A variety of fatty acids have been detected in the odour of human faeces; the major one of these is ethanoic (acetic) acid, which of course is more commonly known as vinegar. However, its contribution to the overall odour is low. Much more important are the longer chain fatty acids, butanoic (butyric) and pentanoic (valeric) acid. Butanoic acid is actually the main odour-causing component of human vomit, to give you an idea of its aroma. Pentanoic acid has a similarly unpleasant smell, and the two together give a ‘farmyard’-like smell. This is also contributed to by a compound that doesn’t fit into the three categories, para-cresol, which is also a major player in the odour of pigs.