Sweet and Salty

Over at the Times website, Harold McGee takes a question on salt and baking:

Q: Is there any truth to the old cook's adage that adding a pinch of salt brings out the sweetness in sugars? If so, can you please explain the science behind it?

Harold McGee replies: I'm not sure that salt makes sugar taste sweeter, but it fills out the flavor of foods, sweets included. It's an important component of taste in our foods, so if it's missing in a given dish, the dish will taste less complete or balanced. Salt also increase the volatility of some aromatic substances in food, and it enhances our perception of some aromas, so it can make the overall flavor of a food seem more intense.

I don't want to quibble with Harold McGee, as I'm such a big fan, but I think there's a better explanation for why salt makes sweet things taste a little bit sweeter. Although the sweet taste receptor seems to be a G-protein coupled receptor, there's also evidence that applying a sodium-channel blocker (TTX) can dramatically inhibit the activity of all taste receptors, suggesting that sodium plays a key role in the cellular detection of every taste (and not just the taste of salty things). Perhaps, and this is a pretty big perhaps, the extra concentration of salt when added to a dark chocolate souffle or a slightly bitter caramel makes it easier for the sweet taste receptors to fire an action-potential, since there are more sodium ions floating around the apical membrane. If so, that would explain why pastry chefs always add a pinch of salt to bring out the fullness of the other flavors. Just a thought.

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I'd agree that a bit of salt tends to intensify most flavors. I'm not sure about salt making foods actually taste sweeter, but it sure does improve the overall taste of some sweet foods. I'm particularly fond of a bit of salt on a slice of fresh cantaloupe and on chocolate ice cream.

That's funny you mention cantaloupe. I always thought I was strange as a kid for adding a touch of salt to summer melons. But then I went to Italy and realized that the Italian custom of serving cantaloupe with prosciutto is basically another way of using sodium to exaggerate the sweetness and flavor of the fruit.

I like the idea that the filling-out of flavor by salt is a product of increased extracellular sodium and membrane excitability. One could presumably figure out how much extra sodium is necessary to increase the excitability of gustatory neurons and then see how well this corresponds to the pinches of salt we add while cooking.

If you subbed KCl for NaCl, would you predict a dampening of our sense of flavor? If that worked, it would suggest that wonderful culinary alchemy of salt all boils down to the modulation of the resting membrane properties of primary sensory neurons. Salt substitutes contain KCl. Do they dampen flavor?

This is something I'd always wondered about but hadn't really thought about mechanistically, so I thought I'd and some specificity to your argument. TTX is widely used in electrophysiological research to block action potentials because, as you mention, it blocks voltage-gated sodium channels. Its action and these sodium channels, however, are not restricted to taste cells but apply to all neurons throughout the nervous system, since the sodium channel is essential for every single action potential. It's not surprising then that TTX would also inhibit taste cell firing. So while I think your explanation that sodium makes it easier for taste cells to fire an action potential could be correct, your invocation of TTX clouds up the picture. Perhaps a more accurate mechanism for your explanation would be that when sodium passes through ENaC channels on fungiform taste buds, this lowers the threshold for firing when the same cell's T1R (GPCR) sweet receptors are activated. The situation is obviously a lot more complicated with variants of all these channels and overlapping expression patterns and such, though. On the other hand, it could also be something more along the lines of what McGee says, which is that it is a perceptual effect. Bypassing everything happening in your mouth, once the signal for sweet reaches your brain the simultaneous salty signal could provide contrast to the sweet signal, making the sweet one appear sweeter in relation. In any case, whoever thought to put these things together, like chocolate and peanut butter, was a freaking genius.

Salt is very important in bread; bake two batches of bread, one with the normal pinches of salt, the other without any salt. Unsalted bread is bland, tasteless.

I was taught to add most of the salt late in the preparation, at the last rising, since the yeast doesn't thrive as well in salty dough.

Is it the subtle contrast that makes the food more interesting and palatable even if the trigger isn't identifiable? Sweet can be quite bland, like Arthur's unsalted bread. We need that bitter chocolate sauce. Add a bit of cayenne or hot sauce to a cheese dish and the cheese becomes more intense, but no one will guess it's from cayenne. And then there's lemon juice making an appearance in numerous desserts. Taste receptors are akin to the rest of our receptors, and being lulled means being dulled.

Salt is very important in cooking. It's like the salt of the earth.

By Mustafa Mond, FCD (not verified) on 15 Aug 2008 #permalink