PESAH: What is and what is not Hamets?

איסור אכילת חמץ
During Pesah we are not allowed to eat, benefit from or even posses anything Hamets. But, what exactly is Hamets? Hamets (or Chametz) is any fermented substance -solid or liquid- that comes from one of the following five grains: wheat, rye, spelt, barley and oats.
The Hamets fermentation takes place only after eighteen minutes have passed  from the moment one of these grains or one of their by-products comes in contact with water.
The following three factors need to be present simultaneously for a food to be considered Hamets.
(i) Flour or any other product derived from one of the above mentioned five grains.  A fermented food product which is not or that does not contain any element coming from one of the five grains, like rice or corn, is not Hamets.
(ii) Water: when flour coming from one of the five grains is mixed exclusively with pure fruit juice, without water, the dough is not considered Hamets but Matsa Ashira.
(iii) Time.  The Matsa  has two of the three elements that makes a product Hamets: flour and water.  But it is entirely baked before eighteen minutes have passed from the moment water and flour came in contact.  Time, therefore, is the main difference between Hamets and Matsa.
Some common examples of Hamets products are: Bread, pasta, crackers, cookies, fiber-drinks, cereals, baby foods, whiskey and beer.
The prohibition of Hamets includes another element, a food additive which the Tora calls se-or (שאור).  Se-or (grain yeast) is a catalyzer of the fermentation process. While Hamets is intended for eating se’or is a leavening agent in preparing to prepare Hamets foods. In other words, once you have a dough, flour and water, mixed together the fermentation process could take place in 18 minutes, or you can add se-or / yeast and the fermentation process will be faster.  All the prohibitions of Hamets apply to se-or as well.
Rice, corn and any seed, legume or grain which is not one of the five grains (wheat, rye, spelt, barley and oats) are not Hamets and do not become Hamets even if they undergo a fermentation process. The Ashkenazi custom, however, is to forbid the consumption of legumes and grains like rice or corn during Pesah.  Let us see the case of rice. Obviously, the Ashkenazi tradition does not consider that rice or any other grain besides the five above mentioned grains is Hamets. The reason for the abstention from rice on Pesah is that it was common to find grains of wheat mixed with grains of rice. Either because the fields where rice was grown were usually nearby or within the same fields where wheat was grown, and/or because whole rice-grains and wheat-grains look alike. In the old markets, it was not uncommon to find grains of wheat within bags of rice or other grains and legumes.  And, as we will later explain, the prohibition of Hamets during Pesah is so severe that even one grain of wheat would render a whole food forbidden for Pesah. The stringency of refraining from rice is not an exclusive Ashkenazi tradition.   Sephardic Jews are divided on the issue of rice. Moroccan Jews and other Jews from North Africa also avoid eating rice during Pesah, while Persian, Syrian and other Middle East Jews consume rice during Pesah. However, to avoid the possibility of the accidental presence of a grain of wheat in the rice, the Persian and Syrian custom is to check the rice three times before using it for Pesah.
The custom to refrain from eating rice and any other kind of seeds during Pesah is called isur qitniyot (the prohibition of legumes).   Now, even though the Ashkenazi custom forbids the consumption of qitniyot during Pesah, qitniyot products could be kept during Pesah at one’s home and there is no need to throw them out or sell them (Rabbi Eliezer Melamed, Penine Halakha, following Sh. ‘A. Rama, 453:1).  Also, as pointed out by Rabbi Eliezer Melamed,  although the Ashkenazi custom  is to forbid qitniyot during Pesaḥ, that is  only when the qitniyot constitute the majority of that product (more than 50%) but if qitniyot are present in a smaller proportion (corn-syrup, corn-starch, etc), the food is not forbidden (see Mishna Berura 8-9).  You can read the book of rabbi Eliezer Melamed, Penine Halakha online, see here.
Sephardim are allowed to keep and/or use during Pesah any product which is non-edible, i.e., unfit for human or animal consumption, even if that product might contain Hamets  For example: Cosmetics, glues, shampoo, deodorants, perfumes, soaps, detergents or any other cleaning products, etc. In all these cases there is neither a need to check for the absence of Hamets, nor for any kind of Kosher for Pesah certification.The consumption of medicines in the form of capsules or non-chewable pills is also allowed. Why? Because when a medicine comes in the form of a hard non-chewable pill it is considered “non-eatable Hamets” (eno ra-ui leakhilat keleb) therefore, even if it would contain a Hamets by product (which is highly unlikely, as I will explain), one can take that medicine during Pesah. However,  chewable pills, syrups, powder-drinks, supplements drinks and/or any other flavored and/or chewable medicines or vitamins, should be certified Kosher for Pesah, or one should make sure that they do not contain any Hamets ingredient in their composition.Rabbi Eliezer Melamed explains that this Halakhic criteria is also followed by many Ashkenazi rabbis in Israel. He also clarifies that today, virtually all medical pills are made with potato or corn-starch and almost none with wheat-starch (among other reasons because wheat-protein, gluten, is harmful for celiac patients).
A similar criteria is followed by Rabbi Gedalia Schwartz, from the Chicago Rabbinical Council (cRc), as you can see in this letter.It is advisable then, when possible, to ask the physician to prescribe for Pesah medication in the form of non-chewable pills.

Needless to say that in case of a serious medical condition, any necessary medicine should be taken.

Food that is suitable for animal consumption is forbidden to keep during Pesah. If one has a pet, therefore, one should get pet-food that does not contain any Hamets product.   Click here to read the Star K article on pet products Kasher for Pesah.