Shulchan Aruch (YD 118:1) writes that
chalav Yisroel milk that was left in the possession of a
nochri must be sealed. Otherwise, there is a possibility that it was tampered with and adulterated with
chalav akum. We are only concerned that a
nachri would tamper with a food if there would be a benefit to do so. This is known as “
neheneh b’chalipin” (benefiting from switching). In the case of the Shulchan Aruch, what would the benefit be? The
nachri must return a whole bottle of milk to his Jewish friend? What would the
nachri gain by switching
chalav akum for
chalav Yisroel? The answer is that the
chalav Yisroel milk may be fresher. In the scenario of our original question, the
nachri owns all the milk. What would he gain by switching the two bottles? The answer is that as the
chalav Yisroel milk is used, he might replenish the bottle with regular milk which is cheaper. Also,
chalav akum may be more easily available than
chalav Yisroel. Therefore, one who is strict to only drink
chalav Yisroel may not use the open containers of milk that are provided by the store.
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