When we add a good amount of salt on the external surface of raw vegetables like cucumbers, aubergines or salad, after a short time we see water appear. It almost seems like the vegetables “sweat”, just like we do when we run. It’s not our sensation: the water is really leaving the plant cells of the vegetable. What we notice is the consequence of a chemical-physical phenomenon well known to science and which you may have heard of: theosmosis. Simply put, salt creates an environment around your cells very concentrated: in response to this condition, to restore the balance between the outside and inside of the cell, water moves from where there is less salt (the inside of the vegetable) to where there is more (external salty surface). This mechanism is not only valid in the kitchen: it is the basis of industrial techniques used for store fruit and vegetablescall osmotic dehydration. Ultimately: salt releases water from vegetables because it literally alters thewater balance between inside and outside of plant cells, forcing water to move outwards.
Osmosis: why water moves by itself
Vegetable cells are like small bags filled with watersurrounded by a semipermeable membrane. This allows water to pass through easily, but solutes, such as salt dissolved in it, much less. When we add salt to the outside, the environment surrounding the cell becomes more concentrated in solutes compared to the inside. This is where theosmosis: water (the only one that can freely cross the membrane) it moves spontaneously towards the area where there is more saltthe external surface, in an attempt to restore a balance between inside and outside.
No energy is needed: it is a direct consequence of the physical laws that regulate the movement of water through biological membranes. To better understand the phenomenon, let’s consider a fictitious example: two balloons, one full of gas and the other almost empty. If we decided to join the two balloons via a connection (for example, a tube passing through the two blowing slots), the gas would immediately go from the full balloon to the empty one. We observe the exact same concept for salt and vegetables, only the physical entity changes: in the case of balloons we are talking about pressure differencefor vegetables we talk about difference in concentration of salt.
From the kitchen to the industry: osmotic dehydration
What we observe when we salt vegetables is, in a small way, the same principle used in the osmotic dehydration, studied in food science. In this process, fruits and vegetables are put in contact with very concentrated solutions of salt or sugar. The result is always the same: water leaves the plant tissues towards the outsidereducing the internal water content.
But what is the point of removing water in these cases? Well, less water = lower microbial activity and so greater shelf life.
Obviously, everything must be done with the right balance. If osmosis is exceeded, the risk is that of excessively soften the vegetables. In fact, the water inside plant cells also serves to keep them “swollen” and rigid. In technical terms we talk about “cellular turgidity”. When water comes out due to osmosis, turgor decreases: cells yes they deflate slightly and the vegetable becomes softer. And who likes a soggy cucumber? The solution is not to overdo it with salt!
