Robiola / Bondon / Buttermilk cheese coagulation questions

[stgagnon]

Hi,
I have a question about getting curds from buttermilk and rennet.
Short version:
If I add buttermilk starter culture and rennet to milk at the same time, at 86 degrees, I *always* get a nice cuttable curd after letting it set overnight or 24 hours.

If I add buttermilk start culture to milk first, let it culture overnight (it turns into thick buttermilk), reheat it to 86 and add then add rennet, I never get a curd that can be cut, even if I wait 24 hours.  It looks good on top, but if I cut into it, the knife is coated with creamy buttermilk.

Why is this?
I am starting with 1 gal raw goat milk, pasteurizing it with my pressure cooker as if I were going to make yogurt.
Then I add 1/4 t CaCl, sometimes before the culture, and sometimes after.

I know I must be missing something fundamental!

Longer version:
I want to make the Robiola recipe from cheesemaking dot com. (https://cheesemaking.com/products/robiola-cheese-making-recipe )
This recipe calls for the latter approach: milk + starter, wait for ripening, add rennet, wait some more,  cut, and scoop into baskets. 
I depart from this 1-gal recipe in the following ways:
I am precooking the milk as described above.
I get the temp back to 86 degrees, not 72 degrees.
I am using 2 1-oz frozen starter buttermilk cubes instead of a packet of dried buttermilk culture. 
I am letting it ripen a lot longer than 4 hours.. usually at least 12, while I am at work during the day or overnight.

I have not yet achieved a good curd.  I just end up draining the whole batch in a cloth. It takes a very long time to drain.  It is DELICIOUS (sooo much tastier than what I get from the direct-set chevre packets).  But I want to put it in the baskets and get little cheeses! 

I have followed another recipe using buttermilk from the Home Cheesemaking book: Bondon.  In this recipe the starter and the rennet go in at the same time and it sets for 24 hours.  I used twice as much starter as the robiola: 2 oz for 2 qts.   (but still half what the bondon recipe called for, 2oz meso starter for 1 qt)  This works perfectly every time. 

I think I may have just answered my own question: 2x the starter is going into the Bondon method (2oz/2qt vs 2oz/1gal).  Could this be the problem?  Am I not putting enough starter into the Robiola? 

Or is there something else that I don't understand.

Our milk supply is slowing down, I may not have many more gallons to experiment with until next summer, so I'd like to get it right soon.  I may just end up putting the nice bondon curds into the baskets and calling it something else

Thanks for reading..
-Suzanne in VT

[mikekchar]

Just to restate the question: why does the milk not set with rennet if you wait too long after adding buttermilk culture?  This is an excellent question and I hope I can explain it well :-)  In case you just want the answer without the explanation: because the milk is too sour to set a curd with rennet.  If you want to understand *why* that matters, then read on.

The thing to understand is that there are *2* mechanisms for making curds in milk.  The one cheese makers are familiar with is by adding rennet.  Rennet is an enzyme.  Enzymes are chemicals that cut other chemicals in pieces (usually proteins or carbohydrates).  You are probably familiar with the protein called casein.  Casein is the main protein that makes cheese curds.  Most people don't realise that on the scale of molecules, casein is pretty big.  You can probably think about it like a big ball of string.  This big ball of string is called a "micelle" (which means nothing other than it's just a whole bunch of stuff stuck together :-) ).  On the outside of the casein micelle (remember -- ball of string) is a whole bunch of "hairy" proteins.  So now our micelle is kind of like a big ball of string with lots of furry hair sticking out.

The hair has a "positive charge".  That might sound complicated, but it's not.  It's just like a magnet.  Remember that in a magnet, there is a positive end and a negative end.  If you hold the positive end next the the negative end, they will stick together (often really hard!).  But if you hold the positive end next to another positive end, then they will *repel* each other -- you can't stick them together, no matter what you do (as a kid I always had fun trying to force magnets together the wrong way around -- yes, I'm *that* kind of person!).  Anyway, same goes for negative to negative -- they won't stick together.

So you've got all these casein micelles (big balls of string) floating around in the milk.  Each one is covered with this positively charged hair -- so each one repels each other!  You can't get the casein micelles to stick together, no matter what you do.  That's why milk is a liquid.  This is where the rennet comes in.  Remember that rennet is an "enzyme" and enzymes cut other chemicals into pieces.  In this case, rennet is a very special enzyme -- it cuts the hair off of the casein micelles!  So now the micelles are no longer positively charged!

Except there is one small problem.  If the hair is positively charged, how is it sticking to the micelle?  If you guessed, "Because the micelle must be negatively charged and it sticks just like a magnet", then you would be exactly right!  Once the rennet "cleaves" (a fancy word for "cuts") the hair off  the micelle it is now negatively charged -- and negatively charged things *also* don't stick together.  But, we have a secret weapon!  Dissolved calcium is positively charged.  If we dissolve some calcium in the milk (say by adding calcium chloride), then we'll have both negatively charged casein micelles and positively charged calcium "ions" (in this context, an ion is just half of some chemical that's dissolved in a liquid -- in our case we dissolved calcium chloride and it broke up into calcium and chloride ions).  As an aside, in unpasteurised milk, you usually don't need to add calcium chloride because there is enough dissolved calcium phosphate to give you the calcium you need.  When you pasteurise the milk, the heat causes the calcium ions and phosphate ions to stick together and literally fall out of the milk, so we have to add extra calcium.

OK, I'm finally getting to the point (I promise!).  We have a "bald" casein micelle which is negatively charged and a calcium ion which is positively charged.  They stick together just like a magnet.  Furthermore, you will get another casein micelle sticking to the other end of the calcium and another calcium ion sticking to the end of that.  It just starts sticking together like magnetic Lego.  And that's when the curd forms.  Rennet set cheese curd is just a gel of "cleaved" casein micelles stuck together with calcium.

That's the first kind of cheese curd (which most cheese makers take advantage of).  There is another kind of milk curd.  It turns out that when a liquid gets very sour, the "charge" on the particles gets weaker.  I actually don't know why it does this and I've been meaning to look it up, but I'm lazy.  In any case, the point at which the charge goes away completely is called the "isoelectric point" (fancy word that literally means "the point at which the electric charges are the same).

When the "isoelectric point" (which I'm going to call the "yogurt point" for ease of typing) is reached, the casein micelles no longer repel each other.  They kind of stick together, but it's not a strong bond -- not like the bond between the casein micelles and calcium.  Remember that the bond with calcium is like a magnet -- it's super strong.  You practically can't pull it apart.  But micelles sticking together because they hit the "yogurt point" are more like sand in a sand castle.  They stick together just because the shape and position happens to be nice.  If you try to move it around (by stirring or something), then it will either flow (like yogurt) or fall to pieces (like ricotta cheese).

Now to answer your question: When milk is sour enough to hit the isoelectric point (yogurt point), there is no electric charge left.  It doesn't matter if you cleave (cut) the hair off the micelles, because it is not positively nor negatively charged.  It won't stick like a magnet to the calcium, no  matter what you do.  And because it won't stick like a magnet, it won't make a curd.  The best it can do is to thicken the milk to make a kind of yogurt.

Note: if you add a strong acid to milk quickly, it will make a stronger curd -- but still nowhere near as strong as rennet formed curds.   The buttermilk is kind of liquidy because the culture is making it sour slowly -- this means that the "sand castle" bits are really, really small.  There is tonnes more to say on this topic, but I've probably outtyped my welcome as it is.  I hope it was interesting for you.

[stgagnon]

 

Fantastic read!  Thank you Mike!

That is really really helpful.. I believe that also explains why my yogurt is nice and thick when it is fresh,, but several days later it is very soupy.

I feel like a (nerdy) kid in school.. experimenting, following recipes.. enjoyment in the magic of the resulting cheese and then at last a (slightly) deeper understanding of (one of) the processes behind the magic.

You have energized me today to begin my next cheese-of-the-week! 

Thanks again!
-Suzanne

[feather]

Mike, I had to give you a cheese for that. Well done! I can visualize the whole thing now.