Oil Degradation

Deep-frying

Frying is the process of cooking foods in contact with oil held at temperatures above the boiling point of water. One can identify two main types of frying processes, surface frying and deep-frying (or immersion frying). In surface frying, foods fry on each side separately by floating in an oil bath (e.g. frying of doughnuts) or in contact with an oil film (pan frying). Deep-frying involves complete immersion of the frying food in the cooking oil, and as a result the food is cooked from all sides simultaneously. Deep-frying is the most common type of frying in industrial and catering operations.

What changes occur in the oil during the frying process?

During deep-frying of food at temperatures in the region of 170oC– 180oC and the numerous chemical reactions occurring during frying result in changes to the physical and physico-chemical properties of the oil or fat (see Figure below).

The chemical reactions that take place during deep-frying include:

1. Hydrolysis – Occurs when foods are present (and therefore water is released from the food being fried in the form of steam bubbles). Hydrolysis products are, free fatty acids (FFAs), monoglycerides (MGs), diglycerides (DGs)and glycerol.
2. Oxidation – Triglyceride molecules in the frying oil undergo primary oxidation to unstable hydroperoxides which rapidly form secondary oxidation products comprising non-volatile and volatile compounds. The volatile compounds coming form the oil along with volatiles from the food being fried are responsible for the typical fried flavor. Some of the above oxidation products polymerize, increasing the oil viscosity.
3. Thermal Polymerisation – High temperatures of the frying operation produce high molecular cyclic fatty acid (FA) monomers, and TG dimers and oligomers.