Meat hygiene practice - Gracey's Meat Hygiene - page 179

used to produce shoes, garment leather, upholstery
leather and accessory leather. Pig skin , the second most
common leather-making raw material worldwide, is
thinner than cattle hides and is primarily manufactured
into garment leather. The largest producer of pig skins
is the People's Republic of China.
The quality of leather to a large degree depends on
the techniques used for hide removal (flaying) and the
processing that follows.
Bovine hides can be removed manually, which requires
great skill to avoid damage, but nowadays in commercial
meat plants, hides are removed mechanically by hide-
pulling machines following initial knife work.
Pig skins are scalded and de-haired, and this generally
makes them unsuitable for further processing. Pigs can
be skinned with a knife, but considerable skill is required
owing to the softness of the fat. Mechanical pulling of the
hides is gaining popularity because of energy and labour
savings when compared to scalding. However, it results in
a 6-8% loss in carcase weight, and it is generally slower
(150-300 per hour) than scalding (150-850 per hour).
Goat skins are more valuable than sheep skins because
they are larger and produce a better-lasting leather. Sheep
skins require a longer time (up to several hours) to cool
after slaughter than other hides owing to the insulating
properties of the wool and the presence of grease.
As well as for leather, hides and skins are used for
non-food purposes such as cosmetic ingredients and
medicinal prosthetics such as skin grafts and sutures.
After flaying, hides and skins should be chilled or
cured quickly to arrest bacterial and enzymatic decom-
position or spoilage. In areas with low relative humidity,
they may be air-dried for preservation, but salt is mainly
used as the curing ingredient. Salt-pack curing involves a
flesh-side up stack of hides, usually 90-130 cm high,
with an equal weight of salt to hide spread evenly over
the flesh-side of each hide in the stack. This draws mois-
ture out of the hides. Preservatives are often used with
salt-pack curing. Curing takes 20-30 days for cattle.

For purposes other than human consumption, blood
can be collected very easily by placing troughs below the
carcases. They should be designed so that they are easily
dry-cleaned with rubber squeegees. It is essential that
water does not come in contact with the blood for two
reasons: (i) water will cause haemolysis of the red blood
cells and prevent the adequate separation of plasma and
red blood cells and (ii) should blood get into the plant
effluent system, it will greatly increase the biological
oxygen demands (BODs) by around ten times and the
suspended solids by three times.
Before being allowed to enter the collecting tank, ined-
ible blood should be strained to exclude foreign matter.
Drying of blood meal by removing the water is carried
out in three main ways as follows:
1 Direct batch drying carried out in batch dryers similar
to rendering batch coolers. The raw blood is dried to
2-10% moisture by simply boiling off the water.
2 Batch coagulation followed by batch drying . The raw
blood is initially coagulated by injecting direct steam
into an open tank containing the raw blood. The coag-
ulum, which is around 25% total solids (TS) is then
separated by draining or hand pressing and dried in a
batch dryer.
3 Continuous coagulation before drying . This is the most
common method of processing blood. Strained blood
from a blood holding tank is pumped into an interme-
diate pre-heating tank equipped with a low-speed agi-
tator and the blood is pre-heated to 60°C by steam.
The blood then passes to the coagulator and as a result
of steam injection nozzles, positioned at several points
in the coagulator, at the exit, the blood is at an opti-
mum temperature of 90°C. A decanter then separates
the solids, which are dried, and the liquid.

Bone in pet food
Some, mainly pig bone and chicken necks, are emulsified
and used in pet food.
Calcined bone, obtained by roasting in air, is used in
the manufacture of high-class pottery and china, in the
refining of silver and in copper smelting. Bone charcoal
is utilised in bleaching, sugar refining and case-harden-
ing of compounds in the manufacture of steel. Special
bone powders are employed for the removal of fluorine
from drinking water.

Hide curing
Ox hides arrive at the tannery either fresh from the abat-
toir, chilled or more usually salted and dried to prevent
putrefaction. Tanning is defined as the hardening of hides,
using vegetable tanning agents, chromium salts or other
substances such as aluminium salts, ferric salts, silicic
salts, aldehydes and quinones, or other synthetic harden-
ing agents (EC 853/2004, Annex III, Section XIV, 2).
After soaking in water to cleanse and soften the hides,
they are placed in tanks filled with milk of lime for 1-4
weeks to loosen the hair and open up the fibre. The hair
on the outside, and flesh and meat on the inside, are then
scraped off and, after removal of lime by washing in

Hides and skins for leather production
Hides and skins constitute the most valuable material
removed from the animal carcase. Skins come from
smaller animals, for example, sheep, calves and goats,
while larger skins such as those from cattle, horses,
elephants are called hides . Leather from cattle hides is