The Torah states, “And you shall observe (u-shemartem) the matzot” (Shemot 12:17). The Sages interpreted this to mean that the matza must be guarded from becoming ĥametz. This refers specifically to the matzot eaten on the Seder night in fulfillment of the mitzva, for the very next verse states, “in the evening you shall eat matzot.” The other matzot one eats on Pesaĥ are like any other food: they may be eaten as long as there is no concern that they may have become ĥametz. However, we have been commanded to guard carefully the matza used for the mitzva on Seder night from becoming ĥametz. Such matza is called “shmura” matza.
Ideally, the wheat that will be used for the matzot should be guarded from when it is harvested. It is customary to harvest the wheat while it is still a bit moist, for if it were to dry out completely and then get drenched by rain, it would become ĥametz. Similarly, later on the grains must be stored where they will not come into contact with water.
One may also fulfill the mitzva of eating shmura matza with matzot that were guarded from the time the wheat was ground into flour. As long as the wheat kernels showed no signs of becoming ĥametz and nothing has happened that would undermine the presumption that they are not ĥametz, one need not be concerned that they had become wet and became ĥametz. However, this is not the best kind of shmura matza since it was guarded only from the time of grinding.
In extenuating circumstances, when flour that has been guarded from the time of grinding is also not available, one may buy regular flour and fulfill the mitzva by guarding the dough from the time of kneading (SA 453:4). Even where it is customary to rinse grains of wheat lightly before grinding them, it is still permissible in extenuating circumstances to buy regular flour in the market, since a brief rinse is presumed not to be enough to cause the grains to become ĥametz. However, where it is customary to briefly soak the grains in water, one may not buy flour for matza in the market, since it is presumed to be ĥametz (MB 451:24). Therefore, in practice, one should not buy flour that is not certified kosher for Pesaĥ since the wheat is often soaked in water and may have become ĥametz.
In practice, today’s custom is to be scrupulous about shmura matza; matzot that have been guarded from the time of harvest are used to fulfill the mitzva of eating matza on the Seder night. This practice has taken root so thoroughly that people have come to call matza that has been guarded only from the time of grinding “non-shmura matza,” even though it is considered shmura halakhically, and one may use it to fulfill the mitzva of eating matza.
. According to Rif, Rambam, and other Rishonim, the wheat needs to be guarded from the time it is harvested; according to Rosh, from the time that it is ground, since the mitzva only begins when the wheat encounters water, and the wheat was usually ground in water mills. The implication of Rosh’s opinion is that as long as one grinds the wheat without using water, the mitzva of guarding the wheat begins at the time of kneading (MA 453:7). However, several Rishonim write that the wheat needs supervision from the time of grinding regardless of whether a water mill is used; among them are Rashi, Shibolei Ha-leket, and Smak.
According to geonic responsa cited by many Rishonim, if one neglected to guard the wheat from the time of the grinding, he may purchase flour from the market be-di’avad. According to Birur Halakha 40a, one may certainly recite the berakha over matza if it was guarded from the time of grinding (BHL 453:4), because according to Ran and many other Rishonim, the position of Rif and those who agree with him is that guarding matza from the time of harvesting is merely the best way to fulfill the mitzva. According to Kaf Ha-ĥayim 482:1, in an extreme situation one may even use matza that was only supervised from the time of kneading and recite a berakha on it. It seems that most poskim concur that be-di’avad one may use matza that was guarded only from the time of kneading.
Tur and Beit Yosef state that according to Rif, Rabbeinu Yeruĥam, Ran, and Magid Mishneh, the need to guard the matza pertains only to the matza used to fulfill the mitzva on the first night of Pesaĥ. All other matza eaten on Pesaĥ need not be guarded (MB 453:21). (Others maintain that one must use shmura matza for the entire holiday, either out of concern that non-shmura matza will become ĥametz or because they believe that there is a mitzva to eat shmura matza all Pesaĥ; see below, section 5.) According to Baĥ and those who agree with him, the need to guard the matza is rabbinic in origin, and the rabbis merely supported their enactment using a verse from the Torah. However, according to most poskim, this mitzva is from the Torah (BHL 460:1 s.v. “ein”, citing Rashba, Pri Ĥadash, and other Rishonim and Aĥaronim).