This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
What constitutes quality in potatoes? For baking and boiling it is usually accepted that the best potatoes are those that yield a white, comparatively dry, and mealy texture. The homemaker does not like for boiling a potato that sloughs off badly during cooking, though this often occurs with some of the best baking potatoes. For deep-fat frying, dicing for salad, and other purposes a potato that gives a waxy and firm consistency is considered best.
Some investigations on potato quality. It is rather commonly believed that to have mealy potatoes the starch content should be high and that the swelling and bursting of cell walls during cooking produces the white, fairly dry, flaky appearance. Sweetman states that the starch grains do not swell sufficiently to burst but the cooking renders the starch granules readily separable. That ease of cell separation is one of the causes of mealiness is unsubstantiated by other investigators. Whittemore and Juschke decided that potatoes were more mealy when fertilized with a high rather than a low quantity of potash (potassium carbonate).
Cobb states that the parts of the tuber having the greatest concentration of starch are mealiest when cooked. The rank in descending order is cortex, external medulla, and internal medulla. Cobb in a series of investigations covering 5 years found that baking gave more mealy potatoes than boiling.
Baked potatoes often lost 25 per cent of their weight by evaporation, whereas boiled ones lost none.
Environmental factors. Cobb concluded that if proper cultural conditions were given any type of soil may grow good potatoes. His studies showed that temperature during growing and variety were the most important factors affecting potato quality. A low-average temperature favored better quality. This suggests that one reason why Maine and Idaho are noted for their potatoes is the low-average temperature during growth of the potatoes. Small, immature, or ill-shaped tubers were observed to have poor quality.
Stevenson and Whitman state that a variety producing good-quality potatoes under certain conditions may produce poor ones under other conditions, but a good-cooking variety tends to maintain better quality over a wider range of conditions than a poor-cooking quality.
Starch granules. It is possible that two factors determining whether a potato produces a mealy or waxy consistency when cooked are the size of the starch granule and its phosphorus content. See Starch, Chapter XL Large granules swell and gelatinize at a lower temperature than small granules. Thus, potatoes giving a dry, mealy texture may have a comparatively larger percentage of large granules. Smaller granules are supposed to have a higher phosphorus content than large ones, and the paste-forming qualities of starch are increased when the starch is combined with phosphorus. If paste-forming qualities mean increased adhesiveness, then waxy potatoes may have either smaller granules or a higher phosphorus content or both.
Storage temperature. Wright et al. found that the storage temperature affected the quality of the cooked potatoes. They stored Irish Cobbler potatoes harvested August 3 for 8 weeks at the following temperatures: 32°, 36°, 40°, .50°, 60°, and 70°F. In general potatoes stored at 40° were considered fair in quality when cooked; those from storage at 32° and 36° were poor; those from 50° good; those from 60° very good, and those from 70° good. The baked and boiled products from potatoes stored at 40° were slightly sweet and watery; those from 36° storage were inclined to be soggy or watery, with a distinctly yellow color and sweet flavor; those from storage at 50°, 60°, and 70° possessed no undesirable sweet flavor and were of mealy texture and cream in color.
Potato chips. The most important factor in making potato chips appears to be the sugar content of the potato. With a high sugar content the chips become too brown, with production of a more or less disagreeable astringent flavor, before they are sufficiently cooked. The concentration of sugar increases in potatoes stored at a low temperature. At ordinary temperatures the sugar is used up in respiration; at lower temperatures metabolism is retarded but sugar production by enzymes proceeds, so that sugar accumulates.
Peacock and Brunstetter have reported a very simple test for predetermining the cooking value of potatoes because of accumulated sugars. One cubic centimeter of a saturated aqueous solution of picric acid and 1 cubic centimeter of a 20-per cent sodium carbonate solution are placed in a test tube. To this is added a piece of potato of definite length cut with a cork borer. The solution is heated carefully to prevent boiling over for 1 minute. If the solution does not become much darker it indicates that the potato has little sugar and is suitable for both chips and "French fries." With increasing sugar content the solution becomes darker and darker when time of heating and size of piece of potato are kept standardized. The darkest shade given by Peacock and Brunstetter is a dark garnet-brown; but Swalley tested some potatoes that gave shades two degrees darker in color than dark garnet-brown, the darkest being nearly black. Potato slices fried for a definite time in fat at a definite temperature become browner and browner as the sugar content increases and give perfect correlations with the picric acid test. In general Swalley found that the longer the potatoes were stored at 32°F. (storage was from 11-14 to 12-15), the longer the time required for storage at room temperature (those stored at 32° for 31 days required 45 days at room temperature) before good chips were obtained.
Wright et al. found that potatoes stored at 32° and 36° were not suitable for French fries, as they browned too quickly and had a burned flavor. To some extent this was also true of potatoes stored at 40°. As the storage temperature increased the time necessary for cooking increased. Potatoes stored at 60° and 70° gave fries of attractive golden-brown color, mealy texture, and good flavor. Similar results were obtained with chips.
Preparation of chips. Because it is difficult or impossible to slice the potatoes evenly by hand, they should be cut with a slicer. If salt is added to the soaking water the slices are partially dehydrated during the soaking; thus a shorter time is required for cooking and no addition of salt is necessary after cooking. For 1/4 pound of potato and 1/2 cup of water about 8 grams of salt are required.
Cooking chips. The cooking time and temperature are dependent to a certain extent on the quantity of potatoes added to a given amount of fat. For 1/4 pound of slices about 3 minutes are required when the initial temperature of the fat is 190°C. and the quantity of fat is about 2 to 21/2 pounds. Smaller quantities of potatoes cook in a shorter time. The chips are done when bubbling of the fat ceases, which indicates the major portion of the water has evaporated.