These grains are prohibited if they’ve had contact with water/moisture for longer than 18 minutes, which leads to rising or “leavening.” Leavening agents, like yeast and sourdough, are also considered chametz.
You might be thinking– but, isn’t matzo made from wheat flour? Yes! Most matzo is made from wheat, but the process used to “cook” the matzo from the time liquid is added to the flour to the time the matzo is baked can last no longer than 18 minutes. The wheat is closely supervised to ensure that no water touches it from the time of harvest to the time it is baked. This is to make sure that no leavening occurs.
This is all technicality, and it may seem confusing to somebody who is new to keeping kosher for Passover. The most important thing you need to remember is that grain-based products… even beer!… are generally not considered kosher for Passover. Matzo, matzo meal, and matzo cake meal are used as substitutes for bread, bread crumbs, and cake flour, respectively.
In addition to chametz, Ashkenazi Jews traditionally prohibit the consumption of kitniyotduring Passover. These items, known commonly as rice, corn, millet and legumes, have been banned for centuries by Ashkenazi Jews. The origin of the ban is unclear; it’s thought that perhaps kitniyot were considered too similar to grains. Grains and kitniyot often shared the same storage bags, so it’s possible there was concern that chametz might accidentally be mixed in with the kitniyot and consumed during Passover. Another theory is that kitniyot expand when immersed in water, which may have been perceived by the early rabbis as a form of rising or leavening. Whatever the reason, most Ashkenazi Jews today stick to the ban on kitniyot, even though it’s not technically prohibited by Torah law.
Sephardic Jews do consume kitniyot during Passover. They are, however, careful about ingredients co-mingling. In the Middle East, it’s not uncommon to see Sephardic women sifting through bags of rice and lentils repeatedly, going through grain by grain to ensure no chametz is mixed in. While you won’t generally see kitniyot dishes at an Ashkenazi Seder, they are quite common at Sephardic Seders. Kitniyot items include:
KITNIYOT
Rice
Corn
Millet
Dried beans and lentils
Peas
Green Beans
Soybeans
Peanuts
Sesame Seeds
Poppy Seeds
Mustard
Most Jews in America do not consume kitniyot during Passover. So, unless you’re absolutely sure that the people at your Seder observe Sephardic traditions, it’s safest to avoid cooking any kitniyot items during the Passover holiday.
In addition to these restrictions, many Jews avoid eating lamb during Passover because of the paschal sacrifice and Passover symbolism of the lamb shank bone. Some Jews also avoid garlic, though this is more rare. Neither of these rules are observed universally, however.