Biosynthesis of sulfur volatile compounds in broccoli seedlings stored under anaerobic conditions

Sterile broccoli seedlings were used to investigate the effects of anaerobic atmosphere on biosynthesis and emission of sulfur volatile compounds which are responsible for off-odors in controlled and modified atmosphere stored broccoli. The identification of methanethiol, dimethyl sulfide, dimethyl disulfide and hydrogen sulfide in the head space of anaerobically stored broccoli seedlings confirmed that undesirable odors in broccoli are of plant origin. Evidence in support of the enzymatic origin of these volatiles was also obtained by using aminooxyacetic acid which is a potential inhibitor of pyridoxal phosphate-dependent enzymes; the production of sulfur volatiles was delayed and reduced by more than 95%. However, an anaerobic atmosphere had no inducible effect on the activities of cystine lyase, S-alkylcysteine lyase and cysteine desulfhydrases. These pyridoxal phosphate-dependent enzymes are thought to be involved in the production of volatile sulfur compounds by catalyzing the respective breakdown of cystine, cysteine and S-methyl-L-cysteine. The results suggest that these enzymes are not limiting factors in the production of sulfur volatiles in broccoli. However, the analysis of the contents of free sulfur amino acids and their derivatives showed an important increase in potential substrates for the synthesis of volatile sulfur compounds such as cysteine, methionine and S-methylcysteine as a result of anaerobic treatment. This increase usually preceeded or corresponded with the increased emission of sulfur volatiles. Results obtained by monitoring individual free amino acid content variation as a function of storage time, under anaerobic conditions, suggested enhanced conversions among sulfur amino acids. Other sources of sulfur amino acids, such as protein degradation and de novo synthesis, might explain the increase in these amino acids under anaerobiosis. (C) 1998 Elsevier Science B.V. All rights reserved.