In the name of Allah, the most Beneficent, the most Merciful.
The definition of Whey Powder is that watery factions that form along with the curd when milk coagulates. The whey is removed from the curd during the process of making cheese. (Encyclopaedia Britannica p.620 v.12)
Whey powder carries the same ruling as animal rennet. If rennet is taken out of the stomach of an animal slaughtered in the name of Allah, there is no harm in using it. The meat, fat of an Islamically slaughtered animal are permissible. But, in the event they are taken from the stomach of an animal slaughtered unislamically, there is difference of views among Muslim jurists. Imam Abu Hanifah and Imam Malik consider it clean and pure while Imam Abu Yusuf, Muhammad and others call it unclean and impure.
There is a strong likelihood that rennet from unislamically slaughtered animals is used in cheese made in non-Islamic countries. Under the juristic position taken by Imam Abu Hanifa and Imam Malik, leeway exsists. However, it is more precautious to act upon the views of Imam Abu Yusuf and Muhammed and refrain from it. (Maariful Quran p.428 v.1)
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham