UNDER THE MICROSCOPE: OILS AND FATS

SOURCES. Oils are essential constituents of animal and vegetable tissues and as such are found in a wide range of foodstuffs. The main sources of large quantities of oil and fat are in the energy‐storing tissues. Normally, these stores are present as oils but may solidify on cooling. Hence animal fats which exist naturally at body temperatures tend to be solid at room temperatures, whilst vegetable and marine oils are often liquid at room temperature. Chemically, these differences are related to the chain length and degree of unsaturation of the fatty acid residues; shorter chain lengths and higher levels of unsaturation lead to lower melting points. Microscopically, solidification is seen as an increase in the amount of crystallinity in the fat and this can be detected by polarised light microscopy and by electron microscopy techniques. Differences in crystallinity can be found in fats from different animals — for example, beef fats are generally more crystalline than pork fats — and from different anatomical locations within a single animal — for example, exterior fats such as pork jowl fat are generally less crystalline than interior fats such as mesenteric fat.