### Author:  [@astro](https://boards.straightdope.com/u/astro)
#### Post date: [July 1, 2003, 1:35am UTC](https://boards.straightdope.com/t/why-does-sour-milk-taste-sour/185475/3 "2003-07-01T01:35:00Z")
> [@](#):
>
> \*Originally posted by Captain Amazing \*
> \*\*Because if sour milk tasted sweet, it wouldn’t be very logical to call it “sour milk”, would it? \ \*\*
Ummm… no…Cap’n I guess it wouldn’t :dubious:
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### Author:  [@bizzwire](https://boards.straightdope.com/u/bizzwire)
#### Post date: [July 1, 2003, 1:38am UTC](https://boards.straightdope.com/t/why-does-sour-milk-taste-sour/185475/4 "2003-07-01T01:38:06Z")
conversion of lactose (milk sugar) to lactic acid by…um…cooties.
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### Author:  [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)
#### Post date: [July 1, 2003, 1:45am UTC](https://boards.straightdope.com/t/why-does-sour-milk-taste-sour/185475/5 "2003-07-01T01:45:22Z")
**bizzwire** speaks true. Except for the “cooties” part. Digestion of lactose by bacteria produces lactic acid, which tastes sour, just like formic, acetic, and citric acid. They all activate the ‘sour’ receptors on the tongue.
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### Author:  [@Nametag](https://boards.straightdope.com/u/Nametag)
#### Post date: [July 1, 2003, 4:32am UTC](https://boards.straightdope.com/t/why-does-sour-milk-taste-sour/185475/6 "2003-07-01T04:32:21Z")
QtM, what, precisely, do you think cooties _are_? And you call yourself a doctor!
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### Author:  [@Jon\_the\_Geek](https://boards.straightdope.com/u/Jon_the_Geek)
#### Post date: [July 1, 2003, 4:04pm UTC](https://boards.straightdope.com/t/why-does-sour-milk-taste-sour/185475/7 "2003-07-01T16:04:22Z")
This could be answered in the opposite direction, you know. It tastes sour because we developed sour receptors in part to be able to avoid spoiled milk. What’s happening on a chemical level didn’t matter to evolution, just the fact that the really bad sour stuff tended to make people sick or dead.
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### Author:  [@astro](https://boards.straightdope.com/u/astro)
#### Post date: [July 1, 2003, 4:38pm UTC](https://boards.straightdope.com/t/why-does-sour-milk-taste-sour/185475/8 "2003-07-01T16:38:18Z")
> [@](#):
>
> \*Originally posted by jharmon \*
> \*\*This could be answered in the opposite direction, you know. It tastes sour because we developed sour receptors in part to be able to avoid spoiled milk. What’s happening on a chemical level didn’t matter to evolution, just the fact that the really bad sour stuff tended to make people sick or dead. \*\*
What about yogurt and buttermilk then?
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### Author:  [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)
#### Post date: [July 1, 2003, 8:34pm UTC](https://boards.straightdope.com/t/why-does-sour-milk-taste-sour/185475/9 "2003-07-01T20:34:24Z")
> [@](#):
>
> \*Originally posted by Nametag \*
> \*\*QtM, what, precisely, do you think cooties _are_? And you call yourself a doctor! \*\*
Good Grief, **Nametag**! Cooties are vermin! Complex multi-cellular animal organisms, not prokaryotic single-celled critters which don’t rise even to the level of a plant!
[http://defcon.sdsu.edu/3/objects/visual/concepts/conceptpart/d2.gif](http://defcon.sdsu.edu/3/objects/visual/concepts/conceptpart/d2.gif)
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### Author:  [@Nametag](https://boards.straightdope.com/u/Nametag)
#### Post date: [July 1, 2003, 8:50pm UTC](https://boards.straightdope.com/t/why-does-sour-milk-taste-sour/185475/10 "2003-07-01T20:50:36Z")
I’d think a **medical man** would be able to distinguish between the critter _called_ a cootie by the backward, the ignorant, and the profit-motivated, and the genuine infectious cooties that are spread by grade-school girls.
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### Author:  [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)
#### Post date: [July 1, 2003, 10:53pm UTC](https://boards.straightdope.com/t/why-does-sour-milk-taste-sour/185475/11 "2003-07-01T22:53:38Z")
> [@](#):
>
> \*Originally posted by Nametag \*
> \*\*I’d think a **medical man** would be able to distinguish between the critter _called_ a cootie by the backward, the ignorant, and the profit-motivated, and the genuine infectious cooties that are spread by grade-school girls. \*\*
Hey! I’ve isolated cooties and examined them under the microscope! Cuter 'n a bedbug, they are!
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### Author:  [@Fish](https://boards.straightdope.com/u/Fish)
#### Post date: [July 2, 2003, 12:57am UTC](https://boards.straightdope.com/t/why-does-sour-milk-taste-sour/185475/12 "2003-07-02T00:57:51Z")
Looking at it from **jharmon** ’s backward perspective, would it be correct to say that mold and rot smell more strongly, or should we say we’re just more sensitive to the scent of them?
That is, how many parts-per-million we can detect of mold? How does that compare to, say, our ability to detect the fresh article? Does mold actually produce more scent?
FISH
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### Author:  [@Jon\_the\_Geek](https://boards.straightdope.com/u/Jon_the_Geek)
#### Post date: [July 2, 2003, 1:16am UTC](https://boards.straightdope.com/t/why-does-sour-milk-taste-sour/185475/13 "2003-07-02T01:16:49Z")
> [@](#):
>
> \*Originally posted by Fish \*
> \*\*Looking at it from **jharmon** ’s backward perspective, would it be correct to say that mold and rot smell more strongly, or should we say we’re just more sensitive to the scent of them?
>
> That is, how many parts-per-million we can detect of mold? How does that compare to, say, our ability to detect the fresh article? Does mold actually produce more scent? \*\*
I don’t know the numbers… but it also works in the opposite direction. Things like fresh bread (or, for an example that has had longer to evolve, fruits) smell good because it’s good for us to be able to find them, so we can eat. Of course, the plants evolved to make them smell better so we’d find them and distribute their seeds, so the process is somewhat circular.
And, of course, it’s helpful to molds if we DON’T eat them, so I guess that probably evolved circularly as well.
The milk thing doesn’t really matter to the bacteria in the milk, though… many of them could live perfectly well in our bodies–until they killed us, of course. Sour milk tastes/smells nasty to us so we won’t drink sour milk. I’d even guess that sour milk is one of the major reasons we have a sense of “sour” in taste/smell, and its other uses (making some things actually taste good) came about as a refinement of that sense.
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### Author:  [@Dr\_Paprika](https://boards.straightdope.com/u/Dr_Paprika)
#### Post date: [July 2, 2003, 2:31am UTC](https://boards.straightdope.com/t/why-does-sour-milk-taste-sour/185475/14 "2003-07-02T02:31:10Z")
I think milk may taste sour, IIRC, due to butyric acid in additon to lactic acid and cooties. The same applies to breast milk.