The system for flavour production in the onion consists of the enzyme alliinase and the flavour precursors, the S-alkyl-L-cysteine sulphoxides. Flavour production in the onion and in the callus were compared by looking at each part of the system in turn. Measurement of the specificity and Km values of the alliinase from onion bulb and callus showed the enzymes to be identical. The total amino acid level was lower in the callus tissue than onion bulb tissue and the composition was altered. However, this change in amino acid metabolism did not appear to affect the synthesis of the flavour precursors since addition of the important amino acids L-cysteine and L-valine to the medium did not lead to the production of an onion smell from the callus upon crushing. The most noticeable difference was in the composition of the flavour precursors. In the callus S-methyl-l-cysteine sulphoxide was present at a low concentration while the major precursor of onion flavour S-trans-prop-1-enyl-l-cysteine sulphoxide, was absent altogether. It was suggested that the biosynthetic pathway for S-trans-prop-1 -enyl-l-cysteine sulphoxide was not functioning in the callus.
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