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 3/4 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)................

233

0.07

Snowdrift.............................

232

0.06

Crisco................................

231

0.13

Leaf lard..............................

221

0.15

Butter fat.............................

208

0.28

Leaf lard heated 5 hours................

207

0.34

Bulk lard.............................

194

0.51

A much-used lard......................

190

0.61

Olive oil..............................

175

0.92

Peanut oil, I

162

1.10

Peanut oil, II............

149

1.64

Coconut oil.........

138

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.

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 proportions was added, its smoking temperature being lowered with the increased oleic acid content.

Surface exposed. They report that the smoking temperature for a particular fat is lowered when the surface exposed is increased. Thus the wider the diameter of the cooking utensil used, the more the smoking temperature is lowered. They advise using a vessel with as small a diameter as is convenient for deep fat frying.

Foreign particles. They have found that another factor that lowers the smoking temperature is the presence of finely divided foreign particles in the fat. Flour and small particles of other materials accumulate in a fat used for cooking purposes. Hence, a much-used fat has a lower smoking temperature than the original fat.

Smoking temperature and fat absorption. The cooperators in fat investigations at Iowa State College found that absorption of fat by doughnuts is influenced to a greater degree by the smoking point of the fat than by any other chemical or physical factors investigated. Fat absorption was greater the lower the smoking point of the fat or oil.