Question: Yesterday I made bread based on a recipe I found on the flour bag. It contained both rye and wheat flour but also three tablespoons of vinegar. I've never seen it used in bread before, why is it there?

Answer: 
If it contained a high amount of rye flour, an acid would be needed for the bread to leaven. This is because bread with lots of rye rises due to polysaccharates called "pentosans" (if i remember correctly) being sticky and holding in the carbon dioxide bubbles. With heat, an enzyme in rye called "amylase" will start eating up the pentosans, unless the amylase is deactivated with acidity (wheat flour uses a protein called "gluten" to trap bubbles, and its amylase is deactivated with heat anyhow). The acidity is traditionally lactic acid, produced by lactobacilli bacteria in sourdough, but could be vinegar.

If it's mostly wheat flour (i.e. if you have to knead it), the vinegar would just be for flavour.
When I asked this question - [Is there anything I can add to homemade bread to preserve it?](https://cooking.stackexchange.com/questions/7804/is-there-anything-i-can-add-to-homemade-bread-to-preserve-it) - Arafangion said that one commercial bread company now uses vinegar as a preservative. This is his answer to my question: 

> 
> "One commercial bread company has
>  switched preservatives... They use
>  vinegar (I suspect ordinary white
>  vinegar).
> 
> 
> Maybe you could give a little bit of
>  vinegar a go and see how that works?
>  You can still smell it if you sniff
>  and sandwiches do have a faint vinegar
>  flavour, but it seems to work well
>  enough for the company and it
>  apparently hasn't sabotaged the
>  product line...
> 
> 
> Then again, it /is/ commercial
>  bread..."
> 
> 
>