Sour, but not in a bad way.

[maltyapples]

Late last winter I brewed a porter. I took 1 gallon of the batch and aged it on top of chocolate. As I drank this porter, I found that there was a sour finish to it, although it's not something I originally intended. All of the regular porter is now gone, and I just tapped into the chocolate reserve as I have labeled them (huzzah sticky notes!) and they are even more intensely sour at the end. I'm curious as to what has caused this. They don't taste infected or like vinegar, and the roast is still there at the beginning. It's a lot like the sourness at the end of a Guinness. Did I do something wrong with the specialty grains? If they get too hot does it make the end product sour? Do certain grains have a sour finish to them? I'd like to avoid this in the future.

Merry Christmas!!

[Mill Rat]

There are 3 common souring agents, lactobacillus and pediococcus bacteria, and brettanomyces yeast (pardon any misspellings). Lacto produces lactic acid, pedio makes acetic acid among others, and brett produces all sorts of stuff that can range from pleasant Belgian funk to eau de manure. The acid bacteria are self-limiting, and are responsible for the Guinness bite. Anything that was in contact with a brew that has hinted of a sourness should be thoroughy cleansed (this is the one thing for which I'll use a strong bleach solution) before you make anything else with it.