Ossein liming

Different types of gelatine cannot be mixed as they have different isoelectric points and these different types are referred to by their origin and the agent used for hydrolysis, e.g. acid pigskin gelatine. Limed ossein gelatine is normally produced from cattle bones. 

Alkali treatment reduces shrinkage temperature (Fysh, 1958), and Courts (1960) suggests a minimum figure of approximately 44°C for limed ossein (demineralized bone). At this stage few intermolecular cross-links can be present because the material can be dissolved in water at 60°C. Gustavson (1962) has shown that treatment with periodate also lowers the shrinkage temperature (Ts) by as much as 20°C. This could suggest that alkali and periodate attack the same cross-link, but other side reactions under the conditions used will also have some effect. 

In the alkali process, demineralized bones (ossein) or cattle skins are usually used. The animal tissue is held in a calcium hydroxide (lime) slurry for a period of 1-3 months at 15-20°C. At the end of the liming, the stock is washed with cold water to remove as much of the lime as possible. The stock solution is then neutralized with acid (HGl, H2SO4, H3PO4) and the gelatin is extracted with water in an identical manner to that in the acid process. 

In the acid process, the bones and skins are treated in a vessel containing a dilute solution of acid for a predetermined period of time. Then, the acid is washed out with cold water. In the alkali process, the demineralized bones (demineralization is mostly done with acid solutions to remove calcium and other salts from the bone to prepare the collagen-rich bone material known as ossein) are placed in liming pits and soaked in a lime suspension for longer than 60 days. For the hides or skins, a caustic soda solution is used for a shorter period of time. After this treatment, the raw material is washed thoroughly to remove any residual lime. The acid pretreatment is mostly used for skin, while the alkali pretreatment is mostly used for bones (Petersen and Yates, 1977). 

Gelatin and animal glues (glutines) are extracted from waste bones and hides [30.10, 30.11], Type A gelatin is produced by an acid extraction, while Type B uses an alkali — usually slaked lime. Demineralised bone (ossein) and cattle hide pieces are soaked in lime water at ambient temperature for up to 20 weeks. Additional milk of lime is added to the liquor to maintain the alkalinity. 

Croda 50 Bloom Gelatin] Croda 60 Bloom Alkaline Processed Gelatin] Croda 160 Bloom Limed Gelatin] Croda 190 Bloom Acid Ossein... 

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