Bacillus thuringiensis Insecticides: Source of 2-Aminoacetophenone Formation in Wine?

Abstract

2-Aminoacetophenone (AAP) is mainly responsible for the “untypical aging off-flavor” (UTA) in Vitis vinifera wines, a negative change in the wine bouquet, which is described by odor taints such as ‘furniture polish’, ‘wet wool’, ‘acacia blossom’ or ‘mothball’. AAP formation can be caused by photooxidation of tryptophan (TRP), which was previously shown under model conditions, by exposing free and lysozyme–bound tryptophan to UV light to be stored in model wine containing tartaric acid, sulphur dioxide and ethanol. The aim of the present study was to demonstrate the formation of AAP from Bacillus thuringiensis (Bt) as TRP source and, therefore, to identify Bt as a possible reason for UTA appearance in wine. Bt produces toxins and spores during its sporulation phase. The toxins are proteins, which include a number of TRP residues and which are, besides the spores, responsible for its toxicity towards insects. In viticulture, Bt based products are used to fight the European grape vine moth. Applied on grapes, Bt is exposed to sunlight, where Bt-bound TRP might be photooxidized yielding AAP precursors, which are released into the grape must and transformed to AAP during acidic wine storage. To prove this hypothesis, UV irradiated and non-irradiated Bt spray liquids were first transformed into model wines, which then were stored in the absence of light. Non-irradiated Bt spray liquids produced AAP, but prior UV light exposure prevented AAP formation. In a second step, harvested grapes were studied, non-sprayed or sprayed with Bt, treated with or without UV irradiation and processed into wines. Similarly high AAP concentrations were obtained in all wines independent of the treatment with Bt or UV light, indicating that AAP precursors were already present on the studied grapes. However, the application of Bt in this study did not further enhance AAP formation, suggesting that additional factors such as aging processes of Bt residues in the vineyard might be involved.