Why the seal on a mayonnaise jar hiss when you open it?

When a jar of mayonnaise is opened, the vacuum seal is broken, and air rushes in. This creates turbulence and friction which causes the oxygen-rich air in the jar to react with the unsaturated fatty acids in the mayonnaise, causing them to oxidise and release gaseous breakdown products. These breakdown products are mainly pentanal and hexanal, the aroma compounds that give mayonnaise its characteristic flavour, together with ethanoic acid, which produces a sizzling sound similar to a hiss.