What is the best whey to make Halloumi

Sometimes I make great Halloumi and sometimes it all melts when I boil it.  I am wondering retrospectively if it has something to do with the whey.  I am very good at feeding the whey to the pigs only to remember that I need the whey to boil the Halloumi.  The result is that make another cheese and use that, usually feta (to which I add bacteria), but sometimes cottage cheese or ricotta with vinegar or citric acid added.  I am wondering if the more acidic whey will be better or worse for stopping Halloumi melting when boiling.