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What does the expiration date on milk cartons mean?

I have different experiences with the date:

  • There is still about a week before the expiration date but the milk has gone bad
  • It is 1-2 days after the expiration date but the milk doesn't smell or taste bad

In both case, the milk was kept in the fridge.

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3  
Also, even if the milk's sour (as opposed to rancid), you can still use it in baked goods. (some even call for sour milk; use it as a replacement for buttermilk, or adjust your leavening to replace baking powder with baking soda to compensate for increased acid) –  Joe Jul 18 '10 at 23:56

9 Answers 9

up vote 16 down vote accepted

There is a lot of variability in how fast milk will go bad.

  • How long the milk has been opened
  • Pasteurized vs ultra-pasteurized
  • Temperature the milk is kept at
  • Thermal cycling: how long and how often is it kept above 40º
  • Where in the fridge it is kept. The door will have more thermal cycling than a shelf, higher shelves tend to be warmer than lower ones.

All of these influence how long milk stays good. The big date on the top is used by grocers to determine when it can't be sold. If you look closer there will be text to the effect of, "Use within 7 days after opening". This isn't a guarantee. Leaving the carton out while you cook will allow it to go bad faster, as will keeping it in the door. Also if your fridge is too warm for some reason, things will spoil faster.

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9  
I'm of the opinion that how long it's been opened has more impact than the date itself. –  Joe Jul 18 '10 at 23:49
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The type of container also seems to have an impact. I've noticed that, on average, the cartons with a "spout" don't seem to last as long as the ones with screw-tops. –  Aaronut Jul 19 '10 at 2:41

The given date is a sell by date, like others said. What determines the date the milk will actually expire, is the conditions it was stored in. If it was a bit warmer than usual it can expire a week early. Other times, it can expire after the given date. It also matters when you actually opened the bottle. This is why every time you use milk you should smell it (or taste it) before use. the first thing that goes off in milk is the taste - this is because the germs cause fermentation before it is actually unsafe to consume.

In short, if it smells good, use it.

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1  
+1 I use this rule for pretty much everything in my fridge. –  Ruben Steins Jul 19 '10 at 8:42
    
@Ruben: This rule doesn't apply to everything in your fridge though. You should watch out with any prepared food (as the spices outweigh the bad taste), and with lots of other foods which go off before they taste bad. –  configurator Mar 13 '11 at 17:37

The date stamped on milk cartons is neither an expiration date, nor a best by date (at least in the USA). Milk cartons are stamped with a sell by date. This is distinctly different. This is used solely to designate when the store must sell it by. The date the milk goes bad is some time after that.

From personal experience opened milk keeps in my refrigerator for 7-10 days beyond the sell by date. Smell your milk before drinking or using it. Obviously if it's chunky it's also well past bad.

With regards to sell by dates they are generally specified to give the "average" consumer time to consume the product once purchased.

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In my comment above on the question I specified USA. –  hobodave Jul 19 '10 at 2:49
    
I clarified. "International" doesn't mean exclude the USA ;) –  hobodave Jul 19 '10 at 2:59

In addition to the things others have mentioned, how the milk is handled during shipping can have an impact on whether or not it lasts to the best before date. Most of the time it goes from one refrigerator to another, but if a driver gets busy and leaves it sitting out for a while and if the store also leaves it out for a while, it can warm up enough to shorten the lifespan.

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The rule of thumb a few decades ago was: pasteurized milk has 2 weeks from out of the cow until spoiled. I imagine that depending on different variables (such as those listed by sysadmin1138) and improved technology, you might get up to about 3 weeks total.

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Yep. The ultra-pasteurized stuff has a longer shelf-life than the pasteurization in use a few decades ago. This is popular with those of us who don't go through a carton of milk in a week. –  sysadmin1138 Jul 19 '10 at 16:58

It means nothing. Go by smell. If it smells normal or even sweet, you're fine. IF it's slightly acidic, you're getting close. If it's sour or worse, don't even try it.

I give the milk a quick swirl to get rid of the gas which accumulates at the top of the bottle and can be a bit stronger. Texture is the final determinant. If it's not smooth, run.

p.s. I have never used UHT milk, so I'm curious if it spoils with the same pattern.

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Like most other dates on food - it is a best before date, that means that the packager/store guarantees that, if stored properly, it will be good until that date.

So if it goes bad before then, as it may, as you are dealing with real-world objects, you can return it to the store.

After that, a store is less likely to refund.

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No milk carton I have ever seen has had a "best before" date. It is clearly marked as a sell by date. –  hobodave Jul 18 '10 at 23:28
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Perhaps it depends on where. Mine is definitely a "best before" on every carton in my fridge (in Toronto, Canada) –  sdg Jul 19 '10 at 0:32
    
Sigh. Why can't everyone just do what the USA does. ;) –  hobodave Jul 19 '10 at 3:01

We get our milk delivered so it doesn't normally have a date on it and the bottles get mixed and i forget what order i'm supposed to open them in.

So I think the best way to tell is to use your nose and or take a little taste.

if its not tasting good or smelling proper then bin it. unless your going for sour milk to cook with.

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Keep in mind that an expiry date applies only to an unopened container.

That commercial caesar salad dressing with an expiry date 18 months in the future? Throw it out 60 days after opening.

High acid foods like pickles, mustards etc, will last along time, as will "Italian" dressings, but anything creamy has a pretty short life span.

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