Helical structures from neutral biopolymers - Physical Gels from Biological and Synthetic Polymers - page 187

α

Bone
Pig skin

2,5

2,0

1,5

Subunits

Microgels

1,0

β

γ

0,5

0,0

10 4 10 5

M W (g mol − 1 )

10 6

Figure 7.3

Molecular mass distribution obtained by chromatography of first extracts of gelatin from beef
bone (basic extract) and pig skin (acid extract).

hydrolysis of the protein (subunits of the initial strand) and incomplete breakage of the
cross-links (branched high molecular mass components) occur. The molecular mass
and its distribution resulting from extraction are important parameters in determining
the ability of gelatins to form a gel.
In Figure 7.3 we see that the molecular mass distribution of gelatin differs signi cantly

depending on the source. The large central peak of the distribution, the so-called α chains,

is a single tropocollagen strand; chains have, respectively, two and three such

strands covalently bonded. Larger molecular masses (known as microgels) are also
observed. Besides, degraded

β and γ

chains appear as subunits corresponding to lower and
lower molecular masses. The pig skin (

α

(first extract) has a much more even distribution,
with no distinct major population. In the samples in Figure 7.3 , average molecular masses
are M w = 157 000 g mol − 1 for the bone extract and M w = 179 000 g mol − 1 for the pig skin
extract. The indices of polydispersity are respectively M w /M n = 1.89 and 2.17, so similar
average values of the molecular masses and polydispersity indices may correspond to
very distinct gelatin distributions.
The molecular mass (and to a lesser extent its distribution) affects the viscosity of the
solutions and is an important parameter in industrial applications. Another important
factor for applications is the gel

as determined by the industry Bloom test,
measured under standard conditions. Bloom strength is quoted in grams and the higher
this value, the greater the modulus. Usual values range from 300 g to 50 g, with low
molecular mass gelatins having a low Bloom value. Very low Bloom values are observed
for

' strength '

fish skin gelatins, for reasons explained below, related to their gelation temperatures.
In industry, gelatins with low Bloom values are used in food preparations, or technical
applications such as paper manufacturing, while intermediate values are used for phar-
maceutical capsules. The highest values from acidic extracts are required in confection-
ery and, from basic extracts, for hard capsules.