Raw materials : Gelatine is obtained by hydrolysis of the collagen that is contained in animal connective tissues. Commercially, skins or bones of different animal species, such as beef, pork, fish and poultry, form the main raw material for gelatine production. These raw materials are collected from animals approved for human consumption by ante- and post-mortem veterinary inspection. Skins can be frozen if there is a long delay between sourcing and eventual use in the gelatine manufacturing process. Some raw materials, mainly bones, are pre-treated by grinding, degreasing and drying before they are used in gelatine production.
Two main processes are used to extract gelatine from the raw material: the acid process for gelatine type A and the alkaline process for gelatine type B. Some other processes can also be used, but these are on a smaller scale.
The first step of the process is to reduce the size of raw material, if needed, by cutting or grinding. For bones, the grinding operation is done during the degreasing operation to reach a bone chip size from 2 to 25 mm. Skins are reduced to 30–150 mm. The ground or cut pieces of raw material are washed with clear water for some hours before
any further specific treatment begins. For bones, this washing is done after the degreasing operation.
Degreased bone chips are pre-treated by hydrochloric acid to remove the mineral matter, calcium phosphate, inside the bone. At the end of this operation, only organic matter containing gelatine remains. This is called ossein.
A. Acid Process: An acid pre-treatment on skins or bones is affected by adding the raw material to an acid bath at ambient temperature. The acidulation time and the concentration of acid vary with the type of raw material used. Different acids, including hydrochloric acid, phosphoric acid, sulphuric acid or even organic acids, may be used.
B.Alkaline process: The second type of process consists of a pre-treating the raw material with an alkaline chemical agent. Lime or sodium hydroxide solutions are usually used for this operation.
Depending on the alkali and the concentration used, the treatment time takes from 30 days to more than 70 days at ambient temperature, which is normally less than 18 ◦C.
After the chemical pre-treatment, the raw materials are washed to remove all excess acid or alkali and adjust the pH for extraction. Following pH adjustment, the raw material is put
into a cooking tank with hot water, for continuous or batch extractions, to render the gelatine soluble. In batch processes, different extracts are removed as temperatures are progressively
increased from 50 ◦C to 90◦ C over 3–6 h.
The solution from the cooking tanks contains around 4–7% gelatine with some fines of bones or skins. These physical impurities are removed by filtration, including earth, cardboard or membrane filters, or by centrifugation. After removing solids, the salt content is reduced
by passing the gelatine solution through ion-exchange resins. This operation removes the cations and anions of mineral salts contained in the gelatine. This step can also be done by specific ultra-filtration treatment.
After purification, the gelatine solution is concentrated to 30–50% dry matter using vacuum multi-effect evaporators and/or membrane technologies, depending on the viscosity of the solution. Before being gelled, the solution is treated in an ultra-high temperature (UHT) system at a minimum temperature of 138 ◦C for 4 s to guarantee the bacteriological properties of the final product. The concentrated solution is cooled in a votator and extruded in the form of gelatine
‘noodles’. The noodles are spread on the conveyor of a continuous belt dryer and the water is removed through hot air flow to a moisture content of around 10%. After drying, the gelatine is ground and then homogenised, which results in semi-finished lots of dry gelatine.
At this point, all the physical, chemical and bacteriological properties are checked in the laboratory. Different lots of semi-finished gelatine are blended and packaged for shipment and delivery.
Hydrogen Bonds: Hydrogen bonds (Cooper, 1971; Finer, 1975; Engelet al., 1977; Ledward, 1986) play a predominant role in gelatine structure; they stabilise the collagen and are involved in the gel-forming process, for example in the shrinkage of tissues.
Collagen is insoluble in water and its hydrolysis results in gelatine. The thermal denaturation of collagen takes place by heating the collagen after an acid or alkali treatment. Then the fibres and fibrils dissociate into tropocollagen units, mainly by the loss of hydrogen bonds. Now it is commonly acknowledged that the thermal stability of the different
collagens is closely dependent on the total amount of pyrrolidine (proline and hydroxyproline). However, the role of hydrogen bonds in the stability of collagen is arguable (Cooper, 1971).
In tropocollagen, the presence of proline residues gives the peptide chain a helical structure that differs from the α-helix of globular proteins by the lack of hydrogen bonds inside the α-chain. However, glycine residues are located inside the α-chain and are able to create hydrogen bonds. X-ray studies by Bella (1998) showed that the formation of hydrogen bonds involves the participation of water as well as the hydroxyproline groups (Fig. 7.4). Water plays a critical
role in maintaining the conformation of collagen molecules and the mechanical properties of collagen fibrils.
The thermo-reversibility of gelatine gels makes them unique. They have a melting point below 37◦C, which means that they melt in the mouth and are easily dissolved. The gelatine gel is a network of polypeptide chains with junction zones (Fig. 7.5). In a heated solution of gelatine, the triple helices are widely disorganised. Cooling this solution results in the nucleation of helical regions. Gelation is the consequence of a partial return to the collagenlike triple helix structures. It is generally accepted that the junction zones (triple helices) are stabilised by inter-chain hydrogen bonds similar to those found in native collagen.