Food Fights, science style

I've been very frustrated for the past couple of months with a craigslist poster who maintains that there's no way to remove excess salt from a soup/dish. Felt like we all were getting bullied around, yet I couldn't quite frame my argument. I'm sure at least one of you have heard the trick of using a potato to absorb excess salt. Or, if you don't use a potato, you may have heard of being able to mask small amounts of oversalting by balancing it with a bit of sugar. Haven't tried the potato thing, but I've definitely done the latter and it works. I know the sugar doesn't remove the salt, but who cares if you don't perceive a dish as being unpleasantly salty anymore? Anyway, s/he still gets in a huff about it when ever the topic is breached. Infuriating.

This weekend I was talking with a teammate of Ken's (who has his doctorate in chemical engineering and specializes in thermodynamics) and told him of my frustration. We talked about it for a while and then I got this today. Love it.

Hi Lisa,

I can assure you that ClayMonkey is full of s**t (and I don't mean salt)! His (faulty) logic is that the potatoes absorb both salt and water in the same proportions as exist in the soup, leaving the concentration within the soup unaffected. Well, we know the potatoes remove salt from the soup, because they end up tasting salty. But if they were to remove a proportionate amount of water from the soup as well, they would have to swell in volume considerably. That is simply not observed. There may be an exchange of water between the soup and the potatoes (which have their own initial, salt-free liquid content, a point that ClayMonkey has obviously overlooked), but there is essentially no net movement of water from the soup to the potatoes. Since the potatoes remove salt from the soup, but no net water, the soup's salt concentration must decrease.

Let's run a few numbers to make sure. Suppose we put 100 grams of salt into a liter of water, for an initial concentration of 100 g/l. Now suppose we add a liter of potatoes. Let's assume, for argument's sake, that the potatoes are 50% solid and 50% liquid by volume, and that the potato solids absorb salt only 1/5 as readily as the liquids do (you can change these assumed values to see what happens, but the conclusions will remain the same). Okay. We now have 1.5 liters of water, 0.5 liters of potato solids, and 100 grams of salt in the pot. At equilibrium, the concentration of salt within the liquids should be essentially uniform, both inside and outside of the potatoes. Let X represent that concentration (in g/l). The concentration within the potato solids will then be X/5. The total amount of salt will be 1.5 liters times X plus 0.5 liters times X/5, which must equal 100 grams. Therefore, X, the final salt concentration within the liquids, must equal 62.5 g/l. Remove the potatoes, and the remaining broth will still have a concentration of 62.5 g/l, which is 5/8 the original concentration. Case closed.

Of course, as ClayMonkey would point out, you could also reduce the salt concentration to 62.5 g/l by simply adding another 0.6 liters of fresh water to the original liter of broth. But then you'd have 60% more broth than before, and you'd have to add 60% more of every other ingredient (except the salt, of course) if you didn't want to water down the recipe. You won't have that problem, though, if you use the potatoes to lower the salt concentration. Assuming that the potatoes do not expand or shrink by more than a negligible amount, the final volume of the broth will be essentially unchanged as well. Now, for even more fun, let's return to our example. The broth ended up with a salt concentration of 62.5 g/l, but if that's still too salty for our tastes, we can just repeat the process with another equal volume of potatoes. The second batch will end up at 39.1 g/l, the third will be at 24.4 g/l, the fourth at 15.3 g/l, and so forth. In fact, each repetition will reduce the concentration to 5/8 of its previous value, so with enough patience, you can reduce the concentration to practically zero. The bottom line is that ClayMonkey can go ahead and eat his soup with a 100 g/l salt concentration if he likes, but he's an idiot if he thinks he has to. Feel free to tell him that a Ph.D. chemical engineer said so!

Bon appétit!

Aaron

Classic. I get this a few hours later.

Hi Lisa,

After I sent you my earlier treatise on soup and potatoes, I realized that I could have been more precise with my argument that the potatoes do not take on additional water. Originally, I said:

>Well, we know the potatoes remove salt from the soup, because they
>end up tasting salty. But if they were to remove a proportionate
>amount of water from the soup as well, they would have to swell in
>volume considerably. That is simply not observed.

It would have made a stronger argument to have said:

Well, we know the potatoes remove salt from the soup, because they end up tasting salty. But if they were to remove a proportionate amount of water from the soup as well, they would have to increase in mass considerably, which would be accompanied by either a considerable increase in volume (like a balloon filling with liquid) or a considerable increase in density (like a dry sponge soaking up liquid). Well, these effects are simply not observed. Potatoes are nothing like balloons or sponges.

Now I feel better. I didn't want to leave any wiggle room for additional debate.

Chow (instead of Ciao, get it?)!

Aaron