Why do crispy fries contain the chemical lab gels are made from?

Because frying starchy food above 120°C makes acrylamide, and acrylamide is the building block of the polyacrylamide gels used in labs. In food, it forms in the browned, burnt areas of starchy foods like potatoes cooked at high heat. In industry, most acrylamide is produced to make polymers, especially polyacrylamide, a water-soluble material of very low toxicity used as a thickener and flocculating agent; polyacrylamide gels are routine tools in medicine and biochemistry, and in molecular biology they are used for electrophoresis.

In food, acrylamide comes from an unexpected route: the amino acid asparagine reacts with glucose, one of the Maillard reactions, and further steps and heating then generate some acrylamide. That's why it turned up mainly in starchy foods such as potato chips, French fries and bread heated above 120°C, why its production depends on temperature, and why it was not found in boiled or unheated food. It even appears in roasted barley tea, though at lower levels than fried potato snacks.

The discovery in 2002 caused international health concerns. The International Agency for Research on Cancer and two US agencies classify acrylamide as a probable carcinogen. Yet epidemiological studies as of 2019 suggest that acrylamide in the diet does not significantly increase people's risk of cancer, food industry workers exposed to twice the average level do not have higher cancer rates, and Cancer Research UK classes the idea that burnt food causes cancer as a "myth."