cooking fresh, frozen, canned vegetables

July 1, 2006 3:33 PM

I've decided to eat more vegetables, and I have a few questions about the necessity of cooking them, the nutritional content of frozen vegetables, and how cooking affects the nutritional content.

1) What vegetables need to be cooked before eating? What happens if you don't cook them first? How long do you have to cook them (boiling, steaming, stir-frying or whatever method)? I'm not looking for fancy recipes. Just the vegetables by themselves pretty much.

2) I've heard rumors that frozen vegetables can often be more nutritious than "fresh" vegetables which have actually been sitting around quite a bit by the time you buy them. Any vegetables in particular? Are there any canned vegetables that are more nutritious than their fresh or frozen counterpart?

3) And if you have more to add to an old Mefi thread, Does cooking affect the nutritional value of vegetables?.

posted by mrkohrea to food & drink (17 comments total)

Some veggies do well with some cooking: beets, asparagus, snap peas. Anything tough or bitter will soften or sweeten with cooking.

The less time you cook a vegetable, the healthier the results. Cooking breaks down proteins and vitamins.

Stir frys are a good example: you don't need to cook vegetables very long to bring out taste but preserve most of the nutritional value.
posted by Mr. Six at 3:41 PM on July 1, 2006

1) Asparagus is just about impossible to eat unless you boil/steam it, but otherwise most vegetables are fine raw (also more nutritious and arguably tastier - I loves me a good salad)
posted by Popular Ethics at 3:41 PM on July 1, 2006

I recommend picking up The Enchanted Broccoli Forest, which contains friendly instructions on how to prepare and cook various types of vegetables. It was from that book, for example, that I learned that broccoli is best when cooked to a bright green.
posted by bingo at 4:17 PM on July 1, 2006

Carrots are more nutritious cooked.

I actually won a trivia contest at my fitness center because I knew that. Apparently no one else did, which shocked me.
posted by konolia at 4:18 PM on July 1, 2006

Most vegetables are washed one or more times in pre-processing before you buy them, if for no other reason than to remove field soil discolorations. The solutions used in these washes can vary from plain water to mild soaps and anti-bacterial solutions, and can be used in equipment ranging from industrial conveyorized spray system with single pass filtration, to wash tubs operated by hand, which build up considerable soil between refilling cycles. In many stores, fresh vegetables are frequently "spritzed" by water spray systems, whose reservoirs and water droplets can be good places for bacteria to culture. Finally, open produce at supermarkets is frequently handled by many people, not all of whom have perhaps the highest regard for personal hand sanitation. So, if you're going to be eating lots of raw vegetables, make sure to wash them well yourself before consuming, at least.

Some forms of high temperature, quick cooking, such as blanching, are recommended for many vegetables which can still be served cooled and crisp after blanching, but for which the brief surface exposure to cooking heat is sufficient to kill surface bacteria, and inhibit short term surface oxidation and discoloration.

Frozen vegetables can be of higher quality than the same vegetable in so called "fresh" form, but this depends on the proximity of the freezing processing plant to the crop, and the turnover of the items through the distribution chain. Some items are better candidates for frozen consumption than in fresh form, because in the best case, freeze processing can happen in time to prevent chemical changes that are otherwise hard to arrest. If you, like me, are a fan of Silver Queen cut corn, and aren't growing your own, or picking from a pick-it-yourself garden, you'll appreciate the value of quick freezing this sweet delicate treat over the next couple of weeks, as this season's crop begins to hit the freezer section of your supermarket. Cutting and freezing the corn within hours of picking stops the conversion of sugar to starch in the kernel, and makes frozen cut corn a versatile favorite addition to many dishes I make, more than 6 months a year.
posted by paulsc at 5:17 PM on July 1, 2006

Canned vegetables are waterlogged and mostly junk. One exception is good-quality canned tomatoes to use in pasta sauces and soups.

Frozen vegetables all have a kind of samey taste and texture. If you're going to take an interest in vegetables, it's worth figuring out where to get good-quality fresh ones.

A fairly simple technique works on a lot of cookable veg: cut the veg up into pieces, peeling off any hard skin. Bring a pot of water to the boil and put the veg in when it's really boiling. Cook this way for no more than 5 minutes. Meantime, put a little olive oil, a couple of smushed garlic cloves and a pinch of salt into a flat-bottomed wok and saute the garlic gently. Drain the blanching veg as well as you can, then add the drained veg to the wok, giving them a stir-fry in the oil and garlic for a few minutes. At the end, if you like, add a light spritz of lemon juice or balsamic vinegar and quickly toss the veg around. Delicious with broccoli, rapini, cauliflower, asparagus, green beans and so on.
posted by zadcat at 7:35 PM on July 1, 2006 [1 favorite]

1) Some foods are dangerously toxic when raw, e.g. buckwheet greens, rhubarb, kidney beans. Some others just don't taste so good or are too tough.

2) The point of freezing and canning is to preserve food in a consistent, standardized method. The key word being consistent. Fresh food will usually be better, locally-produced organic certainly so, but such food spoils and you sometimes get bad batches.

3) Cooking almost always reduces nutritional value. But the remaining nutrition, assuming a western diet, will be more than adequate to sustain you in a healthy way.
posted by randomstriker at 7:53 PM on July 1, 2006

Many of my favorite veggies I prefer roasted. Spritz pan with olive oil, slice veggies, roast in oven until tender, sprinkle with salt after. This includes sweet potatoes, white potatoes, onions, eggplant, beets, pumpkin, tomatoes (also more nutritious cooked), carrots.

Almost everything else that I really like needs mimimal steaming or sauteeing or can be eaten raw: corn, asparagus (peel the stems of thick asparagus so that they steam quickly), spinach, arugula, edamame, snap peas.
posted by desuetude at 8:52 PM on July 1, 2006

FoodFit.com has a very nice quick guide to vegetables and fruits arranged by season, with nutrititional information, what to look for when buying, and how to store and prepare (and even a couple of recipes for each one). I've linked to the Fall Vegetable guide here because, unfortunately, the Summer vegetable and fruits guides are snafu-ed - but I've emailed them, so I hope they will fix that because these are really nice little thumbnail infobits.

The Worlds Healthiest Foods has extensive entries for veggies. Not the best for super-quick info, but packed with helpful tips and facts you probably didn't know. For example, kidney or gallbladder problems, or low on calcium? Skip the Turnip Greens and other foods with oxalates. Spinach has 13 different compounds that function as antioxidants and anti-cancer agents. Click the "How to Enjoy" links on individual entries for preparation information.

Also, just as another useful link, The Cook's Thesaurus offers photos and general information on just about every food item, which is very handy when you come across an unknown ingredient - and best of all, it suggests substitutes, which I find incredibly helpful.
posted by taz at 1:07 AM on July 2, 2006 [1 favorite]

Some foods are reduced in nutritional content by cooking, but others, like spinach, are healthier cooked. The iron and calcium in spinach cannot be absorbed by your body until after it is cooked.

A friend of mine had a book that compared the nutritional value of food, included raw vs cooked, but I'm sorry, I don't remember the title.
posted by jb at 3:30 AM on July 2, 2006

Green beans start losing their nutritional value about a day after they've been picked. So, unless you know for sure that the beans are that fresh (you'd probably have to live on a green bean farm to be sure -g) I'd go with the frozen. Our family has decided that Trader Joe's is some of the best tasting we've had.
posted by Taken Outtacontext at 5:02 AM on July 2, 2006

You probably know this, but don't eat raw potatoes. They're toxic when not cooked.
posted by mikeh at 11:02 AM on July 2, 2006

Turnip greens are not high in oxalates at all and they contain lots of calcium.

Some nutrients are destroyed by cooking, others are increased. Lycopene in tomatoes and beta-carotene in orange and green vegetables are examples of the latter. There is actually more lycopene in canned tomatoes than in raw tomatoes. But there are also other mechanisms that increase absorption of those nutrients. Lycopene and carotenoids do better with a little fat (add nuts to your salad, for example) and most nutrients are better absorbed when you chew really well.

As for cooking methods, steaming appears to be best. In general, the lower the temperature, the shorter the cooking time and the less water, the more nutrients remain in the food.

jb, I would be interested in a reference that says that the iron and calcium in spinach cannot be absorbed when it is raw.

I wouldn't worry too much about specifics and just enjoy a variety of both raw and cooked vegetables. They are really good for you in whatever way you eat them.
posted by davar at 2:32 PM on July 2, 2006

jb, I would be interested in a reference that says that the iron and calcium in spinach cannot be absorbed when it is raw.

And I'd give it to you, except it was in a one-off reading for a graduate class on agrarian studies two years ago (we studied agriculture, but also food), and I don't remember the title of the book. Sorry. Maybe someone else knows of a good ref.

Basically, the iron and calcium in spinach is bound up with another chemical that we find hard to digest - that chemical is broken down by cooking, releasing the iron and calcium so it's easier to digest. It's probably that's it's hard to digest raw, not impossible. I'm not a chemist, I don't know the chemical, but the example stuck in my mind (because I'm quite fond of spinach, raw or cooked). It was in a chapter on the fad for raw food, as an example where raw does not necessarily mean healthier, though obviously other nutrients are destroyed by cooking (I believe vitamin C is one).
posted by jb at 3:37 PM on July 2, 2006

Thanks jb. It seems like the book was talking about oxalates also. It is controversial how much cooking helps (it has to be boiled, not steamed or stir-fried, because soluble oxalates dissolve in the water, which you should then throw away). One way cooking definitely helps is that it is much easier to eat 10 ounces of cooked spinach than it is to eat 10 ounces of raw spinach.

Spinach is rather high in oxalates, but it is also very high in calcium (spinach contains 430 mg calcium per 100 calories; milk only contains 188 mg calcium per 100 calories) and iron, so the net effect may not be as bad (at least for iron), if you eat spinach and other leafy green vegetables, in combination with a diet that is very rich in vitamin C (which helps iron absorption) and low in tannins (which hinder absorption). Spinach is a good source of vitamin C, unless, of course, it is cooked. Cooking also destroys much of the folate in spinach.
posted by davar at 6:21 AM on July 3, 2006

No one is likely looking at this thread, but I would like to correct a misconception above. Raw potatoes are NOT toxic. If there is a lot of green on the potato and you eat them (raw or cooked), you might get sick....but you have to eat a lot of it to get a toxic dose. Modern varieties are bread to have less of the toxic chemicals anyway. In any case, you can just trim out any green parts.

I've personally been eating raw potatoes my entire life (nothing like a russet sliced up and salted lightly...) I would certainly be dead by now or have serious medical issues if raw potatoes were toxic!
posted by R343L at 3:53 PM on July 13, 2006

thanks, davar! (for the details, which I had largely forgotten). I had been frying spinach, but I will just boil from now on. My husband won't touch cooked spinach, but I can feed him raw and he can have some vitamin C instead. (I'd rather have an orange.)
posted by jb at 6:39 PM on July 18, 2006