Kale leaves shrink when they’re cooked, eggplants turn into a delicious mush and pumpkins can be transformed into a rich puree. Broccoli on the other hand doesn’t change size at all when cooked. It will still be a stem with little flowers at the top. No noticeable shrinking.
Vegetables are so diverse in their texture and colour which makes preparing them exciting, but challenging. Just as for meat, some types need long cooking (like a stew), others very short (like a steak). That’s why this time we’ll have a look at broccoli, how it’s best cooked while preserving its colour & getting the desired texture.
What happens when cooking broccoli?
Broccoli is a vegetable and as such is built up of cells with firm cell walls. Thanks to this structure vegetables which are fresh and healthy have ‘turgor’ in their cells. This means that the cell is filled up with water and presses against the cell wall. This is what makes a vegetable firm and crispy. When the vegetable dries out or starts spoiling, cell walls break down and water is lost which changes the texture.
The stem of broccoli is quite strong. Stems contain a lot of fibers to keep them strong and hold up the rest of the plant. The floret consists of a lot of little flowers. These flowers have less fibers and a softer.
Cooking broccoli for this article means heating broccoli, whether it’s in an oven, or in a pot of boiling water. The heat transforms the different parts of the broccoli. Heat breaks down the carbohydrates that make up a big part of the cell walls. As a result, turgor is lost and the broccoli softens. The top softens more than the stems, since those stems contain fibers that don’t completely break down during cooking.
Colour changes – losing green
Broccoli is green, bright green. It’s the chlorophyll molecules inside its cells that give it this colour. Chlorophyll is the energy factory inside a plant, it uses sunlight to make glucose, energy for the plant.
Chlorophyll is very similar in structure to hemoglobin, the molecule that gives red meat its colour. In the image below you can find its chemical structure. Notice especially the magnesium ion (Mg2+) at the top. This ion is essential for the green colour. When it disappears from the molecule, the bright green colour is lost. Other chemical reactions that change the structure will also decrease the green colour.
Cooking a vegetable, will break down the cells. As a result, acids that resides in the liquid can come into contact with chlorophyll. The acids (protons) will cause the magnesium ion to be displaced, turning the vegetable brownish. Cooking in more acidic water, breaks it down more rapidly.
You may now want to cook your broccoli in alkaline (opposite of acidic) water only. But wait before you do that. The high pH-value can make your broccoli taste metallic (probably not desirable) and it will accelerate breakdown of vitamin C (ascorbic acid).
Chlorophyll can also be broken down by enzymes (chlorophyllase & magnesium dechelatase), again making it lose its colour. Some of this happens during storage (which is why old broccoli turns yellow) and it can also happen to some extent during cooking. However, high heat also breaks down the enzymes, so this process will only be short lived.
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Colour changes – Turning brown
Broccoli can turn an undesirable olive green/brown. This is very different from a desirable brown. Desirable brown is caused by the Maillard reaction. This type of brown also gives a lot of flavour to the broccoli. Since the Maillard reaction happens a lot faster at higher temperatures and needs a drier product to happen onto, you will really only see this when grilling your broccoli, more on that later.
When you cook broccoli you generally don’t want to cook your broccoli into mush, nor do you want it to lose its colour. However, you want it to soften slightly and maybe develop some additional flavour.
For that purpose we tested 5 ways of cooking broccoli and evaluatd the results.
- No heat treatment, aka raw broccoli, we just left it on the counter
- Grilling in a toaster oven for about 30 minutes at 180C (350F)
- Boiling in water:
- Place in cold water, bring to the boil and leave in for 5 more minutes once boiling
- Place in boiling water and leave in boiling water for 5 minutes
- Place in boiling water and leave in boiling water for 20 minutes
Grilling in an oven
Grilling in the oven gave the most flavourful broccoli. It was great to eat just by itself, probably thanks to all those delicious Maillard reactions taking place. It did take longest to be properly cooked and soft.
Since grilling does not involve adding any water, grilling broccoli results in a decent amount of moisture loss in the broccoli. This in turn allows the Maillard reaction to take place and it makes the florets a little crispy.
Grilled broccoli has the darkest green colour at the end, being close to brown. Chemical reactions, such as those that result in browning of chlorophyll require a minimum amount of water to take place. Since the broccoli dries out while breaking down into softer textures less moisture is available for this which may explain the limited loss of colour.
Boiling – dropped in cold vs. hot water
Placing broccoli in cold water and then heating it up to boiling should theoretically give the enzymes a lot more time to do their destructive work. However, we did not notice this effect. The broccoli that was brought to the boil still had a bright green colour. Also it’s texture was still fine, being very similar to the one dropped in boiling hot water.
We would have expected the broccoli that was dropped in boiling water to be greenest of all. Theoretically, the enzymes will be stopped immediately and the short time in hot water will limit any reactions taking place.
For the amounts we tested these two methods do not give any differences in texture & colour. For larger quantities (e.g. if it takes a long time to bring the water to the boil), bigger differences may become apparent.
Boiling – 5 vs 20 minutes
Boiling broccoli in already boiling water for 20 minutes gives a very very soft broccoli. It was so soft that any tough will turn it into mush. There is not texture left at all and the flavour has diluted considerably. If you’re looking for a bland broccoli puree this is perfect, otherwise, it’s not particularly nice. This sample was also whitest in colour of all of them.
Cooking for only 5 minutes turned the broccoli soft enough to eat, while not breaking down every single cell wall.
The first 30s in hot water – blanching
Those enzymes that can break down chlorophyll are particularly important for those who want to freeze their vegetables. Enzymes will continue to break down chlorophyll, be it slowly, resulting in a loss of green colour. This is why producers of frozen vegetables will blanch their vegetables before freezing. Blanching consists of a short (<60s) time in boiling water. It is enough to break down and stop the enzymes, but not enough to fully cook the vegetables and make them lose their texture.
Blanching can even improve the green colour of broccoli. The short heat treatment cause some of the air to disappear from the broccoli. As a result, the light is reflected slightly differently, resulting in a brighter green colour.
Best way to prep broccoli
If you want to eat broccoli by itself, as a side or as a snack and have some time, grilling is the way to go. It gives a lot of flavour to the broccoli.
However, if the broccoli is part of a bigger dish boiling for a few minutes is your best bet. It’s quick and it is then ready to be part of your pasta, quiche or whatever dish you’re planning!
- Ricardo cuisine, link
- Scientific American, How good cooks keep green veggies from going brown, link
- Cristina Efimovna do Couto, Chlorophyll and green colour stabilization on vegetable homogenates, 2016, linkc
- Amy Christine Brown, Understanding food: principles & preparation, link, p.273
- Jeana Gross, Pigments in vegetables, link, p. 49-52
- Michael E.J. Lean, Fox & Cameron’s Food science, nutrition & health, link, p. 231