the value of potatoes as food

THE VALUE OF POTATOES AS FOOD.

By C. F. LANGWORTHY, Ph. D.,

Office of Experiment Stations.

INTRODUCTION.

The potato, called in different regions white potato, Irish potato, English potato, or round potato, was first introduced into Europe between 1580 and 1585 by the Spaniards, and afterwards by the English about the time of Raleigh's voyages to Virginia. It is commonly believed to be a native of Chile. Wild potato plants closely resem- bling those cultivated to-day are still found there, though it is a fact worthy of mention that, as the potato has been modified by cultivation, it has largely lost the power of producing seeds, and the cultivated potato differs from the wild in seldom producing seed-bearing fruits. When first visited by Europeans, the aborigines in Chile and adjacent regions cultivated the potato for its edible tubers and had apparently long done so. It was probably introduced into the United States, especially into Virginia and North Carolina, toward the end of the sixteenth centuiy. It is not surprising that the new food stuff should have grown rapidly into popular favor when we remember its prolific yield, superior keeping qualities, ease of propagation, and agreeable flavor.

STRUCTURE OF THE POTATO.

The potato tuber is in reality a modified stem, being shortened and thickened to serve as a storehouse for reserve material for the propagation of new plants. The outer skin, which is dry in appearance and usually gray or brown in color, corresponds to the bark of the rest of the plant. The portion underneath the skin when exposed to the sunlight turns green and gives the potato an unpleasant flavor. The outer and inner skin are usually removed when the potato is peeled. The flesh makes up the bulk of the potato.

When a section of the potato is carefully examined, it will be seen that it consists o.f three more or less well-defined portions, namely, the skin, cortical layer or fibro-vascular layer, and the flesh, which is made up of the outer and inner medullary layers. The cortical layer, which is the portion lying immediately beneath the true skin, and which is sometimes designated as the inner skin, is slightly colored, containing practically all the coloring matter normally present in the potato, and, as already stated, is the part which turns green on continued exposure

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338 YE AEBOOK OF THE DEPAETMENT OF AGEICULTUEE.

to the sunlight. This portion has some resemblance to the skin in general appearance, and is usually removed with the skin in preparing potatoes for the table. Fig. 43 shows a transverse and a longitudinal section of .the potato.

FIG. 13.—Transverse and longitudinal sections of the potato: a, skin; h, cortical layer; medullarv layer; d, inner medullary layer.

COMPOSITION OF THE POTATO.

The valuable qualities of the potato were speedily recognized, and there are early records of attempts to determine its food value by means of chemical analyses. In 1795 Pearson reported '' Experiments and observations on the constituent parts of the potato root." Einhof, in 1805, published analyses of the potato, as did also Vauquelin in 1817. In America, analyses of the potato were reported some fifty years ago by Emmons. These investigations were useful at the time, although they were not made by the methods generally followed to-day. This was necessarily the case, as the subject of the chemistry of nutri- tion is of comparatively recent growth. In later years many studies of the composition and food value of the potato have been made in this and other countries.

As shown by recent analyses, the skin of the potato constitutes on an average 2.5 per cent of the whole and the cortical layer 8.5 per cent. It is difficult to peel potatoes so that the skin only is removed.

THE VALUE OF POTATOES AS FOOD. 339

Whether both skin and cortical layer or only the former should be called refuse in our current sense of the word is perhaps a question. As potatoes are commonly eaten, a good deal of the flesh or edible portion IS rejected with the skins. When they are baked with the skin on, much of the flesh is apt to be thrown away with the skin. When they are boiled with the skin on, the amount of edible portion thus thrown away may be very small. When the}^ are pared for boiling, the amount wasted is apt to be much larger. When they are rough from defects in growth, or from shrinking and shriveling after keeping over winter, the amount of flesh cut off in the peeling is larger still. Just how much this loss of the edible portion of potatoes will average in ordinary households no one can say exactly. In the tables of analyses published in late bulletins of this Department, the amount of refuse and edible portion rejected with it is estimated at 20 per cent of the whole and the edible portion left as 80 per cent. Doubtless, in many cases the rejected portion is very much larger. The loss of actual

FIG. 44.—Composition of the potato: a, fat; b, crude fiber and other carbohydrates, exclusive of starch; c, protein; c?, ash.

nutriment of the potato by the rejection of so much of the edible portion with the skin is a much more important matter from the stand- point of nutritive economy than people generall}^ realize.

The edible portion is made up of 78.3 per cent water, 2.2 per cent protein (total nitrogenous matter), 0.1 per cent fat, 18.4: per cent carbohydrates (principally starch), and 1 per cent ash or mineral matter. Of the carbohydrates, 0.4 per cent is made up of crude fiber and materials which in some of their modifications constitute the cell walls of plants and give them a rigid structure. The above figures, like others for composition of food materials, represent general averages, from which there are wide variations in individual specimens. Though the skin, cortical layer, and flesh differ somewhat in composition, they all resemble more or less closely that of the whole tuber. Fig. 41: shows in graphic form the composition of the potato.

When potatoes are stored they undergo a shrinkage. According to tests made at the Michigan Agricultural Experiment Station, this amounted to 11.5 per cent when they were kept in storage from

340 YEARBOOK OF THE BEPAETMENT OF AÖEICULTUEE.

September 30 to May 1. This shrinkage is probably due to a loss of water by evaporation.

The Connecticut State Agricultural Experiment Station has made a special study of the proteids of potatoes as well as of many other vege- table products. According to these investigations, the potato contains two proteids, a globulin, to which the name ''tuberin" is given, and a proteose, the latter occurring only in very small amounts.

It will be remembered that food serves the body in two ways: (1) It is used to build and repair body tissue, and (2) it yields energy for internal and external muscular work and for maintaining the body temperature. Carbohydrates and fat are sources of energy and can not serve for the building and repairing of the body tissue. Protein is necessary for this purpose since it alone of the nutrients contains nitrogen, the characteristic element of the body tissue. In addition to this, protein also serves as a source of energy, and thus while the body could not be nourished on fat or carbohydrates alone, it could be, theoretically at least, on protein, since this nutrient combines the two functions of food. The potato contains some protein, but as the principal nutrient in it is starch, it may be fairly classed as a carbohydrate food. As is the case with all carbohydrate foods, it is chiefly valuable in the diet to supply the body with energy. The potato has a fuel value of 385 calories to the pound, that is, when burned in the body (as all foods must be when utilized), it yields energy equal to the amount named.

Of the total nitrogenous matter present in the potato, that is, the so-called protein, onl}^ a little over half, on the average, perhaps about 60 per cent, consists of true proteid, the portion of the protein group which actually builds the body tissues and helps to keep them in repair. The remainder consists of amids and other compounds of inferior food value. Thus, if 100 pounds of the edible portion of the potato contain 2.2 pounds of total protein, the amount of true proteid will be only 1.3 pounds. This deficiency of proteids in the potato is another matter which people generally do not appreciate. It helps to explain why large numbers of the country population of Ireland and Germany, whose food consists largely of potatoes, are so poorly fed. It is not so much the insufficiency as the one-sidedness of the diet. This is an illustration of the fact that no single article of diet is fitted properly to nourish adult man in health.

As appears from the figures quoted above, potatoes contain a large amount of water. It is largely present in the juice, which consists of water holding various salts and other bodies in solution. In their high water content potatoes resemble turnips and cabbages, which, on an average, contain, respectively, 89.6 and 91.1 per cent water. Ordinary roller process flour contains only about 12 per cent water and a much larger proportion of protein, fat, and carbohydrates than potatoes.

THE VALUE OE POTATOES AS FOOD. 341

Rice also contains about 12 per cent water. Although the protein content of rice is much higher than that of potatoes (being on an average 8 per cent), carbohydrates make up the bulk of the total nutritive material, and, like potatoes, rice is properly classed as a carbohydrate food. If the value of a food is judged solely by its chemical composition, a wrong impression may be obtained. For instance, potatoes as purchased consist of one-ñfth and rice of seven-eighths nutritive material. The first inference is that rice is more than four times as nutritious as potatoes. In one sense this is true, that is to say, a pound of uncooked rice contains more than four times as much nutritive material as a pound of raw potatoes. But if we take about 4 pounds of pota- ' toes, that is, the amount necessary to furnish as much nutritive material as the pound of rice, the composition and nutritive value of the two quantities will be just about the same, while from a pecuniary stand- point the advantage would be on the side of the potatoes. The chief difference in the two foods before cooking is that one is juicy and bulky while the other is dry, and therefore more concentrated. In cooking rice we mix water with it, and may thus make a material not very different m composition from potatoes. By drying potatoes they can be made very similar in composition and food value to rice. Con- sidering the two articles as ordinarily purchased, 4^ pounds of raw potatoes and a pound of uncooked rice contain nearly equal weights of each class of nutrients and have about the same nutritive value.

If a potato is grated and inclosed in a linen cloth and pressed, a large amount of juice will be obtained. The juice thus prepared is a dark- colored liquid which has an acid character, commonly said to be due to citric acid with more or less tartaric and succinic acids. The mineral matter present is very largely in the form of potash salts. The juice also contains some albuminoids (that portion of the total protein of most value as food), asparagin, and other organic compounds. If the grated potato is mixed with water, starch falls out from the broken cells and settles to the bottom of the vessel, and may be removed in the form of a white deposit. Starch is manufactured to a large extent from potatoes by methods which are similar to the above in principle.^

THE COOKING OF POTATOES.

Although the potato owes its nutritive value principally to carbohydrates, it will be remembered that it contains some nitrogenous matter also. According to the investigations of Lawes and Gilbert, the juice of the potato contains more proteid or albuminoid nitrogen than the flesh. This is an important matter, since albuminoid nitrogen is more valuable for the body than nonalbuminoid nitrogen. In general, it may be said that 85 per cent of both protein and mineral matter

^ For an extended account of the manufacture of starch from potatoes, see Bulletin No. 58, Division of Chemistry, U. S. Department of Agriculture.

342 YEAEBOOK OF THE DEPAETMENT OF AGßlCüLTUBE.

io the potato (the latter being yahiable for dietetic reasons, though not a nutrient) is in the juice. More or less of the juice of any food may be accidentally lost when it is prepared for the table; and the possibility of loss in cookings due to this and other factors, is a matter of importance. Any sugar or other soluble carbohydrates might be removed if potatoes were cooked b}^ boiling. No considerable loss of starch as such is to be expected, since starch is insoluble in water. Some starch is changed to a soluble body, dextrin, a sort of sugar^ by the action of dry heat, possibly also when water is present.

The principal ways of cooking potatoes are baking, boiling, and try- ing^ or some modifications of these processes. The objects sought are principally to soften the tissues and render them more susceptible to the action of the digestive juices and to improve the ña vor. Just whj^ cooking changes the flavor as it does has apparently never been made the subject of investigation. In potatoes, as in other foods, the cooked starch is more agreeable to the taste than the raw. Possibly also there

FIG. 45.—Changes of starch cells in cooking: a, cells of a raw potato with starch grains in natural condition; t>, cells of a partially cooked potato; c, cells of a thoroughly boiled potato.

are volatile bodies of more or less pronounced flavor, which are removed or produced by the heat of cooking. The physical condition of the potato is much affected hy heat. In the raw potato the separate starch grains are inclosed in cells with walls composed of crude fiber, a material resistant to digestive juices. If potatoes were eaten raw, the digestive juices would not reach the starch as easily unless the cell walls happened to be ruptured mechanically, as in mastication. Heat, however, expands the water present, ruptures the cells, and breaks up the starch, expanding the gTanules, which, when raw, consist of tightly-packed concentric layers, to a mass of much less solid structure. These changes are shown in fig. 45.

The albuminoids in foods are coagulated by heat, and so are ren- dered insoluble in water in which food is cooked. This explains why foods, meat especially, should be plunged into boiling-water if it is desired to retain the albuminoids. The heat at once coagulates the albumen on the surface, thus preventing more or less completely the extraction of materials in the inner portion. It seems probable that


this reasoning would apply to potatoes as welt as to meat, although they contain much less albumen. The effects of cooking potatoes by boiling in different ways were tested not long ago at the Minnesota and the Connecticut (Storrs) Agricultural Experiment stations. The potatoes were boiled in distilled water, lime water, and alkaline water; part were boiled in water hot at the start and part in water cold at the start. In some cases the iX)tatoes were peeled before boiling and in some cases this was not done. In two tests the peeled potatoes were soaked before boiling. The total loss of material (dry matter) ranged from 6.5 per cent of the total amount present in the case of the peeled potatoes soaked before boiling* to 0.2 or 0.3 per cent in the case of the potatoes boiled with the skins on. The greatest loss of total nitrogen and ash was also found y/hen the peeled potatoes were soaked before boiling; least when this was not done. Whatever the method of boiling, little of the carbohydrates was lost. From the experiments as a whole, it may be said that when potatoes are boiled with the skins removed there is a very considerable loss, not only of organic nutrients, but also of mineral salts. To obtain the highest food value, potatoes should not be peeled before cooking. When potatoes are peeled before cooking and placed directly in hot water and boiled rapidly, less loss of materials is sustained than when they are cooked in water cold at the start. If potatoes are peeled and soaked in cold water before boiling the loss of nutrients is quite considerable; in the case of proteids, being equal to one-fourth of the amount present. The loss in a bushel of potatoes thus cooked would be equivalent to the albumen in a pound of sirloin steak. When potatoes are boiled with the skins removed the greatest actual loss of nutrients seems to be due to the mechanical abrasion of some of the soft outer portions while cooking. In the experiments at the Connecticut (Storrs) Agricultural Experiment Station it was found that nearly 3 per cent of the carbohydrates and 4 per cent of the albuminoid material were lost when potatoes were thus cooked. When the potatoes were boiled with the skins on the loss of nutrients was ver}^ slight, consisting chiefly of nonalbuminoid nitrogenous substances and mineral matter. It is therefore evident, if it is desired to boil potatoes with as little loss as possible, that the skins should be left on.

Comparatively speaking, there are probably few cases in which it is necessary to take accoimt of the losses due to different methods of boiling potatoes and where the possibility of loss would outweigh the liking for them prepared in some particular way, but in institutions where a large number must be provided for, and, in fact, under any condition where rigid economy is necessary, the matter may assume considerable importance.

An extended study of the relative composition of large, medium, and small potatoes, and of the different parts of the tubers and of the

344 YEAEBOOK OF THE DEPARTMENT OF AGEICULTURE.

taste and culinary properties, was recently reported by Condon and Boussard, two French scientists. The authors believe that the culinary value of potatoes is directly proportional to their nitrogen content and inversely proportional to their starch content. The different varieties of potatoes were found to vary greatly in their resistance to boiling, some retaining their form completely, while others were almost wholly disintegrated. The opinion was advanced that resistance to boiling depends principally upon the relative amount of albuminoids present. No definite relation was observed between chemical composition and early maturitj^ Generally speaking, the early varieties contained more water and nitrogenous materials and less starch than the late varieties tested.

As regards chemical composition, it may be said in general that boiled potatoes contain a little less water than raw potatoes, and except as this changes somewhat the proportion of nutrients, they differ little in composition from the raw. Mashed potatoes, if they are not seasoned, must necessarily have the composition of the unmashed boiled potato, making allowance for the small proportion of water which would probably be lost by evaporation in mashing. When milk, cream, or butter is added to mashed potatoes in preparing them for the table the nutritive value is increased, though the chief reason for adding such materials is doubtless to improve the flavor. This is also the reason why salt and pepper are added. Baked potatoes have practically the same composition as the uncooked, some water being lost by evaporation. When potatoes are fried, as in making potato chips, they lose by evaporation much of the water present and absorb more or less fat. They therefore have a higher nutritive value, pound for pound, than raw potatoes. Potato chips have been found by analysis to contain 2 per cent water and 39.8 per cent fat, as compared with 78 per cent water and 0.1 per cent fat when raw. The many ways of cooking potatoes, with or without the addition of other materials, which are described in books' devoted to cookery, are in principle modifications of those already alluded to. The wholesome- ness of potatoes cooked in different ways is largely a matter which each must decide for himself, the general experience being that for men in health most of the methods followed are satisfactory.

Evaporated potatoes are now on the market, being especially recom- mended for provisioning camps and expeditions. As compared with fresh, the evaporated potatoes have a high nutritive value in proportion to their bulk. This is the case with all evaporated foods, such material having been concentrated by the removal of a large proportion of the water originally present.

DIGESTIBILITY OF POTATOES.

In considering the nutritive value of any food, the digestibility must be taken into account, for it is what the body can absorb from any given


material as it passes through the digestive tract, rather than chemical composition, which determines food value. The digestibility of potatoes has been frequently studied. Some years ago Rubner kept a Bavarian soldier who was used to eating large quantities of potatoes on a potato diet for two days. The potatoes were boiled and eaten with salt or with vinegar and oil as a salad. The carbohydrates, the principal nutritive material in the potatoes, were quite thoroughly assimilated. As is usually the case, the digestibility was determined by deducting from the total nutrients present in the food the quantities excreted in the feces. The protein was not well digested. Similar results were obtained by a later German investigator, though in this case the protein was somewhat more thoroughly digested. In this experiment about three-fourths of the crude fiber present was found to be digestible. A number of experiments on the digestibility of potatoes were also made in St. Petersburg by a Russian investigator with three healthy men. Each test was divided into two periods. In one a simple mixed ration was eaten; in the other the food consisted of cooked potatoes only—baked, boiled, or fried in oil. It was found that 93 per cent of the total dry matter (which in the potato consists largely of starch) was digestible; of the total nitrogen present, 59 per cent; and of the albuminoid nitrogen, 41 per cent. In experiments on the digestibility of potatoes by man, recentl}^ made at the Minnesota Agricultural Experiment Station, it was found that 71.9 per cent of the protein and 93 per cent of the carbohydrates were digested or assimilated. In this experiment the potatoes were eaten with some eggs, milk, and cream, so the conditions may be assumed to be about normal. How far the digestibility of any food is influenced by the foods eaten with it is a matter on which little reliable information is available.

The experiments cited above show little, if anything, regarding the ease or rapidity of digestion, and, indeed, trustworthy data of this sort regarding any food are not abundant. According to some investigations made a number of years ago by Beaumont, and which have been frequently quoted, roasted or baked potatoes require somewhat less time than bread for digestion in the stomach (conversion into chyme); boiled potatoes require the same time as bread, namely, three and one-half hours. Too much importance should not be attached to these differences, asno account was taken of digestion in the intestines. As the digestion of starch takes place in the intestinal tract after the food has left the stomach, this is a matter of great importance in the case of starchy foods like the potato. The total length of time the potato remains in the body probably does not vary much with the difl'erent methods of cooking. Furthermore, if a person is in health it does not necessarily follow that there is any advantage in digesting one food more rapidly than another.

1 A1900—23

346 YEAEBOOK OF THE DEPARTMENT OF AGRICULTURE.

FLAVOR OF POTATOES.

The flavor of potatoes depends chiefly on the substances which are dissolved in the juice. These include various mineral matters, citric acid, and other organic bodies in different combinations. It seems probable that the character of the soil and the amount of moisture it contains exert an influence upon the flavor, as the same varieties of potatoes grown under different conditions vary more or less in this respect. It has been found, for instance, that potatoes manured with muriate of potash yield watery tubers. That such causes may have a marked effect, is shown by the extreme case in which potatoes grown in very wet soil sometimes have small tubers above ground in the axils of the leaves instead of under the surface. Such tubers have an unpleasant flavor, and for this reason, if for no other, are unfit for food. The strong, unpleasant flavor of potatoes which have grown at the surface of the ground, and more or less exposed to the influence of light, is familiar. The green color of such potatoes is due to the chlorophyll formed under the influence of light. The unpleasant flavor is attributed to solanin. Potatoes which have been touched by the frost possess a sweetish flavor. According to analyses of normal and frozen potatoes, this is due to the conversion of some starch into sugar. Normal potatoes were found to contain from 13 to 16 per cent soluble material and from 84 to 87 per cent insoluble material, while the percentage of the former in the frozen potatoes ranged from 14 to 20 and of the latter from 80 to 86 per cent. In the normal potatoes starch constituted 67 to 76 per cent of the total insoluble material and sugar 0.3 to 0.7 per cent of the total soluble material. In the frozen potatoes only 58 to 72 per cent of the total insoluble material was starch, while the sugar amounted to from 0.4 to 1.7 per cent of the total soluble material. This change of starch into sugar is attributed to the action of a ferment present in potatoes. The explanation seems reasonable, since it is known that ferments play an important part in the chemical processes which take place in plants.

It is undoubtedly true that many persons select fruits and vegetables on the basis of size and appearance,'large fruit or vegetables of fine color being given the preference without regard to their flavor. There are, however, many who realize that different varieties vary greatly in flavor, and are governed by this fact in their selections. Such dis- crimination has developed, for instance, a special market for certain finely flavored varieties of fruits. It is undoubtedly much less com- mon for the purchaser of potatoes to be governed by flavor in his selection of them, and yet the different varieties, or the same variety grown under different conditions, vary greatly in this respect. A smooth potato of good form and size does not necessarily possess a flavor superior to one in which these characteristics are less marked, yet it would perhaps.almost always be given the preference by most


purchasers. Good flavor in potatoes is a matter worth attention. If purchasers demanded this quality as well as attractive appearance and size, growers would without doubt meet the demand.

PLACE OF POTATOES IN THE DIET.

According to statistics obtained in the large number of dietaiy studies made in this country, potatoes constitute about 13.7 per cent of the total food consumed by the average American family, and furnish not far from 3,9 per cent of the total protein and 10 per cent of the total carbohydrates.

The potato is a staple article of diet in almost every household. The universality and extent of its consumption would seem sufficient to prove it to be a wholesome and nutritious food. The statement, however, is frequently met with in popular articles that potatoes are not wholesome. So far as can be learned this is purely a gratuitous assumption. While it is possible that there are persons with w^hom they do not agree, or who for some reason are compelled to forego starchy foods, there is no reason to suppose that potatoes are not as a rule a useful and wholesome article of diet.

The potato is essentially a starchy food, and eaten alone it would furnish a very one-sided, badly balanced diet, which would probably prove unwholesome to most people, as it has been estimated that man in health, performing a moderate amount of muscular work, is bes-t maintained by a diet furnishing daily 0.25 pound protein in addition to fat and carbohydrates enough to make the total fuel value 3,500 calories, while a man without active physical labor might be well nourished with 0.20 pound of protein and 3,000 calories of energy. When the potato is eaten with meat, eggs, fish, etc., which are essentially nitrogenous foods, a w^ell-balanced diet, which is most conducive to health and vigor, is secured.

Scientific investigation shows that the practice, which has become so general, of serving potatoes with meat and other similar foods which contain liberal amounts of protein is based upon correct principles, one food supplying the deficiencies of the other.

Potatoes and other foods containing carbohydrates are sometimes objected to on the ground that they are starchy foods and do not supply much nitrogenous material. It should be remembered, however, that the potato does contain a by no means inconsiderable amount of protein, and further that carbohydrates are an essential part of a well- regulated diet. The digestion experiments referred to above show that potatoes properly cooked furnish such material in a digestible form. They have been a staple article of diet for many years without harmful results, and therefore the conclusion that under ordinary circumstances they are other than a useful and wholesome food seems unwarranted.

348 YEAEBOOK OF THE DEPARTMENT OF AGRICULTURE.

POSSIBLE DANGERS FROM EATING POTATOES.

Although under ordinary circumstances potatoes are unquestionably a wholesome food for most persons, illness is sometimes caused by eating them. There are undoubtedly some persons in health with whom potatoes do not agree, just as there are those who can not eat strawberries without distress. This is due to personal idiosyncrasy and not to the harmful character of the food. Reference can not be made here to the conditions of ill health in which potatoes or other starchy foods are forbidden, since this is a subject which pertains to the practice of medicine rather than the subject of dietetics. Cases of actual poisoning b}^ potatoes are by no means unknown. So far as can be learned the abnormal symptoms in such cases were caused by the pres- ence of solanin in the potatoes. Several years ago 357 soldiers in a battalion of the Austrian army showed symptoms of solanin poisoning. The potatoes used for food were examined. Those which were fresh contained a small amount of solanin, while those which had sprouted contained much more, still larger amounts being found in the sprouts than in the tubers themselves. The potatoes undoubtedly caused the poisoning in this case. Potatoes a 3^ear old which have lain in a cellar and shriveled and small potatoes which have sprouted without being planted are considered especially dangerous, and should not be eaten. If perfectly fresh potatoes contain any solanin, the amount is so small that it does not cause harm.

the value of potatoes as food

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