Beans and hummus cannot be eaten by Ashkenazim on Passover because they are what is known as kitniot. What is kitniot? Kitniot are pulses that are usually ground up and soaked in water, at which time they swell up. A decree was enacted by certain Rabbis to forbid kitniot on Passover because it shares this characteristic in common with leaven. This decree was accepted by Ashkenazic Jewery, hence the prohibition. But it is only applicable to Ashkenezim. There is, however, a fundamental difference between kitniot and leaven, and that is the amount of the protein, Beta Amylase, that each contains. In the five species of grains there is a very large concentration of this protein, whereas in kitniot products there is not. This is the vital point that distinguishes kitniot from chametz. What is clear is that the prohibition against eating chametz is from the Torah, whereas not eating kitniot is a Rabbinical prohibition.
