pKa
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Casein from Milk
First off: Hello all! First post on the forum!
Question: I can pull casein from milk quite easily enough- heat and add an acid (I just use vinegar). It separates into curds and whey. I strain out
the whey and I have what is more or less mostly casein. (Also known as Ricotta cheese.)
The question I have is about what's going on in there. Is it a case of lowering the pH forces the casein micelles apart, or is it the acid reacting
with the calcium phosphate molecules that form the centers of these micelles?
Thanks in advance.
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hissingnoise
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Welcome to SciMad, pKa. . .
AFAIK, acidic solution reduces casein's solubility.
Acetic acid doesn't react with Ca(PO4)2 or Ca(H2PO4)2.
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chemkid
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I believe there is some degree of denturation in the hydrophilic parts of the protein which cause it to precipitate
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chemoleo
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Oh, no, this time it is not denaturation (although your guess, Chemkid, would be usually correct)
To quote from wiki
http://en.wikipedia.org/wiki/Casein
| Quote: | Casein (from Latin caseus "cheese") is the predominant phosphoprotein (αS1, αS2, β, κ that accounts for nearly 80% of proteins in milk and cheese. Milk-clotting proteases act on the soluble portion of the
caseins, K-Casein, thus originating an unstable micellar state that results in clot formation. When coagulated with rennet, casein is sometimes called
paracasein. Chymosin (EC 3.4.23.4) is an aspartic protease that specifically hydrolyzes the peptide bond in Phe105-Met106 of κ-casein and is
considered to be the most efficient protease for the cheese-making industry (Rao et al., 1998). British terminology, on the other hand, uses the term
caseinogen for the uncoagulated protein and casein for the coagulated protein. As it exists in milk, it is a salt of calcium. Casein is not
coagulated by heat. It is precipitated by acids and by rennet enzymes, a proteolytic enzyme typically obtained from the stomachs of calves.
The enzyme trypsin can hydrolyze off a phosphate-containing peptone.
Casein consists of a fairly high number of proline peptides, which do not interact. There are also no disulfide bridges. As a result, it has
relatively little secondary structure or tertiary structure. Because of this, it cannot denature. It is relatively hydrophobic, making it poorly
soluble in water. It is found in milk as a suspension of particles called casein micelles which show some resemblance with surfactant-type micellae in
a sense that the hydrophilic parts reside at the surface. The caseins in the micelles are held together by calcium ions and hydrophobic
interactions. There are several models that account for the special conformation of casein in the micelles (Dalgleish, 1998). One of them
proposes that the micellar nucleus is formed by several submicelles, the periphery consisting of microvellosities of κ-casein (Walstra, 1979;
Lucey, 2002). Another model suggests that the nucleus is formed by casein-interlinked fibrils (Holt, 1992). Finally, the most recent model (Horne,
1998) proposes a double link among the caseins for gelling to take place. All 3 models consider micelles as colloidal particles formed by casein
aggregates wrapped up in soluble κ-casein molecules.
The isoelectric point of casein is 4.6. The purified protein is water insoluble. While it is also insoluble in neutral salt
solutions, it is readily dispersible in dilute alkalis and in salt solutions such as sodium oxalate and sodium acetate. |
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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pKa
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Well thanks for the info.
My guess was that the acetic acid exchanged the ion at the centers of the micelles. Regardless- my project is about plastic. Just for poops and
giggles (rules on swearing here?) I'm trying to make a very simple caulking compound that can be kept in a zip-lock bag and dispensed by cutting a
corner in it. My thinking, and I'm trying it today when I get home: To precipitate out some casein from milk and then, because it's full of water and
vinegar, and possibly other unknown materials- wash extensively with 95% isopropanol to azeotrope with the water (for quicker drying) and wash away
the acetic acid and other organic materials.
Then I'll put what I have left in a zip top bag and see how it works out. My guess is it won't work so well as a glue, per say, but it might work in
an injection mold.
I have half a gallon of milk past its sell-by (and for that matter drink-by) date, why let it go to waste?
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chemoleo
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I don't think just any protein will do for making sticky tacky substances. Aside from the protein sequence, it is also the nano-alignment that
matters, forming beta sheets, helices or amorphous regions and the like....
Perhaps you should look into spider silk:
http://en.wikipedia.org/wiki/Spider_silk
[Edited on 19-11-2008 by chemoleo]
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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watson.fawkes
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| Quote: | Originally posted by pKa
Then I'll put what I have left in a zip top bag and see how it works out. My guess is it won't work so well as a glue, per say, but it might work in
an injection mold. |
Casein was used as the binder in old milk paints. Not exactly a glue, but at least a
little related.
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unionised
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Casein glue used to be used a lot in woodwork before synthetic polymers came on the seen. I don't think just any protein would work, but this one
will.
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