Why it is not recommended to mix the milk of different days? 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.