Sweaty, smelly Saturday - review of odor processing from the nose

It's Saturday, and the heat has reached Orange County. MOH and I went out for our weekly trip to the farmer's market and it was blistering hot. It's not normally too hot in the OC, so when it spikes above 80, things get sweaty real quick. Needless to say, this week we will not be eating my favorite summer soups - no malabar spinach, no amaranth soup. It's stir fry and pasta this week!

This week my baby sister is visiting us and my parents have sent us some great homecooked viet food that's always a treat. My parents sent along dried fish, a gallon of honey (so good - it's really pure, granulated honey that tastes great with ANYTHING), tom kho tau (sauteed prawns cooked in its roe - sounds weird, but really delicious), and finally..... drumroll..... lemongrass tofu! After reading this list, you're probably thinking.. of everything listed, you're most happy about lemongrass tofu?!?! No, this tofu is amazing. It's very simple in principle - get tofu packed in water, pat tofu dry and slice thick squares of tofu. Then, deep fry tofu to a crisp (don't use tofu that's already been deep fried - frying tofu that's been sitting in water is more effort, but tastes far superior, trust me) and fill with stuffing of your choice. For me, I love a garlic-lemongrass sautee mixture, but tofu will go great with any type of filling (eggplant shallot mixture would taste just as good!).

This dish is very aromatic and lends to the fragrance of the lemongrass and garlic filling. To get the filling, my mom simply sautees garlic, lemongrass, and thai chilis, seasoning with a bit of salt. Then, when the tofu crisps are cool, you can split the tofu in halves and place filling. It's a very sharp, spicy smell, and it catches your interest right away. 

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So, how is smell accounted for in our brain? How does our brain tease apart the difference between the suffocating B.O. in a crowded gym versus the sweet smell of blueberry shortbread cookies? Extreme examples of two types of smells, but you get the picture.

Our ability to discriminate between various odors is dependent upon the different odorant receptors, or sensors in our nose that are sensitive to specific odor molecules (more comprehensive review in Mori et al., 1999). Mammals are equipped with as much as 1000 odorant receptors. Each receptor is specific for sensing a particular odor molecule. 

The odor molecules come in through our nose and bind onto different odor sensors on cells called olfactory sensory neurons (OSNs) along our olfactory epithelium in our nose. 

Each OSN is specialized to be specific to a particular odor molecule, whether it's the scent of a ripe habanero pepper or fresh-out-of-the-oven chocolate chip cookies. Furthermore, each OSN only has one receptor that codes for a particular odor. So what exactly happens with a complex odor, like what you'd experience at a bakery? BTW, there's this awesome bakery in Huntington Beach, the Donuttery, that has the BEST red velvet donuts. I would pass up a red velvet cupcake for one of their donuts. If you're stuck with the late night munchies, or just need to satisfy that sweet tooth, run here. They're open pretty late.

Lots of great concoctions, but red velvet (right hand side, fourth from back) is the best!

Imagine that you're actually at a bakery -  you have wafting scents of flavors (blueberry, strawberry, maple, and chocolate),  pastry varieties (donuts, bagels, croissants), and beverages (coffee, tea, milk). The different odor molecules (like blueberry versus coffee) bind onto their specific receptors on the OSNs. In turn, the odor signal received by the OSN travels to the first "smell" region of your brain, the olfactory bulb, where it terminates on the glomerulus, a specific cell structure that is responsible for acting as a gatekeeper for all the odor-information processed by your nose. The glomerulus acts as a relay center where it receives all the signals from your OSNs, bundles this information, and sends it up to the olfactory cortex, where all the information comes together for you to perceive particular smells. 

(this Science cover features staining of the olfactory bulb - the dots in blue are the individual glomerulus. The long lines stained are the connections that originate from the OSNs.The lines represent where odor information travels before it hits the glomerulus.)

While the actual storage and synthesis of olfaction-mediated memories are performed in higher brain regions like the orbitofrontal cortex and amygdala (see previous posting), our brain dissociates different odors by what combination of OSNs are activated in our noses. So the next time you're at your local bakery or hitting up your favorite food truck scene and you get a whiff of all your favorite scents, think back on how precisely your nose receptors can parse out different scents. Amazing, right?

Happy dining all! 

PS. Be on the lookout for my next posting: culinary experiment 1 - quinoa sliders > meat sliders? Stay tuned for the results! Granted, the poll will be conducted with an n of 3, results will be reported nonetheless!

References:

Mori et al., 1999. The olfactory bulb: coding and processing of odor molecule information. Science. 286 (5440): 711-5. 

Costanzo RM. 2005. Regeneration and rewiring the olfactory bulb. Chemical Senses. 133-4.