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The methods that I know to test cold milk is to either boil it and look for the break, taste it (yuk!), or hope it smells bad enough to know it's bad.

Is there an easy and scientific way to know whether milk is still good? and for how much longer it's likely to stay good?

The expiry date alone is not a good indicator since it assumes some conditions about the storage temperature and we've all seen good milk past expiry and bad milk before expiry.

I'm also curious about the particular moment beyond which we consider milk 'goes bad'. Can one tell how far away it is from the current seemingly healthy condition of milk?

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I suspect there is no one particular moment when milk goes bad. Like most biological processes, it is a complex interaction of time and temperature controlling the growth of microfauna. Other than laboratory tests to detect a particular level of acidity, or a concentration of microfauna, I don't think you will get an objective measure. –  SAJ14SAJ Apr 15 '13 at 20:34
    
@SAJ14SAJ you're right. Looks like it just keeps getting sour and clotty until it becomes unbearable. –  MandoMando Apr 15 '13 at 20:44
    
@SAJ14SAJ There's no question that you're right. However, bacterial populations increase exponentially, so I wouldn't be surprised if there were a tipping point where it goes from "not much bacteria" to "way too much bacteria" in a couple of days. –  octern Apr 5 at 17:57
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3 Answers

Use your nose. The smell of sour milk is overpoweringly wretched. My belief is that if you can stand to smell it, you can stand to drink it.

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But often, the upper half of the bottle will stil be OK, while at the bottom you will find those dreaded stinking lumps... –  Cerberus Apr 15 '13 at 19:07
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shake, inspect, smell.... –  Cos Callis Apr 15 '13 at 19:14
    
Smell is not very reliable unless the milk is totally off. I've updated the question. Looking for a more definite test here that doesn't rely on subjective things. –  MandoMando Apr 15 '13 at 19:32
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@MandoMando Sadly, I think the method is you wait until it is off, then use your time machine to go back three days :-) –  SAJ14SAJ Apr 15 '13 at 19:53
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@SAJ14SAJ yeah, I was worried we'd invent a time machine before a milk testing machine. looks like there are multiple tests, and some easy ones. –  MandoMando Apr 15 '13 at 20:34
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up vote 7 down vote accepted

Looks like FAO (Food and Agriculture Organization of the United Nations) does a formal testing hand book here.

One of the easier methods:

2.4.5.3. The Alcohol Test

    The test is quick and simple. It is besed on instability of the proteins when the
levels of acid and/or rennet are increased and acted upon by the alcohol. Also increased
levels of albumen (colostrum milk) and salt concentrates (mastitis) results in a positive
test.

Procedure:

The test is done by mixing equal amounts of milk and 68% of ethanol solution in a small
bottle or test tube. (68 % Ethanol solution is prepared from 68 mls 96%(absolute)
alcohol and 28 mls distilled water). If the tested milk is of good quality, there will
be no coagulation, clotting or precipitation, but it is necessary to look for small
lumps. The first clotting due to acid development can first be seen at 0.21-0.23%
Lactic acid. For routine testing 2 mls milk is mixed with 2 mls 68% alcohol.

Simplified for home use:

This test is likely to be adaptable to household alcohol drinks such as voda. For example the number of teaspoons of Vodka an ounce or two of milk can hold before it clots.

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The other answer is actually pretty good, I think--the human nose is remarkably good at these things. It evolved for the purpose as it were :-) –  SAJ14SAJ Apr 15 '13 at 20:50
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I can only suggest you check the expiry date and/or smell the milk or taste it? Also if you are using the expiry date as an indicator make sure you store the milk correctly as it will mention on the carton or container, it should say what temperature the milk should be stored at and where you should store it.

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