The sensory pleasure of a good soup represents a complex synthesis of many molecular and cellular inputs. Here’s a taste of the action.
The first major snowstorm of the winter has hit the Northeast, which has put many of us in the mood for a warm bowl of chicken soup. The sensory pleasure of a good soup at this time of year represents a complex synthesis of many molecular and cellular inputs. Let’s take a look at the state of the science of eating that satisfying bowl of soup.
Dipping into a steaming bowl of chicken soup on a winter’s day doesn’t always bring the pleasant sensation that one expects. Most of us in our first attempts to prepare a soup from scratch end up with a disappointingly bland mix. Fortunately, that is an easy mistake to fix with a dash or two of salt. That brings us to our first molecular sensing player: an ion channel known as ENaC (an acronym for Epithelial Sodium Channel) mediates the pleasurable sensation of the right amount of salt (or more precisely, sodium chloride) in the soup. However, too much of a good thing definitely applies to salt in the soup. I can recall sitting in a diner and taking a spoonful of a perfectly good-looking and –smelling bowl of soup, and feeling like I took a gulp of seawater. Salt at high concentrations evokes an aversive response, which curiously is not well understood at the molecular level, but appears to be mediated by the cells of the taste bud that respond to sour or bitter flavors.
Just the right amount of salt is a necessary component of a good winter soup, but it is not at all sufficient—who wants to warm up with a nice bowl of salted warm water? The flavorful broth that the salt complements turns out to activate another component of the sense of taste, the sensation of savory (or umami, as labeled by the Japanese scientists who first characterized it). The molecules eliciting the umami sensation include monosodium glutamate (MSG) and nucleotides such as guanosine monophosphate and inosine monophosphate. These umami flavorants are detected on taste cells by a molecular sensing receptor (from a class known as G protein-coupled receptor, or GPCR) consisting of two subunits, T1R1 and T1R3. Interestingly, these umami flavorants are not enough to generate an umami sensation. An aroma, perceived by the olfactory sensors of the nose, is required as well, which may explain why eating chicken soup when you have a cold, with blocked sinuses, is something you do only when your mother tells you to.
As Goldilocks experienced with the Three Bears’ porridge, a rather narrow range of warm temperatures is needed for full satisfaction from our winter soup. And it turns out that another ion channel—actually a group of ion channels known as TRP (short for Transient Receptor Potential) channels—mediates temperature sensations in the mouth and skin. Two channels, TRPM3 and TRPV1, are activated at painfully hot temperatures. In fact, TRPV1 can also be activated by capsaicin, the molecule that delivers the spicy hot kick of chili peppers. On the other hand, TRPM8 is activated by cool temperatures, and also by menthol, whose minty cool sensation characterizes throat lozenges, Ben-Gay and mentholated cigarettes. Not such a good sensation for our soup though. And for the “just right” warmth, TRPV3 and TRPV4 appear to be activated at warm temperatures that do not reach the pain threshold.
As many of the molecular mechanisms of our soup sensations were only elucidated in the past 15 years, a number of unanswered questions still puzzle the scientists of the senses. One sensory input missing from the discussion above is that of texture (or mouthfeel, as the wine connoisseur would say). Some folks can warm up with a cup of broth, but most of us need some chunky meat and/or vegetable in our perfect winter soup. Virtually nothing is known about the molecular sensors of texture in the mouth, although a pair of ion channels known as Piezo1 and Piezo2, which mediate mechanosensation of skin and bladder, appear to be good candidates. Fortunately, such detailed mechanistic knowledge is not at all necessary to enjoy the perfect bowl of soup on a snowy winter day. Bon appetit!