Whey in butter (Chema't akkum)?

Question

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). 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'm 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.

Answer 1

Indeed in the US and Canada adding whey cream to butter is now being done, as such commercially produced butter needs kosher supervision to ensure it doesn't contain forbidden ingredients (since whey is a byproduct of cheese production).

As R Yehuda Spitz writes in his excellent book Food, a halachic analysis

Butter has of late been increasingly produced with cream recovered from whey, rather than exclusively with the traditional and less kashrus-sensitive sweet cream [...] Since the dairy companies are still allowed to label the product "100% Grade A butter" and even sometimes "100% Grade AA butter", it no longer seems simple or recommended to rely on purchasing commercially produced butter with no specific hashgacha and assume that it does not contain any inherent kashrus issue.

European regulations appear to prevent that and lists of kosher products in Europe often include plain butter without the need for specific supervision (except for those eating chalav Yisrael).

For more on this topic, see the publications from the OU here, here or there, or here from the OK.