A vendor supplies a half liter of milk daily. Some times, due to variety of reasons, I arrive at a situation that I need to handle milk of 3 consecutive days i.e., 1.5 liter.

Elder and experienced members of my family asks me to deal each 1/2 liter of milk differently. But, they fail in providing a valid reason for not mixing them.

Is there any issue with mixing milk of different days and use them for my purposes?

Note that the milk is not too old. All the individual milks are edible.


Even when kept refrigerated, milk gradually spoils due to micro organisms. In case the first day's milk was on the threshold of spoiling, you are effectively bringing the third days milk also to that condition by mixing it with the first day's milk (which by now may have significant amount of microorganisms).

First day's milk bottle/vessel might have been retrived from the fridge, opened, and kept at room temperature (very short durations), and kept back multiple times by the time the third day's milk arrives. This gives microorganisms opportunity to enter and multiply even if the original milk was Pasteurized and sealed airtight in a sterile packaging.

In fact, curd is propagated by mixing fresh milk with leftover curd. Same with sourdough starter.

  • Elder members might be basing this caution, on a time when refrigeration and sterile packaging of milk might not have been common. – AJN May 7 at 5:37
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    People also suggest not directly breaking eggs into a common vessel just in case one of them turned out to be bad. You would then have to discard everything in the vessel. It is suggested to break eggs one by one into a separate bowl, inspect, and then add to the common vessel. – AJN May 7 at 5:43
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    And, if you take your three-day-old milk and mix it into your one-day-old milk, then two days later you mix your three day-old-milk with your new one-day-old milk, you've now turned your day-old into five-day-old milk. – nick012000 May 7 at 6:16
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    I reckon @nick012000's point is the true answer. If you mix just before consumption (or just before cooking if you're cooking it) there will be no problem – Chris H May 7 at 8:02
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    Seconding Chris H's point: you shouldn't consolidate your milks into I've container, but mixing them in your bowl of cereal or recipe shouldn't be a problem if they are individually fine. – dbmag9 May 7 at 12:23

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