This is a LINK to david's other pages.
I would add a few comments here David.
You will see that on one of his recipe pages he talks of "floating curd".
As he says, this indicates the presence of gas forming bacteria in the milk. If you break the curd pieces you will find that they are full of tiny pin-prick sized holes.
These are caused by the gas released by E.coli bacteria. For the majority of people this is no great problem (makes one fart maybe) but, if just a few of these E.coli are E.coli 0157, they could be very harmful to susceptible consumers, particularly young children, and can cause renal failiure and even death.
If your curd doesn't sink when you stop stirring, or if lots of small pieces of it insist on floating.......... DUMP IT!
E.coli is a feacal contaminant. It only breeds in the gut ... of animals and man. 0157 can produce powerfull toxins in the gut when consumed even in very low numbers.
If it is present in your milk it will almost certainly have originated from dung at milking time.