Why is kitniyos not a prohibition whose reason is common knowledge and no longer applies? It is a minhag that developed, which should, I would think, make it even easier to break when the reason does not apply to a d?rabbanan anymore.
Am I correct in assuming that a commercial business that produces rice almost certainly has a large area (probably someone in China) that does nothing but rice? There probably isn?t wheat or anything else even close by. Similarly, these companies are probably so huge that each type of bean, etc· is grown in large fields that do nothing but that type of bean.
I've never understood the whole thing in the first place, since even back then, a tiny piece of wheat making its way into the rice fields would be bitul b'shishim as long as everything is harvested and prepared before Pesach, right?
avrohom adler, usa
The first thing we must remember about Pesach is that we take the Isur of Chametz very seriously and accept stringencies upon ourselves that are Lifnim me'Shuras ha'Din. The Shulchan Aruch (OC 442:6) records the Minhag of certain very careful people to scrape walls and chairs that may have come in contact with Chametz. The Mishnah Berurah says that one should not make fun of this custom by saying it is a Minhag Shtus, because it has a basis in the Yerushalmi and even though it is not obligatory nonetheless Yisrael Kedoshim Hem v'Nahagu l'Hachmir Afilu b'Mashehu. The ARI Z"L (Mishnas Chasidim, Meseches Nisan, 3:4), declares that anyone that guards himself from Chametz during Pesach with all the "Chumros sh'Machmirim ha'Machmirim" will benefit his soul greatly during the coming year. In that spirit, the Ashkenazim accepted upon themselves the stringency of not eating Kitniyos.
You are right that, in our day and age of big agrobusiness, when things are grown and packed in huge quantities, there is less chance of different foodstuffs getting mixed together. Nonetheless, the annals of Kashrus supervision are filled with stories of mishaps in food factories, and we shouldn't assume that it is impossible for Kitniyos to be adulterated with grain. One large packing plant may have a packing line for beans and another for grains, and somehow some grain finds its way into the beans.
You are also right that a "tiny piece of wheat" would be nullified in the Kitniyos if it got mixed in before Pesach. But who says we are only worried about tiny pieces and only about pre-Pesach mixtures? Maybe more than a sixtieth of wheat could find its way into the Kitnyos before Pesach, or a tiny piece during Pesach.
And don't forget that this is not the only reason for the Minhag. The second major reason mentioned by the Poskim (see Mishnah Berurah 453:6) is that bread is also made from Kitnyos - cornbread, for example - and if we allow Kitnyos bread, then the less-than-knowledgeable may come to eat Chametz bread as well. This reason would seem to be as relevant as ever, especially considering the many newcomers to Torah-observant Judaism.
You raised the issue of a Takanah d'Rabanan being discontinued when the reason for the Takanah no longer exists. Whether this is true or not is a complicated matter and is debated among the Poskim. It is beyond the scope of this reply.
Also, the fact that only the Ashkenazim accepted this Minhag may or may not make it less binding than any other Rabbinic law. The MAHARIL says that one that disregards this custom has violated the Torah prohibition of Lo Sasur and is Chayav Misah b'Yidei Shamayim - terminology usually reserved for d'Rabanan laws (see Sha'arei Teshuvah 453:1).
A final note: there does seem to be one "exemption clause" in this Minhag. The Aruch ha'Shulchan (OC 453:1) says that if we have a bad agricultural year and people are going hungry, we can eat rice and legumes that year (assuming that there aren't other foodstuffs such as potatoes to alleviate the problem).
Kol Tuv,
Yonasan Sigler
While we're on the topic......According to this second reason....What about all of the modern-day products that are made from pesach matza meal? other than a slight variation in taste/texture (depending on who cooks...sometimes there is no noticeable difference) many of these are indistinguishable from chametz products. For example, pesadich breakfast cereal, muffins, noodles, etc. Why don't we say that one will err and have the chametz by mistake? Understandably this would not apply to those who avoid gebructs, but for everyone else, especially newcomers to Torah-observant Judaism?
Chanuka Sameach.....
I didn't find an authoritative written source that addresses your question, but I did ask one prominent Posek here in Israel. His response was that you can't forbid Matzah meal because you can't forbid Matzah. It seems that it's all or nothing; either you forbid that type of food altogether - even in its whole state - or you don't forbid it at all. One reason for this might be that it is too difficult to determine how finely that food can be ground before it becomes forbidden. We see, for example, that it is difficult to pin down exactly how fine vegetables may be chopped on Shabbos before one runs into the Melachah of Tochen (see Biur Halachah, "ha'Mechatech", 321:12). Kitnyos, on the other hand, we can live without for a week, so they were forbidden altogether.
Another possible answer to your question is that the first reason of the Gezeirah doesn't apply to Matzah (whole or ground up), i.e. we're not worried about grain getting mixed up with Matzah, and only things - like Kitnyos - to which both reasons were applicable, were included in the Gezeirah. (Whereas to possibly be freed from the Gezeirah, once something was forbidden, would require that neither reason apply any longer.)
Kol Tuv,
Yonasan Sigler