Formation of methyl ketones during ripening of Cheddar cheese

Abstract

SummaryCheddar cheeses made using single strains AM2and HP ofStreptococcus cremorisas starter were analysed for methyl ketones during ripening at 7°C. The level of odd-numbered C3–C15methyl ketones increased very slowly in all cheeses and at maturity represented only 15–20% of the level potentially available from the degradation of esterified β-ketoalkanoic acids of milk fat in cheese. The potential for formation of the methyl ketones in unripened cheese curd was similar to the sum of free and potentially available ketones in mature cheese which suggests that the oddnumbered ketones once formed are not reduced or otherwise degraded by the normal cheese microflora. Significant variations were observed in the level of 2-butanone, which, in some cases, far exceeded the quantity which could be formed by breakdown of β-ketovaleric acid. Organoleptic evaluations and comparison of the levels of methyl ketones present in mature cheese with published threshold data suggest that while the methyl ketones may contribute to flavour they do not in their own right impart a typical mature Cheddar flavour to cheese.