Changes during heating oil as the heat transfer medium

6 Changes during heating oil as the heat transfer medium 

Frying is a process in which food is heated in contact with hot oil. In the case of shallow frying, the layer of oil in the pan is only 2-20 mm thick, so that food is only partially immersed. The frying oil is not usually used for further frying, but it is used for flavouring the fried meal. 

Changes of antioxidants in frying oil are usually very pronounced as it is used repeatedly for frying, sometimes for several days or even weeks. In the case of intermittent frying, hot frying oil is left to cool without further 

The destruction of antioxidants in fried food is much lower than in frying oil as the contact of food with frying oil is only short. Antioxidants are oxidised only in the surface layers. The deep frying takes only a few minutes so that only the outer layers of deep fried food are heated to 150-180°C, but the inner layers are not oxidised to more than 100°C in such a short time. Therefore, losses of antioxidants in the inner layers are negligible. Many food components, such as proteins, flavonoids or tannins, inhibit the destruction of antioxidants in fried food during the frying operation (Pokorny, 2000). 

Oxidation reactions of fried food occur rather intensively on storage as the thin film of flying oil left on the surface of fried food is easily attacked by oxygen from the air. The stabilisation of fried food against oxidation on storage thus offers a more important problem than does oxidation during the flying operation. 

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