5.65 pH (approximate)
Because Yeast has an approximate pH of 5.65, Yeast is considered acidic, but is out of the red and might be eaten in moderation while having gastritis or reflux symptoms. It's always advisable to try eating just a little one day and see how you do before eating more. Come back and let us know how you did in the comments below.
Remember, everyone is different and there might be reasons besides pH that make this food intolerable, triggering of reflux, or beneficial. Check out comments below to see what other people's experiences have been with eating Yeast and please consider sharing your own knowledge to help others.