Abstract
During the processing of onion, pink-red colored pigments are often formed. The process is believed to be a multistep process including enzymatic and non enzymatic reactions. In order to investigate this process, we developed a reaction system, where pink-red pigments (‘pinking’) can be formed by reacting amino acids with onion thiosulfinate formed by reacting an isolated garlic alliinase and (+)-S-1-propenyl-L-cysteine sulfoxide (1-PeCSO) in the natural onion juice. The unknown pink-red pigments formed during this process were separated and detected using a high-performance liquid chromatography (HPLC) and a diode array detector (DAD) at 515 nm. Fractions collected from this separation were further analyzed using liquid chromatography (LC) and tandem mass spectrometry. Similar head group structure, two 3,4-dimethyl pyrrolyl rings that were cross linked by allyl group, was determined for all conjugate-pigments formed from different amino acids based on the accurate and tandem mass spectrometry. However, the tail group attached to the N-terminal of pyrrole ring differed for each pink-red pigment depending on the amino acid used. In addition, in most cases more than one pink compound were identified for the same amino acid used. We presumed that the complexity of the pink-red pigments was due to the involvement of 21 natural amino acids and other derivatives of the products. Finally, we suggest that the pinking process in crushed onion is very similar to the greening process in crushed garlic, emphasizing that thiosulfinates from flavor precursors and free amino acids are absolutely necessary during the discoloration.