For the source text click/tap here: Nedarim 52
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MISHNA: One who vows that milk is forbidden to him is permitted to partake of whey [kum], the liquid that separates from milk when it is made into cheese. But Rabbi Yosei prohibits him from partaking of whey. If one vows that whey is forbidden to him, he is permitted to partake of milk.
What is the difference between milk and whey?
We explore the chemistry and antiquity of whey and whey products including the kashrus ramifications of lactose and whey products.