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!
In general vegetables and fruits have a tendency to become a lot softer after cooking or when spoiling. Why’s that? A bread or a piece of meat don’t tend to do this.
It has all to do with some very fruit and vegetable specific textural properties. Most of it all starts with the so-called turgor. It’s something that’s pretty plant specific, meat and dairy don’t have this special property! Understanding the texture of fruit and vegetables will help you in your cooking and product development skills. It warns you what to watch out for . So today, we’ll be diving into your high school biology lessons, discussing : plant cells and vegetable and fruit texture!
Vegetable & fruit texture and Turgor
Vegetables and fruits are plants, so, they are build up of plant cells. Plant cells are pretty different from animal cells. The main difference is the cell wall plant cells have and animal cells do not. Here is where the characteristic texture of fruits and vegetables starts.
Besides the cell wall the plant cell also has a cell membrance around itself through which certain molecules can travel. Plant cells also have a nucleus (which isn’t very important today) and a vacuole, a big bag of water inside the cell (which is very important for the fruit and vegetable texture. Below is a simplified drawing of such a plant cell.
Plant cells have a very unique property, they can build up a pressure, called turgor. The turgor is nothing more than a well filled bag of water (vacuole) pushing against the cell wall. The cell wall is pretty strong and will be able to handle this pressure. 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 poched pear, a melting carrot in the soup or a wilting piece of lettuce.
In the image below you can see how this works. Water can travel in or out of a cell through the cell wall and the cell membrane. The direction the water flows is driven by a process called osmosis. In this process water will try to keep the concentration of solutes (this could for instance be salt) the same in- and outside of the cell. If there is a lot of salt out of the cell and not that much inside, the water will travel out to make sure that the concentration is similar on both sides.
Maintaining 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.
The first essential thing to do is to make sure that that cell wall remains in place. Once the cell wall collapses the water bag will have nothing to press against. Thus, no pressure can be built up. Think of it like a balloon, if the balloon pops (=cell wall), there is no pressure difference anymore.
Second of all, you have to make sure that enough water stays inside the cells. In other words, you should make sure the products don’t lie in a very salty bath, that will make a lot of water seep out through osmosis. But not only osmosis makes you loose water. Simple evaporation of water also decreases the water content in the cell and might lead to a reduction of turgor.
Getting rid of turgor – Breaking plant cell walls
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 wall 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.
Non turgor fruit and 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.
Function & composition
These differences occur because of the difference in molecules present as well as the function of a fruit or vegetable. Even though they are all built up of plant cells, the composition of these cells greatly depends on the variety and the function of the cell. We shortly mentioned celery stalks above. These stalks have to be able to stand up straight, so their cells contain a lot of cellulose which gives them this strength. A pear on the other hand is meant to be eaten, so should be a lot softer and has far less of this molecule. An onion on the other hand is meant to store ‘food’ and its layered structure leads to a very different texture. Simply a thinner layer makes it behaves different in your mouth compard to for instance a potato.
Also, the amount of air in a fruit or vegetable influences the texture. An example mentioned by Harold McGee (in On Food and Cooking) is the difference between apples and pears. 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.
Vegetable and fruit texture evolves
Last but not least, the texture of fruits and vegetables will continuously change. Remember, they are living products. They respire and a lot of chemical processes continue to happen inside them. Therefore, they keep on changing, how the change proceeds and how fast greatly depends on things like temperature and acidity (pH).
It also depends on the type of fruit or vegetable. Some fruits for instance keep on ripening after they’ve been harvested. Enzymes will keep on converting complex molecules into smaller ones, for instance, starches are converted into sugars. Think about an apple, a fresh apple is crunchy, an old apple will become mealy.
So next time when you’re making poached pears, vegetable soup or looking at your wilted lettuce. Think of what’s happening in your food. Is there a way you can use this knowledge to improve your cooking and product development? For sure, there are a lot of tricks to maintain (or lose) turgor and other textural properties?! Acids and certain salts containing calcium can influence all these processes. It’s a matter of testing. Let me know how you go and what you’d like to learn further!
Want to learn more? Here’s a far more in depth article on plant cell walls. Also, On Food and Cooking from Harold McGee has two very extensive chapters on fruits and vegetables.