That irresistible golden crunch from a basket of freshly fried chicken or spring rolls is tough to beat. But a Taiwanese family medicine physician is urging Hong Kong food lovers to look closer at the pot of oil behind that satisfying texture. Dr. Lee Sian, popularly known as Szu Szu Doctor, is warning that repeatedly heated cooking oil—sometimes called “eternal oil” in local food culture—can silently accumulate cancer-causing compounds with every batch it fries.
The Science Behind the Sizzle
Dr. Lee points to a revealing study that analysed vegetable oil used for continuous deep frying. After frying potato batches sixty times, the level of dangerous polar compounds in the oil surged to nearly 28 percent. This is alarmingly above the 25 percent international safety threshold, the point at which oil is officially considered unfit for consumption and should be discarded.
The real concern lies in chemicals like 3-MCPD and glycidyl esters, both of which form when oil is repeatedly blasted to high temperatures. These substances are classified as potential carcinogens by food safety authorities worldwide. Regular ingestion of these compounds places a significant toxic burden on the body’s internal systems, raising the stakes for anyone who frequently eats deep-fried fare from street stalls or restaurant fryers.
What Happens Inside the Body
The damage is not merely theoretical. Animal studies shared by Dr. Lee reveal stark consequences. In one experiment, rats fed oil heated to 300°C showed clear signs of liver damage, including chromosomal injury to liver cells and early pre-cancerous lesions. A separate study tracked the metabolic fallout: total body inflammation increased, “bad” LDL cholesterol and triglycerides climbed sharply, while protective HDL cholesterol dropped. The animals also developed fatty liver and leptin resistance, a metabolic red flag that strongly predicts cardiovascular disease and diabetes.
On a molecular level, high heat forces oil to break down through oxidation, hydrolysis, and polymerisation. Oxygen and moisture from food attack the fatty acid chains, generating free radicals. When consumed, these degraded fats overwhelm the liver’s natural antioxidant defenses, driving a cycle of chronic inflammation that damages tissues over time.
Spot the Warning Signs: 3 Checks Before You Fry
While diners cannot monitor the oil at their favourite cha chaan teng or hawker stall, Dr. Lee offers a simple checklist for home cooks—and a useful guide for anyone ordering takeaway.
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1. Colour and Clarity. Fresh oil is golden and clear. If the oil in the pan looks dark brown, almost muddy, or appears cloudy with visible sediment, it is heavily oxidised and must go straight into the discard bin.
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2. Foam That Lingers. A gentle sizzle when food hits the oil is perfectly normal. But if the surface erupts into thick, soapy foam that refuses to subside, the chemical structure of the oil has severely broken down. This is a clear signal that your oil has reached the end of its usable life.
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3. The Rancid Stench. This is the most reliable indicator of all. Fresh cooking oil has a clean, neutral aroma. If you detect a sharp, acrid, or stale rancid odour—often described in Cantonese as having a distinctive lui mei or yi mei character—the oil is chemically spent. Do not use it under any circumstances.
A Broader Lesson for Hong Kong Food Culture
Dr. Lee’s insights arrive at a time when convenience and thrift often dominate kitchen habits. Many home cooks hesitate to discard oil after one or two uses, and busy eateries may stretch fryer oil to cut costs. But this research makes it clear: the savings are not worth the long-term health price.
For food lovers across Hong Kong, the takeaway is straightforward. Enjoy your crispy treats, but stay vigilant. Look at the oil. Give it a sniff. Ask questions about how often your local shop changes their fryer. Building better awareness today can help protect your liver, your metabolism, and your overall health for years to come. For more guidance on healthy cooking practices, check out our related features on air fryer safety and how dietary choices impact metabolic health.