This question comes in some form every year, and it’s always worth answering because the answer is, well, fun. Any time you set out to buy dried or candied fruit you can count on seeing labels that say things like “Naturally Unsulphured!” which has to be good, right? But then what is “sulphuring” anyway, and is it really a bad thing?
The process of sulphuring has to do with plant enzymes. Enzymes are and always have been a major headache for fruit eaters, since they cause picked fruit to brown and soften. And that’s a bummer if you’re the type of person that likes to make picked fruit last. Humans have expended lots of time and energy over the millennia trying to figure out ways to shut enzymes down (even if they didn’t realize what enzymes were). But then what are these browning enzymes doing in fruit in the first place? The answer is because they’re a key part of a plant’s natural pest control system.
Imagine for a moment an ant biting into a nice juicy pear on a tree. Once the cell walls in the pear’s flesh rupture, several very interesting things happen. First, ethylene gas, an anesthetic that doubles as a plant hormone, is released to make the ant dopey and slow, and trigger accelerated ripening. Next, an enzyme called polyphenol oxidase — normally kept within special structures inside the cells — starts to mix and bond with molecules called phenols like chemicals in a glow stick. As these compounds continue to react and combine with oxygen, brown pigments are created. The fruit flesh also starts to soften as cell walls start to collapse. The whole chemical cascade is designed to cause the fruit to shrivel and fall off the tree in a hurry, thus keeping the ant infestation from spreading.
Of course browning and softening also happen as a result of normal ripening. And while human beings love fruit, our appreciation of bruised, mushy fruit flesh only goes so far. Which is why lots of different schemes have been developed over the years to defeat the whole process.
Heat is a great way to destroy enzymes. The problem there is that heat cooks fruits flesh and turns it to mush. But what if there was a way to stop enzymatic action without exposing fruit to heat? Enter sulfur dioxide, a gas which prevents browning by bonding to the phenols in the fruit’s flesh before the enzymes get a chance to do it. This is what “sulphuring” is.
Is it artificial? You could argue that the process is artificial, though sulfur dioxide is a compound that abounds in nature. It’s created any time something containing sulfur is burned. Volcanoes produce a lot of it, so do forest fires. But then so do cars, since gasoline contains a fair amount of sulfur. And then I suppose when you really get down to it, sulfur dioxide is one of the main components of acid rain, since it bonds to water vapor in the atmosphere to create sulfuric acid.
Is sulfur dioxide toxic? Definitely not in the miniscule amounts that are used in processing fruit. The one drawback to it is that depending on how liberally it’s applied, it can give dried and candied fruits a faintly chemical flavor.