When oil smokes during cooking, what happens to the geometry of the fatty acids?

• A. It is changing shape from Trans to Cis
• B. It is changing shape from Cis to Trans
• C. Nothing, it is normal for oil to smoke in high temperature cooking
• D. Nothing, when oil smokes, that is how you know it's ready for frying

Answer: (B)

Heating unsaturated fatty acids can cause geometric isomerization. The cis double bonds that give many oils their kinked, fluid shape can rearrange under high heat to the trans configuration, producing a more linear molecule. When oil reaches its smoke point, this cis-to-trans shift changes the geometry of the fatty acids. That’s why the best answer is a shift from Cis to Trans. The other statements don’t reflect the directional change that occurs with high-heat isomerization, nor do they describe what the smoking point indicates about fat structure.