Formation of methyl ketones as artifacts during steam distillation of Cheddar cheese and butter-oil

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Abstract
Summary: A complete range of methyl ketones with odd numbers of carbon atoms from C3 to C15 was isolated by steam distillation at atmospheric pressure from fresh butter-oil, cream and Cheddar cheeses of various ages. Evidence was produced to show that the greater part of these methyl ketones was formed during the heat treatment of milk fat. The maximum quantities of methyl ketones obtainable from butter-oil, determined by exhaustive steam distillation at atmospheric pressure, averaged from 14 ppm. for undecan-2-one to 46 ppm. for pentadecan-2-one. Some artifact formation of methyl ketones also occurred, although to much less extent, when dairy products containing milk fat were steam distilled under reduced pressure at 40 °C. Possible mechanisms of formation of the methyl ketones are discussed.Since it proved possible to extract methyl ketones in low concentration from mature cheese at room temperature, it is evident that milk fat may contain precursors which break down to methyl ketones slowly during cheese ripening, this breakdown being accelerated at higher temperatures. Methyl ketones may therefore play a part in Cheddar flavour, but a previous report from this Institute that the time of first appearance of certain methyl ketones of odd numbers of carbon atoms above C5 in steam distillates, at atmospheric pressure from Cheddar cheese, coincided with the appearance of the typical Cheddar flavour was not confirmed.