Will Raw Potato Absorb the Excess If you Put Too Much Salt When Making Soup?

Almost everyone has heard that advice: Throw in some chunks of raw potato, simmer them for a while, and they will absorb some of the extra salt.

But as with so many common beliefs, this one, to my knowledge, has never been tested scientifically. I took that as a challenge and set up a controlled experiment. I simmered raw potato in salty water and with the help of a chemistry professor colleague’s laboratory assistant, measured the amount of salt in the water, both before and after the potato treatment.

Here’s what I did.

I made up a couple of too-salty mock soups, actually, just plain salt water, so there would be no other ingredients to mess things up with their own saline predilections. But how salty should I make my samples? Many recipes begin with about a teaspoon of salt in four quarts of soup or stew, with more salt added “to taste” at the finish. So I made my soup sample No. 1 with one teaspoon of table salt dissolved in each quart of water, while soup No. 2 contained one tablespoon of table salt per quart of water. That’s about four and twelve times the usual recipe-starting saltiness, respectively, and perhaps two and six times the saltiness of a soup that had already been salted “to taste.”

I heated each of the two mock-soup samples to boiling, added six ¼-inch-thick slices of raw potato, simmered gently for 20 minutes in a tightly covered pan, removed the potato, and allowed the liquid to cool.

Why did I use slices of potato rather than chunks? Because I wanted to expose as much surface area to the “soup” as possible, giving the spuds every opportunity to live up to their salt-sucking reputation. And I used the same amount of potato surface area (300 square centimeters, if you must know) in both samples.

Of course, I also simmered the same amounts of the two liquids in the same covered pot on the same burner. Scientists, as you must be thinking by now, are absolute maniacs about controlling all conceivable (and even some inconceivable) variables except the one they’re comparing. Otherwise, they’d never know what caused any differences they might observe. It always annoys me when a person tries something once under completely uncontrolled circumstances and then goes running around saying, “I tried it and it works.”

The concentrations of salt in the four samples, the two salt waters both before and after being simmered with the potato, were determined by measuring their electrical conductivities. The idea is that salt water conducts electricity, and the conductivity can be directly related to the salt content.

And what were the results? Did the potatoes really reduce the concentrations of salt? Well…

First let me tell you about the taste tests. I reserved the potato slices after they had been simmered in the salty waters. I had also simmered potato slices in plain water (same amounts of potato and water). My wife, Marlene, and I then tasted all of them for saltiness. She didn’t know which samples were which. Sure enough, the potato simmered in plain water was bland, the potato simmered in the one-teaspoon-per-quart water was salty, and the potato simmered in the one tablespoon-per-quart water was much saltier. Does this mean that the potato actually absorbed salt from the “soups?”

No. All it means is that the potatoes soaked up some salt water; they didn’t selectively extract the salt from the water. Would you be surprised if a sponge placed in salt water came out tasting salty? Of course not. The concentration of salt in the water, the amount of salt per quart, would not be affected. So the salty taste of the potatoes proved nothing, except that for more flavor we should always boil our potatoes, and our pasta, for that matter, in salted water rather than in plain water.

Okay, now, what were the results of the conductivity measurements? Are you ready? There was no detectable difference in the salt concentrations before and after being simmered with potato. That is, the potato did not lower the concentration of salt at all, either in the one-teaspoon-per-quart or in the one-tablespoon-per quart “soup.” The potato trick just doesn’t work.

There are other saltiness-reduction dodges that one hears about, such as adding a little sugar, lemon juice, or vinegar to reduce the perception of saltiness. Are there, then, any reactions between saltiness and either sweetness or sourness that could diminish the sensation of saltiness? After all, it’s the salty taste that we want to diminish, even if the salt is still there.

It was time for me to go to the taste experts, the scientists at the Monell Chemical Senses Center in Philadelphia, an institution devoted to research in the complex field of human taste and smell.

First, as far as potato effects are concerned, no one I spoke with could think of any reason that a potato or its starchiness would reduce the perception of saltiness. But Dr. Leslie Stein helpfully supplied me with a 1996 review paper in the journal Trends in Food Science & Technology by Paul A. S. Breslin of the Monell Center on the interactions among flavors.

Can one flavor suppress another? Yes and no. It depends on both the absolute amounts and the relative amounts of the interacting flavors. “In general,” Dr. Breslin writes, “salts and acids [sour flavors] enhance each other at moderate concentrations but suppress each other at higher concentrations.”

That might indicate that adding a fair amount of lemon juice or vinegar to a quite salty soup could indeed make it taste less salty. But, Breslin points out, “there are exceptions to… these generalities.” In the particular case of salt and citric acid (the acid in lemon juice), he quotes the results of one study in which citric acid reduced the perceived saltiness, one study in which the saltiness was unaffected, and two studies in which the perceived saltiness was actually increased.

So whatcha gonna do? Add lemon juice? Vinegar? Sugar? There is really no way to predict how they will act in your particular soup containing your particular amounts of salt and other ingredients. But by all means try any one of these measures before feeding the stuff to your dog.

It appears that there’s only one sure way to rescue a too-salty soup or stew: Dilute it with more stock, unsalted, of course. It would skew the flavor balance toward that of the pure stock, but that can be corrected.

There were a few interesting sidelights to this experiment that I’ll record for you science buffs. (The rest of you may go on to the next question.)

First, it turned out that the conductivities of the salt waters after simmering with potato were slightly higher than, not lower than, those of the untreated waters. So potatoes alone must contribute some electrical conductivity to the water in which they are boiled.

That took me by surprise, because on first blush one would think that only starch comes out of the potatoes into the water, and starch doesn’t conduct electricity. But potatoes contain a lot of potassium, about two-tenths of a percent in fact, and potassium compounds do conduct electricity, just as sodium compounds do. At any rate, I corrected for that effect by subtracting the potato’s conductivity contribution from the conductivities of the potato-simmered salt waters.

Second, if, in spite of the tight cover and gentle simmering, any substantial amount of water had been lost from the pots by evaporation while cooking the potatoes, the conductivity of the water would have gone up, not down, and no such effect was found after correcting for the conductivity provided by the potato itself.

I think it’s an airtight case, don’t you?