Equipment and Methods for Skinning Carcasses: Hide Removal Process Explained

Hide removal — separating the hide from the carcass — is one of the most labour-intensive operations in slaughter processing, accounting for roughly 11–40% of the total labour involved in dressing a carcass. The work must be done carefully, without cuts, and without tearing meat or fat from the carcass surface. The hide is removed in two stages: siding (manual partial removal) and mechanical pulling. Siding is the hand removal of the hide from areas such as the head, neck, limbs, shoulder blades, and abdominal cavity.

The siding area depends on the animal species, fatness, and several other factors. On cattle carcasses the sided area equals 20–25%; on pigs it is 30–50% depending on fatness; and on small ruminant (sheep and goat) carcasses it is 30–40%. Figure 1 — Air inflation of a cattle hide: a — into the area of the fetlock joint; b — into the area of the supraorbital arches; c — near the xiphoid cartilage of the breastbone; d — near the base of the tail.

How is compressed air used before hide removal?

Compressed air is injected under the hide before removal to reduce tearing of meat and fat from the carcass, prevent damage to the hide, and ease the workers' labour (Fig. 1). Clean compressed air at a pressure of 0.3–0.4 MPa is used for the purpose.

The air is delivered with a gun fitted with a hollow needle 12–20 cm long and 6–8 mm in diameter, the end of which is cut at a sharp angle.

When removing the hide from a cattle carcass, the needle is inserted into the subcutaneous tissue at five points, each for a set time:

• 1 — into the fetlock joints of the front legs from the inner side along the cannon bones — for 2 s;
• 2 — into the area of one of the supraorbital arches, directed from one eye toward the other — for 5 s;
• 3 — into the area of the xiphoid cartilage of the breast section along the white line of the abdominal cavity — for 4–5 s;
• 4 — into the area of each hock joint of the hind legs from the inner side along the small shin bones — for 2 s;
• 5 — into the base of the tail from the inner side along the sacrum — for 2 s.

To remove hides from small ruminant carcasses, air at a pressure of 0.4–0.5 MPa is delivered into the area of the fetlock joint of the hind limbs, into the root of the tail, and into the lower fold of hide located beneath it.

For pig carcasses, inflation is carried out at a pressure of 0.4–0.6 MPa into the abdominal cavity for 5–7 s in the groin area. As a result, the carcass takes on a rounded shape, the hide stretches, and the folds smooth out. During mechanical hide removal from such carcasses, the amount of fat trimming is reduced and the commercial appearance improves.

The integrity of the internal organs is not disturbed in this process. After the hide is removed, the air is released by making a puncture with a knife in the groin area.

How is the hide removed mechanically?

Mechanical hide removal from the carcasses of different animal species is carried out in a defined sequence (Schemes 1, 2, 3). When processing pig carcasses, the heads are left attached to the carcass after hide removal until the post-slaughter veterinary and sanitary inspection is complete.

Depending on the anatomical and histological structure of the hide, the force needed to remove it varies. The magnitude of the force is influenced by the species, sex, and fatness of the animal, and by the area of the carcass from which the hide is being pulled. Scheme 1 — Sequence of operations during hide removal from cattle carcasses Scheme 2 — Sequence of operations during hide removal from small ruminant carcasses Scheme 3 — Sequence of operations during hide removal from pig carcasses

During mechanical hide removal the subcutaneous layer is torn, and the force applied to the hide is transmitted through that subcutaneous layer and the superficial fascia onto the muscle or fat tissue.

In areas where the hide is connected to the superficial fascia through the subcutaneous layer — whose strength is the same in all directions — the hide can be separated by tearing regardless of the direction in which the force is applied.

Where the dermis of the hide is connected to the superficial fascia through the muscles along the vertical axis of the carcass (in its front part), the force must be directed across the muscle fibres or along the fascia fibres.

Under this condition the fascia fibres are not torn but split apart, since splitting requires less force than tearing, and the carcass surface is not damaged. The tear method of hide separation has become the most widespread, and the operating principles of mechanical hide-removal equipment are based on it. Figure 2 — Knives for removing hides from animal carcasses: a — knife for removing hides from the legs and detaching heads, Ya2-FIN-2; b — knife for undercutting the hide, Ya2-FIN-3; c — knife for hide removal.

For the final removal of hides from cattle carcasses, plants use installations of the A1-FUU and FUAM types (batch operation with mechanical carcass fixers) and continuous-action installations of the RZ-FUV type (Figs. 3–5). Figure 3 — F1-FUU installation for removing hides from cattle carcasses: 1 — hydraulic press; 2 — fixer; 3 — hide-removal mechanism. Figure 4 — FUAM unit with a rotating fixer for removing hides from cattle carcasses: 1 — fixing clamps; 2 — vertical shaft; 3 — movement levers; 4 — bevel gear pair; 5 — gearbox; 6 — V-belt drive; 7 — electric motor; 8 — traction chain drive; 9 — drive sprocket; 10 — chute for lowering the dressed carcass; 11 — frame; 12 — tensioning sprocket; 13 — profile guide; 14 — traction chain; 15 — turning sprocket; 16 — tray Figure 5 — RZ-FUV installation for removing hides from cattle carcasses: 1 — conveyor for the limbs; 2 — conveyor for hide removal; 3 — conveyor for transporting the hides

When hides are removed on batch installations, the carcasses must be taken off the main conveyor. In addition, mechanical dirt present on the surface of the hide gets onto the carcass.

These drawbacks are eliminated in the continuous-action installation, whose output — depending on the speed of the conveyor unit — ranges from 65 to 132 head per hour. The unit is 12 m long and is placed on a single floor, whereas batch installations are 7.55 m high and therefore require tall premises.

Mechanical hide removal from small ruminant carcasses is carried out on drum-type FSB installations (Fig. 6), which are made in two versions: for removing hides top-down — from the tail end to the neck — and for removing them bottom-up — from the neck section to the tail. The separation angle is about 15°. Conveyor installations of the F1-FUU type are also used to remove hides from small ruminant carcasses. Figure 6 — FSB installation for removing hides from small ruminant carcasses: 1 — drum; 2 — pin; 3 — chain; 4 — fixer.

From pig carcasses the hide is removed either completely or partially (cropping/crupon removal), or the carcasses are processed with the hide on. For complete hide removal, siding is performed (in the same way as for cattle, except for the head and legs). The siding area for lean pig carcasses is 25–30%, and for fatty ones up to 50%.

After bleeding, the Achilles tendons of the hind legs of pig carcasses are exposed, and the carcass is hung on the overhead rail using rollers. The carcass is then fixed immovably by the lower jaw with a pedal-operated tensioning device.

The hide is gripped with a loop of chain whose end is hooked onto the winch hook, and the hide is torn away from the carcass in the direction from the head toward the rear.

The tearing speed is 3–5 m/min for fatty carcasses and 10–12 m/min for lean ones. The hide is torn off at an angle of 0°. During separation the hide is held back by hand to prevent the back fat from being torn off.

A continuous-action FShN unit has been developed for removing hides and crupons from pig carcasses and hides from small ruminant carcasses. It consists of a chain elevator for hide removal, set at an angle of 41° to the overhead conveyor rail, and a carcass-fixing conveyor positioned parallel to the overhead conveyor rail. The unit's output is 100 hides per hour.