Experimental cookery

Heat decomposition products o£ glycerol. Some free and uncom- bined fatty acids and glycerol are usually found with the fats. When heated to a high enough temperature the glycerol decomposes, losing two mole- cules of water and forming acrolein. Acrolein is an aldehyde that has a sharp odor and is irritating to mucous membranes of the nose and throat and to the eyes. The odor and irritation are familiar to all who have had experience with "burning fats." 548 FATS AND OILS Free fatty acids. The amount of free fatty acids found in some fats and oils is greater than in others. As previously stated, upon hydrolysis, which may be brought about by enzymes, the lipases, or by boiling with alkali, fats and oils split into glycerol and fatty acids. In cooking materials in fat, particularly foods with considerable water content, the percentage of free fatty acids is increased. During heating in the presence of moisture some hydrolysis occurs, yielding glycerol and free fatty acids. The Smoking Temperatures of Fats When fats or oils are heated to a high temperature decomposition occurs and finally a point is reached at which visible fumes are given off. This point is called the smoking temperature. Some fats decompose at lower temperatures than others; those that smoke at low temperatures are not pleasant to use because of the odor and irritating effect of the fumes. The decomposition also gives an unpleasant and disagreeable flavor to the food. Therefore, it is preferable to use fats with high smoking temperatures for sauteing and deep fat frying. Blunt and Feeney investigated the smoking temperatures of edible fats. They found the smoking temperatures of various fats when tested in a small evaporating dish having a diameter of about 3^ inches to be as follows : TABLE 60 The Smoking Temperatures of Some Edible Fats {Blunt and Feeney) Kind of fat Smoking temperature, °C. Free acid as oleic, per cent Cottonseed oil (Wesson) Snowdrift Crisco 233 232 231 221 208 207 194 190 175 162 149 138 0.07 0.06 0.13 Leaf lard 0.15 Butter fat 0.28 Leaf lard heated 5 hours Bulk lard A much-used lard 0.34 0.51 0.61 Olive oil 0.92 Peanut oil, I 1.10 Peanut oil, II 1.64 Coconut oil 1.90 The following factors cause variations in the smoking temperatures of fats and oils: (1) the amount of free fatty acids present, (2) the surface exposed, and (3) the presence of foreign particles. TYPES OF FAT SPOILAGE 549 Free fatty acids and smoking temperature. Blunt and Feeney have determined the amount of free fatty acids in the fats they used and report as shown in Table 60 that the fats with the highest percentage of free fatty acids have the lowest smoking points, from which they decide that the smoking temperature of the fat is closely dependent upon its acidity. To confirm this they removed the free fatty acid from some of the olive oil and found its smoking point to be 234°C. and its free acid, as oleic, 0.03 per cent. To some of the cottonseed oil, oleic acid in different pro- portions was added, its smoking temperature being lowered with the in- creased oleic acid content.