For Hong Kong food lovers, that golden, crispy fried pork chop or order of chips may be hiding a serious health threat. According to Taiwanese family physician Dr. Li Sixian, cooking oil repeatedly heated and reused—dubbed “eternal oil”—can accumulate cancer-causing compounds. New research shows that oil fried as few as 60 times can surpass international safety limits, exposing consumers to harmful chemicals that damage the liver and disrupt metabolism. Experts now share three unmistakable warning signs to tell when fryer oil has turned toxic.
The Science Behind “Eternal Oil”
Dr. Li, a popular health educator on social media, cited a study in which vegetable oil was used to fry potatoes up to 60 consecutive times. After that many rounds, the polar compound content in the oil skyrocketed to nearly 28 percent—exceeding the internationally accepted discard threshold of 25 percent. Polar compounds are byproducts of oil breakdown and are linked to inflammation and disease.
But polar compounds are just the beginning. Repeated high-temperature frying also generates dangerous substances including 3-MCPD and glycidyl esters, both classified as potential human carcinogens. These pollutants accumulate each time the oil is reheated, meaning the longer a fryer runs without a full change, the higher the toxic load.
Animal Studies Reveal Alarming Health Effects
Laboratory experiments on rats have shown devastating consequences from consuming repeatedly heated oil. Dr. Li highlighted two key studies:
- Liver damage and precancerous changes: When rats were fed used oil, researchers detected polycyclic aromatic hydrocarbons (another carcinogen) that were not originally present. Liver cell chromosomes showed increasing damage with dosage, antioxidant enzyme levels dropped, and early signs of cancerous transformation appeared.
- Metabolic syndrome precursors: In a separate experiment, oil heated to 300°C for 30 to 240 minutes was fed to rats for 45 days. Longer heating times led to higher LDL (bad) cholesterol and triglycerides, while HDL (good) cholesterol decreased. The animals also developed fatty liver and leptin resistance—both warning signs of cardiovascular disease.
Why Does Oil Turn Dangerous?
Cooking oil, when exposed to high heat, air, and moisture from food, undergoes oxidation, hydrolysis, and polymerization. Healthy fatty acid chains break apart, forming free radicals and polar compounds. Consuming these degraded fats overwhelms the liver’s antioxidant defenses, triggering lipid peroxidation and chronic inflammation throughout the body.
Three Warning Signs to Watch for
While consumers cannot control how often restaurants change their oil, Dr. Li advises paying close attention to these indicators when buying fried food—or frying at home:
- Dark, cloudy oil: Fresh oil is clear and golden. If the oil appears deep brown, black, or murky, it has severely oxidized.
- Persistent foam: A little bubbling during frying is normal, but if the oil produces a thick, long-lasting foam that does not dissipate, the oil has degraded.
- Pungent, rancid smell: Good oil has little scent. A sharp, acrid “off” odour (often described as stale or glue-like) means the oil must be discarded immediately.
What This Means for Hong Kong Diners
Fried food is a beloved staple of Hong Kong’s street food culture—from siu mei shops to dai pai dongs. Yet the invisible quality of the frying oil can vary wildly. Dr. Li’s message is clear: even if you cannot avoid fried treats entirely, being aware of these visual and olfactory clues helps you make safer choices. When oil looks dark, foams excessively, or smells bad, it is time to skip that batch.
For home cooks, the simplest rule is to never reuse oil more than two or three times, and to strain it after each use to remove food particles that accelerate breakdown. Store oil in a cool, dark place and smell it before each reuse.
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Source: Dr. Li Sixian’s Facebook page, referencing peer-reviewed studies on cooking oil degradation.