Editor's note: This article is a reprint. It was originally published on June 5, 2017.
Knives have many uses in the kitchen. If you want to get really fancy, using the right knife and the right technique can help you make a perfect julienne or chiffonade. But using the right knife can also do other things you may not know about. One of the most important is increasing the nutrients that food provides to your body.
Not long ago, aspiring chefs would simply tear leafy greens without getting bruised. Chopping lettuce and spinach for salad was considered a dud. An article in The Conversation captured the chef’s dilemma well.1
“For a long time, we have believed that tearing vegetables, especially salad leaves, is the best way to preserve nutrients. The idea is that tearing the leaves destroys fewer plant cells than chopping them. When you cut the cells directly, their contents flow out. That means nutrients, especially minerals like potassium, can leak out.”
Chopping vegetables releases more polyphenols
However, some argue that chopping vegetables (at least some vegetables) increases polyphenols. Polyphenols, which are found only in plants, play a number of roles, including providing color to plants and protecting them from ultraviolet rays.
When certain vegetables, such as celery, parsnips, and lettuce, are bruised or cut, they produce more polyphenols to protect against further damage. For example, if an animal were to try to eat them, the bitterness of the polyphenols might prevent them from taking a second bite. For humans, chopping or slicing vegetables makes the biologically active compounds in them more bioavailable, and the health benefits can be derived from the “synergistic” combination of phytochemicals.2
Antioxidants are important compounds that help protect against harmful free radicals and inflammation. Polyphenols are amazing, by the way. According to the American Journal of Clinical Nutrition, polyphenols are the most abundant antioxidant in the American diet, providing 10 times the daily intake of vitamin C and 100 times the daily intake of vitamin E or carotenoids.3
Today's nutrition4 It cited a number of health benefits attributed to the polyphenols in foods, most notably “a reduced risk of three common health conditions: urinary tract infections, cardiovascular disease, and type 2 diabetes.” But a caveat from the same report was that these benefits were not yet “established science.”
According to one study, “bruising” or cutting vegetables may increase their antioxidant capacity, depending on the type of fruit or vegetable tissue. Zucchini, white and red cabbage, iceberg lettuce, celery, carrots, parsnips, red radishes, sweet potatoes, and white potatoes were all evaluated for a variety of nutritional properties.5
“Changes in phenolics ranged from a 26% decrease to a maximum of 191% increase, and changes in antioxidant capacity ranged from a 51% decrease to a maximum of 442% increase. Reduced ascorbic acid was reduced by up to 82%, while changes in anthocyanins and carotenoids were less evident.”
Cutting vegetables affects their nutrients and color.
When chopping vegetables, there is sometimes a problem that the fine-tuned chef tries to solve carefully, and that is enzymatic browning. You have probably seen it. Peeled potatoes turn an unsightly brown before the second peeling. This happens because the enzyme polyphenol oxidase breaks down polyphenols. Sometimes oxygen hits the freshly cut vegetables, and the polyphenol oxidase mixes with the plant chemicals polyphenols.
The phenol compounds turn brown, which gives some cut foods a rust-like color. Believe it or not, the knife you use can make all the difference. It’s all about the speed at which the browning occurs, and that depends on the knife you use.
Iron and copper speed up browning, and most stainless steel knives are made with this along with other metals. When cutting vegetables, they are exposed to a triple shock, a mixture of oxygen and the metals mentioned above.
But there are alternatives. Ceramic and plastic knives are said to be “chemically inert” and slow down the browning process. In tests, Cook's Illustrated6 I found that using a plastic knife delayed browning by a full day longer than using a metal or hand knife. I also tested a ceramic knife and a stainless steel knife, and while neither completely prevented rust, the ceramic knife gave noticeably better results.
Rather than replacing all your other knives, you can use ceramic blades as a complement to other cutlery.7 These knives are best for cutting fruits, vegetables, and boneless meats, but are not suitable for cutting frozen foods, meats with bones, cheese (which tend to stick), or for crushing foods such as garlic.
Taste, Texture and Refrigeration
Chopping can change the texture, making some foods softer and more watery, and it can also change the taste. Refrigeration can slow down both of these processes. The Conversation says:8
“Refrigeration can help slow down this browning reaction and preserve potentially beneficial polyphenol content. This works because the cold temperature of the refrigerator slows down the chemical reaction that normally breaks down polyphenols…
However, the act of producing polyphenols in vegetables (such as chopping them) often involves depleting vitamin C. So nutritionally, it may be a case of 'rip Peter off and give Paul the money', with no overall benefit.”
Chopping vegetables can change their flavor, making them a little bitter. But “bruising” leafy greens and herbs before adding them to a recipe gives them a flavor that you can’t get from just cutting them off the stems, and you lose the nutritional benefits of chia. According to The Kitchn:9
“When you chop herbs into tasty little pieces to add flavor to your dishes, only the cells that are hit by the knife will release their oils into the food. If you're adding herbs to something like a sauce that's been simmering for a long time, don't worry; the oils, combined with the heat of the stovetop, will eventually seep out.”
5 Tough Vegetables to Cut and How to Cut Them
Sometimes it’s nice to broaden your horizons and try something a little different for dinner. If you’re feeling unsure about what to do with a vegetable, these simple steps will help. Using a specific knife can make the job easier, as can a cutting board.
Artichoke
It may look like a big, rubbery flower, but what you're really trying to get is a soft heart, and that takes a bit of expertise.10
guideline
- When you pull off the outer green leaves, bright yellow leaves are revealed.
- Using a serrated knife, cut off the top third of the artichoke (exposing the tops of the tightly folded purple artichoke “petals”) and the lower part of the stem.
- Using a paring knife, peel away the yellow top and sides until you reach the heart, then trim the outer part of the stem, similar to peeling a broccoli stem. Trim carefully so that the heart doesn't start to peel.
- Now, carefully remove the faint bulb, leaving behind a feathery purple “chalk”, using a small knife and spoon to complete the process.
bit
Sometimes called beetroot, this versatile vegetable is flavorful and delicious in cold salads or hot soups, but there's a trick to keeping its color.11
guideline
- Cut off the leaf portion with a sharp knife, but leave about 1 cm of leaf stem at the top.
- Wash the beets thoroughly, boil them in enough boiling water to cover them, prick with a fork to check for “doneness,” then plunge them into cold water to stop the cooking process.
- After cooling, cut off the stem with a small knife, then rub the skin with kitchen paper and it will peel off easily. Cut into slices or cubes.
Spaghetti Squash
To prepare this vegetable, you will need a sturdy knife, fork, and spoon.12
guideline
- Use a knife to poke the entire shell to allow steam to escape while cooking.
- Bake whole in a preheated 400 degree F oven for 10 minutes, or until tender enough to cut into halves.
- Place the pumpkin on a cutting board and cut it lengthwise.
- Use a spoon to remove the seeds, then scrape the pumpkin with a fork so that it falls into spaghetti-like strands and is placed on a plate.
Colabi
Using a sharp, sturdy knife and vegetable peeler, you can remove the white flavor inside, which is quite similar to the core of cabbage or broccoli.13
guideline
- Cut off the stems and leaves of the kohlrabi and cut into quarters.
- Cut out the core (about 1 inch square) from the center and discard. Remove the skin with a peeler.
- Cut the slices into stick shapes, i.e. julienne or cube shapes.
Celery root
Celeriac, also known as “knob celery,” looks like a tough, fibrous bulb of a clump of roots, but when you look at the core, it has a strong celery flavor. It's great in soups and stews, and you can even add it to salads. All you need is a sharp knife.14
guideline
- Cutting off the top, including the leaves and bottom, will allow the “handle” to lie flat on your cutting board.
- Using a sharp knife, scrape the sides open, leaving as much of the white vegetable inside as possible.
- Chop as desired, cut into julienne or slices, and soak the pieces in cold water with a little lemon juice to prevent browning.
Note on vegetables
Unfortunately, only 27% of Americans are eating the foods they need for optimal health.15 The bottom line is this: Chopping, slicing, or otherwise “bruising” your vegetables releases more polyphenols than simply biting into them, but you need to eat moderate amounts of fully cooked vegetables to get the health benefits, including polyphenols.
