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?
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.
Another reason you can use vinegar in a bread recipe - to produce a sourdough. Under traditional methods of making a sourdough bread, one keeps a 'sour' (sponge) or a piece of dough from the days previous production, which acts as a starter for the current bread. In many of todays commercial bakeries (grocery stores, etc.), sours are not kept from day to day, and powdered substitutes are used. They are normally just a glorified acetic acid (in a powdered form). So, you can just use vinegar to produce the same results.
When I asked this question - 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..."
In making faster no-Knead bread Jim Lahey suggest adding 4 drops or up to 1/4 tsp of red wine vinegar to the basic recipe. Improve gluten development and flavor. See the video with Mark Bittman here: http://www.nytimes.com/2008/10/08/dining/08mini.html?ref=dining