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If I look on nutritiondata.self.com for the nutrition value of raw lentils (as an example), it tells me the protein value per 1 cup is 50g. However, 1 cup of boiled lentils is good for 18g of protein!

I looked up the question, however the popular answer is that it doesn't make much of a difference. But judging by those numbers, I think that it does!

So, what should I do?

Lentils being just an example! This applies to other foods too..

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2 Answers

Lentils (and other foods) expand when cooked - its not the same amount of lentils (and other food).

Some vitamins and such are destroyed by heat, but to be sure on exactly what, only a lab could determine.

For macronutrients such as proteint, fats, etc - whatever goes in, comes out.

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Ok, so if my bag has a nutrition value for 1/4 cup, is that before or after being cooked? –  Martin Mar 27 '12 at 23:27
Based on your questions vs nutritiondata.self.com, it appears to be precooked –  rfusca Mar 27 '12 at 23:34
That still leaves the question.. how do I figure out how much nutrients are in the food after I've cooked it? –  Martin Mar 27 '12 at 23:49
@user9692 only a lab really can. –  rfusca Mar 27 '12 at 23:50
@user9692: If the main effect is expansion, just measure before you cook and you'll know what's in the finished product. If you really care, measure the volume after too. –  Jefromi Mar 28 '12 at 1:25
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If you are really serious about this, you can calculate it.

First, obtain the macronutrient counts per 100 g of ingredient you are adding. For example, lentils have 26 g of protein per 100 g.

Second, weigh everything you are adding. Let's say you want to cook 200 g lentils.

Third, calculate the total amount of nutrients you added. With my nice round numbers, it will be 52 g proteins from the lentils.

Fourth, weigh the prepared dish. (it doesn't help to sum up the raw ingredients only, because water will evaporate during cooking). Let's say you end up with 700 g of cooked lentils.

Fifth, calculate the new protein amount per 100 g. 52 g protein per 700 g lentils makes it around 7.4 g protein per 100 g.

Now you are ready. You only get a small amount of error when some nutrients are destroyed by cooking, for example a maillard reaction will involve both proteins and carbohydrates as input and produce carbohydrates only. But it is a very small error; the variability of nutrient levels between batches of the same food are much greater than this error. (You don't believe that every lentil kernel around the world has exactly 26% protein, right? The data you see on labels is an average of a few measurements, and the single batches can vary a lot).

Note that you can't do this for micronutrients, because many of them are affected by cooking.

Also, don't even attempt to do it using volume measures. Once you mix two different liquids, the volume of the mixture isn't always the same as the sum of the two volumes you started with. Also, solids can absorb liquids without changing their volume much, etc.

And of course, you have to do it separately for each recipe to get a reasonable degree of precision. You can't just rely that lentils cooked in pure water will absorb the same amount of water as lentils cooked in a, say, soup soured with tomato juice.

If you still think this all is worth it, go ahead and do it. Most people don't need such a minute control over their diet, especially when they cook by themselves and know (roughly) what goes into their meals.

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This isn't really answering what is asked - all you're doing is showing how to calculate the macronutrient density before and after. Which doesn't change the nutrition at all. If I put a 1/4 cup of lentils at X amount of protein, cook them, and then eat the entire dish its still X amount of protein. –  rfusca Mar 28 '12 at 0:48
@rfusca I interpreted the insisting in the comment to be about the final nutrients count in a portion of prepared food, not about the loss of nutrients through cooking - which is close to nothing for proteins, carbs and fats. I guess that the OP has to come back and say which interpretation is correct. –  rumtscho Mar 28 '12 at 1:00
Thanks! I'm trying to find out more about the nutrients, actually. My insistence about the prepared food is because I'm not sure how much the preparation destroys the nutrients. Knowing that the macronutrient loss is practically nil is basically what I was trying to find out. Is there a way to know anything about the micronutrient loss as well, or is that different depending on each nutrient/source/cooking method? –  Martin Apr 16 '12 at 17:05
@martin the reason you can do the calculations I described is exactly because there is no loss. If you are talking about nutrient loss through heat, you have to know how much is lost, this is not something you can calculate. Probably not even specialists can calculate it, given how many variables are involved - it is easier to measure an average value for the remaining nutrients in a certain type of food and use it as a ballpark figure. (But of course, it is not especially precise - you don't know how much you had to start with, the duration of cooking, the acidity of your food, etc.) –  rumtscho Apr 16 '12 at 17:17
Alright, thank you! –  Martin Apr 16 '12 at 18:33
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