Throughout the world fats of different origins are being produced in ever greater quantities. At the same time the conventional market for fat has been decreasing, largely because of the introduction of synthetic detergents; hence a surplus of fat is available at a low cost for the feeding of animals.
Fat is described by its origin, melting point (titre), amount of free fatty acids, colour and impurities. Animal fats are described as tallows when they are solid above 40 C, as lards when they are solid between 20 C and 40 C, and as oils when they are liquid below 20 C. Generally, tallows come from cattle or sheep, lards from hogs, horses or bones of all kinds, and oils from marine animals or vegetables. The lard from pigs and poultry can be highly variable depending on the diet. The water content of lard should be checked, as it is possible by using certain chemicals to make it take up extra water.
Fish-liver oil, especially cod-liver oil, was formerly widely used as a source of vitamins A and D. It has been found, however, that it oxidizes very quickly in mixed feed and loses its potency within a few days; thus its inclusion in mixed feeds is wasteful. Fish-liver oil is of value in animal nutrition, but to retain its vitamin content it must be kept in a cool, airtight container and away from strong light.
Palm oil is very rich in beta-carotene. The amount varies greatly depending on the method of processing and stage of maturity of the fruit. Unripe fruit contains about 1 mg of beta-carotene per kilogram, while ripe and overripe fruit have about 3 mg and 2.5 mg per kilogram. For cattle this corresponds to 600, 1900 and 1600 i.u. of vitamin, respectively.
Soapstock is a by-product of the refining of crude vegetable oil, which contains free fatty acids and traces of protein that must be removed before the oil is sold for eating. After adding sodium hydroxide, the oil is heated. At a certain temperature, the sodium reacts with the free fatty acids and combines with heat-coagulated protein in globules which settle to the bottom of the vessel. The clear refined oil is siphoned off the top. This material is known as soapstock because it is used in the production of soaps. It is rich in free fatty acids, usually containing more than 35%. Soybean soapstock also contains xanthophyll. In one experiment 6% soybean soapstock gave egg yolks a good colouring. Small amounts of soapstock have been included in pig and poultry diets. It has no harmful effects and can be used in the same way as fat.