I made a chocolate stout a while back that's kind of astrigent. I'm not sure I would call it sour, but it makes you pucker a bit. It had 8 oz of Trader Joe's Tumaco cocoa powder added in primary, no secondary. I've read that alkalized or Dutch processed cocoa powder reduces the acidity and astrigency, so I think it was the cocoa powder that caused it. The base recipe was the partial mash oatmeal stout from Brewing Classic Styles. No lactose involved because I have lactose intolerant and allergic friends & family.
Has anyone else had a similar experience? Do you think non-alkalized powder could be the culprit? Would that make Dutch processed better for adding to beer?