If re-boiling does not curdle the milk, can that be taken as an indicator that the milk hasn't developed the bacteria (and their toxins)?
Fresh milk does not separate when boiled. I use it in sauces and pan gravies and it always boils and never separates. 

My suspicion is that you are separating it on purpose, for example to make cheese. In that case you have to add acid or rennet to separate it. If you then re-boil it to separate it again, you get very little solids. I have made cheese at home then tried to 'ricotta' the left over whey and all I get is a tiny bit of solids.