Temperature at which smoke comes continuously from the surface of fat.
Development of free fatty acids by hydrolysis of fat during frying causes a decrease in the smoking point.
Fat that has had repeated or prolonged use will begin to smoke at a temperature too low for frying.
Suspended matter such as flour or batter particles lowers smoking point
Greater the surface of fat exposed the lower is the smoke point.
Fats heated in shallow wide pans with slightly sloping sides begin to smoke at low temperature than those heated in smaller pans, with vertical sides.
Smoking point of soya, cotton seed, pea nut and corn oils is 2300C.
The smoking point of hydrogenated fats is 221-2320C.
Shortenings containing monoglyceride as an emulsifier smoke at a lower temperature of about 1760C.
Changes in fat on heating
There are four main ways in which oil decomposes and all of these lead to a diminution of smoke point and operating temperature and lowers cooking quality, if the oils are kept for a long time.
Pyrolysis (thermal break down)
The action of heating oil , whether or not it is in contact with food, causes a break down in its molecular structure
When fat begins to smoke, its chemical break down begins and free fatty acids and acrolein are formed from glycerol.
If the fat is allowed to smoke for any periods of time , the acrolein causes irritation of eyes and nostrils.
Oxidation
Reaction of hot oil surface with oxygen in air causes molecular break down and development of rancidity.
Hydrolysis
As water is boiled in large quantities from food during frying, some of this water will tend to decompose the oil.
Reaction with food residues
Food fragments left in oil after cooking undergo chemical reaction particularly if the oil is heated without removing residues. Straining oil after use reduces this type of deterioration.