Why rennet anyhow?

[Lenomnom]

 And while we a gabbling away on the subject, did you ever wonder what a casein micelle looks like?





This came from https://www.uoguelph.ca/foodscience/book/export/html/1608. The page has a fistful of links and some really interesting info on micelle structure.

[Lenomnom]

   Ok I'm on a roll, and I think I have found an answer to the debate Mike and I were having about the charges on caseins.


in the following diagram:





we see some arrows (top) pointing from about pH 7 to the isoelectric point and from there off into low low pH territory. What they are trying to tell us is that in fresh milk all the caseins exist as the negatively charged parts of salts such as Calcium caseinate. At very low pH casein stops grabbing electrons and starts donating them, existing again as salts but now as the positive ion, as in casein chloride. At an intermediate point, called the isoelectric point, caseins essentially exist as equal amounts of both types, the net charge on the average casein is zero, and precipitation occurs."AHA" you say "Since the effective charge is zero there is no electostatic force keeping the micelles apart, and they can bond and precipitate as curds!

Which just goes to show you that for every complex, difficult problem there is a solution that is simple, elegant and wrong. Electrostatic repulsion actually has about zip to do with it, it is something called steric repulsion that gets nuked by this process and it's, um, difficult.

Anyone know what steric repulsion is?

[mikekchar]

Ha ha!  I have no idea what any of that means! 

[awakephd]

Mike, are you repulsed by this discussion?

[mikekchar]

To be fair.  Lenomnom ninja'd me.  I was responding 3 posts back :-)  But I'm still not quite sure  what it means ;-)

As for steric repulsion: https://www.physicsforums.com/threads/steric-repulsion-simple-explanation.883156/ Lenomnom, why do you think steric repulsion is involved?

Here's a nice explanation of how the isoelectric point is achieved: https://www.khanacademy.org/test-prep/mcat/biomolecules/amino-acids-and-proteins1/v/isoelectric-point-and-zwitterions.

I *think* that diagram is actually conflating 2 concepts.  Casein micelles are not composed of only casein.  They are bound up into balls by calcium phosphate.  This is where we get into a theoretical area because as far as I can tell nobody actually knows how they are composed.  However, as the pH decreases (referring back to the video), the casein (statistically) starts picking up protons (hydrogen ions) on the carboxilic acid group.  If I understand correctly, this liberates the calcium phosphate from the micelle.  This is why the diagram is saying "Dissociation of Ca from the micellar complex".

Incidentally, the release of the calcium phosphate "relaxes" the micelle, which increases its size.  It also allows it to stretch, which is important in how cheese melts/stretches.

I think the discussion of "casein salts" and "caseinates" is a bit of a red herring, however in saying that It made me realise something.  My assumption was that the casein micelles are  suspended in the solution.  This is, of course, incorrect.  The key to understanding that is that the isoelectric point is the point where the protein *precipitates*.  In other words, the casein exists in a dissociated form and when it hits the isoelectric point (average charge 0 across the protein), it precipitates out.

It's also interesting that at the isoelectric point, all of the bound up calcium phosphate is discharged, which makes a certain amount of sense to me if it was relying on the negative charge to stay bound up in the micelle.  I still can't quite wrap my head around how it's composed in there, but... oh well.

[Lenomnom]

As for steric repulsion: https://www.physicsforums.com/threads/steric-repulsion-simple-explanation.883156/ Lenomnom, why do you think steric repulsion is involved?

The link you gave explains a number of different steric effects, but it never really explains how steric repulsion works. The reason I think steric repulsion is involved is because the page I took the micelle photo from says it's the principal effect.  Secondly, if electrostatics were dominant in water solutions, then in brine all the salt ions would simply pair up and nothing would dissolve. The reason the ions are free and remain so is that any charged species in water causes nearby water molecules to align like iron filings in a magnetic field, that alignment creates an opposing field and tends to kill the attraction at close ranges.

If you start with a raw casein ball, the charges on each molecule add up and the whole becomes highly charged. The water caging effects will neutralize that field at short ranges. I think the steric effect is simply due to the fact atoms have positive nuclei surrounded by much larger electron clouds. Thus, molecules are similar. When two molecules get close there is a tendency for the clouds to repel, even if both molecules are uncharged. Now if you change the pH, this is the repulsion that is neutralized at the isoelectronic point. With steric repulsion gone, and electrostatic attraction ineffectve, all that is left is Van der Waals forces, which are attractive, and random variations in momentum which might cause things to bang into each other quite hard, hard enough to penetrate the water cage and allow bonding to take place.

Sure it's all pretty speculative, but it's the best I can do with a sophomore understanding and some internet support.

[Lenomnom]

     The rest of your post I have not dealt with, cuz it's deep, difficult and I need time to absorb so many new concepts. However I put it to you that the casein micelles are in fact suspended in the solution. If they were not, they'd all be sitting on the bottom of the quart of milk in the store. This is not to say you can't make em do that, of course. A little acid and heat and voila, a precipitate of cheesy comestibles appears.

    One of the side benefits of all this is I have sort of accidentally developed a greatly improved mental model of how things dissolve in water. Hopefully this will be useful, since cheesemaking is essentially the reverse process.

   We are ranging far and wide so here's an odd question for you. You probably already know that pure water exists in equilibrium with H+ and OH-  So there is always a small amount of OH- and H+ in any water sample. Or if you like, [H+][OH-]=10 exp -14. So if this occurs, why doesn't the OH- further dissociate into another H+ and an O-- ? Hmmmm?