In practice, many poskim rule in accordance with the view that when ḥametz is batel be-shishim before Pesaḥ it is not ḥozer ve-ne’or, and therefore it is permissible to eat such a mixture on Pesaḥ. This is because, according to the Torah, ḥametz is batel be-shishim even during Pesaḥ. The Sages added the stringency of rendering a mixture forbidden because of even a drop of ḥametz. This means that the dispute about ḥozer ve-ne’or relates to a rabbinic prohibition, and when in doubt about a rabbinic dispute, the halakha follows the lenient opinion. This is the position adopted by most Sephardic Jews (SA 447:4).
Some rule that if the ḥametz that was batel be-shishim before Pesaḥ was fluid (laḥ), the halakha follows the lenient opinion, and the ḥametz is not ḥozer ve-ne’or. If, however, it was solid (yavesh), the law follows the stringent view, and it is ḥozer ve-ne’or. For example, if a drop of beer falls into another beverage, it blends with the liquid and ceases to exist independently. As a result, after being nullified, it is not ḥozer ve-ne’or and does not render the mixture forbidden. However, if a crumb of ḥametz falls into a solid food, because it continues to exist independently and does not blend with the mixture, it has a degree of significance. Therefore, when Pesaḥ arrives it is ḥozer ve-ne’or and renders the entire mixture forbidden (SA and Rema 447:4, based on Terumat Ha-deshen). This is the approach adopted by Ashkenazim and some Sephardim.
Flour, because of its fineness, is considered a fluid mixture. This is because the distinction between fluid and solid depends principally upon whether or not the forbidden food blends completely with the permitted food. In a fluid mixture, the forbidden food blends completely with the permitted food, and in a solid mixture the forbidden food remains independent. Accordingly, there is no need to check the wheat grains before they are milled and baked into matzot, because after the wheat is milled, the flour produced from the leavened grains will be nullified and blend completely with the rest of the flour, and when Pesaḥ arrives it will not reawaken to render the mixture forbidden (SA and Rema 453:3).
Based on this principle, some poskim say that it is best to bake matzot before Pesaḥ so that if even a tiny quantity of flour or dough becomes ḥametz during the kneading process, it will blend with the rest of the dough and be batel be-shishim before Pesaḥ. This assures that it will not reawaken and render the matzot forbidden during Pesaḥ. Likewise, with respect to machine matzot, sometimes tiny particles of dough get stuck in the tines of the machine during kneading, and they remain there long enough to become ḥametz, whereupon they fall back into the dough. However, because the pieces of dough that became ḥametz blend completely with the rest of the dough, it is considered a fluid mixture, and since the ḥametz is batel be-shishim before Pesaḥ, it is not ḥozer ve-ne’or.
All of this is be-di’avad, but le-khatḥila we take special care to bake matzot in which there is no concern that even the tiniest bit of ḥametz got mixed in. Those who are meticulous take care to eat matza from wheat that was guarded from the moment it was harvested, which is acceptable even according to the stringent view that ḥametz is ḥozer ve-ne’or even in a mixture of fluids with fluids (below, 12:5, n. 5).
The view of Shulḥan Arukh warrants closer study. SA (442:4) cites Rambam that “tiryaka” (“theriaca,” a type of medicine) is prohibited on Pesaḥ since the concoction contains a drop of ḥametz, as it is ḥozer ve-ne’or once Pesaḥ begins. This seems to contradict the lenient ruling in SA 447:4. According to Rema, SA retracted what was written in §442 and permitted ḥozer ve-ne’or. Pri Ḥadash explains that ḥozer ve-ne’or applies in a case where one intentionally mixed ḥametz into the mixture. Taz states that tiryaka is forbidden because the ḥametz in it acts as a stabilizer (davar ha-ma’amid) in the mixture.
It is also important to note that even according to the lenient opinions that ḥozer ve-ne’or is permitted (according to SA – in all cases; according to Rema – in liquid mixtures), it is forbidden to intentionally blend ḥametz into a mixture before Pesaḥ and annul it in a sixty-to-one ratio in order to eat the mixture on Pesaḥ (as per Pri Ḥadash’s explanation of SA and MB 447:102 at the end). Only ex post facto, when the ḥametz was mixed in unintentionally, may it be eaten le-khatḥila. According to the stringent opinions, since the mixture may not be eaten, it also may not be kept in one’s house, though be-di’avad, if he kept the mixture in his house over Pesaḥ, he may eat it after Pesaḥ (MB 447:102).
MB 453:32 states that according to Taz, if a bit of ḥametz was already mixed in before Pesaḥ and there are less than sixty parts of permissible food in the mixture to annul the ḥametz, one is permitted to add more permissible food to the mixture to nullify the ḥametz. However, MA and most poskim maintain that this is forbidden, since it appears that he is intentionally trying to nullify a forbidden food. In extreme situations one may rely on the lenient opinions.
. Terumat Ha-deshen 1:114 states that flour is considered a fluid mixture, and this is the opinion of the majority of poskim, as MB states in 447:32. However, Baḥ states that according to Smak and Raavyah, flour mixed with flour is considered a solid mixture; therefore, one should preferably take care that no leavened flour mixes with the matza flour. This is also the view cited in MB 453:17 and SHT 25. Additionally, it is clear that le-khatḥila one should take into consideration the opinions that all ḥametz reawakens on Pesaḥ, whether solid or fluid.
There are three views regarding the rationale behind ḥozer ve-ne’or. According to the most stringent view, even in the case of a fluid mixture, and even if the ḥametz itself has been removed and only a minuscule amount of it was absorbed into the mixture and is not discernible, it is still ḥozer ve-ne’or. In contrast, MB 453:17 mentions the more lenient view of Olat Shabbat and Eliya Rabba that the ḥametz is ḥozer ve-ne’or only if one re-cooks the mixture, for only then does the ḥametz contribute more taste to the mixture. If it is not re-cooked, it is not ḥozer ve-ne’or. The mainstream view, as MB states ad loc. based on MA, is that ḥozer ve-ne’or applies to solid foods, i.e., only when there is actual, substantial ḥametz remaining in the mixture.