Meal to mill
Figure 6. Wet pressing low-temperature-rendering system.
The two important fats of (land) animals used commercially are lard (fat from pigs) and tallow (fat from sheep and beef animals). Lard is made from specified, clean, sound tissues of healthy pigs, whereas rendered pork fat can be made from any fatty tissue. Edible tallow (dripping) is manufactured from specified edible fatty tissue. Oleo stock (premier jus) is a high-grade tallow prepared by low-temperature wet rendering of the fresh internal fat from beef. Inedible tallows (usually defined as fat with a melting point >40°C) and greases (melting point <40°C) are produced in many grades from inedible fatty tissue and dead stock.
Many different grades of tallow can be produced, and most countries have their own trading standards for tallow and grease. Tallow price reflects the prices obtained for other oils and fats, especially soy and palm oil. Vegetable oils are usually used for edible purposes and tend to be higher priced than tallow, which is used mainly for inedible processes such as soap manufacture, oleochemical production, and as an energy source in animal feed. Beef tallow is the largest component of bath bar soaps because of its economics and availability and its chemical composition. Fats are a useful component of animal feeds, as they have about twice the energy content of protein and carbohydrates. They also reduce dust, improve color and texture, enhance palatability, increase pelleting efficiency, and reduce machinery wear during feed production. Tallow can be used as an oleochemical feedstock. The resulting derivatives can be used to make abrasives, candles, cement additives, cleaners, cosmetics, paints, polishes, perfumes, detergents, plastics, synthetic rubber and water-repellent compounds. Tallow can be fractionated into its various components by crystallization, supercritical extraction, solvent extraction, or detergent fractionation. Tallow and its methyl esters can also be used as a fuel oil. Consider able amounts of tallow are still used for soap making. Glycerine, a coproduct of soap or fatty acid production, can be purified and used as a chemical in its own right.
Edible tallow and lard can be used in margarine, shortenings, and cooking fats. Tallow tends to give a better flavor to fried foods and is more stable during the cooking process than vegetable oils. However, because hard fats such as tallow can be associated with heart disease, the overall consumption of edible tallow has declined.
The material remaining after water and fat have been removed can have one of many names, depending on the rendering method and/or raw material used. These names include tankage, meat-meal tankage, digester tankage, wet-rendered tankage, or feeding tankage (finely ground, dried residue from wet-rendering material low in hair, hoof, horn, manure, and paunch contents), digester tankage with bone, meat and bone meal digester tankage, meat and bone meal tankage, or feeding tankage with bone (of higher phosphorus content than feeding tankage); meat meal (usually from dry rendering processes); and meat and bone meal or meat and bone scrap (of higher phosphorus content than meat meal). Feather meal is made from finely ground, wet-rendered feathers. This material is very digestible, but not well balanced nutritionally. Steamed bone meal (from wet rendering), or bone meal (from dry rendering), is defatted, dried, and ground bones suitable for animal feeding. Poultry meal has a similar composition, appearance, and nutritional value to meat meal but is made from poultry by-products. Fish meal is another high-quality meal similar to meat meal.
The type of raw material rendered and the rendering process used influence the composition of the meal pro duced. Meals made from proteinaceous material are high in nitrogen (the crude protein content is usually >50%) and also contain calcium, phosphorus, and fat. Bone meal and low-quality meat meals are made from material with a high bone content and have a relatively small market. One main use is as a fertilizer, but these meals are now being replaced by mineral fertilizers. Meals with a higher protein content are sold as meat and bone meal for use in animal feed formulations. They have a typical composition of 50% minimum crude protein, 4 to 10% moisture, 8 to 16% fat, and 20 to 30% ash. The price received for a given meal usually depends on its crude protein content, although some buyers may specify a minimum digestibility and availability of amino acids.
Bovine spongiform encephalopathy (BSE), commonly known as mad cow disease, was identified in the 1980s. Because of the belief that BSE transmission is via feeding-rendered ruminant protein to other ruminants, some countries introduced legislation banning the use of ruminant-derived animal proteins in animal feeds. This can have a great economic impact on the rendering industry. Researchers are developing economic sterilization processes and/or developing methods to monitor the sterilization effect of rendering processes (13,14).
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