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Teil 3 Seite 6-13
tissue from the bone. The contents of the degreasing vessel are separated to give hard
bone, sinew (soft bone) and liquids containing tallow and water.
The hard bone is washed using hot water, to give a final moisture content of around
10 %. The sinew is pressed to remove fat and water before being dried with the hard
bone to give a final moisture content of 14 %. Drying at 85 °C takes 45 minutes.
The dried bone and sinew are sieved at 2 mm and 5 mm to give bone meal (< 2 mm
fraction), intermediate bone pieces (2 - 5 mm fraction) and degreased gelatine bone/sinew
(> 5 mm fraction).
The tallow/water mix is separated using centrifuges to give purified tallow and process
water. The liquids are maintained at a temperature of 85 °C for 30 minutes during separa-
tion.
Fine solids removed from the liquid during separation, together with fine solids from the
pressing of the sinew are combined and dried to give a moisture content typically < 10 %.
The product temperature achieved in the drier is around 110 °C for at least 45 minutes.
Cyclones are used to remove the air and to separate the fines from the larger particles
destined for the gelatine manufacture.
Bone chips are graded by density, using a hydrocyclone, because higher density bones
require more processing than low density bones, both to demineralise and to extract the
gelatine. They are then dried in a band oven, with a starting air temperature of around
350 °C and an exit temperature of 150 °C. The chips are only in contact with the hot air
for a short time and they are also cooled by the evaporation of the water, so their tem-
perature will not normally exceed 85 ºC. The drying time varies from 20 – 60 minutes.
The dried bones are then classified by size, using a densimetry table, comprising an in-
clined screen with an upward air-draught on to which the bone chips are dropped.
The amount of dried degreased bone chips obtained from 1 kg of bone is usually about
200 g.
If hides or skins are used this pretreatment stage is not required.
B Demineralisation/acidulation
The demineralisation process is the removal of the inorganic component, which com-
prises mainly natural phosphates and calcium carbonate. The defatted bones are placed
in a strong hydrochloric or sulphuric acid solution at pH 1 - 2, where the tricalcium phos-
phate is converted into the soluble monocalcium salt, which is run off in solution, for later
conversion to dicalcium phosphate, soluble calcium chloride and carbon dioxide. The
chemical reaction with hydrochloric acid is shown.
Ca3(PO4)2 + 4 HCl -> Ca(H2PO4)2 + 2 CaCl2
CaCO3 + 2 HCl -> CaCl2 + H2O + CO2