broccoli just before being cooked in several way

Investigating 5 ways to Prepare Broccoli – Broccoli science

How you prepare broccoli doesn’t just affect its flavor. It also affects color, texture, and more. Do you cook it short or long, hot or dry? The final results can be very different! Using science, we compare 5 different ways to prepare broccoli.

Vegetables are so diverse in their flavor, texture, and color. This makes preparing them exciting, but sometimes challenging. Every vegetable type may have its own best way to be prepared. We’ll demonstrate this, using broccoli as an example.

Introducing you to: Broccoli

Let’s first have a closer look at broccoli.

Family of kale and Brussel sprouts

Broccoli belongs to the Brassica oleracea species. A very diverse group of vegetables all belong to this group. Apart from broccoli, it includes kale, cabbage, Brussel sprouts, cauliflower and more. As such, just like Brussel sprouts, broccoli contains a high amount of glucosinolates. These strongly influence flavor – and are also found to have beneficial properties for health, which we won’t discuss in further detail.

broccoli florets

Structure & Cells

Broccoli has a thick green stalk with green ‘flowers’ on top. The stalk especially contains considerable amounts of cellulose and lignins, two fibers that help give strength to the broccoli.

Broccoli is a plant. As such is built up of plant cells. A unique feature of plant cells is that they have a strong sturdy cell wall, unlike animal cells. This gives strength to the cells.

Additionally, plant cells contain a water ‘reservoir’. When this reservoir is full the inside of the cell presses against the cell wall. This gives a little pressure and is what we call turgor. However, when the cells lose water, the turgor disappears. This causes leaves to wilt and turn soft.

In broccoli, this is especially important for the flowers and the leaves. These don’t have the additional strength from fibers to help them keep their shape. They’ll wilt when the turgor disappears. The stalks on the other hand aren’t as much affected. Their strong internal structure will hold up.

broccoli just before being cooked in several way
Five samples of broccoli florets, ready to be cooked.

What happens when cooking broccoli?

When you want to prepare, thus heat, broccoli, a few things start to change.

It softens

First up, the heat breaks down some of the complex carbohydrates that make up a big part of the cell walls. As a result, the turgor is lost and the broccoli starts to soften. A lot of the carbohydrates, such as pectins, may also dissolve or break apart. This again helps to soften the broccoli, making it easier to eat.

The longer you heat broccoli, the softer it becomes. At some point, it will be so soft you can compress it with a light touch of a fork.

But doesn’t shrink!

Broccoli doesn’t shrink during cooking. It might shrivel a little, but it will remain more or less the size it was at the start. This as opposed to its leafy relative kale. Broccoli has its fibers and strong structures to thank for this.

Color changes – losing green

Broccoli is green, bright green. Chlorophyll molecules are responsible for this. However, chlorophyll isn’t the most stable molecule.

When broccoli is added to a hot pan or boiling water you may at first notice it turns even brighter in color. This is not because more chlorophyll is made. Instead, it’s because the chlorophyll that is already there has become more visible.

However, when you continue to heat the broccoli it will again lose that bright green color. Chlorophyll molecules will react to become pheophytins, or leach out of the broccoli. This happens even more quickly if you’re cooking the broccoli in an acidic environment.

Natural colors in our fruits and vegetables aren’t particularly stable, chlorophyll isn’t the only one. Boiling red cabbage for too long will also result in the loss of its bright purple/red color.

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The role of enzymes

Some of the reactions can happen without an intermediary. However, other reactions that result in the breakdown of chlorophyll need a little help. This is where enzymes come in.

Enzymes are a very specialized type of protein. They can catalyze chemical reactions. That is, they help these reactions take place, without participating themselves. Various enzymes in broccoli can result in the breakdown of chlorophyll.

In fresh raw broccoli, these enzymes don’t get in touch with the chlorophyll. But when broccoli is heated and broken down, they can start to catalyze reactions and break down chlorophyll even faster.

Chlorophyll_a
A chlorophyll-a molecule

To maintain the green color you could cook broccoli in alkaline water. However, that can give your broccoli a metallic taste and accelerates the breakdown of vitamin C.

Color changes – turning brown

Losing the bright green color is undesirable. But, just as when you’re grilling a steak or baking bread, creating a brown color can be a good thing.

Chlorophyll doesn’t play a role in these reactions. Instead, the brown colors and accompanying smells and flavors are created through the Maillard reaction. This is a reaction that takes place at high temperatures in dry-ish conditions. As such, it won’t happen when you boil broccoli in water, or steam broccoli. But it can happen when you roast, grill, or stir fry broccoli.

During the Maillard reaction sugars and proteins in the broccoli react with each other. This results in the formation of brown molecules. But it’s not just about color. A lot of desirable flavor molecules are formed as well.

Flavor changes

As is the case for its relative Brussel sprouts, cooking will change the flavor of broccoli. This is partly because the defensive system of broccoli is activated!

As we now know, broccoli cells contain enzymes. There are a lot of different types of enzymes, and some of these can catalyze reactions that cause the formation of flavor molecules.

Broccoli also contains so-called precursors. These are molecules that form the starting point for a specific reaction. In whole, healthy broccoli the precursors and enzymes are stored in separate ‘compartments’. That way, they don’t react with one another. However, upon heating the broccoli, cells and compartments break down. This causes the molecules to find each other and react.

Preparing broccoli

So heat definitely impacts the structure, color and flavor of broccoli. But how to translate that theoretical knowledge into practice?

For that purpose, we investigated 5 different ways to prepare broccoli:

  1. No heat treatment, aka raw broccoli
  2. Grill in an oven for 30 minutes at 180°C (350°F)
  3. Place in cold water, bring to the boil (takes 5 min.) and cook for an additional 5 minutes
  4. Add to boiling water, cook for 5 minutes
  5. Add to boiling water, cook for 20 minutes
grilled broccoli
Broccoli grilled in the oven

Grilling broccoli – For added flavor!

As expected grilling in the oven resulted in slightly browned broccoli. The oven is hot and dry, ideal circumstances for the Maillard reaction to get going. This also resulted in a different flavor profile than any of the other broccoli tests. The types of chemical reactions that took place are clearly different. In our opinion, this was the most flavorful broccoli, but of course, that’s a very personal opinion.

Grilled broccoli has the darkest green color at the end. Apparently, chlorophyll can better withstand these conditions or becomes more visible.

But it takes longest

That said, cooking broccoli this way does take the longest.

It’s what we expected though. Water is a more efficient heating medium than the air in the oven. Water can bring energy into the broccoli more effectively. Air on the other hand doesn’t contain as much energy and isn’t as efficient. It’s a similar mechanism when comparing deep-frying vs. oven-baking.

broccoli cooked in several ways
Broccoli prepared five ways
Top left: raw (1). Middle: grilled in the over (2). Bottom left: added to cold water, then boiled for 5 minutes (3). Top right: boiled for 5 minutes in boiling water (4). Bottom right: boiled for 20 minutes in boiling water (5).

Adding broccoli to cold vs. hot water – Minor differences

Remember how enzymes can play a role in both the breakdown of color as well as flavor development? Well, enzymes are very heat sensitive. They are inactivated if they come into contact with hot boiling water. As such, we expected that adding broccoli to hot boiling water would immediately inactivate enzymes, whereas that would happen more slowly when you add broccoli to cold water that’s heated up.

Theoretically, that could result in a difference in final color and flavor.

However, we did not notice this effect. Both 5-minute broccoli samples still had a bright green color and a very similar texture.

Keep in mind though that we tested small quantities of broccoli. If you want to boil a large amount of broccoli, it will take a lot longer for the water to come to a boil. As a result, it takes longer for enzymes to be inactivated.

Boiling too long makes broccoli very, very soft…

Boiling broccoli for a mere 20 minutes results in very soft broccoli. So soft, that even a slight pressure turns it into mush. The flavor is a lot more muted as well. And, this broccoli sample is by far the whitest of them all (you can see that best in the stalks). So it clearly lost most chlorophyll.

If you’re looking for a bland broccoli puree this is perfect. Otherwise, not really so.

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 flavor to the broccoli. If you don’t have as much time, give it a quick dip in some boiling water – or a microwave(!) – to help it cook a little faster.

That said. Personal preferences vary, so you might find your best way to make broccoli is different from ours!

Sources

Bell, L., Oloyede, O. O., Lignou, S., Wagstaff, C. and Methven, L. (2018) Taste and flavour perceptions of glucosinolates, isothiocyanates, and related compounds. Molecular Nutrition & Food Research, 62 (18). 1700990. ISSN 1613-4125 doi: https://doi.org/10.1002/mnfr.201700990 Available at https://centaur.reading.ac.uk/76654/

Radhika Bongoni & Ruud Verkerk & Bea Steenbekkers & Matthijs Dekker & Markus Stieger, Evaluation of Different Cooking Conditions on Broccoli (Brassica oleracea var. italica) to Improve the Nutritional Value and Consumer Acceptance, Plant Foods Hum Nutr, link

Amy Christine Brown, Understanding food: principles & preparation, link, p.273

Michael E.J. Lean, Fox & Cameron’s Food science, nutrition & health, link, p. 231

Jaime Prohens-Tomás, Fernando Nuez, Vegetables I: Asteraceae, Brassicaceae, Chenopodicaceae, and Cucurbitaceae, p. 121, link

Sánchez-Machado, Dalia & Bueno-Solano, Carolina & Núñez-Gastélum, José Alberto & Reyes Moreno, Cuauhtemoc & Lopez-Cervantes, Jaime. (2009). Biochemical composition and physicochemical properties of broccoli flours. International journal of food sciences and nutrition. 60 Suppl 4. 163-73. 10.1080/09637480802702015., link

Scientific American, How good cooks keep green veggies from going brown, link

Yuan GF, Sun B, Yuan J, Wang QM. Effects of different cooking methods on health-promoting compounds of broccoli. J Zhejiang Univ Sci B. 2009;10(8):580-588. doi:10.1631/jzus.B0920051, link

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