Why kale leaves shrink upon cooking – Leafy vs root vs stem vegetables

Long before kale was a super food, and a real hit in foodie land, we Dutch in the Netherlands used to eat it plenty during the winter months. We’d use it to make ‘boerenkoolstamppot’ which is essentially mashed potatoes, mixed through lightly cooked (or sometimes raw) kale, best eaten with some mustard and smoked sausages (for those interested, there’s a recipe at the bottom, but it really is very simple)!

Most Dutch tend to buy their kale leaves pre-cut since you need these huge volumes of leaves to make a decent amount of ‘stampppot’. Once you boil them done, even if it’s just for a few minutes, they will shrink and virtually disappear right before your eyes!

The same will happen to your spinach. However, if your broccoli would shrink right in front of your eyes you’d be sure something went completely wrong there! Broccoli will stay just about the exact same size during cooking, as opposed to your kale. This post dives into the different types of vegetables (root vs. leaf vs. stem) and we’ll figure out where that shrinkage comes from!

The different parts of plants

In order to dive deeper into that shrinking phenomenon we have to look at plants in general. If you’re not familiar with the cellular structure of plants and the concept of ‘turgor’ now might be a good time to read up on that in our post on texture & turgor. All vegetables are made of those same plant cells but how they’re organized exactly depends on where they come from on the plant. Vegetables all somehow come from a plant, but they can come from very different sections.

Root vegetables

Starting at the bottom we can find roots, carrots are a great example of a root vegetable, as are parsnips or beets. (e.g. carrots) Root vegetables contain a lot of nutrients for the plant to grow from, mostly are starches and other carbohydrates. These vegetables can be kept for quite long times without too much trouble. Their main function during the life of a plant is to gather nutrients for the plant to grow from.

Some will group tuber & bulb vegetables (these are your potatoes, onions, leeks) under root vegetables, whereas others group them separately. How exactly you group them isn’t important here though. They do also serve as storage areas for the plant.

Stem vegetables

Going upwards from the root we will find the stems. Stems have to be strong enough to hold on , above we can find stems which support the plant to stand upright. These vegetables have to contain a lot of strong fibers, e.g. cellulose, to hold on to the rest of the plant. Celery and asparagus are great examples of stem vegetables. They have that fibery structure.

Flower vegetables

The first and more obvious example of a flower vegetable would be a zucchini flower. It clearly is a flower in the ‘traditional’ flower sense. However, did you ever realize that artichoke and broccoli are flowers as well?

Fruit vegetables

Not just fruits are fruits, sounds confusing indeed, but there are various vegetables that also serve as the fruit of the plant. They contain seeds since the fruit is meant to be eaten by animals (in most cases at least) and then pooped out somewhere else to grow a new plant. Examples of fruit vegetables are avocados, zucchinis and eggplants.

Leaves

Last, but not least, the vegetables which are essentially the leaves of the plants they come from. Kale and spinach both are the leaves of the plant, just like lettuce. One of the main jobs of the leaves is to produce energy (glucose) from sunlight using photosynthesis. This photosynthesis process is performed by chlorophyll molecules. These are what makes the leaves turn green, hence the presence of chlorophyll in kale.

kale leaves + masher

Structure of leaf vegetables

Leaves are designed in such a way that they are optimized for catching sunlight to produce energy sources for the plant. They have a large surface area and are pretty thin, they don’t need to be thick since only the outside will be able to catch light anyway. Also, they do not contain all the fibers that the stems do, this would only interfer with catching sunlight. In kale leaves though, you will see a solid stem within the leaf. It is centralized though to minimize interference with the leaves themselves.

Once a leaf has ‘caught’ some of that sunlight photosynthesis will start. During this process carbon dioxide (CO2) and water (H2O) are taken from the air and converted into glucose (C6H12O6). In order to perform this reaction the leaves have large air spaces in which the gases can enter for photosynthesis. Also, the glucose will somehow have to be transported back to the plant (e.g. the roots). If you look closely at kale you will see veins running through the leaves. These transport the sugars that are made with photosynthesis back into the plant.

Introducing kale

Kale is on

 

There are a lot of different kale types, some are curly (these tend to be most cumbersome to prepare, but most popular in the Netherlands), others though have flatter leaves. All of them belong to the same family of plants, the Brassica oleracea, which is the same family Brussel sprouts belong to. Again, Brussel sprouts don’t really shrink during cooking either, so it must be another factor that’s important here.

What happens when cooking leaf vegetalbes

A leaf has definitely been optimized for making energy and not to hold on to its shape. It has very little of the structural textures and plenty air. As a result, they break down quite a bit during cooking.

When boiling leaves cell walls in the plant are broken down. This softens the vegetable. A lot of processes can be at play here, but one is the loss of turgor. This happens to most vegetables, not just the leaves. However, fibrous structures (e.g. cellulose) or starch reservoirs (in the root vegetables) will keep there texture somehow and will just soften. Leaves on the other hand don’t have that. Instead, the cells are broken releasing a lot of air from their pockets. As a result they shrink quite a bit, it’s mostly just air that’s gone!

Boerenkoolstamppot (mashed potatoes with kale) recipe

Curious about that recipe we mentioned at the top? Here’s the recipe, it really is very simple but typically Dutch!

Dutch food - Kale leaves + potato mash = Stamppot
 
Author:
Ingredients
  • 300g kale
  • 400g potatoes (feel free to exchange some regular potato for a sweet potato, parsnip or celeriac!)
  • some milk (we use max. 20ml)
  • some butter (we use max. 10g)
  • salt & pepper (to taste)
  • mustard
  • smoked sausage
Instructions
  1. Boil potatoes in water until they are soft and fall apart easily (test this with a knife).
  2. Boil kale in a slight layer of water for about 10-20 minutes (cook for as long as you like to eat it).
  3. Pour of the water from your potatoes and add some butter and milk (you can leave these out). Add salt and pepper to the taste.
  4. Mash your potatoes (see that device in the photo above? that's something nearly all Dutch households have, it's the tool we use to mash potatoes).
  5. Mix in your drained kale.
  6. You're ready to eat! I like mixing in some mustard in my kale 'stamppot' (that's how we call this) and eat it with some sausage (a very typical smoked type, not dry though). Lots of people like to eat this with gravy, however, I'm not a big fan of that.

Sources

  • Lefsrud, M.L., Kopsell, D.A., (2015), Air temperature affects biomass and carotenoid pigment accumulation in kale and spinach grown in a controlled environment, link
  • On Food and Cooking from Harold McGee p.263-264
  • Wikipedia
  • An article on leaf structure
  • Plants in action

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