, fat, and protein content of such foods compares favourably with that of the freshly cooked foods, and the palatability of most canned foods now on the market is good. In fact, many people prefer tinned pineapple, peaches, and apricots to the fresh fruit. A comparison of their retention of vitamins B t and C with that of the freshly cooked food provides a sufficient estimate of their nutritive value. The greatest loss of vitamin B t occurs in the canning of meat and fish, because of their low degree of acidity and the long cooking times required for complete sterilisation. Usually their vitamin B x content is considerably less than that of the freshly cooked meat, and . because of the methods used in preparation the niacin and riboflavin content of canned corned beef in particular is comparatively low. Losses of vitamin Bt in the canning of vegetables do not differ greatly from those in ordinary cooking processes. The vitamin G value of canned vegetables and fruits generally compares favourably ' with that of the freshly cooked food, provided the syrup or vegetable liquor is not discarded. In fact, when high-quality fresh vegetables and fruits are not obtainable, canned products prepared from freshly picked, high-quality foods may have a better vitamin C content than those available. As a general rule, therefore, canned vegetables and fruits may be substituted for freshly cooked ones when desired. Bottle-d fruits of high vitamin C value, such as orange juice, rose-hip syrup, black currants, and gooseberries, are also excellent sources of this vitamin when fresh fruit is expensive or scarce. Other Methods of Preserving Salted foods, pickles, and jams are of value chiefly as supplementary energy foods in the diet. They also provide a welcome change when used as flavouring agents in dishes or for other purposes, but should not be regarded as regular . substitutes for fresh foods or fruits. Though the nutritive value of foods may be decreased by cookery and other forms of processing, if the approximate amount of destruction which may occur is known, the disadvantages can easily be obviated.

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