Gelatin Manufacture

There are a large number of unit processes used in the manufacture of gelatin. The raw materials from which it is derived are demineralized bone (called ossein), pigskin, cow hide, and fish skin. In China, donkey hide is used quite extensively. In theory, there is no reason for excluding any collagen source from the manufacture of gelatin, but the raw materials listed are currently commercially available. Interestingly, in countries where pork is sold with its skin intact, there is no pigskin available for gelatin manufacture.

As mentioned earlier, there are two processes by which collagen is processed to gelatin: the acid process and the alkali process. The acid process (studied in detail by Reich et al. [9]) is mainly used with pigskin and fish skin and sometimes bone raw materials. It is basically one in which the collagen is acidified to about pH 4 and then heated stepwise from 50°C to boiling to denature and solubilize the collagen. Thereafter, the denatured collagen or gelatin solution has to be defatted, filtered to high clarity, concentrated by vacuum evaporation or membrane ultrafiltration treatment to a reasonably high concentration, and then dried by passing dry air over the gel. The final process is one of grinding and blending to customer requirements and packaging. The resulting gelatin has an isoionic point of 7 to 9 based on the severity and duration of the acid processing of the collagen, which causes limited hydrolysis of the asparagine and glutamine amino acid side chains.

The alkali process (studied in detail by Cole [10]) is used on bovine hide and collagen sources where the animals are relatively old at slaughter. The process is one in which collagen is submitted to a caustic soda or lengthy liming process before extraction. The alkali hydrolyzes the asparagine and glutamine side chains to glutamic and aspartic acid relatively quickly (11), with the result that the gelatin has a traditional isoionic point of 4.8 to 5.2. However, with shortened (7 days or less) alkali treatment, isoionic points as high as 6 are produced. After the alkali processing, the collagen is washed free of alkali and treated with acid to the desired extraction pH (which has a marked effect on the gel strength to viscosity ratio of the final product). The collagen is then denatured and converted to gelatin by heating, as with the acid process. Because of the alkali treatment, it is often necessary to demineralize the gelatin solution to remove excessive amounts of salts using ionexchange or ultrafiltration. Thereafter, the process is the same as for the acid process—vacuum evaporation, filtration, gelation, drying, grinding, and blending.

Although gelatin is often considered a commodity like sugar, the descriptions of the processes and raw materials above should indicate that gelatin has the potential for be ing a variable product. Users must ensure that they are using the best product for each particular application. In the past, little emphasis has been placed on the animal age of the raw material, particularly in the case of gelatins from bovines; however, it is now known that this factor plays a significant role in the molecular structure of the derived gelatin. The role of liming in the alkali process used to be considered one of progressive alkali hydrolysis of the collagen, which made it possible to denature the collagen at lower temperatures and thus maximize the yield of top-quality gelatin. Recently, however, it has been shown that the role of liming is limited to the hydrolysis of one collagen cross-link that fluoresces at 290 to 380 nm and that liming has increasing less effect on extractability in older animals. The result is that alkali treatment times have been greatly reduced. One of the less well-recognized effects of alkali treatment is the "opening up" of the hide collagen, as it is called in leather manufacture, or the destruction of the proteoglycans associated with the collagen fibrils. This probably results in a more pure gelatin via the alkali process, as is indicated by electrophoresis of the gelatin proteins (6).

At present, enormous developments are being made in the understanding of the structure of collagen and the changes occurring with senescence, and these developments are bound to have an impact on the appreciation of the variables in gelatin, particularly at the molecular level.

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