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If we let yogurt for more days, the yogurt become more and more sour why?

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Lactic acid is produced by 'probiotic' bacteria breaking lactose into lactic acid. Over time, more lactose is converted, producing more byproducts, thus more sourness.

The byproducts of this reaction are responsible for the distinctive flavor of yogurt.

See a more complete description here: Lactic Acid Fermentation

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For a more basic description, see here: en.wikipedia.org/wiki/Curd#Formation – logophobe Jul 29 '14 at 16:17
    
If you have something to add to your answer, please just edit it rather than adding a comment - it makes it easier for people to see! – Jefromi Jul 29 '14 at 17:59
    
whups! :-) didn't see the edit link, thank you – jim Jul 29 '14 at 18:21
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Lactic acid is, itself, sour -- you don't need "byproducts" to make it sour. – David Richerby Jul 29 '14 at 19:57

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