Sweet and Salty

I've got a thing for things that are sweet and salty. Caramels with sea salt. French fries with plenty of ketchup. Peanut butter and strawberry jelly. Melon with prosciutto. Is there anything better to eat on a hot summer day than a ripe cantaloupe dressed with some cured meat, thinly sliced? I think not.

But why do sweet and salty sing so well together? Why do we add a pinch of salt to chocolate cake, or not fully taste the sweetness of a tomato until it's been sprinkled with sodium chloride? And why does bread without salt taste so bland?

The first thing to understand is how we perceive saltiness. It's an elegantly simple setup: sodium ion (Na+) channels are perched on the surface of the tongue. When sodium chloride enters the mouth, it dissolves in saliva and then directly alters the membrane potential of the taste cell. The end result is an influx of positively charged sodium ions, which depolarize the cell and trigger an action potential. The brain is told that we've tasted something salty.

Sweet is a more complicated taste sensation. It begins when a sweet tastant (glucose, fructose, sucralose, etc.) binds to a G-protein coupled receptor, which then triggers an enzymatic cascade. But the fast cellular process ends up in the same place, as the taste receptor cells start to depolarize. (That's a fancy way of saying they increase the amount of positive ions on the inside of their membrane. This accumulation of voltage is what allows them to pass on the electrical message to other neurons.)

But back to the salty-sweet conundrum. Why does melon and prosciutto taste so good? 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). While previous work has emphasized the role of potassium ions and calcium ions in the depolarization of sweet taste receptors, I wonder if sodium ions might also play a role. Perhaps, and this is a pretty big perhaps, the extra concentration of salt when added to a dark chocolate souffle or a 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. This would also explain why food without any salt is so hopelessly boring: it might be literally harder for our various taste receptors to get excited. Just a thought.

The other alternative is that the amplification of sweet by salt occurs downstream, at some later stage of sensory processing. Who knows? All I know is that I'm having a watermelon and feta salad for lunch.