An ester molecule is a combination of several smaller molecules linked together: an acid or acids with an alcohol. In fats the alcohol is glycerol, more familiar under its common name of glycerine. The glycerol molecule has three sites for connecting to fatty acids, and in a fat all three are filled. The fat molecule is therefore called a triglyceride molecule.
There is a wide assortment of fatty acids, and all fats and oils contain a mixture of them. Fatty acids can exist on their own, and may be detached from the glycerol which holds them. Their molecules vary considerably in length and weight. Those with the shortest molecules are volatile and have strong smells, noticeable when a fat breaks down and releases them, as happens when the fat becomes rancid. The shortest are butyric acid, one of the main smells of rancid butter; caproic, caprylic, and capric acids, which all smell ‘goaty’; and lauric and myristic acids, which are components of the smell of bay and nutmeg. Longer, heavier molecules are not volatile and have no smell. A fat with a majority of short and medium length acids is liquid at normal temperatures; one with predominantly long acids is solid.