It is beyond the scope of this article to go into all the intricate details pertaining to whey, but I will try to give a short synopsis of the issues involved in products that contain whey.
One popular food in today’s market that contains whey is ricotta cheese. To sum up the process in a nut shell, after the milk is coagulated, it is cut into small pieces and heated to allow the non- coagulated fluid to be separated. The non-coagulated fluid is then drained off as liquid whey. There is a discussion amongst contemporary poskim as to whether or not there is an issue of gevinas akum, as far as this whey is concerned. Some say that there is no concern at all with this process being done by a non-Jew and mashgiach supervision is not necessary. Some even take this a step further and say that the whey is allowed even when heated up together with the cheese, during the cheese production (even though the cheese itself is not allowed, due to the issur of gevinas akum).
The basis for this opinion is that they maintain that the gezeira mandated by chazal was limited to the cheese itself and does not apply to any other by-product. Other poskim do not accept this extra leniency and will only allow the whey if it was cooked with the cheese at a temperature that is no hotter than 120 degrees Fahrenheit (i.e., yad soledes – the halachic temperature that causes absorption). In conclusion, there is a question whether the gezeira mandated by chazal applies to other by-products of cheese. The common practice of the major kashrus organizations is to follow the latter opinion and not allow the cheese to cook at a temperature hotter than 120 degrees Fahrenheit.
