I see that many recipes for mozzarella call for the use of an acidifying adjunct (citric acid or lemon juice or vinegar for example), but I found a recipe that recommends that I add a culture and wait about 48 hours for the culture to ripen the milk and so acidify it. I like the idea of "ripening" the milk with kefir and using that as the basis for mozzarella but is this a preferred or a dis-preferred method of getting the milk down to the appropriate pH for stretchability? I ask because to date I have not had much success with mozzarella although one batch that would not stretch and which I tried to salvage by pressing under a few pounds of weight for a day or so was very capable of melting quite deliciously after a few days of quiet storage in a cheese drawer in my fridge (I prefer to eat my failures and not compost them)..
The secret to Mozarella is hitting the exact acidity needed to get a smooth stretch -- right around 5.3 pH. One way to get there is to add acid -- but like you, my results doing that have been iffy. The other way to get there is to culture the milk. Whether or not you should wait 48 hours depends on how quickly the cultures work. With commercial freeze dried cultures, it doesn't take nearly that long; even with the kefir, I'd be checking it regularly to see how far along it is.
My 'fast' lemonjuice-acidified mozzarella works kinda OK, but has nowhere near the flavour development of a bacterially ripened one. I really think bacteria (including keffir) is the way to go, along with a good pH meter.
Good solution to using the failed curds too: if the special thing about mozzarella is the texture when fresh, if its going on a pizza or lasagne or toast, the original texture is forgotten, and its really only about toasty brown-ness, strings and fat. At least you end up with 2/3!
Many thanks to both responses. Much appreciated.