Why does sugar not grow mold?

Q_Goest

Ok, here's a quick question (or maybe not so quick). I was making coffee this morning and brought supplies in to work, one of which is sugar. The last bag of sugar is almost gone and has been sitting here for quite a few months now, but it's still just as clean and pure as the day I brought it in.

Why doesn't sugar grow mold on it or decay in any way? I have to believe there are microbes in the sugar, the same as those that lay on any surface or drift through the air and land on food. Why doesn't sugar decay?

Ryan_m_b

Why would you expect it to? A bag of sugar pretty much only contains sugar. Without access to all the other nutrients, minerals and biomolecules that organisms need they aren't going to thrive.

Andy Resnick

Interesting question. Refined sugar is almost pure (99%) sucrose, which can be broken down by mold and yeast. There have been studies of bacterial contamination:

http://onlinelibrary.wiley.com/doi/1...668.x/abstract

And apparently refined sugar is very sensitive to mold:

http://www.tis-gdv.de/tis_e/ware/zuc...el/wuerfel.htm

My guess is that your sugar stayed dry enough so that extensive contamination couldn't occur.

Borek

Most microorganisms require water to survive. Water on the surface of sugar crystals creates a high concentration syrup with very high osmolality, not easy to survive is such conditions. Google plasmolysis.

Ygggdrasil

Indeed, the answer has to do with osmotic pressure. Highly concentrated solutions of sugar prevent microbial growth because they essentially dehydrate the microorganisms that try to grow in them. I regularly make 50% (w/v) solutions of glucose in lab, store them at room temp, and never see any microbial growth in them.

Q_Goest

Thanks for the responses. Strangely, I think we've all seen mold grow on something as non-nutritious as gypsum board if it is slightly damp, so I guess it has to have something to do with a lack of water and the osmotic pressure explanation seems like a winner!

Pythagorean

Gypsum board is actually "nutritious" to sulfur bacteria and their consumption of it results in the release of hydrogen sulfide:
http://www.odh.ohio.gov/~/media/ODH/...ulfideh2s.ashx

The bacteria that grow on the gypsum may then become food for another organism (or other organisms may also find nutrient content in the gypsum board)

In general, if it's growing there it has nutrition. Nutrition can also come from contaminants (like pollen) finding their way into the material that humans usually don't notice.

epenguin

I was going to say like Ygggdrasil nigh osmotic pressure. I have never dared ask but always guessed that was the reason why mold is quite slow to grow on jam or marmalade where nutrients are surely not lacking.

DaveC426913

I thought it would be simpler to Google dessicant, which I believe sugar is.

Borek

Sucrose is slightly hygroscopic, especially in a powdered form, but it would be not my dessicant of choice. There are many much better substances. And I still think plasmolysis is a better key word here - it precisely shows what will happen to the cells in contact with the concentrated sugar solution.

DaveC426913

Yeah, it's not really useful, but I think it's still the principle as to why critters won't grow in it.

Hypercharge

The same is it with salt, but salt is even more hostile to life.
In general, dry food is very unlikely to grow mold because it's lacking one VERY important thing: Water.

DaveC426913

Not to put too fine a point on it, but - it's not that water is lacking, or that sugar/salt are dry - there's plenty of moisture available in the air - it's that desiccants absorb and lock up that moisture in such a way that bacteria and fungus can't retrieve it.

vampares

I have seen soft drink syrup that was spieled rot into a fair dumpster like nastiness. In this case there is phosphate chemicals, which elicit mold and mildew, ect.

I have grown anaerobic yeast to a point of forming a dark "mold" formation over the course of ~10 days. The "fuel" was entirely confectioners sugar. There was water and I did add some minerals and nutrients: b12 w/ Ca phosphate, squalene, zinc...there was water of course.

Edit: And a bit of proteins. and KCl.

Darwin123

No liquid water.
Dissolve some sugar in liquid water and expose it to open air for a few weeks. Replenish the water that evaporates every few days.
What happens next depends on what microbe from the air first colonizes the sugar solution. Insects may get to the sugar solution first. However, something will grow in the water.
Some molds will both grow and float on the surface of a sugar solution. Leave a sugary drink open to the air outside the refrigerator for any length of time. That drink will go bad.