Journal of Film Preservation - FIAF

Table 2. Typical Property Values of Gelatine 28
Important photographic properties of the gelatine include the physical
properties of high Bloom Value and viscosity together with
transparency and lack of colour. In addition, its purity is critical since
certain trace impurities such us trace metals, sulphur components
(thiosulphate) and nucleic acids can affect the photographic emulsion
behaviour. Other parameters, shown in table 1, like, ash, iso-ionic pH, %
of nitrogen, % hexose sugars reflect the manufacturing route but are
not particularly significant to the photographic gelatine. Cross-linking
helps to controls the swell of the gelatine, which is necessary during
the processing of photographic images. Cinematographic applications
of the gelatine require higher dimensional stability that other
applicatins and, to reduce swelling in processing solutions, different
hardening agents (cross-linking agents) may be used, for example,
formaldehyde or bis(vinylsulfonylmethyl)ether and polymeric
hardeners, polyacrylates with pendant vinylsulphonyl or
2.cloroethylsulphonyl groups.
Enzymatic biodegradation of gelatine
Polypeptides are biodegradable substrates and gelatine is not an
exception. Its biodegradation occurs, like in other polypeptides, as a
result of the fragmentation (hydrolysis) of the peptide bonds present in
its macro-molecule.
These fragmentations are carried out by proteolytic enzymes. In a first
step, proteases catalyse the break of the gelatine structure giving long
peptides segment chains by the scission of specific bonds in the
49 Journal of Film Preservation / 67 / 2004