Contributed by: R. Ezra Mizrahi
There are three times that one must eat Massah on the night of the Seder: Mossi Massah, Korech, and the Afiqomin. Let's explore each Massah-eating time during the Seder with its proper halacha. Before we begin, let's clarify two important points:
A) Kazayit: (pl. Kezetot) Hebrew word which literally means "like an olive". Our Rabbis use this measurement to dictate to us the volume amount of how much Massah one is required to eat on Pesah. Maran writes that this measurement is equals to 27 grams, while the Rambam writes that this measurement equals only 18 grams.
B) Massah: For the purpose of this article, we will be referring to the round hand-made Shemourah Massah, that is approximately 60 grams.
Mossi Massah: Ideally one should eat two Kezetot, a Kazayit from each of the top two Massot on the table. Although regularly one must have a measurement of Kazayit like Maran (27 grams), here for Mossi Massah, one may rely on the opinion of the Rambam (18 grams) for a number of reasons:
1) According to the Rambam, Rif, and Rosh this is the correct measurement for Kazayit, and one is fulfilling the missvah as ideal.
2) If one had only eaten one Kazayit as Maran prescribes at 27 grams, one does fulfill the obligation "bediavad" (post facto). Having two Kezetot as the Rambam prescribes at 18 grams each, one is still eating more than the measurement of one Kazayit of Maran (18+18 = 36 grams which is greater than 27 grams) which is what one minimally needs to fulfill the obligation.
3) The measurement of Kazayit is based on bread which takes up less volume than Massah, so one could have less Massah to cover an obligation of a Kazayit.
4) The entire obligation of having two Kezetot for Mossi Massah, only applies to those eating from the Massah on the table. However, as soon as one dips into the box for more Massah to give to other members of the family, one Kazayit is enough.
[Why is this? There is a halachic argument if one should have a Kazayit from the top Massah which is whole and seemingly better or should one have a Kazayit from the bottom Massah which is already broken (Lehem Oni); therefore our Rabbis decreed that one should have a Kazayit from each. Therefore, the entire requirement of having two Kezetot is only with respect to the Massot on the table.]
In light of everything said above, for Mossi Massah the total is 36 grams, a little more than half of the Massah.
Korech: One should have a regular amount of Kazayit of 27 grams, a little less half a Massah,. However, for those who have difficulty eating Massah, may have 18 grams, a little more than a quarter of a Massah.
Afiqomin: Some say one should have two Kezetot, but the halacha is that one must have one Kazayit. Here, the Kazayit must be 27 grams, a little less than half a Massah.