Deep-fat frying is one of the most commonly used procedures
for the preparation and production of foods. However, deep-fat
frying produces both desirable and undesirable flavour compounds,
and it can change the flavour stability and quality, the
colour and texture, and the nutritional quality of the fried foods.
Hydrolysis, oxidation and polymerisation are common chemical
reactions during the use of frying oil, with the production of volatile
and non-volatile compounds, some of which are potentially toxic
(Choe & Min, 2007). Volatile compounds are lost from hot frying oil,
while non-volatile compounds steadily increase in concentration.
The non-volatile compounds are often of high molecular weight
and/or polarity, and their presence has been used in several studies
to indicate the quality of used or reprocessed oil (Lumley, 1988).
From the nutritional point of view, the non-volatile degradation
products of used frying fats and oils are of particular relevance, as
these can remain in the oil, and they can be retained in the food,
and subsequently ingested. Such non-volatile products include