When making soup, I accidentally put in too much salt. Was there anything I could have done about that? I've heard that raw potato will absorb the excess.
Almost everyone has heard that advice: Throw in some chunks of raw potato and simmer them for a while, and they will absorb some of the extra salt. But as with so many tenets of food lore, this one, to my knowledge, has never been tested scientifically. I took that as a challenge and set up a controlled experiment. I simmered some raw potato in salty water and measured the amount of salt in the water before and after the potato treatment.
I'm not going to tell you how it turned out until I tell you exactly what I did, because I want to get full scientific credit for this ground-breaking research.
I made up a too-salty and a much-too-salty mock soup sample -- 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 teaspoonful of salt in four quarts of soup or stew, with more salt to be 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 Sample No. 2 contained one tablespoon (three teaspoons) of table salt per quart of water. That's the equivalent of about four and 12 times the usual recipe-starting saltiness, respectively, and perhaps two to six times the saltiness of a soup that had already been salted "to taste." (For any chemists out there, the one-teaspoon-per quart sample measured 0.092 molar and the one-tablespoon-per- quart sample measured 0.30 molar.)
I heated each of the two mock-soup samples to boiling, added six 1/4-inch-thick slices of raw potato, simmered gently for 20 minutes, tightly covered, then 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. 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, because scientists, as you must be thinking by now, are absolute nuts about controlling all conceivable (and even some inconceivable) variables, except the one they're trying to compare. Otherwise, you'd never know what caused any differences you might observe. I could very easily sermonize here against those who try something once under completely uncontrolled circumstances and then go running around saying, "I tried it and it works." But that's another column.
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 here is that salt water conducts electricity, and the conductivity can be directly related to the salt content. I am indebted to Carrie Geary of the University of Pittsburgh's chemistry department for performing the measurements.
And what were the results? Did the potatoes really reduce the concentrations of salt? Well, . . .
But 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 as a control. (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. (That's called a blind tasting.) 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. But does this mean that the potato really removed salt from the "soups?"
No. All it means is that the potatoes soaked up some salt water; they didn't selectively extract salt out of the water. Would you be surprised if a sponge placed in salt water came out tasting salty? Of course not. But 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 whatsoever in the salt water 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 sample or in the one-tablespoon-per quart sample. The potato treatment just doesn't work. Period.
Epilogue
There were a couple of interesting sidelights to this experiment that I'll record for you science buffs. (Class dismissed for the rest of you.)
First, it turned out that the conductivities of the salt waters after simmering with potato were slightly higher -- 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 0.2 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 I've got an airtight case, don't you? Nobel Committee, please note.
Labelingo: Perspicacious reader William Maner of Williamsburg reports finding the following ingredient list on a product: Milk, Cream, Sugar, Pecans (Buttered, Salted and Roasted Pecans), Corn Syrup, Non Fat Dry Milk, Whey and Whey Protein Concentrate, Stabilizer (Guar Gum, Mono and Diglycerides), Calcium Sulfate, Polysorbate 80, Locust Bean Gum, Cellulose Gum, Carrageenan, Natural Flavor (Hydrolyzed Soy Protein) Water, Caramel Color, Salt, Artificial Color (FD and C Yellow #5 and #6.)
The product? Food Lion Homemade-Style Ice Cream.
Ah, just like Grandma used to make.
(Have you noticed any silly things on food labels? Send your Labelingo contributions, along with your name and town, to Food 101, Food Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or to the e-mail address below.)
Robert L. Wolke (www.professorscience.com) is professor emeritus of chemistry at the University of Pittsburgh. His latest book is "What Einstein Told His Barber: More Scientific Answers to Everyday Questions" (Dell Publications, $11.95). Send your kitchen questions to wolke@pitt.edu.
