This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
Frying is a process of cooking by which the heat is transmitted by the contact of the food with melted fat, butter, or oil, and not by radiation, as in the case of broiling or roasting. As explained by Williams, the fat does not necessarily boil, for the food, as well as the fatty material itself, may contain a considerable proportion of water which, by being suddenly vaporised, produces the familiar spluttering which accompanies the process of frying.
The boiling point of fats is very much above that of water, and the vaporisation of the latter is complete at 2120 F. Between 3000 and 5000 F. may be required to vaporise the so-called volatile oils, but fats and oils used in cooking do not apply to this class, and when heated above 4000 F. they turn dark brown or black and emit a disagreeable odour and smoke, leaving a non-volatile carbon residue.
The process of frying bears somewhat the same relation to boiling that the broiling of meat does, in that the heat employed is considerably greater. It is suddenly applied, and as a result the external surface of the food mass is coagulated and hardened before the juices in the interior have time to escape. For this reason, delicate fish, like the trout, is much more highly flavoured and palatable when fried than boiled. More or less butyric acid is developed from fat in frying.
The popular idea in regard to frying is that the fat used, whether butter, lard, or drippings, is simply for the purpose of preventing food from adhering to the frying pan, but, from the explanation of the process quoted above, it is seen that this is not the case, and the best frying is done by immersing the food completely in a bath of fat or oil. Even olive or sperm oil may be used for this purpose, and the fish or other food is lowered in an open wire basket or netting into a deep pan which contains the fat, in which it is completely submerged. There is no danger of the fat soaking into the food if it is sufficiently hot and if the process is not too long continued, for, as stated by Williams, "the water amid the fibres of the fish is boiling and driving out steam so rapidly that no fat can enter if the heat is well maintained to the last moment." Fritters cooked in this way are light and puffy from the sudden expansion of the water which they contain into large bubbles of steam, and are consequently decidedly more digestible.
Bacon fries in its own fat.
Frying is less perfectly understood by cooks than almost any other method of preparing meat, and the process as usually carried out results in very unwholesome products. The pans used are too shallow, and the food and fat are apt to become scorched.
When the meat or other material is dipped into hot melted oil or fat, more or less of it clings to the surface of the food, and for this reason may render it unfit for persons with feeble digestive powers. In the case of fish cooked in this manner with their scales, the fat which adheres to them may be easily removed when eaten, and the meat within will be found to be quite digestible; but meat, such as steak, cooked by frying is notoriously indigestible. Salt meat may be cooked first by boiling before frying, as in the case of hams, although the latter may be subsequently roasted instead of fried. Such meats always require prolonged cooking. According to Yeo, the addition of a little vinegar tends to make them more tender.