SA 447:4 rules that when ĥametz is batel be-shishim before Pesaĥ it is not ĥozer ve-ne’or, and therefore it is permissible to eat such a mixture on Pesaĥ. This is because, according to the Torah, ĥametz is batel be-shishim even during Pesaĥ. The Sages added the stringency of rendering a mixture forbidden because of even a drop of ĥametz. This means that the dispute about ĥozer 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.
Rema (ad loc.) rules in accordance with Terumat Ha-deshen (1:124), that if ĥametz that was batel be-shishim before Pesaĥ was a fluid mixture (laĥ), the halakha follows the lenient opinion, and the ĥametz is not ĥozer ve-ne’or. If, however, it was a solid (yavesh), the law follows the stringent opinion, and it is ĥozer 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 ĥozer ve-ne’or and does not render the mixture forbidden. However, if a crumb of ĥametz 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 Pesaĥ arrives it is ĥozer ve-ne’or and renders the entire mixture forbidden. This is the approach adopted by Ashkenazic Jews and some Sephardic Jews.
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 ground and baked into matzot, because after the wheat is ground, the flour produced from the leavened grains will be nullified and blend completely with the rest of the flour, and when Pesaĥ 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 Pesaĥ so that if some flour or dough becomes ĥametz during the kneading process, it will blend with the rest of the dough and be batel be-shishim before Pesaĥ. This assures that it will not reawaken and render the matzot forbidden during Pesaĥ. Sometimes, when machine matzot are being baked, particles of dough get stuck in the tines of the machine and remain there long enough to become ĥametz, whereupon they fall back into the dough. However, because the pieces of dough that became ĥametz blend completely with the rest of the dough, it is considered a fluid mixture, and since the ĥametz is batel be-shishim before Pesaĥ, it is not ĥozer ve-ne’or.
All of this is be-di’avad, but one should preferably take care to bake matzot from wheat that certainly has not become ĥametz and guard against the smallest crumb of flour or dough becoming ĥametz.
. The custom of the Sephardim: Kaf Ha-ĥayim 447:76-78 states that many Sephardim have the custom to be stringent about ĥozer ve-ne’or, and this is the opinion of Pri Ĥadash and Ĥida in Birkei Yosef 447:14. It seems that these authorities were also stringent about liquid mixtures. Kaf Ha-ĥayim adds that Sephardim have the custom to be as strict as Rema, but no more. This is similar to what Zekhor Le-Avraham states: Sephardim on Pesaĥ follow Rema. Nonetheless, SA cites the lenient position anonymously, which indicates that it is completely lenient in this matter. Additionally, Yabi’a Omer OĤ 2:23 expands on the topic and reinforces the lenient opinions, arguing that most Rishonim are lenient about ĥozer ve-ne’or. Moreover, according to She’iltot ĥametz is batel be-shishim even on Pesaĥ, and even those who are stringent maintain that this prohibition is rabbinic, and in a dispute about rabbinic law we are lenient.
SA (442:4) cites Rambam that “tiryaka” (a type of edible medicine) is prohibited on Pesaĥ since it contains a drop of ĥametz that becomes reawakened once Pesaĥ begins, and renders the entire mixture forbidden. This seems to contradict what SA stated in 447:4. According to Rema, SA retracted what was written in §442 and permitted ĥozer ve-ne’or. Pri Ĥadash explains that ĥozer ve-ne’or applies in a case where one intentionally mixed ĥametz into the mixture. Taz states that tiryaka is forbidden because the ĥametz 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 ĥozer ve-ne’or is permitted (according to SA – in all cases; according to Rema – in liquid mixtures), it is forbidden to intentionally blend ĥametz into a mixture before Pesaĥ and annul it in a sixty-to-one ratio in order to eat the mixture on Pesaĥ (as per Pri Ĥadash’s explanation of SA and MB 447:102 at the end). The only time such a mixture is permissible is if the ĥametz was mixed in unintentionally. According to the stringent opinions, since the mixture may not be eaten, it also may not be kept in one’s house, though if one kept the mixture in his house over Pesaĥ anyway, he is permitted to eat it after Pesaĥ (MB 447:102).
MB 453:32 states that according to Taz, if a bit of ĥametz was already mixed in before Pesaĥ and there are less than sixty parts of permissible food in the mixture to annul the ĥametz, one is permitted to add more permissible food to the mixture to nullify the ĥametz. 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, Baĥ states that according to Smak and Raavya, 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 opinion cited in MB 453:17 and SHT 25. Additionally, it is clear that one should preferably take into consideration the opinions that all ĥametz reawakens on Pesaĥ, whether solid or liquid.
Let us note three opinions regarding the rationale for the law of ĥozer ve-ne’or. According to the most stringent opinion, ĥozer ve-ne’or applies even in a fluid mixture, even if the ĥametz itself has been removed and only a minuscule amount of taste was absorbed into the mixture, and even though this taste is not discernible. The second opinion, cited in MB 453:17 in the name of Olat Shabbat and Eliya Rabba, is that ĥozer ve-ne’or only applies if one re-cooks the mixture, which would cause the ĥametz to add taste to the mixture. The third opinion is that of MA, quoted by MB ad loc., that ĥozer ve-ne’or applies only to solid foods and only when there is still actual ĥametz remaining in the mixture.