Quick Mozzarella

[Tedybar]

I am having some issues with my mozzarella...  The last two batches of quick mozzarella I made I had to knead the curd before a little bit before I could get it to stretch.  The final cheese result was perfect as far as texture goes, but the taste was slightly bitter.  My questions are:

1) Is kneading the curd acceptable/normal before stretching?
2) What would cause the slightly bitter taste?