By chance, we noticed a massive fungal growth causing lipolysis of margarine which had been accidentally contaminated with blue cheese. The fungus also produces large amounts of methyl ketones, including 2-pentanone. Cultures from contaminated margarine and cheese grew the same organism, Penicillium roqueforti. Members of many filamentous fungi, including Penicillium sp. and Aspergillus sp., produce methyl ketones when grown on fatty acids as their sole energy source [7]. The cheese industry utilizes products produced by Penicillium roqueforti to give blue cheese its distinctive flavor. Medium-chain fatty acids are shortened by one carbon unit during conversion to a range of methyl ketones, generally with an abundance of 2-heptanone > 2-nonanone > 2-pentanone > 2-undecanone, although differences between cultures There are 2-pentanone differences [8-10]. Despite its long-standing commercial application, the biochemical pathways involved have not been clearly demonstrated. From the collective evidence, methylketones appear to be formed by a futile cycle associated with incomplete β-oxidation of fatty acids. It has been suggested that β-oxidation normally proceeds to the formation of a medium-chain 3-oxoacyl-CoA intermediate, but is subsequently stalled in the medium-chain acyl-CoA thiolase reaction. Thioesterase releases coenzyme A (CoASH) from the accumulated intermediate, decarboxylation of 3-oxoacids to methyl ketones. CoASH may then be recycled to initiate the oxidation of more fatty acids, but their metabolism will again be bottlenecked by thiolase enzymes [7, 8, 11, 12]. This process likely occurs via the peroxisome β-oxidation pathway rather than mitochondrial β-oxidation. Penicillium rosenbergii has peroxisomes (microorganisms) and methyl ketones produced by other fungi are associated with a large increase in the number of peroxisomes characterized by catalase [13]. Figure 1 shows the proposed pathway for the production of 2-pentanone from hexanoic acid.