The bitter ingredient in hummus is tahini. This is what makes the hummus taste rich. However, there are many different types of tahini and some are much more bitter than others. So if your hummus is coming out bitter, try a different brand of tahini before throwing in the towel!  Another thing that might make your hummus bitter is zesting your lemon too deep. If you are getting into the white part of the lemon skin then you are getting too deep. This part is usually pretty bitter.