Despite the simplicity of the actual gesture, that of adding the right quantity of salt is one of the most difficult and risky operations in cooking. No need for lengthy explanations as to why: even a bit too much salt in a dish risks compromising the flavour altogether. It is particularly likely to occur when preparing first courses and side dishes, in which salt tends to dissolve more, but we have all experienced an excessively salty chicken or quiche Lorraine. Awful isn't it?
Much has been said and written in scientific terms on how to eliminate or at least to mitigate the problem and it goes without saying that numerous household remedies have been devised which are supposed to neutralise an excessively salty dish. One of the most widespread fixes is that of adding a potato to the dish, either whole or cut up into pieces (depending on whoever is explaining the method, often backed up by pseudo-scientific notions worthy of a science fiction film). According to this theory, the potato soaks up the excess salt like a sponge, enabling you to recover what would otherwise be destined for the waste bin.
Then, around this quick fix, a whole lot of methods have sprung up regarding quantities and timing, which tend to make it sound even more credible.
For example, some claim authoritatively that the correct proportion is that of “a potato for soup” (as though “soup” were a quantity) and for a length of time equivalent to 25 and 35 minutes, so as not to risk removing all of the salt. If you browse the web, or chat with friends who are keen on cooking, you will find out that this method is really quite popular, so much so that it will be fun to explain that it has no scientific grounds at all and actually belongs to the rich and variegated world of culinary myths. And it is our mission to debunk all such myths.
Why potatoes do not reduce the saltiness of a dish
There is no explanation as to why this method does not work, for the simple reason a scientific approach obliges you to prove the contrary, that is to say, that a certain technique or procedure works. The difference is subtle but important: if the addition of a potato to an excessively salty dish does not adjust the flavour, it means that the fix does not work, without any need to beat around the bush.
So, why not try it out for yourselves: add a potato to a very salty soup and... the soup will still be salty but, if anything, you will now have a nice tasty boiled potato. Some have even attempted to approach the problem scientifically by carrying out a proper experiment and, with the right equipment, you can repeat it yourselves.
How to fix a salty soup?
Take two water samples of 400 ml and add the same quantity of salt to both, let's say 35 grams. Then, without stirring boil the two samples for the same amount of time until the salt dissolves. At this point, add a peeled potato of about 80 grams to one of the samples. Finally, after 30 minutes, remove both samples from the heat and filter them. Follow this up with an analysis using a refractometer. Both samples produce an identical result, which shows that the potato has not reduced the salt concentration. If you are wondering why the trick does not work, ask yourselves instead what chance it has of working, seen as potatoes are unable to absorb any substance selectively.
So what can be done to adjust a salty soup? Dilute it by adding a liquid of some type, such as water or milk. Alternatively, add a sweet ingredient which may partly balance the flavour. On the other hand, I always recommend potatoes as an excellent side dish.