This section is from the book "The Potato: A Compilation of Information from Every Available Source", by Eugene H. Grubb, W. S. Guilford. Also available from Amazon: The Potato: A Compilation Of Information From Every Available Source.
The shaded portion represents the average loss of nutrients when boiled
Transverse and longitudinal sections of the potato: a, skin; 6, cortical layer; c, outer medullary layer; d, inner medullary area. From Farmers' Bulletin 295, United States Department of Agriculture.
Kind Of Food
Sugar, | Crude Starch, Fibre etc. Per ct. Per ct.
Potato, as purchased
Potato, edible portion
Potato, mashed and seasoned
Potatoes fried in fat, "Potato chips"
The corky skin of the potato makes up about 2.5 per cent of the whole, and the cortical layer 8.5 per cent., leaving 89 per cent. for the medullary areas. Theoretically, the skin is the only refuse or inedible material in the potato, but in practice a considerable part of the cortical layer is usually removed with it.
The edible portion of the potato - i. e., the tuber without the corky skin - holds on an average about 78 per cent. water, and so only about 20 per cent of the whole tuber has a direct food value.
The illustration shows very plainly that the bulk of the potato tuber is water. The stage of growth and other conditions affect the proportion present, young tubers being more juicy or watery than those which are fully developed.
The carbohydrates are by far the most abundant of the nutrients. Of the 18.4 per cent. less than 0.5 per cent. is made up of cellulose, yet one sometimes hears the statement made that potatoes are indigestible on account of the large quantities of cellulose which they contain. In reality there is as much or more in almost all the cereals and other vegetable foods, and such a criticism of the potato has no warrant of fact.
The bulk of the carbohydrates which the potato stores for future use is in the form of starch, which is, of course, insoluble in cold water, and small quantities of such soluble carbohydrates as dextrose, sugar, etc. In young tubers there is a larger proportion of sugars and less starch than when they have become mature. As the tuber lies in the ground the starch content increases. When it begins to sprout, however, part of the starch is converted by a ferment in the tuber into soluble glucose. Thus, young or early potatoes and old ones both have a smaller proportion of starch and more soluble sugars than well-grown but still fresh tubers. 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.
Other carbohydrates in the potato are the so-called pectose bodies, the substances which cause fruit jellies to stiffen, and when the tubers are large and pulpy pectoses may make up 4 per cent. of the tuber, though they usually occur in much smaller quantities. They are believed to have about the same food value as starch.
Fat, or ether extract, appears in such small quantities in potatoes that it may be practically neglected in discussing their food value, especially as the greater part occurs in the inedible skin in the form of a wax-like body.
The protein bodies are rather scanty, as compared with those of cereals and such vegetables as peas and beans. Only about 60 per cent. of the total amount present is true protein - that is, in a form which can be used for the building and repair of body tissue. This means that a pound of potatoes furnishes only about 1.3 per cent. or 0.2 of an ounce of true protein, and emphasizes the statement already made that potatoes alone make a very incomplete diet, as the proportion of nitrogenous material would be very small in a quantity sufficient to supply the body with all the energy-yielding material required.
These potato proteids have been studied by the Connecticut Experiment Station and found to consist of a form of globulin, for which the name 'tuberin' is suggested, and a proteose, part of these nitrogenous constituents being dissolved in the juice and part stored with the starch in the cells, especially in the cortical layer.
The nonproteid forms of nitrogenous substances in the potato are asparagin and small quantities of amido acids, occurring mostly in the juice. If they have any food value it is indirect and due to the fact that they protect the true proteids from waste during digestion. It is possible that they may aid digestion in some way or serve a similar purpose. There is a larger proportion of protein compounds, and especially of the more soluble forms, in young potatoes than in old.
The most important mineral matters found in potatoes are potash and phosphoric acid compounds. There are several organic acids (as citric, tartaric and succinic acid), which vary in tubers of different ages and account in some measure for the flavor of potatoes.
If peeled potatoes are exposed to the air the outer surface turns brown, just as does the flesh of many fruits. Such change is due to the action of enzyms or unorganized ferments naturally present in the plants. In the presence of the oxygen of the air they work upon tannin-like bodies in the tuber or fruit in such a way that the latter change color. In the case of potatoes this browning may be prevented by putting the peeled tubers into salted water or even into cold plain water.
In the condition in which they are purchased potatoes resemble such succulent carbohydrate foods as turnips and beets, with an average water content of 90 per cent., more than they do such dry carbohydrate foods as flour or rice with an average of 12 per cent. The condition in which foods are eaten should also be taken into account, for if the value of a food is judged solely by its chemical composition as it is found in the market a wrong impression may be obtained. For instance, potatoes as purchased consist of one fifth 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 four pounds of potatoes - 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 standpoint 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 in composition from potatoes. By drying potatoes they can be made very similar in composition and food value to rice. Considering the two articles as ordinarily purchased, 4.5 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."