Author Topic: When proteins bind with direct acid versus cultured acid versus rennet  (Read 1277 times)

Milk Maid

  • Guest
Can one of you remarkable cheese people explain to me what the difference is when casein binds because of a direct acid (ricotta), a cultured acid (yogurt), or simply rennet (queso fresco)? If direct acid a strong bond between the proteins? Are all of the bindings similar, just different intensities? I know I am asking a huge question and should probably go find a book to read. But thought I would ask and see what people say.

Thank you!

Milk Maid

meyerandray

  • Guest
Well if this were a test, I would get a C at best, I should go back to G.Caldwell's "Mastering Artisan Cheesemaking" and review, but what I remember (I don't have the book down here right now):

The casein micelle when in liquid milk have hair-like casein "hairs" protruding, and they are negatively charged, so negative repels negative, and they remain separate.
Rennet physically cuts these hairs, or the negatively charged protrusions, (I can't remember how/why...), and also makes them hydrophobic (not water loving), which encourages them (the protein clusters) to bond together. 
Ok, so maybe a D on the test...

Acid coagulation essentially melts the micelle's structure, the H+ ions in the acidic coagulant neutralize the negatively charged hairs, which takes away their ability to repel one another.  The acid somehow strips the general "clustered" structure of the micelle-something with calcium phosphate that I can't remember.

Hmmm, maybe I shouldn't have answered until I looked at the book.  Hope this really bad and unclear info can help until someone wiser can answer.

linuxboy

  • Guest
IIRC, for simplicity, Gianaclis sided with the Walstra view of the world in terms of micellar structure. It has been the most historically popular model since Walstra's 2000 revision. The model is just a tad outdated in some ways, especially in understanding sub-micellar composition, because there's an aspect of Walstra's model that for ease oversimplifies the interaction between submicellar structures and how they form micelles. Or to put it more plainly, the model suggests that micelles are generally similar in size, and sub-micelles are fairly uniform. In reality, recent studies have shown micellar dynamics to be highly variable (eg work that Holt did for the nanocluster model). So what does that mean for our current understanding of the questions raised here... Let's work through it.

First, current model primer to help explain micellar composition and interaction:
- protein aggregates (caseins... as1, as2, b, k) organize around tiny clusters of calcium phosphate. Protein specificity of bonding preference (protein-protein vs protein-calcium phosphate) depends on protein type.
- There's a long "leg" sticking out where the k-casein bonds, points outward
- So you have inter-micellar forces, and a "outer" micellar force due to the k-casein orientation
- Just so happens that k-casein has negative charge at terminal peptide chain, so it suspends the micelle in the the milk.
- There are generally two types of bonding forces... strong and weak.
- Weak bonding forces are all the ones that are not calcium phosphate interactions. Meaning, your hydrophobic ones... ionic bonding, hydrogen bonding, etc. Van der Waals generally not considered a strong force for milk.
- Strong bonds are generally intra-micellar ones, the ones for calcium phosphate.
- And both of these bonds types matter for cheesemaking.
- Even though micelle is fairly stable, it can be destabilized by acid, by heat, and by enzymatic processes (eg chymosin).

Now let's get into each destabilization approach.
enzymatic
Classic rennet curd, eg queso fresco. Follows the 3-phase process for k-casein cleaving from 105-106 bond disruption (or similar in that vicinity depending on enzyme type... eg pepsin and some plant enzymes different). Pretty straightforward... k-casein destabilization after enzymatic cleaving means there's no more repulsion. So previously hydrophobic micelles can now bond together. Also applies to the quasi-rennet, the semi lactics. It's a continuum. Two forces here. One is steric repulsion from negative k-casein charge. Other is electrostatic repulstion from high pH (isoelectric point important here). Need calcium to create bridges so micelles bond.

acidic
Acidic is straight up micellar disturbance from decreased electric charge. Isoelectric point is around 4.6, so as you approach it, micelles break up. Is not only about k-casein. Is also about intra-micellar forces. Here you get the quality difference. EG, if have 4.2 and try to bond sub-micellar elements, you get a kind of gritty lactic. If go with higher range, 4.5-4.8, you have moderate intra-micellar disruption and a gel will form to a more stable lattice. Hydrophobic interactions drive the bonding. So if you have something crazy, like super low pH, or prolonged acidity before bonding, will not be able to re-bond into smooth gel.

heat and heat-acid
Possible to destabilize micelles through heat. Easier to destabilize through heat and acid. Situation here is similar as lactic... destabilized micelles, new re-aggregation of micelles due to lack of repulsion (hydrophobicity). But here, other dynamics happen. Dynamics are whey protein adsorption, different isoelectric point of whey proteins (5.2 instead of 4.6). And of course heat. So there's a similarity here to lactic curd more than enzymatic. Has to do more with the bond types and charges for how a micelle is stable, vs outright enzymatic cleaving and reaggragation.


NB: scattered thoughts. Did not double check the literature, just what I recall offhand.

to comment on Celine's post

Quote
he casein micelle when in liquid milk have hair-like casein "hairs" protruding, and they are negatively charged, so negative repels negative, and they remain separate.
Rennet physically cuts these hairs, or the negatively charged protrusions, (I can't remember how/why...),
Is enzymatic model for bond separation, at Phe-met bond

Quote
and also makes them hydrophobic (not water loving), which encourages them (the protein clusters) to bond together.
Pretty much that, right. Hydrophobicity describes physics, not so much chemistry of the bond. It's due to weak bonding. Exact nature not known still
 
Quote
Acid coagulation essentially melts the micelle's structure, the H+ ions in the acidic coagulant neutralize the negatively charged hairs, which takes away their ability to repel one another.
Not only, but also. k-casein neutralization would account for only moderate attraction. Micellar isoelectric point is function of entire micelle. And acid bonds typically happen after intra-micellar degradation. Can't just say enzymatic and acid styles are similar due to k-casein, and k-casein destabilization is matter of enzymatic vs acidic. 
Quote
The acid somehow strips the general "clustered" structure of the micelle-something with calcium phosphate that I can't remember.
Yeah, basically, that's the gist of it. The idea of the outer k-casein "shell" is a little misleading. It's not like a coated M&M. More about protein shape and orientation after a micelle stabilizes

xo,
Pav

[edit] Might as well attach a decent paper
« Last Edit: July 01, 2013, 09:21:50 PM by linuxboy »