There is a fundamental question regarding the laws of kashrut on Pesaĥ: what is the status of foods that are not normally made with ĥametz all year round? Are they kosher for Pesaĥ as they are, without any special supervision, or must we consider the possibility that they were somehow mixed with ĥametz and should not be eaten on Pesaĥ?
According to Shulĥan Arukh, as long as there is no real concern that some ĥametz fell into this food or that it has absorbed the taste of ĥametz by being cooked in a pot in which ĥametz was cooked recently, there is no need to suspect that the food contains ĥametz.
However, Rema writes that Ashkenazic custom is preferably to avoid eating specific products without special supervision for Pesaĥ. This is because ĥametz is used throughout the year, and we are not generally cautious about it, so we suspect that some of it may have fallen unnoticed into these particular foods. We are also concerned that the foods may have been unwittingly cooked in ĥametz pots.
In practice, all kashrut organizations today tend to follow the stringent ruling of Rema and do not certify foods for Pesaĥ unless due caution to avoid ĥametz was exercised during the food’s preparation. Perhaps this is the way one must act today even according to Shulĥan Arukh, because all industrially produced foods contain a variety of ingredients, and there is concern that one of them is not kosher for Pesaĥ. Therefore, during Pesaĥ, one must be careful not to eat any factory food products that are not certified kosher for Pesaĥ.
Where the facts of a case are not in doubt, there are still often practical differences between the rulings of Shulĥan Arukh and Rema. Although fundamentally the law accords with Shulĥan Arukh, the tendency today is to be stringent so that food will be kosher for all communities. This is the appropriate practice when it is not overly difficult to be stringent. However, when stringency causes significant loss, there is room to support those who follow Shulĥan Arukh.
. The issue of ĥozer ve-ne’or lies at the heart of this question. Those who are stringent are concerned that a crumb of ĥametz fell into a food before Pesaĥ, and that when Pesaĥ begins ĥametz “reawakens” and causes the entire mixture to become forbidden (Rema 447:4 based on several Ashkenazic Rishonim, and Radbaz 1:487). However, according to the opinion that ĥametz is not ĥozer ve-ne’or, even if a crumb had fallen into the mixture, it would have been batel before Pesaĥ and does not reawaken on Pesaĥ. Moreover, Pri Ĥadash states that even according to the opinion that ĥametz is ĥozer ve-ne’or there is room to be lenient in this case, since there is no reason to suspect that a crumb of ĥametz fell into the mixture. Furthermore, all agree that the prohibition of eating a food into which a drop of ĥametz fell is rabbinic, and according to She’iltot, even ĥametz is batel be-shishim.
There is another concern that one cooked the food in ĥametz utensils, and the food absorbed some of the ĥametz taste from the utensils. However, those who are lenient hold that there is no reason to suspect this, since it is assumed that most utensils have not been used within twenty-four hours and would then not influence the taste of the dish in a positive way (and would not make the dish prohibited). Furthermore, even if the utensils had been used within twenty-four hours of cooking the dish, ĥametz before Pesaĥ is permissible, thus the cooked dish is a “nat bar nat” (the pot absorbed the taste of the ĥametz and in turn, passed the taste on to the cooked dish) of permissible food, which is permitted; and see Yeĥaveh Da’at 1, 11 and in the notes. This fundamental dispute is dependent on other issues, including ones that involve sharp foods (davar ĥarif) and their interaction with absorbed tastes; and see MB 447:5.