The starch content in potatoes in Wisconsin is discussed in "Farmers' Bulletin 65" of the United States Department of Agriculture:

The value of a potato crop to the grower depends mainly upon the yield and the size, form, and healthy condition of the tubers. Perfect tubers find ready sale at the best prices, while the yield in itself has no effect on the transaction, and the chemical composition of the potatoes is, as a rule, disregarded by the buyer, unless they are to be used in starch making. In every 100 pounds of average potatoes there is seventy-five pounds of water. Of the remaining twenty-five pounds about twenty pounds is carbohydrates (starch and sugar, etc.) and two pounds protein. The chief value of the potato for food as well as for starch making lies in the starch which the tubers contain.

The protein content is low and the carbohydrates high, and, therefore, potatoes are especially valuable for use in connection with foods rich in protein, such as lean meat, eggs, etc., to furnish a well-balanced diet. The subject of the starch content of potatoes is thus seen to be one of great importance, and during recent years it has attracted increased attention from American and European investigators.

An interesting study of the conditions affecting the starch content of potatoes, begun in 1889, is reported in the Annual Report of the Wisconsin Experiment Station for 1895. In these investigations the starch content was approximately determined by means of the specific gravity of the tubers. Since by far the greater part of the potato tuber is starch and water, and since starch is heavier than water, it is evident that the variation in starch content will affect the specific gravity of the tuber.

Among the forty-six varieties of the crop of 1889 the variety Zenith showed the highest starch content, 22.9 per cent., and Rural Blush the least, 13.1, the average for all varieties being 16.2 per cent. In 1890 thirty-one varieties, mostly different from those tested the year previous, had an average starch content of 14.3 per cent., Burbank showing the highest, 17.7, and the Kidney, a potato from Germany, the least, 11.4 per cent. All these varieties of potatoes were grown on the same kind of soil and under practically the same cultural conditions. Still, the variation in starch content was as much as 9.8 per cent.

The starch content was found to vary with the season with different tubers of the same variety. Pronged and regular tubers of four varieties were tested separately, and in each case it was found that the percentage of starch in the pronged tubers was smaller than in the regular tubers. This seems to be one of the causes of the variation in the starch content above referred to "A test of different-sized tubers of the same variety proved that there was practically no difference in the starch content of large and small tubers.

In studying the influence of the depth at which tubers grow in the soil upon the starch content, it was found the first year that in every case the percentage of starch was largest in the deeper-growing tubers and smallest in those growing nearest the surface. When we consider the slight variation in depth at which the tubers grow in the soil these facts are significant. The next year the experiment was repeated with a trial of level and hill culture. The level culture gave higher starch content than hill culture, and the variations with the depth were greater in the hill culture than in the level culture. These facts suggest a possible explanation of the depth influence - viz., that it acts through the temperature of the soil. The deeper tubers are in a cooler medium than the shallow ones, and soil that is hilled is warmer in warm, dry weather than that which is not hilled. The variation of temperature in the deeper and shallower layers would naturally be greater in the hilled soil.

In experiments in planting at different distances the starch content increased as the distance between the plants decreased. This seems to accord with the results of the tests of depth of planting, since close planting promotes shading of the ground and thus tends to reduce the soil temperature.

A test of scabby and healthy tubers of the Delaware variety showed a higher starch content in the scabby tubers than in the healthy ones, thus showing that scabby potatoes are not necessarily poorer in starch than those free from scab."

The manufacture of starch from sweet potatoes is discussed in "Farmers' Bulletin 334" of the United States Department of Agriculture as follows:

With a view to the more complete and profitable utilization of the sweet-potato crop the South Carolina station several years ago began investigations to determine the starch content of different varieties of sweet potatoes and those most promising for starch making, how much starch can be recovered commerically from the potatoes, and the quality of the starch for commercial purposes.

As it is usually managed at the present time, only a fraction of the crop is disposed of, all unmarketable potatoes being usually a dead loss, and frequently, through inability to market the crop promptly, great loss is suffered through damage by rotting, etc. Where the crop could be disposed of to starch factories the grower would have the following advantages: (1) All potatoes could be sold, regardless of their size. (2) No barrels or containers would be required in marketing the crop. They could be loaded into a wagon in the field and hauled directly to the factory, or to the nearest railroad and loaded into cars for shipment. (3) Grown on such a large scale, modern machinery could be employed in planting, cultivating, and harvesting the crop, thus reducing the cost of growing. (4) Heavier yielding varieties could be grown, which are the ones most valuable for starch production.

Up to the time when the study of the question of producing starch from sweet potatoes was begun at this station, it was a subject that had received practically no attention in this country. It is true, starch was made from this plant on a small scale in the Southern States during the war, but the starch obtained in this way was a comparatively impure product and intended only for home consumption.

It would seem that the sweet potato could be profitably used for this purpose, as it contains a larger percentage of starch than the Irish potato, yields a heavier crop, and can be grown more cheaply. Another advantage it has over the Irish potato is the fact that the vines of the former make a good food for stock - some varieties being very palatable, making good hay and excellent silage. In composition they compare favorably with other forage crops.

The development of cotton manufacturing in the South has created a demand, which is continually increasing, for starch used in 'sizing' yarn and 'filling' cloth. At present every pound of this starch is brought from other states, principally from the cornstarch factories of New York and Illinois. The experiments which we have had carried out show that for use on cotton goods the starch produced from sweet potatoes is better than cornstarch, and fully equal to the best grades of Irish potato starch. The annual production of sweet potatoes in the South Atlantic and Gulf States is about 60,000,000 bushels, but this might be easily increased tenfold. The theoretical amount of starch produced per acre from a good crop of sweet potatoes is from one and one half to four times as much as from corn, wheat, or Irish potatoes per acre.

The variety most in demand for a table potato is not necessarily the one best suited for the manufacture of starch. In fact, we can say almost conclusively that it is not, as the variety containing the largest percentage of starch is apt to be dry and insipid. Then, too, for starch production we want a prolific potato, and as a rule the heaviest yields are not of the best quality for the table. These are the most essential requirements: (1) High starch content; (2) prolificness; (3) flesh light, or white in color. The following come nearer possessing these requirements than any we have thus far examined: Providence, Southern Queen, and Triumph. Further work may show that there are other varieties better suited for this work than any of those mentioned.

The machinery used in the station experiments was similar to that used in making starch from Irish potatoes.

Four varieties of potatoes were used - Southern Queen, Providence, Triumph, and Red Nansemond. The first three were chosen for their high starch content and light color, and one test was made of Red Nansemond to see if the color would interfere with successful starch making.

The results of experiments carried out for two years in succession show the entire practicability of the manufacture of starch from sweet potatoes, but 'the data accumulated is yet insufficient to make any positive statement as to whether engaging in this enterprise will prove a paying investment.'

At the price at which sweet potatoes are sold at the present time their manufacture into starch alone would not be profitable. It must be remembered, however, that, grown on the scale which would be necessary to run one or more starch factories, there are a number of expenses which could be eliminated.

In the conducting of a factory the following plan suggests itself as a feasible one: The factory to take over all potatoes from the farmer, select the best and even-size ones to be shipped to market for table use, and make starch from the small, over-size, and ill-shaped ones. All operations being controlled in this way by the factory on a large scale, the product could be utilized and marketed to the best advantage. In case of dull market conditions, instead of shipping the potatoes they could be canned, for which there is a great and increasing demand at the present time.

A successful method has also been devised for evaporating sweet potatoes. In this condition they will keep indefinitely, and, owing to their concentrated form, can be shipped long distances at comparatively small cost. They would, no doubt, be quite popular if better known.

As is the case in all paying enterprises, it would be necessary to watch carefully the byproducts and utilize them to the best advantage. A method could undoubtedly be devised for collecting the water with which the potatoes are treated in the grinding operation. This would contain the greater portion of the sugars and could be added to the pulp - from which starch has been extracted - and all sugars, starch, and fermentable matter remaining could be converted into alcohol. It has been shown that, theoretically, fifty gallons of 95 per cent. alcohol could be produced from the residues from 100 bushels of potatoes.

It is practically settled that the starch produced from sweet potatoes is of a high grade and suitable for use in many operations where a high grade starch is required. In all of the tests we have had made not a single adverse report has been received.

In practical tests for laundry purposes, for sizing yarn, filling cloth, thickening colors, etc., the starch gave highly satisfactory results."