Expert: Dana Krempels, Ph.D. - 6/10/2007

Question
Hi:

I notice that many fruits [excluding apples] emit foul odors when ripe. What chemicals are responsible for this? I've done as much research as I can on this but not gotten anywhere. This isn't a homework assignment. I am asking these questions out of personal interest.

I hate those odors. That why I like to eat apricots, peaches, and similar fruits when they are sour, hard, and greenish. When sour, hard, and greenish, most fruits smell pleasant. When they are too ripe, they become excessively sweet, grossly-soft up and turn mucus-yellow; this is when they start to stink.

What causes those immeasurably-foul odors?

It could not be putricine. Putricine smells like rotting flesh, which is also a foul odor but totally different from that of ripe fruits. To my nose, over-ripe fruits don't have a smell that even nearly resembles rotting flesh. Both are equally bad odors, though.

Its also not ethylene - a chemical used to speed ripening. Ethylene has a sweet pleasant smell to it. I have smelled it myself in a lab. It's beautiful.

Butyric acid smells like stinky cheese [including Swiss], smelly feet, sweaty shirts, dirty socks, neck-sweat, back sweat, filthy scalp and unwashed hair. So it definitely isn't butyric acid. In fact, since these foul odors occur after ripening [a process which uses up the acids]; I doubt that any acid or acidic substance is responsible for the foul odor of ripe fruits.

I notice the stink especially in canned fruits. Most fresh fruits don't have as much of a strong stink even when ripe. However, canned fruits [often dripping in syrup] have an unbearable stench to me. Maybe it is something to do with the sugar? I don't know.

Why do canned ripe fruits stink more badly than fresh ripe fruits?

Also, it can't be ethanol. I like the smell of ethanol.

I've asked similar questions in science newsgroups, and they think I have an olfactory perception disorder causing me to perceive odors differently from other humans. I don't believe this at all.

I have tried tiresomely searching on google but there are no websites that have an answer to my question.

Also, I've noticed that most ripe fruits do not have to be rotten in order to give off the foul odors I sense. Simply being ripe causes the odor.

Any assistance is greatly appreciated.

If this is out of your expertise would you please give me an idea of who could answer my question?


Thanks,

Green

Answer
Dear Green,

Interesting question!  

I don't think you have an olfactory perception "disorder."  But I do think you may be much more sensitive to certain chemicals or chemical combinations in ripe fruit than the average person.  My guess would be that the chemicals that you find so disgusting are a combination of very slight traces of butyric acid and perhaps ketones.  Though you enumerate a list of chemicals associated with particular smells, these "typical" smells can change with concentration and in the presence of *other* chemicals.

The most interesting question might be:  why do you find these smells so repulsive, when most other humans don't?  It's certainly adaptive to be able to smell spoiled food, and it could be that you have inherited the evolutionary baggage of very picky eaters who just couldn't be fooled by food that was a little bit spoiled, and hence more risky to eat (since spoiled food harbors not only bacteria, but potentially dangerous things such as ethanol).

The only way to know for sure which of the components of the extremely complex chemical brew comprising the smell of ripe/rotting fruit would be for you to find a willing organic chemist who could let you whiff a few test tubes filled with some of those components until you recognize what's gross.  Because that is probably something unique to you.

If it's any consolation, I don't like the smell of overripe fruit, either.  Ripe bananas are disgusting.  Once they lose their green edges, they're just mulch to me!  ;)  So you're not alone.

Hope this helps!

Dana