Does It Matter How You Cook Your Vegetables?

Are you the type that refuses to eat your veggies?

Maybe you begrudgingly pinch your nose and force them down?

Or are you on the other side of the spectrum, enjoying every leaf and stem, wishing you could get even more out of every bite?

No matter your preference today’s article, written by my student volunteer Chris Miller, will help you maximize your nutrient intake, and enjoy every bite.

Part 1- Making Them Taste Good

Brassica vegetables (broccoli, cauliflower, cabbage, brussels sprouts, kale, etc) are common staple greens in most cultures. Brassicas, also known as mustard greens, are typically low in sugar and high in sulfur and glucosinolate which explains their bitter flavor. Boiling extracts the water soluble glucosinolate and sulfur which is why broccoli, kale, or whatever your preference isn’t as bitter when boiled. This is a missed opportunity however because low sugar content means brassica take longer to burn and need more time/heat for caramelization

Caramelization is the oxidation of sugar. It’s responsible for the sweet nutty flavor you taste in browned vegetables or other sugary foods. Through a series of non-enzymic chemical reactions sugars are converted by heat into caramel, making the dish sweeter. Caramelization requires heat, time, and sugar. Caramelization of most vegetables requires roughly 230F or 110 degrees Celsius which means under normal conditions caramelization cannot happen via boiling or steaming, as the maximum heat retention of water is 100 degrees Celsius. This can change due to pressure or dissolved solutes but we’ll run with the assumption you aren’t creating superheated water with the laboratory you keep in your cupboard. All of this is to say that you’ll require a “dry heat” cooking method such as roasting or frying in order to get the most flavor out of your vegetables.

Glucosinolates are responsible for the rich bitter flavor all brassica have. They have their own flavor, but when mixed with another chemical called myrosinase glucosinolate produces a new chemical called isothiocyanate. Isothiocyanates have a similar but more complex flavor profile than glucosinolates, increasing the bitterness but also the depth of flavor in your veggies. When a brassica is damaged it exposes cells containing myrosinase, allowing it to mix with glucosinolate. This means chopping brassica up prior to cooking provides time for this chemical reaction to take place. Chopping a brussels sprout or broccoli floret in half also increases surface area available for caramelization.

Salt is a well known flavor enhancer. It does this by interacting with all five flavor receptors (salt, sweet, sour, bitter, umami) and altering their activity.

Importantly for the topic at hand salt blocks bitter taste receptors making bitter flavors left over from the cooking process more palatable.

Part 2 Minimizing Nutrient Loss

Cooking is a type of chemistry, it can change nutrient composition of food in different ways depending on the cooking method or nutrient in question. Things to consider are:

  • Heat stability
  • Nutrient solubility (water or fat)
  • What you’re actually making.

The heat stability of vitamin C and to a lesser extent A can be a concern depending on how hot, and how long you’re cooking your vegetables. Vitamin A loss is mainly a threat when cooking for a particularly lengthy period of time and in fact moderate cooking times increase b-carotene availability. Moderate being defined here as enough cooking to soften the vegetable’s texture, but not burn it. Vitamin C on the other hand is much more delicate and prone to loss from heat. Vitamin C can be quickly lost at temperatures below boiling, although the exact temperature and time depends on the vegetable, preparation method, etc.

Vitamins, based on solubility, may be leached off into their cooking medium. So water soluble vitamins (C and B) are at risk when boiling. Research shows there is a large range in the potential percentage leaching of water soluble vitamins into cooking water, up to 55% may be leached from the vegetable. Glucosinolate, the bitter compound mentioned earlier, has been linked to cancer prevention and is also leached into cooking water. Leached vitamins don’t simply disappear when they enter cooking water, they’re still in there and assuming you plan on using the water for something like soup you’ll still receive their full benefit.

Fat soluble vitamins (A, D, E, and K) come with their own complexities. Fat is required for their absorption and storage in the body. This means vegetables should be paired with fats when cooking or serving since they’re typically low in fat themselves. Cooking vegetables in oil or serving them with higher fat foods has been shown to increase absorption of fat soluble vitamins.

Case Study: The Example of Lycopene In Tomatoes

Brassica aren’t the only vegetables affected in this way however, let’s look at lycopene in tomatoes as an example. Lycopene is the pigment that gives ripe tomatoes their bright red colour, it is a non-provitamin A carotenoid and antioxidant that has been proven to reduce your risk of developing cancer or cardiovascular disease. What’s particularly notable about lycopene is that it is more readily absorbed when the tomato has been pureed, such as with canned tomatoes, or when heated. Commercially processing tomatoes involves heating, which converts trans configuration lycopene molecules into cis configuration, which are more bioavailable. Oil has also been found to increase absorption of lycopene. Putting this all together we can see that tomatoes cooked with oil for something like a pasta sauce will provide more lycopene than an equal amount of raw tomatoes served without oil or fat.

So Which Way Is Best?

With all this information in mind we can confer that to get the best flavor and most nutrients out of your veggies, specifically brassicas, they should be roasted or stir fried in oil and lightly salted; unless you’re preparing soup in which case the more vegetables you can boil in the stock the more nutritious the broth will be. If you prefer your broccoli boiled then please don’t let this article stop you, just consider using the cooking water for a nice soup broth.

Now go roast some brussels sprouts and enjoy yourself.

Here’s a very simple recipe for roasted broccoli to get you started

 

Special shout out to my student volunteer Chris who was very open to feedback on his writing and produced a great piece on an important topic.

Andy De Santis RD MPH