I recently read a responsum from R' Tam (brought in Shaarei Dura 78) where he discusses butter that was mixed with whey. While he permits regular butter from a gentile for consumption he nevertheless prohibits butter which was mixed with whey (see the reasons given there). The R' Tam states that if one were to wait four days the whey would evaporate from the butter and even the latter would be permitted for consumption. In the responsa from the Ri, I saw that this was the practice in medieval England.
My questions are as follows:
Firstly, does anyone know how the whey was added to the butter, did they actually mix the whey in the butter (meaning that the whey curdled along with the butter and became part of it), or was it something separate in which the butter sat in (similar to cheese in brine)?
I' asking because if the whey was mixed in the butter and became part of it, it is hard to understand how it was able to evaporate after four days (am I missing something here)?
Secondly, does anyone know if the practice of adding whey to butter is still practiced nowadays so that it will be able to shed some light on this butter process?
If anyone can shed light on any of these would be helpful.