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Poached pears are soft and juicy, whereas a ripe fresh pear has a nice crunch to it. Raw carrots crunch and you need a firm bite to break a bit of, but, the carrots which have been boiling in your soup for more than an hour are beautifully soft and melt away. A fresh crop of lettuce has firm leaves, however, after a few days the leaves might all become soft and start to wilt. Fruits and vegetables can move between being crisp or soft quite easily, giving a wide variety of textures.
These textures of your fruits and vegetables are very different to that of a steak or freshly baked bread for instance. This is because of the plant cells of which all your fruits and vegetables are made. Bread doesn’t have these intact cells any more and a steak has animal instead of plant cells, which behave quite differently. We’ll dive into the world of plant cells, texture and turgor to understand where this all comes from.
Cells are the building blocks of life
The texture of fruits and vegetables starts with its building block, the cell in the plant.
Any living organism (except maybe for viruses, but we won’t dive into that here) is made out of cells which is why cells are often called the building blocks of life. A cell consists of an outer layer (a membrane) which keeps a collection of liquids, molecules and little organelles together. A cell is a tiny factory and by keeping everything together within the membrane can do a variety of things.
Bacteria consist of only one cell, whereas plants and animals are huge collections of millions of cells. These cells all have their own roles within the bigger plant or animal. Cells from bacteria, plants and animals all have a different structure. These structures define whether something is a plant, animals or other type of organism.
Structure of plant cells
Above you can find a simplified image of a plant cell. An important characteristic of a plant cell is the outer cell wall. This is quite a rigid structure and is made up of cellulose, hemicellulose and possibly pectin and lignin. All of these are large molecules, belonging to the carbohydrates.
Inside the cell wall there’s a cell membrane which protects the inside of the cell. Only some components can move freely through this membrane. Within the membrane all the other cell components float around. In the image above you can see for instance the nucleus. This ‘center’ contains all the genetic material, the DNA, of the cell. It is the control center of the cell. Not shown on the image above are the chloroplasts, these are the energy factories of a plant cell. Chloroplasts contain chlorophyll which absorbs sunlight and uses its energy to make glucose using photosynthesis.
Then there’s the vacuole which essentially is a bag of water inside the cell. The vacuole is a storage place for useful molecules and has several other important roles, but the one we’ll be focussing on here is its ability to create turgor in a cell.
Turgor in plants
The fact that plants can create and maintain ‘turgor’ in their cells is what makes them unique and different from animal cells. Both the cell wall and the vacuole play an important role here.
Turgor refers to the fact that plant cells can build up pressure inside their cells. It’s the difference between a wilting lettuce leaf and a firm one. This pressure is created by the vacuole. When the vacuole is full of water is pressed against the cell wall. By pushing against the cell wall it makes is nice and firm. The cell wall is build to withstand this pressure. Think of a balloon as an example. When it’s blown up well it is nice and firm. However, when it starts losing air the balloon shrivels and isn’t as tense anymore.
When the bag of water isn’t full anymore and doesn’t press against the wall the cell wall won’t be as tense anymore. It will keep it’s shape, it won’t collapse completely, but it will result in wilting leaves.
It’s this turgor that makes plants firm. It’s what makes the carrot and pear crunch. However, upon losing the turgor the plant will become limp; resulting in a soft poached pear, a melting carrot in the soup or a wilting piece of lettuce.
How water moves in and out of plant cells
Water can travel in or out of a cell through the cell wall and the cell membrane. There are two main processes which manage this flow of water in your picked fruit and vegetables: osmosis and transpiration.
Fruits and vegetables transpire and slowly lose water. They do this while on the plant, but then the moisture is replenished through the roots. Once the fruit or vegetable is harvested, they will continue to lose moisture, be it slowly. Once too much moisture has left the product, the vacuoles aren’t filled up enough anymore for them to maintain turgor. This is when your mint leaves and apples start wilting and become less crunchy.
Another very important process within cells is osmosis. Osmosis occurs when there is a semi-permeable membrane. In other words, when there’s a barrier through which not everything can move. Cell membranes are an example of such a membrane. Water can travel through freely but many other molecules cannot. Osmosis is the process in which the ‘free’ molecules will move in such a way to even out the concentrations of other molecules (e.g. sugars or salts).
In other words, through osmosis the concentration of solutes (e.g. those sugars or salt) will be the same inside and out of the cell. Below you can find a simplified visualization.
An example of osmosis, transpiration & turgor
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So let’s have a look at a specific example. Imagine you have even slices of freshly slices zucchini. You will store these slices in three different ways:
- Uncovered on your countertop
- Submerged in tap water
- Submerged in a solution of water with a lot of salt
Over time you will notice that the three will behave very differently. The first will slowly dry up and crinkle, it will lose it’s turgor. The second won’t change a lot, it will stay nice and firm. Since all the sugars and salts are still inside the zucchini and within the cells, water will try to enter the cells to even out the concentration within and outside of the cell. It will do so until it has reached turgor. The third one though will not stay nice and firm, it will become softer. Since the concentration of solutes in water is a lot higher in the salt solution the water will move from the cells to the solution to try and even this out.
How to maintain turgor (thus crunch & crispness)
So, for a fruit or vegetable to remain crunchy and crispy and have the right texture, it is important to maintain that turgor inside the cell. In order to do so there are two things you have to maintain.
First of all, the cell wall should stay in place. This serves as the holding container and walls for the bag of water to press onto. Once the cell walls is broken the water will just run out and no pressure can be built up. Cooking breaks up the cell walls, as does slicing or damaging the food.
Furthermore, there should be enough water inside the cell. You do not want the product to dry out, so don’t place it in direct sunlight. Storing it in a slightly humid environment may help here as well. However, be careful, you don’t want moulds and yeasts growing either (they like a moist environment as well!).
Softening fruits & vegetables – Getting rid of turgor
Sometimes though we want to get rid of turgor. We want the poached pear and the carrots in our soup to become nice an soft. In those instances we will try to break down the cell walls to let the water flow freely.
Boiling fruits and vegetables often breaks down this cell wall, though to which extent depends on the vegetable or fruit. The composition of cell walls differs per plant type. It contains large carbohydrates (learn more about carbohydrates) of which the most common are cellulose, hemicellulose, lignin and pectin. Preparing fruits and vegetables has a lot to do with how these molecules react to heat (thus, boiling, frying, grilling, etc.). Cellulose is barely affected by heat (think of celery stalks), but some of the other components, such as pectin, will leak out of the wall structure when they’re heated. This will weaken the cell and water will be able to leave the cell. As a result, the fruit or vegetable loses turgor and they become softer.
Other factors that determine fruit & vegetable texture
Even though turgor is the most important factor when it comes to vegetable and fruit texture, it is not the only one of influence. Just imagine the pear and the carrot, when they’re fresh, they both have turgor, but nevertheless have a very different texture. The pear is a little more mealy and softer, whereas the carrot is probably more crunchy.
These differences have to do with the ratio of other molecules present in your produce. Different products have different plant cells with different molecules. Celery stalks for instance have a lot more cellulose in them than let’s say that soft pear. Nevertheless, all are made of plant cells.
Also, the amount of air in a fruit or vegetable influences the texture. That’s one of the reasons apples and pears have such a different texture, apples contain a lot more air than pears (25% vs only 5%). These air pockets make the pear a lot denser than the apple and influence the way the pear breaks when you bite into it. It is also one of the reasons kale and spinach shrink so much when they’re cooked!
Last but not least, the texture of fruits and vegetables will continuously change. Remember, they are living products and continue respiring. Some fruits and vegetables will also continue ripening after they’ve been harvested. This can also cause tremendous changes in the final texture.
It’s these continuous changes that makes fresh produce interesting, but sometimes challenging to work with. It’s never exactly the same and one day that apple variety might be perfectly crunchy, whereas the next day you’ve got a mealy one.
Here’s a far more in depth article on plant cell walls.
On Food and Cooking from Harold McGee has two very extensive chapters on fruits and vegetables which are worthwhile a read.