Re-heating cooking medium such as sunflower oil could release toxic compounds, which could be linked with brain diseases and some types of cancer.
Researchers from the University of the Basque Country (the region comprising three provinces of northern Spain on the Bay of Biscay bordering france in the north-east) have been the first to discover the presence of certain aldehydes in food, which may be linked with neurodegenerative diseases and some cancers.
"It was known that at frying temperature, oil releases aldehydes that pollute the atmosphere and can be inhaled, so we decided to research into whether these remain in the oil after they are heated, and they do," said study co-author Maria Dolores Guillen, the journal Food Chemistry reports.
Until now these substances had only been seen in bio-medical studies, where their presence in organisms is linked to different types of cancer and neurodegenerative diseases such as Alzheimer's and Parkinson's, according to a statement.
The toxic aldehydes are a result of degradation of the fatty acids in oil, and although some are volatile, others remain after frying. That is why they can be found in cooked food. As they are very reactive compounds they can react with proteins, hormones and enzymes in the organism and impede its correct functioning.
The research involved heating three types of oil (olive, sunflower and flax seeds) in an industrial deep fryer at 190 ºC. This was carried out for 40 hours, eight hours a day in the first two, and 20 hours for the linseed oil.
The latter is not normally used for cooking in the west, but it has been chosen due to its high content in omega 3 groups.