Fat-borne volatiles and sheepmeat odour.

The effects of lamb age and diet on volatiles from fat are described. Rendered fat from ram lambs raised on ewe's milk then a corn-based diet was compared with that from lambs raised on milk and a pasture of grass/clover, six treatments in all. An additional treatment comprised very old ewes maintained on pasture. Helium-borne volatiles of rendered fat were resolved on a DB5 gas chromatographic column and the mass spectra obtained. Long chain alka(e)nes like neophytadiene were dominant in pasture treatments especially where the lamb growth rate was slow. Branch chain fatty acids (4-methyloctanoic, 4-methylnonanoic and an unidentified acid) were also highest in these treatments. Longer chain aldehydes like 2-undecenal were good indicators of a grain diet. Hexanal, commonly associated with rancid odours, was unaffected by treatment. The diketone 2,3-octanedione was an excellent indicator of a pasture diet, as was 3-methylindole (skatole). Phenols showed complex relationships to treatments, but were generally more common in pasture treatments. Benzenethiol (thiophenol) was unaffected by treatment. Inspection of principal component analysis plots identified 10 volatile compounds as contenders for the cause of sheepmeat odour; branch chain fatty acids were confirmed as the leading chemical class. There were indications that puberty or age caused an increase in the odorous 4-methylnonanoic acid. Animal odour-the odour of confined livestock-was clearly causally linked to 3-methylindole, a rumen breakdown product of tryptophan. 3-Methylindole was also responsible for rancid odour, rather than hexanal and its analogues. A hypothesis is advanced that links 2,3-octanedione formation to the enzyme lipoxygenase and linolenic acid, both abundant in green leafy tissue. Overall, the data confirm that sheepmeat odour/flavour is specifically linked to the branch chain fatty acids, and is probably exacerbated by pasture-derived 3-methylindole and alkyl phenols.