When the Torah commands us to eat something, "Ain achila pachot m'kezayit," the minimum amount required is a "Kezayit." Kezayit literally means like the size of an olive. The Mishna in Masechet Keilim formulates the definition as a "Kezayit beinoni," a medium sized olive. Yet, how is this amount calculated in today’s measurements.
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909,Parshat Pikudei and Parshat, Tzav) calculates the Kezayit as a weight measure of nine Dram. We know that one Dram is equivalent to three grams. Therefore, according to the Ben Ish Hai, a Kezayit equals 27 grams. That is why many old siddurim state that the minimum amount of food requiring a beracha achrona is between 26-30 gram.
Hacham Ben Sion Abba Shaul (Israel, 1923-1998, Or L’sion, Vol. 3, Introduction) raises a number of difficulties with the opinion of the Ben Ish Hai. One of the issues he discusses is the opinion of Maran in this matter. In Hilchot Pesach, Maran clearly holds that a Kezayit equals a half of a besa (egg). This is the opinion of Rashi, Tosafot and other Rishonim (Early Commentaries). However, in other places, (Hilchot Erubin and Hilchot Yom Kippur) Maran adopts the opinion of the Rambam that a Kezayit is only one third of a besa.
How can this discrepancy in Maran be reconciled? Hacham Ben Sion concludes that in principle Maran holds like the Rambam; a Kezayit is one third of a besa. However, on Pesach, he was machmir, and adopted the stricter opinion of half a besa.
In practice, with regard to calculating a Kezayit of Masa , Hacham Ben Sion takes a different approach. Whereas the Ben Ish Hai calculated the Kezayit as a weight measurement, Hacham Ben Sion measures it as a quantity of volume. In other words, a Kezayit of Masa is the volume of Masa that will displace a Kezayit (27 cc) of water. He calculated that the volume of 20 grams of our regular Masa, round or square, displaces 27 cc of water. He formulated the measurement in weight so that we would not have to calculate the food volume.
According to this, before Pesach, one should take a food scale and weigh out 20 grams of Masa in order to determine a Kezayit. On the Seder night, we eat Masa three times. The first time, Mosi- Masa, some eat two Kezayit. Another Kezayit is need for Korech and one more for the Afikoman.
Hacham Ben Sion made the calculation for Maror as well. He determined that the Romaine Lettuce we use as Maror has almost the same density as water. 30 grams of Maror displace a Kezayit (27 cc) of water. Therefore, a Kezayit of Maror is 30 grams, which should be weighed out before Pesach to give an indication of the amount that must be eaten.
Hacham Ovadia disagreed with Hacham Ben Sion. He did not formulate Kezayit in terms of volume. He felt that it is too difficult for the masses to make the necessary calculations and conversions. Measuring volume would require conducting experiments involving water displacement and pulverizing foods to eliminate the air pockets, which do not count towards the measurement.
Rather, Hacham Ovadia ruled like the Ben Ish Hai, the Kaf Hahaim and the Petach HaDvir who all formulated Kezayit as a weight. Accordingly, 27 grams of any food will always be a Kezayit, irrespective of the food’s density. To determine a Kezayit, one merely has to look at the food labeling to determine the food weight.
Rav Abitan in his English Yalkut Yosef writes that Hacham Ovadia would also agree to the measurements of Hacham Ben Sion after he already successfully made the calculation. Accordingly, one who ate only 20 grams of Masa, instead of 27 grams, has fulfilled the misva.
It is difficult to decide between these two giants. However, if somebody is looking for an easy way to determine a Kezayit, he should adopt the approach of Hacham Ovadia that a Kezayit is always 27 grams, irrespective of the food density. It is too difficult for us to measure the volume of each type of food. However, with regard to Pesach, since Hacham Ben Sion already calculated the volumes, we can avoid uncertainty and rely on his measurements. Therefore, 20 grams of Masa and 30 grams of Maror equal a Kezayit.
SUMMARY
A Kezayit can be measured as 27 grams of any food, irrespective of its density.
For the Misva of Masa and Maror, one can be lenient and eat 20 grams of Masa.
