Photooxidation of tryptophan leading to 2-aminoacetophenone--a possible reason for the untypical aging off-flavor in wine.

2-Aminoacetophenone (AAP) was recognized as the key compound for the so-called untypical aging off-flavor (UTA) in Vitis vinifera wines. In this study, it was shown that AAP can be formed by photooxidation of free and protein-bound tryptophan (TRP) in combination with a subsequent storage in model wine. Solutions of TRP and lysozyme were exposed to artificial sunlight both in the presence and in the absence of the photosensitizer riboflavin. Aliquots of the irradiation batches were stored in model wine solutions containing tartaric acid, sulfite and ethanol in different combinations. AAP formation could be identified from both free and bound (lysozyme) TRP, while free TRP resulted in higher yields. The presence of riboflavin during irradiation generally favored the AAP formation. AAP formation increased with increasing irradiation times, but AAP was not detectable, if TRP was directly incubated in model wine. Not only the irradiation time but also the storage time of model wines favored the formation of AAP. Concerning the model wine composition, it became evident that the presence of tartaric acid resulted in the highest AAP formation during storage.