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Why Kale Shrinks When Cooked (but broccoli doesn’t)
Long before kale was a super food, and a real hit in foodie land, we Dutch used to eat it all the time during the winter months. We’d use it to make ‘boerenkoolstamppot’ which is mashed potatoes mixed with lightly cooked (or sometimes raw) kale, best eaten with some mustard and smoked sausage.
Most Dutch tend to buy their kale leaves pre-cut since you need these huge volumes of leaves to make a decent amount of ‘stamppot’. Once you’ve boiled them, even if it’s just for a few minutes, they shrink considerably!
The same will happen to your spinach as well as eggplant. On the other hand, broccoli & Brussel sprouts don’t shrink at all. Even though they’re all vegetables, they can behave pretty different and a lot of that has to do with the role of the vegetable on a plant (root vs. leaf vs. stem). It so happens that fibers & air play an important role as to whether your vegetable will shrink or not!
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.
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.
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.
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?
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.
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.
Structure of leaf vegetables
Leaves are designed in such a way that they are optimized for catching sunlight to produce energy sources (glucose) for the plant. They have a large surface area, are green and 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 as much fibers as the stems do, this would only interfere with catching sunlight.
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. While gases are taken up, glucose has to be transported back into the plant. This happens through a system of veins running through the leaves.
Kale is a leafy vegetable
If you look closely at kale you will see those veins running through the leaves. Also, you can see a solid stem within the leaf. This won’t do much for photosynthesis but helps the leaf maintain it’s shape and serves as a large vein. It is centralized to minimize interference with the leaves themselves.
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 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. It is not the family though that is important for shrinkage, it’s that leafy texture with all those air pockets inside!
What happens when cooking leaf vegetables
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!
Even though eggplants aren’t leaves, they also have a very airy texture. The inside of an eggplant is almost like a sponge and so also contains a lot of air pockets. This is why an eggplant also shrinks considerably during cooking (or absorb a ton of oil)!
- 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
- An article on leaf structure
- Australian Society of Plant Scientists, New Zealand Society of Plant Biologists, and New Zealand Institute of Agricultural and Horticultural Science, Plants in action, link