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10.1 Definition of a Byproduct

Items of value produced during slaughter other than the carcass and edible offal are referred to as byproducts. Byproducts are thus inedible material less rejects and waste. Figure 4 amplifies the definition.

Byproduct industries are many and specialized in industrialized countries particularly the USA and Europe. The range of products is also wide and diversified. This is made possible by volume and variety in raw material and, most important, in technology. A list of the major possible uses of byproducts is presented in Appendix I (Table 3).

Less-developed countries conversely have limited yields of slaughterhouse byproducts due to the greater premium placed on non-carcass components as a source of food. Table 2 shows for instance that the average East and West African cattle yield a high proportion of their body weight as saleable meat relative to inedibles, the ratio being about 70 percent : 30 percent. On the other hand, in North America the saleable yield profile of slaughtered steers compared to inedible material is in the region of 60 percent : 40 percent. This ratio is however reversed at the retail level as the carcass is deboned and trimmed of excess fat in response to consumer preferences.

Potentially available byproducts from cattle are thus higher in yield in North America (45 percent) than say West Africa (9 percent), the latter comprising only the hide. Similarly, sheep and goat skins constitute the only body components of value outside edible meat. The yield here accounts for about 15 percent of liveweight in the East African goat. In the West African dwarf goat no byproducts are harnessed when the animal is dressed with the skin on, i.e. by singeing and scraping off the body hair. Consequently, maximal use is made of the animal body, about 75 percent as food, leaving the rest as rejects.

It is possible, nevertheless, to modify slaughter practices in the poorer countries and salvage usual rejects, blood, horns, hoofs, gut contents, and any condemned meat and manure for byproduct processing. Also in cultures where stomachs, intestines, lungs and reproductive organs are banned from diets, much raw material can be retrieved for further use.

It appears, however, that such utilization can be possible only where volume is available (as in large municipal abattoirs) and provided cattle products are included. In this way the opportunity is offered not only for more economic production but also as a means of removing nuisance from the slaughter premises, and assuring a hygienic and sanitary environment.

Apart from hides and skins (which are already being utilized) the following groups or items of byproducts can be considered for processing, taking into consideration their agro-industrial significance:

  1. Soft organs - stomachs, intestines, lungs, carcass trimmings, reproductive structures etc. (where not utilized for food); floor sweepings, drainage trappings and condemned meat - together for rendering into meat/bone meal;

  2. Hard organs - horn and hoof can similarly though separately be processed into horn/hoof meal and used as fertilizer;

  3. Blood - can be dried into blood meal and used in animal feed.

  4. Gut contents and manure (from lairages and kraals) for compost or fertilizer production; another possibility here is biogas production.

The general principles involved in the production of these items (including hides and skins) are outlined below.

10.2 Hides and Skin Curing

Hides and skins have the highest yield and value of all products of livestock other than the carcass, and in some livestock-rich developing countries such as Somalia and the Sudan, they account for substantial portions of export revenue. The approximate yield of green (or fresh) hides and skins in pastoral tropical livestock is as follows:

  Percentage of Liveweight… Aver. wt. kg
Cattle 6–8 18
Large Sheep & Goats 14–16 6
Dwarf Sheep & Goats 10–12 4

Hides and skins are processed into leather by tanneries, hence it is necessary to preseve them for storage and shipment after removal from the animal. The method of preservation is curing, either in free air or by use of salt or both. In each of these methods the preservative principle is the same, namely, removal of moisture from the product to enhance keeping quality. Thus air acts by facilitating evaporation of moisture from the skin, and salt by osmotic withdrawal of water, thus making the moisture unavailable for growth of microorganisms. Salt has an additional protective effect as it penetrates the tissues and with its presence inhibits the growth of deteriorating organisms.

  1. Air-Curing

    Hides and skins must be cured immediately after removal from the animal body. Initially, they are prepared for this process by cleaning off residual meat, fat and manure from the surface in a process called fleshing. They are then washed, drained and trimmed to remove the ears and lips. To air-cure the hide or skin, the ideal method is to stretch it with strings from all sides and angles over a wooden frame or wire loop, and suspend it in the open to allow air to circulate freely around it and dry uniformly (Fig. 5).

    This method is preferred to ground drying which yields a poor quality product with cracks, wrinkles and folds, as well as subjecting the hide or skin to moulding and putrefaction. Hot, dry environments such as prevail in some tropical savannahs are best suited to air drying, but not the humid or the wet-forest type. The main disadvantage about air drying is that shrinkage is high, about 40–50 percent of green weight; hence the finished weights are low, although they incur correspondingly lower shipping costs.

  2. Salting

    Salt-curing processes are of two kinds: dry and wet. In the former process, the simplest method, called green salting, is to rub the flesh surface with dry salt and stack the salted pieces in a pile in a cool place under some weight for 30 days. At the end of the process the hides and skins are removed, shaken of the salt and folded for consignment. (They can, however, be air-dried over a wire or wooden frame after salting). The rate of application of salt is 25 percent of green weight, but fine salt is used for skins and coarse salt for hides. Shrinkage is less when hides and skins are green salted, but increases with additional air-drying.

    Hides and skins are said to have been wet-salted when cured in brine which is a strong solution of salt prepared at 15–20 percent concentration. The term brine curing is also used for the process. For skins fine salt without impurities is used, while rock salt suffices for hides. The immersed products are held in cellars often in the slaughter premises at a temperature of 10° to 16°C (or 50 – 60°F) for 3 to 4 weeks. Shrinkage is fairly low, between 15 and 25 percent. Thus yields are higher, up to 70 percent which in effect makes wet salting the most desirable process in sales returns. The wet-salted hides and skins are also of best quality barring deterioration which can be more pronounced in wet hides compared to the dried varieties.

    Cured hides and skins are graded and purchased according to weight, quality and condition. The weight criterion as explained previously is based on curing yield. Quality basically refers to the class of hide or the breed or type of animal from which it is extracted, while condition relates to physical characteristics such as the extent of damage to the animal, due for instance to disease, branding and flaying methods.


Fig 5

10.3 Meat/Bone Meal Production

The raw materials for quality production of meat/bone meal are all parts of the animal, less the skin or hide, hair, horn, hoof, blood and gut contents. This means that they may include skinned heads and feet, bones, viscera and carcass trimmings which are not utilized for food. Condemned material and relevant parts of freshly dead animals can be included, but not putrefactive material or that in a high state of decomposition. This material should be incinerated or buried in deep pits.

A steam-rendering tank is used for meat/bone meal production. This is an oblong-shaped or vertical cylinder with a cone-shaped base built of heavy steel and fitted with a steam-charging mechanism to provide high temperatures for cooking.

Water is first introduced into the tank, up to about one-third capacity; hence the term wet-rendering which is commonly applied to the operation. (Dry-rendering excludes the addition of water and in fact expels moisture from the system. It is used mainly to extract fat from tissues. Tanks used in dry-rendering are of the horizontal type, the heat being applied at lower temperatures)

When water has been placed in the wet-rendering tank the relatively heavier materials like bones, feet and heads are put in next in reduced sizes at the bottom of the tank. Softer organs such as those of the viscera and carcass trimmings are layered next. Finally, fat is placed on top, allowing a headspace for the boiling action. In practice, the fill does not exceed three-quarters of the cylinder's volume.

With the tank closed, steam is charged through the bottom directly into the tank. This is done under pressure which should keep rising to about 18 kg (or 40 lb) and held there for about 5 hours. It should be noted that the boiling point of water is elevated as pressure increases. Thus at ordinary atmospheric pressure water boils at 100°C (212°F), but with an extra pressure of say 4.5 kg (10 lb) the boiling point is raised to 115°C (240°F) and at 6.8 kg (or 15 lb) to 121°C (or 250°F) and so on. Heat is necessary to break up, soften the tissues and release fat, and importantly destroy harmful microorganisms.

At the end of the heating time, the pressure is lowered gradually and the tank is allowed to cool for 40 – 45 min. During this time, the heavier material gravitates to the bottom. Water collects above this while fat settles on top. The fat is removed first (as a byproduct, tallow) followed by the water. Finally the cooked meat and bone material is discharged. After removing residual moisture, the meat/bone material is dried, milled and bagged. (The cooking water contains some dissolved protein and fat: both are removed separately, the protein being added to the meat/bone meal before drying, and the fat to tallow stock).

Meat/bone meals, sometimes called feeding tankage, are used in animal rations. Each batch should, however, be analysed to determine the nutrient composition as the phosphorus and protein content are important criteria for grading and marketing.

Horn and hoof are prepared similarly to meat/bone meal, but this is done separately. Often the horn/hoof meals are used as fertilizers.

10.4 Production of Blood Meal

Blood is fairly rich in nutrients, especially protein, but being liquid it readily collects dirt once it leaves the animal body. Dirt starts putrefaction which lowers the blood's usefulness, and if drained outside on the slaughterhouse grounds sanitation problems arise by virtue of its clotting property. Other nuisances created by clotted blood are stench, filth, attraction of rodents and the breeding of flies. It is of utmost importance that blood when collected should be handled in a hygienic manner and processed with minimum delay.

(a) Collection and Yield

Blood can be collected directly in metal or plastic drums if the animals are hoisted for bleeding, but if killed on the floor small enamel or plastic bowls can be placed immediately beneath the let-out to receive the blood and empty it into the drum. The estimated yield of blood and blood meal in average tropical livestock is as follows:

  Fresh Blood Dried Blood or Blood Meal, kg
As % of livewt. Weight, kg
Cattle 2.5 6.30 1.26
Sheep and Goats 0.6 0.25 0.05

(b) Small Scale Processing

Where only a few animals are slaughtered in a day, small-scale low-technology processing can be undertaken rather than to spill the blood to waste and create problems of sanitation. Thus from say 10 cows and 3 sheep, approximately 64 kg of fresh blood can be obtained which can yield at least 12 kg of dried blood. To process this the blood is cooked in a tank to coagulate it, and is drained of liquids which collect on top after cooling. The coagulum is then broken up and spread on a tarpaulin or plastic sheeting for drying. Alternatively, the coagulated mass can be placed in a simple solar dryer for drying (Fig. 6).

(c) Wet Rendering

In plants that have steam-rendering tanks, the fresh blood can be mixed with selected non-carcass components of the description given in Chapter 10, paragraph 2, and wet-rendered. In this instance, the blood should substitute for water in the tank. An advantage here is that the protein content of the offal meal will be raised quantitatively with the addition of blood, although some amino acids may be damaged by the strong action of the heat while others may leach into the cooking water.

(d) Commercial Drying

A more productive approach is to process the blood under relatively reduced temperature conditions using a commercial blood drier. In principle, the blood-drier is a dry-rendering tank disposed horizontally and invested with a steam-jacket. Special devices are provided within the tank to prevent blood from coating on the interior walls and reducing drying efficiency.

Blood is introduced into the tank as a coagulated mass, previously obtained by steam action. As much liquid as possible should be squeezed from the coagulum. Heating is initiated at 82°C (180°F) and progressively raised to 94°C (200°F) for about three hours, then elevated to 100°C (212°F) for 7 hours. Drying is complete when the final moisture level in the dried product is about 12 percent. During drying, moisture is constantly and rapidly removed from the tank by means of condensers to which the tank is connected.

Complete moisture removal is not desirable otherwise the final product would darken or char, while above the 12 percent level the residual moisture can cause deterioration and loss of nutrients. The protein content of the finished product is about 80 percent.


Fig 6

10.5 Manure, Compost and Biogas

Digestive and excretory wastes of ruminants, collectively referred to as manure are a mixture of dung and urine and occur in two forms: as sweepings from lairages which are built into heaps outside the slaughter building and collected from time to time in small quantities by small-scale farmers to enrich soil fertility, and as kraal manure which may remain permanent on the holding ground. Kraal manure is less-preferred because it is often sogged with water (from rains) or mixed with earth from treading by the animals as well as straw from bedding, thus creating problems in collection and spreading on farms.

(a) Use of Manure

In either form, the quality and usefulness of manure becomes reduced as exposure to the open without protection or sheds or roofing causes loss of valuable nutrients, e.g. nitrogen by evaporation, and soluble substances (potassium and phosphorus) by leaching during rains. Otherwise cattle dung is a good source of phosphorus while the urine yields liberal amounts of nitrogen and potassium. Furthermore, the organic matter component of the manure remains longer in the earth when applied to soils to provide crops with a steady source of nutrients.

Fresh, straw-free manure with its urine mixture can be collected and held in special sheds or enclosures to decay a little before being put on the soil. If placed on the soil surface without prior decay or improper mixing with the soil, the manure loses considerable nitrogen, apart from physically smothering plant growth. Ruminants are known to avoid grazing close to pastures with dung on them.

(b) Composting

The process of breaking down organic matter in dead plant material, crop residue and leaves by decay before returning them to the soil can also be applied to old manure. The process is called composting. Farm composts are normally heaped above the ground, alternate layers of plant residue being sprinkled with ammonium sulphate, lime and water to facilitate decay. The pile is protected from rain and strong winds by being covered with heavy logs or a mud wall, then left to rot.

For environmental and sanitation reasons, the composting of manure should be done in pits or bunkers instead of stacks and heapings. A pit is an ordinary hollowing of the earth, while a bunker is a chambered structure constructed with cement blocks or bricks above the ground (Fig. 7). Both structures must be roofed or provided with sheds for security against rain. By the same token, water-logged areas must be avoided when locating the structures. The pits and bunkers are filled with alternate layers of kraal and lairage manure which should be wetted slightly with some liquid waste water from the slaughterhouse. They are then topped with leaves and covered with heavy boards or roofing sheets. Breakdown of the material proceeds slowly. After 2–3 weeks the contents should be turned and mixed, repeating the process after 4–5 weeks. In about 8 weeks or less the compost should be ready. Well-rotted manure must be fine textured without much straw in it.

(c) Biogas Production

Compost of even higher fertilizing characteristics is obtained as a byproduct in the breakdown of manure in special devices called digesters for the production of biogas. (Biogas is so called because it is a mixture of gases produced as a result of anaerobic breakdown of organic matter by bacteria. The gases in the mixture are methane, 60 percent which is the main component and a source of fuel; carbon dioxide, 36 percent, and hydrogen, oxygen, nitrogen and hydrogen sulphide making up the rest.)

As a rule, biogas production is not economic, the yield being very low. In animal wastes, for instance, the yield of biogas is lowest for cattle; pigs are intermediary with poultry being highest on the scale. In addition, operational problems exist affecting the charging of the system and continuous flow of gas, not to mention the explosion hazard. Proven commercial plants must be procured if biogas production from animal wastes is contemplated. In this case the digester gas utilization must be based on a practical necessity such as requirement for heating water (by direct burning) to maintain sanitary services in the slaughterhouse.

Because of its low yield, another consideration can be the advantages offered by biogas production in the treatment of organic wastes including the removal of offensive and insanitary influences from the environment. Whichever the application, compost is always produced from the operation, which with treated liquid waste, can be used in vegetable cultivation to yield revenue to offset costs.


Fig 7

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