Characterization of the Reaction of Bisulfite with Unsaturated Aldehydes in a Beer Model System Using Nuclear Magnetic Resonance Spectroscopy

The mechanism and chemical equilibria of addition of bisulfite to unsaturated aldehydes were investigated using NMR spectroscopy. trans-2-Butenal was used to model the behavior of trans-2-nonenal. Reaction products were isolated and their relative stabilities analyzed. In the presence of excess bisulfite, the addition proceeded stepwise, to initially give a carbonyl adduct and ultimately yield a disulfonate as the thermodynamic product. The addition to the carbonyl moiety was rapid compared to the subsequent addition to the double bond. Preparative analytical procedures to liberate bisulfite were unable to revert the addition to the double bond. The irreversible nature of bisulfite addition to the double bond, and the stability of such adducts, did not support the generally accepted mechanism of the release of unsaturated aldehydes from nonvolatile species.