The Chemicals We Get From Potatoes

R. H. Treadway, T. C. Cordon.

The carbohydrates, proteins, and minerals that make potatoes an excellent food for our bodies also furnish excellent nourishment for molds, bacteria, and other microbial forms of life. In the language of the microbiologist, the potato is a good substrate for the growth of micro-organisms. The liquor from boiled potatoes, "potato broth," has long been established as a nutrient medium in experimental microbiological work. That is to say, in culturing, or growing, bacteria and molds in order to identify species, determine numbers, or increase their concentration preparatory to putting the micro-organisms to practical use, potato liquor provides the necessary stimulation for growth.

Potatoes contain about 20 percent solid matter -and 80 percent water. Starch, by far the most important solid constituent, accounts for 65 to 75 percent of the potato on the dry basis. Potatoes ordinarily contain little sugar, but the starch can be converted into the fermentable sugars maltose and dextrose.

About 10 percent of the dry matter is protein, much of which is present in soluble form. Free amino acids, the simple building blocks of protein, are also present. A good part of the protein content of the potato, therefore, is in a readily available condition for utilization by micro-organisms. Essential inorganic elements, such as potassium, phosphorus, and magnesium, occur inadequate amounts among the mineral constituents of the potato. Other necessary growth factors are also present.

Large tonnages of potatoes have been fermented to produce ethyl alcohol, the common beverage and industrial alcohol. During periods when grain is scarce or costly, potatoes have been used to great advantage when they were available in sufficient quantity. In 1946, 29 million bushels of potatoes were used by alcohol distilleries. Most of the potato spirits has gone into industrial alcohol; some has appeared in blended whiskies and liqueurs. In 1947, 13.3 percent of the alcohol produced in the United States came from potatoes (compared to 16.5 percent from grain), with ethyl sulfate a synthetic method starting with petroleum and molasses serving as raw materials for most of the production.

An attempt was made during the Second World War and immediately afterward to popularize the blended whisky containing potato alcohol. Through the period of hostilities, the production of grain alcohol for beverage purposes was banned because industrial alcohol was sorely needed in the war effort. It is likely that people associated the potato alcohol with the idea of an inferior, temporary war substitute. At any rate, when potable grain alcohol again became available, little or no market remained for potato alcohol in beverages. Europeans have long used potato alcohol in vodka and other liquors. Actually the difference in taste between grain and potato alcohol is negligible.

Butyl alcohol, which is valuable in the formulation of lacquers and in the synthesis of organic chemicals, also is obtained by fermentation of potatoes. We do not know how many potatoes have been used in the fermentation of butyl alcohol, but the quantity has been large since 1946. Late in 1948 the price of fermentation butyl alcohol dropped from 32 cents to 17.5 cents a pound on tank-car lots, largely because millions of bushels of surplus potatoes were available at low cost through the Government's diversion program. The price drop placed fermentation butyl alcohol in direct competition with the synthetic grade, an unusual situation. Eighteen percent of the 35 million bushels of surplus potatoes from the 1947 crop went into the production of ethyl and butyl alcohols. From the 1948 crop, 34 percent of the 133 million bushels acquired by the Government were used in the fermentation of those two alcohols.

IN THEORY IT IS POSSIBLE to produce almost any fermentation product from potatoes. For various reasons, however, it would not be feasible to use potatoes in all fermentations. In some, the non-carbohydrate constituents may interfere with fermentation of the sugar, uncontrollable side fermentations may occur, or insurmountable difficulties may arise in isolation and purification of the desired product.

The Eastern Regional Research Laboratory undertook a study to determine the suitability of cull and surplus potatoes for various fermentations. Early in 1947, P. A. Wells, the director, suggested using potatoes in a novel way in fermentations. He pointed out several advantages of potatoes for this use. Potatoes might be of value in culturing and propagating organisms that produce the amylases ( starch-splitting enzymes), which are necessary agents in forming the sugars so that fermentations can proceed. The potato nutrients also would be expected to promote the fermentation of the sugars. Hence, he suggested that the conversion of potato starch to sugars and the fermentation of the sugars to other products might be carried out simultaneously. Chemists and microbiologists joined forces in developing this line of work.

We estimate that if potatoes were carefully graded and only the best grades sold for table use, a supply of subgrade stocks of 50 million bushels or more a year would exist. A fermentation industry based on such a large quantity of raw material would seem entirely feasible. Assurance of a constant supply of raw material is a requisite for the development of any large-scale industrial processing.

The problems associated with utilization of potatoes in industrial fermentations arise from their perishability and the high water content that gives them their great bulk and weight.

Freshly dug potatoes go through a rest period for about 2 months after harvesting. During that time they will not sprout, even at relatively high temperatures. Afterwards, they must be stored in inhibitor cool place or treated with a sprout Inhibitor to keep them dormant and prevent sprouting. In the Northern States, where there are facilities for storage over the winter, processing of fresh potatoes can be spread over about 8 months of the year. The intermediate crop of potatoes which is harvested in midsummer and late summer is stored for only a few months. Practically no early-crop potatoes are stored in the Southern States.

Because potatoes are four-fifths water, they are bulky and heavy to transport and comparatively costly to handle, unload, store, and process at industrial plants. The ideal situation is to have processing plants in the potato area where transportation costs are kept down. It also appears advisable, though, to have facilities for dehydrating potatoes to convert them into stable form. A stockpile of dehydrated potatoes would enable processors to operate the year around through both low- and surplus-production years.

Potatoes have only a low value as raw material for fermentation of ethyl and butyl alcohols, in which they must compete with cheap blackstrap molasses and the synthetic processes. Cuban blackstrap molasses was available in Philadelphia at a cost of only about 4 cents a gallon (in February 1950). At that low cost, the fermentable sugars are worth about 0.6 cent a pound. Potatoes containing 14 percent fermentable carbohydrate would thus be worth only 8.4 cents a hundred pounds, delivered, in competition with blackstrap molasses at 4 cents a gallon. Nevertheless, surplus potatoes costing approximately three times that figure, delivered, were used in Philadelphia in February 1950. Apparently the supply of cheap molasses is inadequate at times.

Ventures in alcohol production from cull and surplus potatoes have failed in Idaho and Maine in the past few years. Because there was little demand for industrial alcohol in the vicinity of each distillery, the output had to be shipped great distances to heavy industries that constituted markets. We believe it is possible, however, that alcohol can be produced successfully in regions of large potato acreages if alcohol comes into common use for supplying automotive power. Maine, Minnesota, North Dakota, and Idaho potato States far from cheap industrial alcohol might then find it economical to use alcohol manufactured from locally produced potatoes for blending with gasoline to provide fuel for cars, busses, trucks, and tractors.

Potatoes admittedly are worth only a trifle now in competition with black-strap molasses in the fermentation of ethyl and butyl alcohols, but research workers are constantly on the alert for new fermentations in which potatoes may have sufficient superiority to command a higher price than their carbohydrate content alone would warrant.

Minor differences in nutrients and traces of essential constituents present sometimes mean a lot in fermentations. For example, lactose (milk sugar) is used instead of the much cheaper sucrose ( cane or beet sugar) in the Penicillium notatum fermentation to produce penicillin. Lactose has the same chemical elements and molecular weight as sucrose, but it gives a better yield of penicillin.

Although blackstrap molasses costs little as a fermentation material, it has so far been unacceptable in its impure state for some fermentations, such as the production of lactic and citric acids. We hope to uncover practical fermentations in which cull and surplus potatoes are an economical raw material.

The production of lactic acid, which was first studied, illustrates what might possibly be done with other fermentations.