Challenges with fats and fatty acid methods.

The content and chemical nature of lipids in feedstuffs is heterogeneous. It has long been known that ether extraction by the Weende procedure inadequately characterizes the fat content of feedstuffs, yet it remains the official method. Diethyl ether (or hexanes that are often used) extracts significant amounts of nonnutritive, nonsaponifiable lipids from forages, and often incompletely extracts lipids of nutritional value, especially fatty acids present as salts of divalent cations. Preextraction hydrolysis of insoluble fatty acid salts with acid releases these fatty acids, and this step is included in the official procedure for certain feedstuffs in the United Kingdom; however, acid hydrolysis increases analysis time and decreases precision. Acid hydrolysis also causes confusion as to the proper definition of the fat content of feedstuffs. A preferred method of fat analysis determines the total fatty acid concentration in feed samples by converting fatty acid salts, as well as the acyl components in all lipid classes, such as triacylglycerols, phospholipids, and sphingolipids, to methyl esters using a simple, direct one-step esterification procedure. Fatty acid methyl esters are then quantified by GLC, which provides information on both fatty acid quantity and profile in a single analysis. Adjustments in conditions and reagents may be necessary to overcome difficulty in quantitatively preparing esters from certain types of fatty acids and their derivatives in commercial fat supplements. After correction for glycerol content, analysis of oils by this procedure provides information on the content of nonsaponifiable material, such as chlorophyll, waxes, and indigestible polymers formed from heat- or oxidatively damaged fats. The correct description of feedstuffs for energy value of fats is the content of total fatty acids.