Unheated Coronopus-tainted milk yielded butterfat from which benzyl methyl sulphide, benzyl isothiocyanate, benzyl cyanide, indole and skatole were isolated by high-vacuum distillation. The first 3 of these compounds were also isolated from the corresponding skim-milk, buttermilk and butter serum by vacuum steam distillation: identification was by gas chromatography and mass spectrometry.
The flavour threshold of benzyl methyl sulphide in milk and butter oil was found to be respectively, 1 part in 108 and 1 part in 107, whereas the amount isolated from butterfat was approximately 1 part in 106.
This evidence, together with the similarity in flavour of the Coronopus taint in milk and butter oil with that of untainted milk and butter oil to which had been added respectively, 1 part in 107 and 1 part in 106 of benzyl methyl sulphide, demonstrates that benzyl methyl sulphide is a principal contributor to the flavour defect. A hypothesis is proposed to explain the origin of the sulphide and related components in the milk. The possibility remains that in commercial weed-tainted butter, which is made from heated cream, other compounds may be present.
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