Retro-aldol degradation mechanism

The alkadienals could be formed from the autoxidation of PUFA and may contribute desirable arenas to freshly prepared foods (19). Further degradation of alkadienals often increased undesirable flavors. Josephson and Lindsay demonstrated that 2,4-decadienal could produce 2-octenal and ethanal (20) and 2,6-nonadienal could produce 4-heptenal and ethanal (21) via retro-aldol condensation mechanisms. Hsieh et al. (22) reported that iscmers of various alkadienals and alkatrienals gave green, greasy and oxidized fish oil odors in crude menhaden fish oil. 

Figure 2. Mechanism for the Formation of 2-Pentenal, Propanal, 2-Propanone and Ethanal from the Thermal Degradation and Retro-Aldol Condensation of Linolenic Acid.

One possible mechanism for the degradation of 2,4-decadienal is the retro-aldol mechanism proposed by Josephson and Lindsay (8) for the oxidation of a,B-unsaturated fatty aldehydes. In their investigation of the degradation products of 2,4-decadienal, Josephson and Lindsay (9) identified 2-octenal and hexanal as primary products. Both 2-octenal and hexanal were identified in the present systems. 

Unsaturated aldehydes with a double bond conjugated to the carbonyl group are easily degraded during the deep frying process (Formula 3.95). Addition of water results in the formation of a 3-hydroxyaldehyde that is split by retro aldol condensation catalyzed by heat. Examples of this mechanism are the degradation of (E,Z)-2,6-nonadienal to (Z)-4-heptenal and acetaldehyde, as well as the cleavage of 2,4-decadienal into 2-octenal and acetaldehyde. 

---