This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Masters and Smith have investigated the character of the changes in fat during cooking. They state that the changes occurring in the fat in cooked pastry are slight, unless the pastry is very thin or over-cooked.
Woodruff and Blunt have reported the changes in fats absorbed by-potato chips and a dough mixture. They state that the iodine number is lowered, and the percentage of free fatty acids increased. These changes are not great, but the fat absorbed by the food undergoes greater changes than the fat in which the food is cooked. Long cooking or use of the fat increases the amount of decomposition of the fat. The higher the temperature to which the fat is heated, the greater the decomposition products formed. Thus during cooking the fat should not be heated higher than the temperature needed for cooking the food.
Morgan and Cozens found that fats used for frying a standard dough showed consistent decreases in iodine number, lowering of the melting point and increases in acidity and refractive index. They state that in general the increase in acidity is greater in those fats showing the larger amounts of absorption.
King et al. used 9 fats, including 3 kettle-rendered lards from animals fed on rations consisting largely of (1) peanut, (2) corn, and (3) brewer's rice, a standard prime steam lard, a hydrogenated lard, a hydrogenated cottonseed oil, and three highly refined oils from corn, cottonseed, and peanuts, respectively, for studying frying of potato chips. The fat absorbed by the chips was about the same for all the fats used. Chemical tests showed deterioration of all fats. After 10 fryings the fitness of the fats, particularly the lards on account of their high peroxide value, for further use was questionable. They found no relation between chemical changes and the flavor of the chips. The percentage of free fatty acids in the fat extracted from the chips was greater than in the frying fat. Based largely upon desirability of flavor the palatability tests indicated oils were preferable to lards for frying potato chips, those fried in peanut oil ranking highest with cottonseed oil second. Of the lards, the hydrogenated and that from animals whose rations consisted largely of peanuts were considered best. Potato chips fried in oils and in most cases those stored in the refrigerator kept fresh longest.