The Torah defines chometz as any mixture that contains flour and water that has been allowed to rise.

When dough rises, it undergoes a process called “leavening.” Whenever bread is baked, the dough is allowed to rise for at least an hour or two, to improve the taste, texture and volume. Once it has begun to change, it becomes leavened bread. In Hebrew, this is called “chometz.”

If you mix such with water, and leave it undisturbed, under optimum conditions it will become chometz in eighteen minutes. If, however, the flour touched hot water, or salted water, it becomes chometz INSTANTLY. And adding yeast to the dough makes it rise immediately, and more efficiently. (Yeast is one of several substances that are called “leavening agents.”)

Also, if flour touches water that is mixed with another type of liquid, it becomes chometz instantly. Ironically, things like apple juice cannot cause flour to ferment, but apple juice mixed with water causes the mixture to ferment and become chometz instantly.

However, if the dough is completely baked before it begins to rise, it is called “matzah.”

Not all grains can actually rise that way. The Torah defines five types of grain that can become chometz when mixed with liquid: grain: wheat, spelt, oats, barley, and rye, or any of their derivatives. (Those are also the only five types of grain from which we may make Passover matzah to eat the first two nights of Passover.)

During the entire Holiday of Passover, it is forbidden to own, eat or handle chometz in any way. It is even forbidden to drive any benefit from chometz during Passover, such as making profiit from it, using it for fuel, or feeding it to your animals. Any benefit whatsoever is forbidden during Passover.

Instead, throughout the entire Holiday of Passover, we eat matzah. However, it is impossible to bake matzah that is kosher for Passover on your own.

You cannot simply buy flour from the store and make matzah with it. In the first place, flour today is processed. It is often washed, which makes it chometz. Grain is usually tempered, which means it is soaked in water to soften it. Many flours are bleached. Any one of these processes make the grain chometz. There are other problems as well.

What about special flour from health food stores? Well, the Torah says “And you shall guard the matzos…” (Exodus 12:17). In other words, you must guard them carefully so that they do not become chometz. Also, we are supposed to guard it during the entire process, even stating out loud that it is being processed for the purpose of making matzah for Passover use.

So even if you buy flour that the miller claims has never been processed with water, you are nevertheless buying flour that has not been guarded by the Torah’s standards.

In point of fact, the flour used for proper Passover Matzos is usualy guarded even before it has been ground into flour. It is guarded from water from the very moment the stalks of wheat are harvested. If they touch water at any time after that, they are not used for Passover. Throughout every step of the process, the flour and the water are carefully (and separately) guarded. They are transported and stored with the same exacting measure of care. They are kept from any warmth until the actual baking takes place, because heat can speed up the chometz process, which we want to avoid.

Every utensil used in baking the matzah, the table, the kneading bowl, the cup that pours the water, the rolling pins, from beginning to end, absolutely every tool is carefully made kosher for Passover each and every eighteen minutes while the matzah bakery is being used. You can read about this process in greater detail at an article called “More on Chametz,” and also in my article “Baking The Passover Matzah.”

Extracts from chometz are also forbidden. Alcoholic fermentation from chometz is forbidden. Many of the foods and snacks you eat all year are chometz. The obvious ones are bread, cake, crackers, pretzels, cereal, noodles (except pure egg noodles prepared entirely with Passover utensils), beer, malt, whiskey, and so forth. Even a food in which chometz has been very diluted on Passover is forbidden. Even non-chometz food prepared in utensils that have been used for chometz are chometz (because some of the taste of food previously cooked in that utensil may get into new food cooked in that utensil) and may not be eaten on Passover, even if you wash the pots first.

During Passover, we may not use any items that have chometz in them. Even a mixture of chometz and permissible products is forbidden. Moreover, it is forbidden to derive any benefit whatsoever from chometz, and even from chometz derivatives. Lists of permissible and forbidden items are available in an number of publications, including Rabbi Blumenkrantz’s Digest of the Laws of Pesach. This book is updated every year, based on careful research, or so I am told. To purchase this guide, call (718) 337-6056, or (718) 337-6144.

However, some things may be made usable for Passover, even though they have been used for chometz during the year. The process is complicated, and that is a subject for another article.