Susan, when making cheese, one very, very, highly important factor is the ratio of protein to fat in the milk. This is because based on the number of fat molecules when compares to protein molecules, the curd structure will be way different. It will have different properties in terms of:
- The size of the openings in the curd matrix. More protein tends to lead to smaller openings because protein molecules are smaller and you get a tighter bond in the matrix.
- The distribution of fat molecules. This also has to do with homogenization, but if you have more fat molecules, they are physically bigger. And with bigger fat molecules, even though the spaces in the matrix are bigger, anything that travels in or out has a longer route to travel. So for example, the whey drains much slower.
- The strength of the curd. This is somewhat self-explanatory. But curd (the gel/matrix) is formed by proteins that are bonded to each other. If there are more proteins, the bonds are stronger. Think of curd like a chainlink fence. That fence is made up of a bunch of casein micelles that are bonded to each other. And then think of the fats like snowballs that you throw at the fence and they stick. And then expand that fence in three dimensions, and that's basically what curd is.
I'm talking about rennet cheeses for all this, enzyme-coagulated.
Commercially, milk is standardized to reduce the variability. There are optimum targets for the protein to fat ratio. This is because most cheese processes are the same... you use a certain starter amount 1%, usually, sometimes more. You use a heat schedule and curd size and floc/set time to get to a specific gel strength and moisture target before knitting curds and pressing them. And you get the time, acidity, and moisture level to coincide for the specific type of cheese you make.
For parm, the moisture target is really low, something like 22-25%. You can't use a full-fat milk for that because in a full-fat milk, the PF ratio is something like .6-.8. That would give you a curd that releases whey slowly, that leeches fat from the curd because the matrix would be weak, and where the final moisture and solids/fat targets would be all wrong. So one way to remedy this is to skim the fat or centrifuge it off in a separator. Or, add back protein to get the PF ratio to be correct. The proper PF ratio for parm is something like 1.25... which is usually 2.2-2.6% fat for normal holstein/holstein-cross milk. But if you add back protein, you can use much higher fat milk, and keep the rest of your make the same. Texture, flavor, maturation, curd behavior, etc will be the same.
+1 Sailor, great ideas on practical PF modification at home. It's very easy to adjust SNF with powder.