Wayde Allen (wallen@boulder.nist.gov)
Mon, 28 Jun 1999 14:19:30 -0600 (MDT)
On Thu, 24 Jun 1999, tshack wrote:
> I'm getting a quote from a firm for some 175 bloom gelatin, and they are
> saying the pH is around 4.5 to 5.2. Is this something that might cause
> a problem for collotype or other gelatin processes?
Not likely, the pH depends on the method used to produce the gelatin and
does affect the gelatin properties somewhat. However, you will ultimately
adjust your process to adapt to the characteristics of your gelatin.
The following quotes are from the book I'm currently reading about
gelatin, A.G. Ward and A. Courts, The Science and Technology of Gelatin,
Academic Press, 1977.
(pgs. 114 - 116)
"1. The effects of pH and added electrolyte (pg 114 - 116)
Even small variations in Ph can give rise to quite marked changes in
reduced (or in intrinsic) viscosity, especially at low ionic strength.
The reduced viscosity is a minimum at the isoionic point, and rises to
a maximum at pH3 and pH 10.5 approximately. The addition of
electrolyte reduced the dependence of reduced viscosity on pH.
Stainsby (1952) has accounted for these results by presuming that the
presence of a nett charge on a gelatin molecule gives rise to repulsive
forces which are a maximum at zero ionic strength. At the isoionic
point the equal numbers of positive and negative charges cause the
maximum folding of the molecule. As the pH is raised or lowered the
nett charge increases and the polypeptide chains unfold, giving an
increase in viscosity. However, to raise or lower the pH alkali or
acid must be added, and the counter ions resulting from these additions
eventually become sufficiently numerous to reduce the forces between
charges. At this stage the unfolding ins reduced again, and viscosity
falls. Addition of electrolyte similarly6 reduces the forces between
charges. Except in the isoionic region this also leads to a fall in
viscosity at any pH. At the isoionic point there is a overall
attraction between the charges on each gelatin molecule: this is
weakened when salts are added and a small increase in viscosity occurs.
The shape of the curve relating reduced viscosity and pH reflects the
variation on nett charge with pH (though the nett charge is not usually
plotted for gelatin as it is calculable from the titration curve only
when the molecular weight is known). In consequence, the curve for an
acid process gelatin differs quite strikingly from that for a gelatin
extracted from alkali-pretreated collagen in the range pH 6-10.
With both types of gelatin, these pH effects diminish with increasing
gelatin concentration because of the increase in counter ion
concentration and, with commercial samples, because of the increase in
electrolyte concentration due to the inorganic ash of the sample."
(pg. 212 - Gelatin Modification Reactions, Experimental Conditions)
"Cast gelatin films can generally withstand greater extremes of
reaction temperature and pH, depending on the amount of water present
in the film."
(pg. 525 - Physical tests for gelatin and gelatin products)
"pH and Salt Concentration - Gelatin solution viscosity is strongly
affected by pH, reaching a minimum at the iso-ionic pH. Minimum pH
effect has been noted in the range pH 6-8 (Stainsby, 1952); therefore,
for comparative viscosity testing the pH of gelatin solutions should be
adjusted to a point in this range, or, for specific applications to the
pH level of the application. The effect of pH is reduced by the
presence of salts."
(pg 530 - Physical tests for gelatin and gelatin products)
"The effect of pH and Salt Content on Gel Melting Point - Stainsby and
Taylor (1958) examined the effect of pH and salt content upon the
gelatin gel melting point. Their results showed that the melting points
of both acid and alkali processed gelatin gels are pH dependent below
pH 5, the melting point decreasing with increasing acidity. Between pH
5 and 9 there is virtually no change in the melting points of
alkali-processed gelatin gels, whereas the melting points of
acid-0processed gelatin gels continue to rise slightly. ..."
Probably more than you wanted to know, but maybe it will help a little.
- Wayde
(wallen@boulder.nist.gov)
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