Identification and Quantitation of Four New 2-Alkylthiazolidine-4-carboxylic Acids Formed in Orange Juice by a Reaction of Saturated Aldehydes with Cysteine

Despite several technological efforts to maximize the quality and shelf life of chilled stored not-from-concentrate orange juice, changes in the overall aroma profile might occur during storage. Besides the degradation of terpenoids, a loss of the aroma-active aldehydes, hexanal, octanal, nonanal, and decanal as well-as of 1-penten-3-one were recently confirmed as a major cause for the changes in the aroma profile of orange juice even during storage under aseptic conditions at 0 °C. To unravel the fate of the aroma-active aldehydes, model experiments were carried out considering the oxidation into the corresponding acids as well as a reaction with free amino acids present in orange juice. The oxidation into the acids could be confirmed by isotope labeling experiments; additionally, the reaction of the four aldehydes mentioned above with l-cysteine yielded four new compounds identified as 2-alkylsubstituted thiazolidine-4-carboxylic acids. Their quantitation in orange juice samples by newly developed stable isotope dilution assays revealed that these acids were already present in the fresh samples but were considerably increased after storage. Labeling experiments in orange juice administered with either labeled octanal or labeled cysteine confirmed that the reaction quickly occurs in the juice. The data contribute another puzzle piece to the loss of aroma-active aldehydes during orange juice storage, which may also be relevant in other foods.