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So I'm a little confused. I made 30 minute mozz with store-bought milk, and it turned out nice, but more like cream cheese. I was advised to add CaCl2, as I understand that pasteurization results in a Ca deficiency that interferes with coagulation. OK but ..... everyone says that you SHOULDN'T add CaCl2 because it interferes with stretchiness. On reading a little more, I see that you should NOT add it to raw milk, but you should add it to pasteurized milk. Am I to understand that too much Ca is what interferes with stretchiness? As in, add it if you don't have enough, but don't end up with too much?

Calcium chloride replaces calcium in the milk that is tied up during pasturization. Calcium is needed to make the curd strong and hold together.

In mozz the strech comes from acid and the over all curd strength comes from rennet and calcium. Need more strech add a little more acid (to a point because if you add too much the cheese won't strech either). If you need more cheese like quality in the end (like what you had when it was like cream cheese) use calicium chloride and if that doesn't fix the issue maybe try different milk or use a little more rennet.

Thanks. So if that's the case, that acidity allows stretchiness, why is it said that calcium chloride is said to reduce stretchiness? I don't think calcium chloride is going to do much to the acidity. It's a salt, so it is neither acidic or alkaline. But I guess if calcium strengthens the curds, then overly strong curds won't want to stretch. Is that what's going on?

No too much calcium shouldn't be detrimental to the cheese other than maybe causing off flavours, it should help the curds stay together a bit better during the make though.

Calcium (with phosphate) is responsible to bind proteins with each other. Acidification solubilise the calcium phosphate that is then replaced by hydrogens ions. This weakens the link between the protein strands that can then slide between each other and allow the curd to stretch. If there is too much acid (low pH), there won't be enough calcium phosphate left to bind protein strains and the curd will break rather than stretch. This is kind of disolving the curd. Not enough acid (too high pH) and the calcium phosphate will bind the proteins too strongly and the curd will not stretch. If you add more calcium to the milk by addition of calcium chloride, you will probably need a little bit more acid to get the same stretch (since there is a little bit more calcium to solubilize) but really not much more. In fact, you can probably use the same amount of citric acid (if you use direct acidification) or target the same pH if you acidify with cultures and get similar stretching than without addition of calcium.

Thank you very much for that explanation!  That was a big piece of the puzzle I was missing for how the calcium phosphate is bound up in the casein and how the stretch happens.

Edit: As usual, I'm looking for more info.  This is good: https://www.uoguelph.ca/foodscience/book-page/structure-casein-micelle as is the page right after it https://www.uoguelph.ca/foodscience/book-page/casein-micelle-stability

What blows my mind is that current models of the casein structure and the role of calcium phosphate is *really* recent.  We're talking only 30 years ago or so.  New papers are being published *now* debating how it works.  No wonder amateur cheese making resources don't talk about it!

Edit 2: And what is increasingly becoming deja vue, here's a nicer article from the University of Wisconsin: https://www.cdr.wisc.edu/sites/default/files/pipelines/2000/pipeline_2000_vol12_01.pdf

I swear I get 99% of my info from U of Guelph and U of Wisconsin...

Excellent. Thank you. Is there some target pH for the mix that would help determine how much acidification to do to get optimal stretch?

Target pH for starter acidified curd is 5.2-5.4
For citric acid acidified curd, it's 5.4-5.6
It's always useful to make a stretching test of the curd by plunging a small piece of curd in hot water for a few seconds and see if it strech correctly. If it breaks apart, you probably have too much acidification or you used bad quality milk, if it doesn't stretch, you need more acidification. If you acidify with culture, let fhe curd acidify a little bit more. If you used direct acidification, do something else with the curd.

I appreciate a specific answer! Thank you.

I can add that I now have some experience with with-and -without calcium chloride (which was written up in another thread). I did 30-minute Mozz, with storebought pasteurized homogenized milk, acidified with 1 3/4 tsp citric acid (per gallon). It came out tasty, but with the consistency of cream cheese. On the advice given to me in that thread, I did it again, but with 1/4 tsp of liquid calcium chloride (as sold for cheese). Very different. The result was kneadable, and formable into a nice solid loaf. When you pushed in on it, it pushed back.

So with all the abundant cautions about DON'T USE CALCIUM CHLORIDE IF YOU WANT STRETCHABLE CURDS, in this case, it just ain't true. I have to assume that this advice applied maybe to raw milk, and was blindly adopted as a general rule.