The quality and safety of oils used for frying in restaurants often degrade quickly, leading to potential health risks, according to a new University of Guelph study.

As a health inspector, Anitta Sebastian often wondered about the quality and safety of frying oils used for deep frying in restaurants. When she started her master’s studies in food science at Guelph, she decided the topic would make a good research paper.

She gathered cooking oil from 20 downtown Toronto restaurants and had the samples analyzed at the University of Guelph. All of the samples showed extremely high oxidation levels, bringing into question the toxicological safety of foods. The results of the study were featured in the Toronto Star and on CTV News.

Sebastian and her adviser, Prof. Alejandro Marangoni, Food Science, found amounts of free fatty acids and peroxide were above acceptable food safety levels.

“Deep frying is very popular, and it can be quite useful in restaurants, but what I was seeing in my visits to these restaurants was that oil would sometimes be used for days at a time,” said Sebastian.

“My job covered a range of areas, but it did not include examining the cooking oil. The oil I cook with in my kitchen definitely does not look like this. I was curious if my concerns about the safety of the cooking oil would be proved right in the lab.”

The restaurants were chosen randomly and were all independent outlets, said Marangoni.

“Chain restaurants tend to have more direction and guidelines for the use and disposal of cooking oil,” he said.

“The more you reuse oil, the lower your costs are. In independent restaurants, the decision of when to throw out oil can be extremely subjective, and is often based on how the oil looks or smells. But there are potential health risks if people consume high amounts of food fried in unsafe oil.”

Free fatty acid (FFA) measurement is a key way to analyze cooking oils. The accepted range noted in a 2008 study for FFA is less than 0.1 per cent. FFA in fresh samples analyzed by the Guelph researchers ranged from 0.05 to 0.08 per cent. The collected in-use samples ranged from 0.25 to 3.99 per cent.

“This indicates the triacylglycerol components were undergoing hydrolytic degradation,” the study said.

Peroxide value is used to assess the oxidative state of oils. Oil is considered rancid when its peroxide value rises above 10 meq/kg oil.

For in-use samples, 35 per cent were above the rancid threshold and as high as 48.1 meq/kg oil. For discarded samples, the highest peroxide value was 247.5 meq/kg oil.

Even more worrying were the very high levels of secondary oxidation products in oils in use. All in-use and discarded samples showed extremely high levels of secondary oxidation products based on the p-anisidine value (p-AV) (7.6 b p-AV b 56.5), bringing into question the toxicological safety of foods prepared with such oils.

“Without doubt, a large proportion of the frying oils used in these commercial establishments were highly oxidized,” the report said.

Only European countries currently test oil safety, using a measure called total polar compounds (TPCs), which analyzes FFAs, peroxide, acids, alcohol, ketones and other substances.

The Guelph researchers found TPC testing often showed incorrectly that oil was within acceptable safety ranges.

“Our testing of individual levels of FFAs and peroxide showed this was clearly not the case, and the TPC test, while easy to do, is not very useful for estimating the degrading of oil,” said Marangoni.

He said a more useful test is a p-anisidine test based on a chemical colour change. The test kit costs $500 to $1,000 and about $100 a month to use.

“We can’t just have a time guideline to cover all restaurants, because each uses different oils and for different reasons,” he said.

“An oil test kit would be more accurate and ensure that customers are having their food cooked with quality ingredients.”

The study, “Quality and safety of frying oils used in restaurants,” appeared in the journal Food Research International.