quick mozz makes w/new pH meter

[Susan38]

I have been (finally) playing around with my new pH meter, and decided to try it out on quick mozzarella makes for faster feedback turnaround times.

My history of quick mozz, in short, is in producing the hard rubbery balls that don't melt well.  I could produce a nice glossy sheen with aligned strands, but had to keep the curds in almost boiling water for long periods, and really pull to get the stretch.  I tried not stretching as much, no kneading, adding lipase, etc. with not much improvement.

Over time and research I was suspecting the pH was not low enough.

Fast forward to this past month, when I again attempted the quick mozz recipe using pH meter for the first time.  At first I was measuring pH on everything in sight, LOL!  And recalibrating often to ensure I was getting good readings.  Following are the pH readings before and after acidification with citric acid (ca).

Raw milk at 92 F:  6.76 (a bit higher than expected)
Pasteurized milk, at 88 F:  6.73
Milk @ 90 F after adding 1.5 teaspoons ca at 75 F then warming to 90 F:  5.71!!!  Aha!  pH too high for stretch/melt!!!  (It is a bit of a relief to find out the milk was the issue (or rather, the issue was me not measuring pH prior to this)).

At this point I decided to add more citric acid to get the pH lower.  So I added a 0.5 teaspoon of citric acid:  pH at 5.45! 

I was aiming for pH of 5.2-5.3, so added another 0.5 teaspoon citric acid.  pH at 5.21!

And lo and behold, I finally got that drapey-off-the-hand-and-spoon stretch that I had been after all this time.  And no more hard rubbery balls of cheese.  Mystery solved.

But now here comes the weird part.  The following week I attempted to replicate the above procedure, sort of testing out how reliable the pH meter was reading over time.  Milk quality was pretty much the same as far as I could tell.  I used the same pH protocol as far as I could tell.

Pasteurized milk @ 90 F:  6.73 (hey, good, that's the same reading as in the last make).

Then I *totally forgot* about my intended scientific approach to replicate the previous make.  I figured I needed at least 2 teaspoons of citric acid to get into ballpark pH range so that is what I added to the 75 F milk.

Milk + 2 t. ca @ 75 F:  5.17!  Whoa!  Kinda overshot the target a bit!
                     @ 90 F:  5.18

Again, got good stretch and nice pliable cheese balls.  Now for the questions:

Just wondering about the chemistry involved about adding acid more slowly vs. all at once and if that made a difference in the pH.  In other words, with both makes I had the same start pH, but slower addition in the first make needed 2.5 t ca, while faster addition in the second make only needed 2 t ca, to get to the approximate same pH). Or does this not make any sense at all, and maybe it was just hooey readings off the meter for whatever reason?

And another question about the second make.  After forming the cheese balls and cooling them in room temperature water, I measured pH of both the whey and the cheese.  Whey came in at 5.19, very consistent with what the milk + ca measurement was, but try as I might, and I did try a lot, including recalibrations, etc.  ... the only curd (cheese) reading I could get was 5.4 @ 75 F.  Seems like that would be too high, even when considering rebound (which is supposed to take a day or so to happen, not within an hour of making).  Could the water it was soaking in influence the pH (i.e. does the surface of the cheese absorb water enough to change the surface pH?)  I did dry the cheese off prior to making measurements.  Or, again, should I chalk the reading off to an inaccurate reading of the meter?

Well anyway, that is my most recent cheese story, overall with a happy ending.  To anyone out there firmly intent on making lots of mozzarella cheese (either quick or traditional), I will repeat what others on this forum have said:  I highly recommend getting a pH meter!

(Apologies for no pictures, but really how does one take photos with gloved hands full of hot stretchy curds???)

[scasnerkay]

A cheese to you for the scientific approach!!

I have always added the citric acid to cold milk so I can't say anything about adding it later on or incrementally!

I do think the mozzarella would absorb some water from the solution it was resting in, and that could change the pH.

My mental image of trying to take pictures with messy gloved hands was a good chuckle!

[mikekchar]

The amount of acid you need depends on the amount of calcium phosphate in solution in the milk/whey.  The calcium phosphate "buffers" the acid.  It's a bit like when you dissolve baking powder in water and then add acid.  As you add the acid, it get neutralised by the baking powder.  So even though you are adding acid, the pH doesn't change (it reacts with the dissolved baking powder and precipitates a salt instead).  Eventually all the baking powder is used up and the pH will move.

With milk it is more complicated.  There is some calcium phosphate dissolved in the milk, but there is a *whole bunch more* wrapped up in the casein micelles (just a reminder: casein protein is wound up like tight balls called "micelles").  In fact, the casein protein is bound together by that calcium phosphate and it is *not* dissolved in the whey.  As you add acid, the casein micelles relax (this is one of the reasons it can start to melt and stretch!).  As it relaxes it allows calcium phosphate to escape.  This calcium phosphate dissolves in the whey and reacts with the acid: causing the pH to stop going down, or even start going up.

It takes time for the micelles to relax and for the calcium phosphate to dissolve.  Also, you can think of it like a tightly wound ball of yarn, where the yarn is caked with chalk.  If you put the ball of yarn in water, the chalk doesn't come out all at once.  At first only the chalk on the outside will dissolve.  As the water gets into the ball of yarn, more and more chalk can mix with the water.  It's the same thing.  So you are slowly releasing calcium phosphate into the whey over time.

If you add acid all at once, then you will use up the calcium phosphate in the milk and eventually the pH will go down.  Over time, though, it will probably start creeping back up.  In traditional mozzarella preparation it is normal for mozzarella pH to "rebound" even up to 0.3 pH.  They will aim at 5.1 (and I even saw a youtube video of a factory where they were measuring the curd at 5.0!) so that a few days later in the shop the pH of the final cheese will be 5.2 or 5.3 (and will still stretch nicely).

It's important to point out that the amount of calcium phosphate in the milk is dependent upon many, many things (season, health of the cows, feed, etc, etc) and so the amount of buffering done by the milk is essentially a random variable.  That's why you can't get repeatable success without a pH meter.

[Susan38]

Mike--thanks for your response, I was looking forward to your explanation, as I've read about your extensive experiences making acid-coagulated cheeses.  I was thinking the calcium phosphate was somehow buffering the pH with the slower-additon method, but wasn't sure, or clear on exactly how/when it was happening.

I have always added the citric acid to cold milk

Susan, I'm glad you mentioned this.  I've read you're supposed to add citric acid to cold milk, otherwise it will coagulate the milk (before you want it to); and I just read in another post that someone says the curd will not stretch if you add it to warm milk.  But.  I'm here to say that I've added citric acid to milk 75-90 degrees and 1) the milk did not coagulate (OK, there were a few tiny flecks of coagulation but it did not seem to affect the curd that was later formed by the rennet addition); and 2)  I could always stretch the curd...especially now since I've made sure to bring it to the correct pH range.

The reason I'm bringing this up is that I prefer to pasteurize the milk just prior to cheese making, and it seems a waste of time to cool it down to say 55 F just to warm it back up to 90 F again.  So I'm wondering just how important it really is to have the milk at a "cold" temperature prior to adding the citric acid.

[mikekchar]

The "isoelectric point", or the point where the milk will gel due to acid is different at different temps.  So it's about 4.9 at 25 C and 5.1 at 42 C.  If you add acid directly to milk at 42 C and aim for a pH of 5.1 it can gel before the rennet gets to it.  This will be bad for the cheese (acid gelling and rennet gelling are different and basically incompatible).  By adding the acid when the milk is cold, you give yourself some safety zone.  If you do it the way you did, by under adding the acid, then you don't have to worry.

[Susan38]

Thanks again Mike.  It looks like I've just barely been able to avoid disaster (i.e. having the citric acid coagulate the milk unintentionally).