The main role in bone tissue is played by the interstitial substance, in which small bone cavities are located, connected by the thinnest bone tubules.

The nitrogenous substance of bone mainly consists of ossein (collagen), which, when boiled with water, turns into glutin, forming a glue that hardens into jelly when cooled.

In addition to ossein, bones contain small amounts of mucin and albumin. The mineral salts contained in bone consist mainly of calcium phosphate and carbon dioxide salts.

Other salts are usually found in small quantities.

The structure of the main (interstitial) substance is determined by the location of collagen fibers in it: the interstitial tissue is built from thin fibers and is much stronger than the coarse-fiber bone tissue of young animals.

Bone fat is contained in bone marrow, which fills the cavities of both tubular and flat bones.

There are mainly two types of bone marrow: red, or lymphoid, and yellow, or fatty.

Red bone marrow is located in the spongy substance of the epiphyses (ends of long bones), for example, spongy bone of the vertebrae, ribs, sternum, bones of the base of the skull. The yellow marrow is contained in the cavities of the long bones and consists almost exclusively of fat cells.

Data on the composition of bone fat (in%) are given in the table

In young animals, the bones are softer and more fragile, with a low content of fat and inorganic substances, but with a high content of water.

The older the animal, the harder its bones become and the more lime salts they contain.

On average, the chemical composition of raw bone is: water - 51%, fat - 15%, nitrogenous substances - 12% and minerals - 22%.

In the same animal, the composition of bone tissue depends on the type of bone

According to their structure, shape and fat content, bones are divided into three main groups:

a) long or tubular bones , in which the length prevails over the width and thickness. These bones have a cylindrical middle part (diaphysis) with an open internal cavity and thickenings at the ends (epiphyses).
This group includes bones of the femur, tibia, humerus, ulna, as well as metacarpals (tarsal bones of the forelimbs) and metatarsals (tarsal bones of the hind limbs).

The bone fat of the femurs, tibias, humerus and ulnas is light yellow. colors; it contains 62.86% unsaturated acid glycerides and 37.14% saturated acid glycerides. The fat of the metacarpal and metatarsal bones is golden-straw in color and contains up to 85-86% glycerides of unsaturated acids.

b) The bones are wide, flat, somewhat curved. This group includes most of the bones of the skull and pelvis, ribs and scapula. They are filled with red brain matter.

c) Bones are rounded or polyhedral. This group includes the bones of the cervical, dorsal, sacral and caudal vertebrae, wrists and tarsals, fetlock joints and fingers.

In industry, bones are divided into tubular bone (tubes and tarsus), passport bone (scapulae, ribs and pelvis) and ordinary bone (all other bones - vertebrae, thoracic bones, epiphyses).

Tubular bone (ornamental) has great strength and homogeneity, can be ground, cut and polished well, and therefore, after degreasing, it is used for the production of various haberdashery products (buttons, combs, toothbrush handles, etc.).

After degreasing, passport bone is used for the production of edible gelatin, and ordinary bone is used for the production of glue, feed meal or fertilizer fertilizer.

The bone must be supplied to the production of edible fats fresh, clean and free of meat residues, no later than 4-6 hours after deboning.

When bones are stored for a long time, the fats they contain hydrolyze and become rancid. Therefore, it is necessary to send the bone for fat digestion as quickly as possible and, if necessary, store the bone until processing in dry, dark, well-ventilated rooms, at relatively low temperatures.

Pre-treatment of bone

Depending on the type and type of bones, pre-treatment methods and methods for isolating bone fat are different.
Since tubular and lantern bones are valuable material used for the manufacture of various haberdashery products, these bones are not crushed and treated with hot water so as not to spoil the bone.

To remove blood and other contaminants, the bone is washed with water in a continuous or batch drum.
A continuously operating drum is a cylinder with a perforated wall for water drainage.
In the front closed end of the drum there is a central hole for loading bones.
Inside, along the length of the drum, there is a pipe with holes through which water is supplied. The drum has a slope of 1.5° and makes 14 rpm. The bone is washed with water (15-20°) for 10-15 minutes. As the drum rotates, the bone loaded at one end of the drum moves slowly and slides into the receiving cart at the other end.
If there are no drums, the bone is washed in vats with running water for 30 minutes.

Since the tubular and lantern bones contain bone marrow containing 88-95% fat, and the bone cannot be crushed, for more complete extraction of fat it is necessary to open the bone cavity, which is achieved by filing the epiphyses (fists) or drilling the lantern bones. The femur, tibia, humerus and forearm bones are subjected only to filing, and the lanterns are filed or drilled.

They use circular saws with a diameter of 250, 300 and 400 mm and a thickness of 2-3 mm. The saw is covered with a casing and equipped with a device for sliding the bone onto the saw blade without the participation of the worker’s hands. The ends of the tubes after filing should be smooth and even, and the bone marrow should be visible at both ends; the fists are boiled together with the ordinary bone.

To speed up the digestion process and increase the yield of fat, the ordinary bone is crushed. In addition, when grinding bone, the boiler capacity of the boiler is better used; The average bulk weight of 1 m3 of ordinary bone is 0.4–0.5 t in uncrushed form, and 0.65–0.75 t in crushed form.
Pre-grinding the bone also reduces steam consumption and produces broths of higher concentration.
Ordinary bone, crushed into pieces and boiled in water at 100° for 6 hours, gives 2% fat; the same bone, crushed in a bone crusher, gives 4% fat.

According to D.I. Lobanov, when boiling 100 g of crushed bones, 12 g of fat is obtained, and when boiling the same bones, but ground, 24 g of fat are obtained.

Grinding the bone too much is due to its loss during crushing and caking in the digester. Experience shows that the optimal size of pieces of crushed bone should be about 20-25 mm.

For bone crushing, so-called bone breaking machines are used, the working part of which consists of a number of stationary knives (combs) fixed in the crusher frame, and a rotating shaft with knives (teeth) located along a helical line. The shaft knives pass between the comb knives and break the bone that falls between them. Machines with one shaft and one comb (single-shaft) produce uneven crushing.

If bone is used as a drainage material, when burning soft raw materials in horizontal vacuum boilers, it is crushed in hammer crushers operating on the impact principle.

Hammer crusher for bone.

A hammer crusher has a horizontal shaft on which are mounted plates connected to each other by rollers. In the spaces between the plates, hammers are loosely placed on the rollers, which, when the shaft rotates under the influence of centrifugal force, take a radial position.

The bone is loaded into the crusher through a hopper, where it is crushed using rapidly rotating hammers and thrown down towards a reflector or grate. Part of the bone, sufficiently crushed, falls through the holes in the grate, and the rest is picked up by hammers and crushed again.

The productivity of such a continuously operating crusher is 3000 kg of bone per hour; engine power 28 hp. With.

Boiling the bone

The release of fat from the bone with modern boiling methods is the result of the heat of the heating steam and water acting on the bone. You can separate fat from bones by simply heating without moistening, but in this case the fat turns out crispy, dark in color, has an unpleasant taste and has little yield.

Therefore, fat is rendered from the bone using the wet method, i.e., with the addition of water.
When fat is boiled from bone in water, collagen fibers are loosened due to the conversion of collagen into glutin, which, unlike collagen, is soluble in hot water.
This makes it easier to release fat from the bone. The amount of adhesive substances extracted (gluten and its hydrolysis products) depends on the cooking temperature: according to Lieberman and Petrovsky, during 1.5 hours of cooking, 0.8% of adhesive substances are extracted from the bone in an open boiler, in an autoclave at a pressure of 1.5 ati - 4.6% and at a pressure of 5 ati - 8.8% of the bone weight.
In addition, the degree of extraction of adhesive substances also depends on the type of bone. Soft spongy bones (ordinary bone) are easier to digest and produce a lot of glue. Tubular bones are much more difficult to boil. According to D.I. Lobanov, when tubular bones are boiled for seven hours, from 6 to 19% of their weight passes into the solution; under the same conditions, the bones of the spine, costal bones and flat bones provide from 16 to 24% of the soluble substance.

As temperature and pressure increase, fat yield increases. Thus, when boiling raw bone in open boilers at 100°; within 6 hours, the fat yield is 4%, the fat yield when boiling bones in an autoclave under pressure of 3-4 atm for 3 hours or 2-2.5 atm for 4 hours 30 minutes is 8%. According to Lobanov, beef hip bones when boiled in an open boiler for 4 hours gave a fat yield of 6.3%, and in an autoclave under pressure of 2 ati for 1 hour - 18.9%; tubular beef when boiled in an open boiler for 4 hours - 11% fat, and at a pressure of 2 ati in an autoclave for 1 hour - 16.3%.

The boiling temperature and pressure depend on the purpose of the boiled bone.
To avoid cracking of tubular and lantern bones and the appearance of stains on them, the temperature of their boiling should not exceed 85-87°. Since the extraction of adhesive substances from the bone increases sharply with increasing pressure, if it is subsequently used for the production of glue, the bone is boiled at a temperature not exceeding 100°. It is most advisable to boil raw bone obtained from meat processing plants in an autoclave under high pressure and then use it to produce feed meal.

The fat yield increases with increasing digestion time at the beginning of the process, and then, upon reaching the optimum, the increase in fat yield almost or completely stops.

Diagram of fat yield depending on the duration of bone boiling.

Basic methods for boiling edible bone fats:

A) boiling under atmospheric pressure in hot water;

B) boiling under atmospheric pressure in boiling water;

B) boiling under high pressure.

A more concentrated glue broth by boiling down fat and bone broth is obtained by using the enrichment method.

Boilers of different designs are used to boil fat from bones:

A) An open, single-walled iron cauldron, with a removable lattice basket made of iron rods, designed to load 300-600 kg of bone. At the bottom of the boiler there is a central drain for broth. Live steam is supplied through a coil located at the bottom of the boiler under the bottom of the lattice basket. When unloading the bone, the doors of the bottom of the basket are folded back. A movable block on the ceiling beam is used to raise and lower the basket. The fat is either scooped from the surface after lifting the basket with the boiled bone, or drained through the lower pipe along with the broth.

B) The same open boiler with a removable basket, but differing from the first type only in that there are two taps on the side: the upper one for draining fat and the lower one for draining broth.

C) An open, single-walled cauldron, with a conical bottom, containing 1000–1500 kg of bone. This kettle allows you to easily adjust the cooking temperature. In the center of the bottom of the boiler there is a pipe for draining the broth. The fat is drained through taps located on the side of the boiler. The bone is loaded onto an iron inclined grid with a hole diameter of 25 mm; The bone is unloaded through the side hatch. Live steam is supplied to a coil located under an inclined grate.

D) Single-walled or double-walled autoclaves.

One of the best devices for boiling bone under pressure is a vertical single-walled autoclave 1 (Fig. 170), which has a loading 2 and unloading 3 necks and a mesh bottom 4 with 10 mm holes. Live steam enters through pipe 5; A water pipe is connected to the same pipe, through which water is supplied after the fat has been boiled down and settled. The water raises the level of fat to the filling neck and allows it to be drained from the autoclave. The broth descends through pipe 6.

A double-walled autoclave is no different from an autoclave for rendering pork and beef fats.

Boiling fat from bones. After loading the bone, water is added to the cauldron so that it covers the bone. The ratio of bone to water is 1 liter per 1 kg. Bone that is not covered with water is strongly dried by steam (evaporating water), becomes brittle and becomes covered with cracks; at a low liquid ratio (0.5 liters of water per 1 kg of bone), the fat acquires a burning smell. Then the contents of the cauldron are heated to 80-85° for the femur, tibia, humerus and forearm bones, to 87° for the metacarpal and metatarsal bones and to 100° for the ordinary bone. Boiling at these temperatures lasts 4-4.5 hours for tubular and lantern bones and 5-6 hours for ordinary bone. Boiling crushed bone in an autoclave at a steam pressure inside the boiler of 4-5 ati lasts about 2 hours and at a pressure of 1.5-2.5 ati - from 4 to 4.5 hours.

After boiling is completed, the contents of the boiler are salted with table salt and left in the boiler for 20-30 minutes. The floating fat is poured into the sump, and the broth into the receiver.

In settling tanks, particles of glue broth proteins suspended in fat, traces of lime soap and other impurities are deposited.

When boiling a tubular bone in hot water, the fat yield is 36-42% relative to the fat contained in the raw bone. The loss in bone weight is 12-13%. When crushed raw bone is boiled in boiling water, the fat yield is 30-45%, and the loss in bone weight after boiling is 13-15%. In autoclaves, the fat yield is 60-80% of the initial fat content in raw raw bone.

The broth after boiling is a cloudy liquid containing a small amount of fat and tissue scraps. To remove fat and impurities, the broth is left to settle at a temperature of 65° for 1.5-2 hours and used for food purposes or for the production of glue. The broth obtained by boiling raw bones in an open cauldron has a strength of 2-3° Baume and contains 3-4% nitrogenous substances. The broth obtained from the same bone in an autoclave contains 10% dry matter.

To increase the yield of fat, it is advisable to subject the bones to secondary boiling in fresh water, in which case it is possible to obtain an additional 2-3% fat. The re-boiling process is carried out at a pressure of 5 atm for about 1 hour and at a pressure of 1.5-2 atm for up to 3 hours.

To extract fat from bone, you can also use vacuum horizontal boilers, provided that the fat is boiled from the bone using a wet method, i.e., with the addition of water.

In these cauldrons, fat and broth are obtained according to the scheme shown in Fig. 171. Fat and broth are drained through a strainer into the broth collector. The boiled bone remaining in the boiler is dried under vacuum to obtain bone meal. The fat from the broth collector is drained through the top tap into the settling tank, and the broth (if it is impossible to sell it in liquid form) is filtered through a suction filter and evaporated in a vacuum horizontal boiler. The dry powdery broth is discharged into a strainer.

The yield of dry broth in powder ranges from 7 to 10% of the weight of the bone. Its composition: fat 2-4%, ash 3-5%, moisture 8-10%, protein substances 81-87%; the color is yellow or light brown, the smell is pleasant. The powder dissolves well in water and, after adding spices (salt, bay leaf, etc.), is used for food.

The yield of flour is 40-12% by weight of the bone.

When boiling bones using the diffusion method, open boilers are connected to each other by pipelines to transfer enriched broths from one boiler to another using a pump or gravity flow levels.

When operating a battery of four boilers, the weak broth obtained by boiling the bone in the first boiler is sent to the second boiler to fill with fresh bone; the more saturated broth is transferred to the third boiler, where an even more saturated broth is obtained. The latter is sent to the fourth boiler, from which, after final enrichment, it is discharged to the receiver.

Boiling this way can give a broth with a strength of up to 10° Baume.

In the tubular bone, after boiling, bone marrow remains inside the canals, and on the surface there are remains of connective tissue, droplets of fat and broth. The remains of the bone marrow are knocked out of the tubular bone by hand, and to remove all impurities, the bone is washed in drums or in the digester itself with hot water at a temperature of 65-85° for 30 minutes. The washed tubular and lantern bones are dried in a room equipped with racks with wire mesh, heating devices and ventilation. The tubular bone is dried at 30--35° for 6 hours and laid out on racks at the rate of 140 pieces per 1 m2.
The yield of dried bone is 75% relative to the weight before drying.
After drying, the tubular bones are sorted by length and weight.

After boiling, the raw bone is sent to the technical semi-finished products workshop.