III. Fats and Oils Technology
THE economic aspects of the fat and oil trade are so interwoven with the uses of these raw materials that some elementary consideration of the technology of fats and oils is necessary for a proper understanding of the economic situation. This is given in the present section.
Commercial Production of Animal Fats
Animal fats -- except butter -- are separated by a process termed rendering or trying-out. The simplest method is to throw the fat-containing tissues into a kettle, heat them over an open fire till the fat has been cooked out, and then strain off the fat from the non-fatty material which is known as cracklings or greaves. More commonly the kettle is not heated by a direct flame but by steam, to avoid charring. This is done in a number of ways. The simplest is with a steam coil which projects into the interior of the kettle and comes in contact with the material to be rendered. Another common method is to use a jacketed kettle. This has a double wall and the heating is done by passing steam through the space between the walls. Sometimes, however, fats are not rendered dry but are placed in the kettle with water and then heated in any of the ways mentioned above; or else steam is blown directly into the water, a procedure wasteful of fuel. At the end of the heating the fat is allowed to rise to the top and is skimmed off.
The commonest rendering method of all is with steam under pressure at a temperature of 12O-13O deg Centigrade. The material to be rendered is placed in a large strong metal cylinder known as a digester, which is then closed tightly. Steam under pressure is passed into it for some time -- often as long as twelve to fifteen hours. The steam is then shut off, the apparatus is allowed to cool, and the fat is skimmed off the surface of the water, some of which was placed in the digester with the material to be rendered and some of which condensed from the steam during the operation. This is an efficient method of rendering, though it does not yield by any means the best product. It is very economical in regard to consumption of fuel and at these high temperatures tissues, even bones, release their fat quite completely. In consequence, the yields by this method are greater than by the others.
In the last few years a new method of rendering, known as dry rendering has been introduced. It is said to be the most economical of all for certain types of fat. The material is first dried to a low moisture content in a suitable machine. A part of the fat is obtained by merely allowing the hot material to drain. The remainder is secured by expression in an expeller or a press (see below, Production of Vegetable Fats).
When the process of rendering is such as to leave cracklings (scrap) or greaves, these retain considerable fat, most of which is often recovered by some form of expression either with a hydraulic press or an expeller.
The best quality of animal fat is obtained by rendering perfectly fresh material with water at low temperatures. It is in this manner that neutral lard and oleo stock (beef fat) are prepared for the manufacture of oleomargarin.
The nomenclature of the commoner commercial animal fats used in commerce is rather complex and there are many grades. Neither the nomenclature nor the grading is the same in all countries or even in all sections of the same country. In a general way, these, together with some of the processes by which they are produced, are described in the following paragraphs.
Hog Fats or Lard
In America the fats from the hog are termed lard (French, saindoux; German, Schmalz) if they are edible, and grease if they are inedible. The grades of lard depend upon the part of the animal from which the lard is derived and upon the method of rendering.
The best grade of lard is known as neutral lard No. 1. It is obtained from the leaf fat of the hog, mainly kidney fat, and fat in the omentum. The omentum is a thin sheet of tissue attached to the intestines; in well-nourished animals it contains a good deal of fat. Before the development of the margarin industry, that is, before 1875, in great part it was rendered separately and marketed as leaf lard, commanding a higher price than other lard because of its great firmness and better flavor. It is still so marketed to some extent by smaller butchers and the small packing houses. In the larger establishments the omental and kidney fat are removed from the carcass at the earliest feasible moment, cooled and promptly rendered with pure water at a low temperature, usually not higher than 50 deg Centigrade. The product is known as neutral lard No. 1 and is used almost exclusively in the manufacture of oleomargarin.
This process does not recover all of the lard in the parts rendered. The remainder is recovered by cooking in digesters at higher temperatures and pressures. It is sold as leaf lard. It should be noted that it is a somewhat different product from the leaf lard of former times and from the leaf lard produced by small concerns that do not make neutral lard, for it contains only a portion of the leaf and kidney fat instead of all of it.
Neutral lard No. 2 is made from the back fat by exactly the same method by which neutral lard No. 1 is produced. The residue, treated in digesters, yields choice kettle-rendered lard.
A third grade of lard is made from trimmings and other fatty parts by digesting at relatively high temperatures and pressures. It is known as prime steam lard. The solid residue in the digesters still contains some lard which is recovered either by expressing in presses or by cooking with a little acid. This product is classed as a grease.
The manner of cooling melted lard greatly affects its appearance. If allowed to cool without special precautions, it is apt to be translucent and to have the appearance which the American consumer attributes to a grease rather than the white, opaque, somewhat granular appearance which he expects in lard. It is therefore customary in packing houses to chill it rapidly. This is usually done with so-called lard rolls. These are large, smooth, hollow, metal cylinders which are revolved on a horizontal shaft. They are cooled from the inside with a current of brine, and the lard is run onto one side of their surface in a thin layer, thereby being chilled quickly. It is then scraped off automatically by a stationary scraper on the other side. The chilled lard drops into a trough in which is a worm conveyor or picker which churns up the lard, thereby giving it the desired color and texture, and conveying it still in the plastic state into storage tanks from which it flows into the containers for shipment. Sometimes the beating up of the plastic lard is so done as to incorporate air which makes it appear whiter and increases its volume though not its weight.
Grease, which in packing-house parlance is merely inedible lard, is rendered with steam under pressure from packing-house offal, and from carcasses that have been condemned by the governmental meat inspectors as being unfit for food for one reason or another. It is graded according to color as white, yellow, or brown grease. White and yellow packing-house greases commonly contain only hog fat. Brown grease may also contain some beef and mutton fat, for it includes the fat from the catch basins. In large packing houses great quantities of water are used which ultimately find their way into the sewers, carrying along in their passage not inappreciable quantities of fats. To recover these it is customary to let the waters settle in basins before they finally flow into the sewer. In these basins fat rises to the surface; it is then skimmed off and combined with the brown grease. As the waters come from all parts of the packing house in which cattle and sheep may be slaughtered as well as hogs, the brown grease naturally may contain beef and mutton fat.
Grease is also recovered from certain of the viscera, other offal, and especially the intestines. In former times, before the practice was restricted by the government, this was known as gut lard and mixed with other lard for edible purposes. It has a characteristic flavor which experts are able to recognize even in admixture. In slaughterhouses under federal inspection all offal of this kind must be worked up for grease.
Greases are sold on the basis of their color and of their chemical composition. The percentage of free fatty acid, of unsaponifiable matter, and of moisture are taken into consideration, and also the titre test. Greases are used chiefly in the manufacture of soap, candles, and lubricants. Before the enactment of the meat-inspection act in 1906 undoubtedly much that is now sold as grease was used for edible purposes. Today the meat-inspection act requires that such greases be denatured by the addition of petroleum products or other denaturant. Denatured white greases, when exported to Europe, have at times been treated there so as to remove the denaturant and they have then been used for food purposes.
Cattle and Sheep Fats or Tallow
The fat from cattle and sheep is known as tallow (French, suif; German, Taig). Sheep fat is rarely used for edible purposes because of the difficulty of removing its strong flavor and odor. It is widely used for soap and candle making and in lubricants. Beef fat is used for all these purposes and for food as well. The fat from the heart, caul, and around the kidney, which corresponds to leaf lard, gives the finest edible tallow. Its best grades are used to produce oleo stock (also known in Europe as premier jus) for the oleomargarin industry by the same process which is used to produce neutral lard. Beef tallow of good quality is also used in so-called lard compounds to give a stiffer consistency. Tallow, except for oleo stock, is usually steam rendered. Inedible tallow is graded and sold on the basis of color -- sometimes also on the basis of its content of moisture, impurities, unsaponifiable matter (abbreviated to M.I.U. by the trade), and free fatty acid. If sold for candle making, the titre test is especially important, since it indicates the yield of solid fatty acid suitable for candle making that may be expected. This in practice is more variable than might be supposed, since in packing houses doing a local business more or less hog fat, which has a low solid fatty-acid content, may be mixed with inedible tallow.
Production of Garbage Grease and Similar Products
While in the larger packing houses under governmental inspection the term grease is generally limited to inedible hog fat, this is not always the case in all sections of the trade; one finds the term grease used for any inedible fat of semi-soft consistency. The trade, therefore, often distinguishes between packing-house grease and all other greases. In small establishments doing a local business, the term grease may be applied to a product of variable origin and containing more or less tallow and fat recovered from retail butchers' scrap and the like.
Garbage grease is one of the more important of the waste fats. In some cities household, restaurant, and hotel offal is separated into garbage proper, consisting mainly of food offal, and into rubbish and trash of all kinds. Grease from the garbage is recovered usually by one form or another of steam rendering (see above, Commercial production of animal fats). It is sold mostly to candle makers. The disposal of garbage presents a serious problem to municipal administrators, and methods of rendering other than by steam, including solvent extraction, have been proposed. Opinions differ concerning the value of different methods. The recovery of garbage grease is sometimes profitable, sometimes not, depending upon local conditions and the general level of prices of fats prevailing at the time the grease is sold. (The several methods of garbage disposal are feeding to hogs, dumping (usually at sea), incineration, and recovery of grease, or combinations of them. The question of the disposal of garbage is so tied up with local politics that it is impossible to determine where the path of efficiency lies.)
In some cities dead animals of various kinds are rendered with the garbage. In others this is done by privately owned rendering companies which work up such material, and at times also the garbage from hotels and restaurants and the scraps from retail butchers' shops. The material produced by them is sometimes known as horse oil which may, however, and usually does, contain fat from other sources than the carcasses of horses.
Production of Vegetable Fats
Vegetable fats, except in special cases, are not produced by rendering. The principal exception is the palm oil produced by African natives in the home of the palm. They boil the crushed and more or less decomposed fruits with water and then skim off the oil.
The commoner method of producing vegetable fats is by expression. This process involves several steps. The first is the preparation of the seeds, which consists of the removal of the shell or hull. This decortication is not always necessary, as in the case of fruits like the olive or small seeds like rape or flax. Sometimes it suffices merely to crack or crush the shell without removing it. Decortication is usually done by special machines; in the case of the coconut, however, it is done by hand. The coconut presents a special case in another respect. The meat which has been separated from the shell and husks by hand is commonly dried either in the sun, or over fires, or by special machines before it is treated further for the production of oil. The dried product is known as copra.
The second step, which follows decortication when this is necessary, is crushing or grinding. This is done either between steel rolls or between mill stones or both. Hence the factories are known as oil mills and the process as oil crushing or oil milling.
The material is now ready for the expression of the oil in presses of various types. This is sometimes done without heating the material, as in the case of olive oil. Such oils are known as cold-pressed oils. Sometimes the residue from the pressing is reground with or without the addition of water, again pressed, and a second portion of oil obtained. The first pressings of olives is known as virgin oil. If the olives from which it is made are of good quality, this oil represents the highest quality.
Since cold pressing does not extract all the oil, it is practiced only in the production of a few special edible oils, the natural flavor of which is highly prized. They are used without further refining other than clarifying and filtering. The oil unextracted by cold pressing is recovered in large measure by grinding up the residue from which the cold-pressed oil has been extracted, cooking it (usually with steam), returning it to the presses, and again expressing the oil. This is known as hot-pressed oil.
The great bulk of vegetable oils -- coconut, palm kernel, cottonseed, peanut, etc. -- are extracted only by hot pressing. All these oils require refining before they are suitable for edible uses.
The expression of the oil is practiced either with presses or with special machines known as expellers. For descriptions of the construction of these machines, the reader is referred to special treatises. (T. W. Chalmers, The Production and Treatment of Vegetable Oils, London, Constable, 1920; Lewkowitsch, op. cit.) Here it suffices to say that the presses are usually hydraulically operated and are commonly capable of exerting very great pressures. They operate discontinuously -- that is to say, they act only upon one charge at a time. After the oil has been expressed from this the press must be unloaded and refilled with a fresh charge of the ground oil-bearing material. Hence the labor costs of operation may be considerable. The expeller, on the other hand, operates continuously and the labor costs are correspondingly low, though maintenance and power charges are said to be high. The expeller is built on the same principle as the ordinary meat chopper or sausage machine which nowadays is to be found in most kitchens. The oil-bearing material is fed into one end of a cylinder within which a power-driven worm conveyor forces the material to the other end of the cylinder and out against resistance, exactly as though it were sausage meat. The pressure exerted in the process squeezes out the oil. In some factories the material, if very rich in oil, is first passed through an expeller to remove a part of the oil, then reground and recooked, and finally expressed in a hydraulic press.
The hydraulic press when skilfully operated removes the oil somewhat more completely than the expeller. However, neither does so completely. From 4 to 8 or even 10 per cent of oil may remain in the residue which is known as oil cake, sometimes also as press cake or pressed cake. A method has been devised to recover this oil. It is known as the solvent-extraction process. It consists of grinding the cake to a meal and then extracting or leaching it with a volatile liquid in which the oil is freely soluble. The extract is then drawn off from the cake into a still where the solvent is distilled off and recovered, leaving the oil behind in the still. The solvent most commonly used is benzene, although carbon bisulfid, petroleum products, and other liquids are also used. (For details the reader is referred to the treatises by Chalmers and Lewkowitsch above cited.) The oil obtained is inferior in many cases to that obtained by expression. It is especially so when the solvent used is carbon bisulfid; such oils are known as sulfur oils.
In Europe the solvent extraction of oil cake for the recovery of residual oil has been practiced quite extensively. In the United States it has been employed principally at times of high prices or for special purposes. The chief difficulty at present is that the ideal solvent remains to be found. Those commonly in use or proposed either introduce a serious fire hazard, or else in time they corrode the equipment, or finally they leave a bad odor or taste in the cake, which impairs its value as cattle feed. It has been proposed to do away with presses and expellers altogether and to dissolve out the oil from original oilbearing raw material by solvent extraction, and there are some plants of this kind in operation in Europe. In America, however, so far as it has been possible to learn, no such plants are now in operation.
Refining has for its object the removal of free fatty acids and other objectionable substances -- principally nitrogenous and mucilaginous matters. The principle involved has already been outlined (see II. Properties of Fats and Oils -- Other useful tests). The melted fat or oil is treated with a little more than the requisite amount of strong aqueous caustic soda solution to convert the free fatty acid present into soap. The oil and the alkali solution are thoroughly stirred together and sometimes warmed. The mixture is then allowed to separate.
The result is that the oil freed from fatty acid floats on top of a layer of soap, alkali solution, and other impurities, which is drawn off. The oil is then washed with water to remove the soap, alkali, and other impurities, when it is ready for the decolorizing or deodorizing process. There are other methods of refining, but this is the one most commonly used in America.
The under layer of soap and other impurities, which is drawn off from the oil, consists of solid matter mixed with some water. It is known as foots, probably because it collects at the foot of the tank. A large proportion of it is soap. It may be sold to soap makers for use in the lower grades of soap, the price being based on the percentage of fatty acid present in it. Hence it is also known as soap stock. Or it may be treated with strong sulfuric acid to set free the fatty acids contained in it. These then float to the surface, are skimmed off, and sold to the soap or candle maker. This product is known commercially as acidulated foots. It pays to produce it rather than to sell ordinary foots whenever the freight from the refinery to the soap maker is considerable, for by converting foots into acidulated foots the weight is reduced about one-half. A third use for foots is to convert it into washing powder by mixing with a suitable amount of soda ash. (Soda ash is soda heated till its water of crystallization has been driven off. Soda is sodium carbonate, Na2CO3; in the crystalline form in which it is found in commerce it contains water of crystallization, and has the formula Na2CO3+1OH2O. Soda ash in contact with moisture tends to take up water and crystallize.) The soda ash takes up the water in the foots and crystallizes with water of crystallization, thereby converting the foots into a hard, dry cake which needs only to be broken up and ground to be salable as washing powder. At the same time the soda ash bleaches the foots and improves the color.
While with the exception of virgin oils (see above, Production of vegetable fats) the great bulk of vegetable oils destined for food use is refined, this is not the case with animal fats. It was formerly the custom to refine the poorer grades of lard and of tallow to make them more suitable for edible purposes. The practice was to wash them in the molten state with a weak alkali solution, or to treat them with alum or other chemicals or with fuller's earth. Such refining is no longer permitted in federally inspected packing houses. About the only practice now allowed is to let the fat settle with the addition of some salt in order to remove traces of water and any fragments of tissue and fibre that may be present, or to treat with fuller's earth or other inert decolorizing agent. It is reported that in many regions of Europe refining of animal fats for food use is not prohibited.
Next: III. Fats and Oils Technology -- Decolorizing or Bleaching
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