Does cooking affect the nutritional value of vegetables?

May 30, 2005 7:14 PM

I like to make stews with rice, lentils, potatoes, and lots of nutritious vegetables (e.g. cabbage, parsley, kale, broccoli, tomatoes, various greens, and so on); it occured to me recently that I might be inadvertently diminishing the benefits of these foods by cooking them for 3-5 hours. Could this be so?
posted by clockzero to food & drink (17 comments total)

Well, unless some nutrients break down from heat (none that I know off the top of my head), everything stays in the pot.

OTOH, many of the veggies you list only need to cook for 30 minutes or so, unless you are using a crockpot.
posted by mischief at 7:19 PM on May 30, 2005

There is a hollywood craze diet that only eats raw foods; it may be fun to read some of their arguments.
posted by michaelkuznet at 7:33 PM on May 30, 2005

Folate is destroyed by cooking, but that's no biggie. There's plenty of that in other stuff.
posted by gramcracker at 7:43 PM on May 30, 2005

The clever way to cook such dishes is to add vegetables at the end of the cooking time. What I do, is add some early (flavor for the meat) and then add the bulk towards the end.
posted by Goofyy at 9:14 PM on May 30, 2005

The Raw Food diet is actually not just a Hollywood craze. There are people all over the world who argue that the nutrients in food break down when it is cooked to a certain temperature. I recently interviewed a woman who has been on a raw food diet for several years and says that she believes food has a higher nutritional value when it is uncooked.

That said, from other research I have done it seems that this is only partially true. Some vegetables actually become more nutritious when cooked at higher temperatures (the tomato is the classic example -- when cooked it breaks down cell walls and actually releases nutrients that you wouldn't get in uncooked tomatoes).

This is what a nutritionist told me when I asked the same question: "It is better to include both raw and cooked foods (in your diet). Some foods you can only eat cooked, for example legumes and lentils need to be cooked before you can eat them. From a food safety perspective cooking can provide quality assurance. Interestingly, a lot of people think that raw foods are higher in nutrients. Often they are because cooking does destroy some of the more water soluble vitamins. It will reduce it by about 20 percent so you're still getting a lot of those vitamins in cooked food."

Generally what I've learned is that you loose the most nutrients when you boil vegetables in water -- all the nutrients go into the water and then down the drain. If you steam the vegetables or cook them in a stock that you will also eat, then you are still getting most of the benefits of the food. I would say that, as long as you include a few raw vegetables in your diet as well, your pot-o-veggies is still very nutritious (and a heck of a lot better than pre-packaged meals).
posted by ebeeb at 9:27 PM on May 30, 2005

Does tooth-cleaning count as a benefit? Raw veggies are good for your teeth. Of course, if you've got good teeth anyway you don't need to worry about it much.
posted by nebulawindphone at 9:34 PM on May 30, 2005

Use vegetables strategically for flavour and texture: the timing other posters have mentioned will also protect some of the nutrients, like folate, that don't deal well with heat.

1. Carnivores only: brown seasoned meat in 2-3 T hot oil; set aside.

2. Take some aromatic vegetables and dice them into 1/4 inch pieces (brunoise). This will be your mirepoix. For me, this always includes onion of some sort, celery and carrots. You may decide to add sweet pepper, mushrooms and/or olives. Sweat your mirepoix in the 2-3 T of oil at moderate heat for 5-8 minutes, until the onion is translucent.

3. Stir in garlic for a minute or so. You don't want to cook it as long as the mirepoix because it could burn.

4. Add 1-2 tablespoons of tomato paste to the mirepoix and garlic, and continue cooking, stirring constantly, for another few minutes. (Processed, cooked tomatoes are a better source of lycopene than raw tomatoes.)

5. Add 1-2 tablespoons of flour to the tomato/mirepoix/garlic and stir and cook for a couple of minutes.

6. Deglaze the pan with wine, beer, tomato juice, stock, water -- whatever you have on hand. Get all the crusty goodness off the bottom of the pan.

7. Carnivores only: add back the reserved meat.

8. Add stock, lentils and grains.

9. Add chopped root vegetables at the appropriate time to allow all ingredients to cook completely together. I find that carrots take longer than rutabaga, which take longer than potatoes.

10. Add sturdy green vegetables at the appropriate time.

11. Add delicate green vegetables and herbs at the end for a few minutes.

You should have a fairly thick gravy from the nearly liquefied mirepoix and the thickening of the flour in the stock. I make mine even thicker by buzzing the liquid with an immersion blender a few times, pureeing some of the mirepoix.

I just learned the mirepoix/tomato paste/flour steps this winter in a cooking class. I now make much better stew than I ever did before.
posted by maudlin at 9:41 PM on May 30, 2005

I've been a rawwie for 2 years now. MF actually has a couple of rawwies on board. It's not a Hollywood craze...not sure where anyone would get that idea, but the comment did make me laugh out loud.
posted by iconomy at 7:27 AM on May 31, 2005

I think heat is the key issue. As mischief mentioned, if you are using a crock pot most of the nutritional elements from vegetables (and other foods) are not lost since it does not reach temperatures that are all that high.

If you are cooking with a pot on a stove or in an oven, for 3-5 hours, then yes.. you are probably removing any and all nutritional value from the vegetables. Do as others have mentioned and add these items later. Cook the broths etc. first, then add the vegetables.

It will probably taste better as well.
posted by purephase at 8:15 AM on May 31, 2005

You can also, of course, split the various components into portions and add some initially and the rest late in the game. I often do this with potatos and celery since both add to the mix in different ways based on early or late addition.
posted by phearlez at 9:24 AM on May 31, 2005

What if you cook in a pot on the stove at the lowest possible heat? (Just the barest flicker of gas on a gas stove, e.g..) Same issue?
posted by kenko at 9:32 AM on May 31, 2005

As long as you eat everything in the pot--the broth, the water--you're getting all the nutrition from your ingredients. This can be hard with steaming for instance, but often if there's a step that includes stock, I'll use half stock and half of the water from the brief steaming or boiling to not waste those nutrients.

Cooking can also aid in nutrient absorption. Macribiotics preaches that cooking beans for long periods of time actually increases the body's ability to absorb the nutrients. This is possibly b/c beans are mostly fiber, so cooking them for longer breaks down the fiber as much as possible so that the nutrients don't get flushed out of the body as fast.

There are also tricks to nutrient absorption that are not obvious, for instance in the case of fat soluble nutrients. If you eat a raw carrot without any fat, you will not get any of the nutrients. The only benefit you'd get is fiber, but no nutrients. Most bright vegetables such as carrots are fat soluble, so always eat them with oil, dressing, something fatty.
posted by scazza at 10:19 AM on May 31, 2005

The argument I've heard from raw foodists is that heating vegetables above 120-ishºF destroys the enzymes. Enzymes are what help to break food down and aid in digestion -- your saliva has enzymes for this purpose. By retaining the food's natural enzymes, your body doesn't have to work as hard to digest the food.

I checked this cookbook out a while ago, and while the food looks really good, the preparation requirements were hellacious. From soaking nuts for several days to make your own "cheese" to substituting mushrooms as steaks, it all sounds well enough on paper, but the instructions require a hefty time commitment.
posted by Hankins at 10:20 AM on May 31, 2005

It's not a Hollywood craze...

From here:

"Well I work with a lot of celebrities behind the scenes and a screenwriter in Hollywood and I think that's what is happening is there's a buzz in Hollywood about raw food."

Just 'cause you're a rawwie and you're not Hollywood doesn't mean it's not a Hollywood craze, right?
posted by languagehat at 11:48 AM on May 31, 2005

purephase or anyone else, can you link to a scientific study or some other sourced material indicating that "any and all" nutrients are lost through long cooking at a specific temperature? I am genuinely curious.
posted by mzurer at 11:59 AM on May 31, 2005

Robert Wolke, who writes Food 101 for the Washington Post, addressed this question in a column a few years back.
posted by grateful at 12:36 PM on May 31, 2005

Gaah! Broken Link, but I found it via google search
posted by grateful at 12:37 PM on May 31, 2005