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hrst
10-28-2009, 03:47 PM
I tried to find some information about how much pig skin food grade gelatin swells at different pH values, as I remember PE saying that photo grade cow bone gelatin has a maximum swell a pH=9 and pig gelatin at pH=3 or something like that.

All I found was http://jgp.rupress.org/cgi/reprint/12/4/537.pdf , at page 4, but it doesn't show pH after 4. So, I made an evaluation myself.

I made six buffered 300 ml pH solutions (using H2SO4, citric acid, fluoric acid and NaOH) ranging from 4.0 to 10.4, cut 1.00 gram of gelatin sheet to each of them, let to swell at 23C room temp for 1 hour 45 mins. Then I drained each sheet for 20 seconds and measured the weight.

Here's the results:

Pig skin gelatin swell at 23C, 1 hour 45 mins
Swelling in gram of H2O per gram of gelatin:

pH = 4.0: 5.41 grams
pH = 5.2: 5.28 grams
pH = 6.4: 5.24 grams
pH = 7.7: 5.13 grams
pH = 9.2: 5.16 grams
pH = 10.4: 5.20 grams.

So, there seems to be no real difference at pH range of 4...10 when washing the emulsion noodles, if you use food grade (pig) gelatin.

Photo Engineer
10-28-2009, 04:24 PM
Here is a plot of Bone gelatin swell vs pH.

I cannot find a pig gelatin figure but basically, rotating the bone gelatin curve on the horizontal axis (X) will give you a rough idea. The references to Pig Gelatin are few to come by due to its lack of use in photographic system design. I did use it a few times as make up gelatin to dilute some things I coated but that was not common.

The minimum swell is about 4.5 for Bone gelatin and about 9 for Pig gelatin. This is related to both the isoelectric point and sometimes the method of preparation (lime or acid).

Unfortunately, Pig Gelatin was pretty much abandoned for photo product manufacture before I did much work in the field of coating and making.

I would also like to point out that using sulfate (sufuric acid) of any sort will skew the test due to the notorious swell inhibiting characteristics of the sulfate ion. In fact, that is why sulfate is used in many hardeners.

No textbook I have clearly states the differences between gelatins probably due to the fact that actual gelatins used were quite secret. In one case, I believe that they used Yak gelatin for one particular product.

I would point out that the referenced article uses Cooper gelatin which was one of the first food grade gelatins and therefore probably a Pig Gelatin. If you look at the figure from that article that I enclosed, I think that you will see that it is the inverse of the Bone Gelatin figure.

My three references are from Mees, Haist, and James and Higgins. The plot is duplicated in Mees, but comes from a web site from years ago.

Pig Gelatin should swell less at alkaline pH and Bone Gelatin should swell more.

PE