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In this question, I learned that one measures the amount of flour used to determine whether challah separation is necessary. Here I learned that there is a difference of opinion between (some) Sefardim and Ashkenazim whether a K'zayit is measured by weight or volume.

My question now is, can one use the amount of ingredients in a recipe to calculate how much of the prepared food is a K'zayit? This would be useful when determining how much is needed to eat to make a blessing over the hand washing, and say the Grace After Meals.

Here's an example which focuses on the volume, but the same question would apply to weight if the igredients were weighed instead of measured by volume. For the sake of simplicity we'll assume that all parts get distributed equally:

I adapted this recipe to make 8 equal size flour tortillas. Here is the recipe I used. I used this website to convert the cups to cubic centimeters, and put the number in parenthesis:

  • 2 cups of flour (473.1764725 Cubic Centimeters)
  • a dash of baking powder
  • 1/2 a cup of oil (118.29411812499998 Cubic Centimeters)
  • 3/4 of a cup of water (177.4411771875 Cubic Centimeters)

According to this article, R' Avraham Chaim Na'eh holds that the stricter definition of a K'zayit is 27 cubic centimeters, while the Chazon Ish holds that it is 47.5 cubic centimeters.

So my question now is, can I add up all the cubic centimeters used in the recipe, divide by 8 and use that information to determine how much of the tortilla is a K'zayit? If I add it all up, the recipe calls for a total of 768.911768 cubic centimeters. This means that each tortilla uses 96.113971 cubic centimeters of ingredients.

Can I therefore say that one tortilla is 2 K'zaytim according to the Chazon Ish, and 3.5 K'zaytim according to R' Avraham Chaim Na'eh? Is this a valid halachic way to determine the volume of a food item, or are there other factors that must be taken into account?

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How much water evaporates? –  Shmuel Brin Mar 15 '12 at 19:18
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Kezayit for what purpose? For some things the halacha may look at the final size while for others it may only matter the amount of flour that went in. It depends on the application. –  Double AA Mar 15 '12 at 19:30
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This could be a simple thing to test. Measure ingredients, cook food, weigh food, see if weight is the same. –  Shmuel Brin Mar 15 '12 at 23:50
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You have to account for the fact that liquid evaporates. Otherwise you could have things that are clearly smaller than an olive but are more than a kezayit. –  Double AA Mar 18 '12 at 16:25
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@Menachem a priori, there are a number of factors to consider. a posteriori: final volume - initial volume = change in volume. –  Double AA Mar 18 '12 at 16:51

2 Answers 2

I propose that that is not a valid method to measure volume for purposes of b'racha acharona for foods generally, though I can't speak to its applicability to tortillas specifically. Here's a counterexample to show its inapplicability in general: coffee, brewed in a stovetop percolator. Put whatever quantity of coffee grounds and water in the percolator, turn on the gas, and wait. The coffee (and grounds) that come out will be considerably less than what went in. Yet search as you might on Google, there are many who discuss how much coffee must be drunk to require a b'racha acharona, but no one seems to take into account the amount of water put into the percolator: they all measure the amount actually drunk. This is not a great proof (lo rainu eno r'aya), but it's somewhat indicative.

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This is basically what I said in this comment above. –  Double AA Mar 18 '12 at 16:57
    
@DoubleAA, and, before you, Shmuel Brill. But I gave a concrete example of coffee. –  msh210 Mar 18 '12 at 16:59
    
Bemchillat kevod Shmuel Brill, he didn't give a svara. I'm not sure how much more conclusive your discussion about coffee is compared to the general problem of things having different shiurim than they have which could have infinite ridiculous nafka minas. What you have not dealt with is Menachem's comment about the flour itself being more than a kezayit per tortilla. –  Double AA Mar 18 '12 at 17:08
    
no one seems to take into account the amount of water put into the percolator: they all measure the amount actually drunk. -- but that's because how much liquid there is has no bearing on whether you need to make a blessing or not. The only factor is how much you drank. A better parallel would be to ask if the amount of water put in to the percolator could be used to determine if there was a revi'is of coffee. –  Menachem Mar 18 '12 at 17:47
    
@Menachem, but the question was about b'racha acharona, and that's what I find a lot of discussion about (for coffee). –  msh210 Mar 18 '12 at 23:07

The question seems to be mixing up three different issues.

  1. How much of a raw ingredient is needed to "tithe" (take halah, trumah or meiser)
  2. How much food needs to be eaten for a certain bracha.
  3. How to measure or decide if the amount of food needed fulfills the halachic requirements.

While issue 3 is a matter of great halachic debate, issues 1 and 2 are not debated at all. Issue 1 has straight forward "amounts" in the halacha. Whether or not you fulfill those amounts is dedpendant on issue 3. Issue 2 however is the main question that you ask here, and all agree that to figure out if you need to make certain brachot, the question is how much went into your mouth. The only way to figure out how many beitzot or kzayit are needed for a bracha is to measure the food that you have eaten. If you know for certain how the food shrinks or grows based on your ingredients, then perhaps you could figure it out from ingredients alone, but that seems much more difficult than just measuring and weighing a single tortilla and figuring out it's size and weight.

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