the total weight of fermentable matter in a bushel of potatoes is 12 pounds, which would yield approximately 6 pounds or 3.6 quarts of alcohol.
Composition.—Starch content: The quantity of starch in American grown potatoes varies from 15 to 20 percent. Probably 18 percent might be stated as the general average of the best grades of potatoes. In this connection it must be remembered that at the present time potatoes are grown in the United States chiefly for table use. Generally, only the imperfect or injured samples are used for stock feeding or for starch making, and this condition will probably continue as long as good edible potatoes bring a higher price for table use than can be obtained by utilizing them for starch or for feeding purposes.
Under the microscope the granules of potato starch have a distinctive appearance. They appear as egg-shaped bodies on which, especially the larger ones, various ring-like lines are seen. With a modified (polarized) light under certain conditions of observation a black cross is developed upon the granule. It is not difficult for an expert microscopist to distinguish potato from other forms of starch by its appearance, which is well shown in Figs. 39 and 40. Many of the granules are quite large, and most of them are ovoid in shape.
The quantity of protein in the potato is quite low compared with that of cereal foods; in round numbers it may be said to be 2.5 percent. The potato contains very little material which is capable of fermentation aside from starch and sugars.
Sugar content: Although the potato is not sweet to the taste in a fresh state, it contains notable quantities of sugar. This sugar is lost whenever the potato is used for starch-making purposes, but is utilized when it is used for the manufacture of industrial alcohol. The percentage of sugar of all kinds in the potato rarely goes above 1 percent. The average quantity is probably not far from 0.35 percent, including sugar, reducing sugar, and dextrin, all of which are soluble in water. In the treatment of potatoes for starch making therefore it may be estimated that 0.35 percent of fermentable matter is lost in the wash water.
One German author, Saare, claims to have found much larger quantities of sugar in potatoes than those just mentioned. The minimum quantity found by this author is 0.4 percent, and the maximum 3.4 percent, giving a mean of 1.9 percent. Ten varieties of potatoes used for the manufacture of industrial alcohol were examined in the securing of these data. It appears that some varieties have a greater tendency to produce sugar than others. The German variety known as "Daber" contains the smallest quantities of sugar, while the variety known as "Juno" contains the largest quantities. The percentages of sugar, as reported by Saare, however, are larger than those reported by other observers, and probably are larger than are usually found.
Average composition: Frazier, of the Cornell station, has collected analyses
