Mad About Science: Smells, sugars and you

By Brenden Bobby
Reader Columnist

What do hydrocortisone steroids, dryer sheets and Febreze all have in common?

Corn cobs.

I’m sure you’ve wondered how aerosol air fresheners work. They’re advertised as trapping and eliminating odors, but how do they do it?

As we know from the law of the conservation of mass, matter cannot be created or destroyed, but it can change shape. Your air fresheners aren’t really destroying the molecules responsible for your smelly trash bin, but they are altering and covering them up.

First, let’s understand how smell works. Most everything has some kind of smell, and that comes from microscopic particles on the surface of the object breaking off and floating through the air, then being sucked into our noses. Smells often originate from bacteria or mold — this is why a plastic cup that’s been left out for a couple of days may smell musty. 

Residual bacterial colonies and mold propagate on the surface while bits fly off when subjected to changes in air pressure, like your sniffer coming close and drawing in air. Other organic materials produce gasses that make funny smells, like hydrogen sulfide, which smells like rotten eggs and is one of the primary reasons your farts stink.

Smell that?

We have specialized cells way up high in our noses called olfactory neurons, which are connected directly to our brains. Each of these has a smell receptor, which samples particles that get sucked into our noses and then sends that information to the olfactory bulb in our brain, which processes that information into something we can understand: a smell.

Our ability to smell is an important sense that helps us understand the world around us without using up too much processing power. It helps us taste and enjoy our food and avoid dangerous places that might contain poison or disease. 

Obviously, we prefer nice fragrances over stinky smells, but what tells us if a particular odor is good or bad?

There’s a lot of debate around this, and I can’t answer it with crystal clarity. It’s likely that we have some genetic memory carried over hundreds of millions of years of evolution that helps us discern good smells from bad ones. We’ve also learned to enjoy certain smells over others, with some perhaps even programmed into our DNA. 

Most things that cause disease, like unsanitary areas with a lot of bacteria, produce smells that signal to our brains that we can get an infection or die from exposure to them — likely one of the major reasons we don’t like the smell of feces or its aerosol sibling, the fart. 

That makes sense, but why is it that we love the smell of flowers?

I can’t tell you with absolute certainty, but it’s likely that many current cultivars have been adapted by humans to smell good for our benefit. The flowers that smell the best have the greatest chance of propagating new generations, so the plants are trained to evolve to suit us. Isn’t that weird to think about?

As far as we know, these flowery fragrances have been used to mask bad smells for as long as civilization has existed. These smells don’t eliminate the bad ones, but if people blanketed the stink with enough perfume, our olfactory neurons would only be able to pick out the prevalent aromas — most of the time.

That changed with the development of human-made aerosolized air fresheners like Febreze. The primary ingredient in these air fresheners is cyclodextrin, a complex sugar harvested from corn cobs. Cyclodextrin has some really interesting properties: it’s a starch that effectively binds to scent-producing molecules; it has a hydrophobic interior, meaning it avoids or repels water; and a hydrophilic exterior, which means it is attracted to and dissolves in water. 

When placed into a pressurized can with an abundance of perfume, it can be sprayed into the air to bond with stinky molecules and ignore the perfume. The cyclodextrin doesn’t destroy the stink, but it bonds with and weighs it down, so only the perfume can reach your olfactory neurons. 

This sounds great in practice, but cyclodextrin is not a permanent solution to a problem. Heat causes the sugar molecules to break down and can release bad smells back into the air. This is actually the premise behind your dryer sheets. Dryer sheets are manufactured with cyclodextrin to trap good smells within the sheet. As the sheet is exposed to heat from your dryer, the cyclodextrin breaks down and allows fragrant molecules to escape and blanket your clothes in a pleasant smell. This also means that your 20-year-old couch, which you’ve never cleaned, is one prolonged sunbeam away from stinking up your apartment.

Cyclodextrin is also used in some medications, including hydrocortisone steroids. The interesting nature of having a hydrophobic interior and a hydrophilic exterior means that it can release the steroids in a controlled or prolonged manner under very specific circumstances. This can prolong the use of medication and ensure less of it is just neutralized and discarded by your body and also make controlled doses last for a longer period of time.

And to think, all of that from a lousy corn cob.

Stay curious, 7B.