Suppression of Volatile Sulfur Compounds from Broccoli by Freeze-Thawing and Their Mechanism with Filtration and Centrifugal Fractionation

The formation of methanethiol and dimethyl disulfide in the broccoli tissue homogenates following filtration and centrifugal fractionation, and their suppression by freeze-thawing was investigated. Methanethiol was formed only in the headspace of the residue but not in the filtrate or supernatant fractions like dimethyl disulfide. Freeze-thawing, however, suppressed their formation in all homogenate fractions. The immersion of the fresh residue in a liquid medium decreased the concentration of methanethiol which indicates that this compound is highly likely soluble in water and produces only in the headspace of the environment wherein the condition is free of liquid medium. Hence, methanethiol formation was not possible except for dimethyl disulfide after addition of the co-factor, pyridoxal 5-phosphate, in the frozen residue. Both compounds were also inhibited by aminooxyacetic acid, a potential inhibitor of pyridoxal phosphate-dependent enzymes, which indicates that MT and DMDS are produced upon the action of cysteine sulfoxide lyase. This enzyme, which yielded strong activity in the residue upon extraction with buffer containing Triton X-100, is highly likely to be a bound enzyme. Suppression of methanethiol and dimethyl disulfide during freeze-thawing is likely attributed to the retardation of the enzyme action due in part to the loss of the co-factor, pyridoxal 5phosphate and its substrate, S-methyl-L-cysteine sulfoxide and owing to the solubility of methanethiol and its affinity for gaseous condition.