In each of the 100 individual food profiles on our website, we tell you the best way to select, store, prepare, and cook each food. Our recommendations are based on our experience with the pleasures of eating—including textures, flavors, and aromas—as well as the research findings about cooking and health.
You may already be somewhat familiar with different cooking methods and their impact on food quality. For example, you are very likely to have seen overcooked vegetables that no longer look vibrant or colorful. But you may not be as familiar with the preparation aspect of vegetable cooking and its potential relationship with food quality.
Many fresh vegetable preparation steps begin with cutting or slicing. Whenever we cut into a fresh vegetable, a variety of changes immediately begin to take place. Some of these changes are chemical (like the exposure of the vegetable's inside portions to oxygen), and others are biochemical (like enzyme-related activity that begins to take place inside cells that have been cut open). By the way, similar types of chemical and biochemical changes take place whenever we put a fresh vegetable into our mouth and chew. Chewing is a natural way of "cutting open" a fresh vegetable and setting into motion many chemical and biochemical changes.
In terms of these very broad and general types of chemical and biochemical changes, cruciferous vegetables are no different than other vegetables. At WHFoods, we profile nine cruciferous vegetables that are especially popular in the U.S.: bok choy, broccoli, Brussels sprouts, cabbage, cauliflower, collard greens, kale, mustard greens, and turnip greens .) In the case of these cruciferous vegetables, however, there may be a special story to tell about preparation steps and their relationship to nutrition and health. This story involves unique phytonutrients in cruciferous vegetables called glucosinolates and their conversion into related phytonutrients that can take place through various kitchen preparation methods. This article will provide you with research-based details in this very specific area of preparation: chopping and slicing. It does not, however, attempt to provide you with a basic background in the area of cruciferous vegetables and their unique sulfur-containing phytonutrients. If you are looking for basic background information about glucosinolates, isothiocyanates, and basic metabolism of sulfur compounds in cruciferous vegetables, please see our articles Eating Healthy with Cruciferous Vegetables and Feeling Great with Cruciferous Vegetables.
Many Internet websites that provide information about cruciferous vegetable preparation and health focus on the activity of an enzyme called myrosinase. This enzyme is found in the cells of cruciferous vegetables and other plants, and it is able to take certain sulfur-containing molecules in cruciferous vegetables called glucosinolates and convert them into related sulfur-containing molecules called isothiocyanates. This conversion may have unique health benefits since greater intake of isothiocyanates can be associated with decreased risk of certain cancers, and with better inflammatory control as well. (It's important to note here, however, that fresh uncut cruciferous vegetables contain both glucosinolates and isothiocyanates, such that we are able to get both sulfur-containing compounds even in the absence of any myrosinaseactivity. And it is also important to mention that both glucosinolates and isothiocyanates have been determined to provide us with unique health benefits.)
The act of cutting into a fresh cruciferous vegetable and its cells can definitely trigger activation of myrosinase, and so it is possible for glucosinolates to get converted into isothiocyanates through cutting, slicing, chopping, dicing, etc. It is also important to add that 10 minutes of cooking will usually deactivate myrosinaseenzymes found in the cells of cruciferous vegetables. In addition, with cooking times between 1-10 minutes, loss of myrosinaseactivity becomes greater and greater with each additional minute of heating (and with any increases in cooking temperature if those increases occur).
Unfortunately, however, this description of the basic principles involving with cruciferous vegetable cutting and sulfur compounds does not give us a complete picture of what happens in real life when fresh cruciferous vegetables are chopped. Myrosinaseis one of hundreds of different enzymes present in the cells of broccoli, cabbage, kale and other cruciferous vegetables, and cutting can cause a wide variety of metabolic changes most of which are not related to myrosinase. In addition, researchers have identified over 150 different glucosinolates in cruciferous vegetables, and these many different glucosinolates have many different metabolic fates when a fresh cruciferous vegetable is chopped. For example, when raw broccoli is chopped and allowed to sit, the primary metabolic conversion that takes places is not from glucosinolates (like glucoraphanin) to isothiocyanates (like sulforaphane), but from from glucosinolates (like glucoraphanin) to nitrile analogs (like sulforaphane nitrile). Also, when raw broccoli is chopped, many glucosinolates remain intact and do not get converted into isothiocynates. Finally, many other types of molecules (both sulfur-containing and non-sulfur-containing) get formed when fresh cruciferous vegetables get chopped. These molecules include epithionitriles and oxazolidines.
In some studies, the net gain in isothiocyanates from chopping and slicing has been determined to be quite low. And perhaps more importantly, the net gain in isothiocyanates from cutting has been shown to be lower in some studies than the potential gain in isothiocyanates that might occur if we left the fresh vegetable intact. That's because bacteria in our lower digestive tract also contain myrosinasebacteria, and under certain conditions, the myrosinasefrom those bacteria could take glucosinolates from cruciferous vegetables and convert them into isothiocyanates. Bacterial activity in the large intestine is a topic of great interest in nutrition and health research, and studies in this area point to greater and greater variability between individuals in their gut metabolism specifically due to differences in gut bacteria. For this reason, it is still impossible to draw any simple conclusions not only about cut versus uncut fresh cruciferous vegetables, but also about other foods in their cut versus uncut form.
It is important to realize that none of the research findings above address the textures, flavors, and aromas of cooked cruciferous vegetables, or the degree of enjoyment we experience when eating them. Our enjoyment of cruciferous vegetables is absolutely critical! Most us of are not going to routinely eat any food that is we do not find enjoyable to eat! In fact, we would argue that the pleasures of taste, texture, and aroma are especially important for a group of foods like cruciferous vegetables, since many people have a preconceived notion of this food group as being less than fully pleasurable. At WHFoods, we are confident that you will discover complex new aromas and hidden flavors in cooked cruciferous vegetables by cutting them into smaller pieces and letting them sit for a short period of time (between 5-10 minutes) prior to cooking. When cruciferous vegetables are included in our recipes, we always recommend this approach because we have seen the results over and over in our own test kitchen. And we also know that there is nothing better than a meal plan that not only greatly benefits your health, but one that is also delicious to eat and brings sheer enjoyment along the way.