About a year ago I bought various titanium offcuts and had a go at the anodising trick. Electricity and a phosphoric acid solution let you turn the surface of titanium different colours, and the finish is very hard-wearing. "Rainbow titanium" gizmoes (like the pen I review here) are common these days.
The easiest way to get your hands on dilute phosphoric acid in this modern world is to use cola, since all cola contains phosphoric acid for flavour. Diet cola is preferable, since it's less sticky. I used Diet Coke.
There was plenty of cola left after I'd satisfied my curiosity about anodising, so I put the excess in a ground-glass-stoppered bottle. It's been sitting in the kitchen next to my radiometer, looking all sciencey, ever since.
There is, by definition, very little food value in a diet drink.
But something still, eventually, managed to grow in the bottle:
I'm not sure what's feeding the mould, but I presume it's the "caramel colour" that's number two on the Diet Coke ingredients list.
Ordinary caramel is just sugar that's been browned by heat, and obviously has plenty of food value; Diet Coke may have "less than one calorie" per can, but they're talking about the dietary "large calorie", which is quite a bit of energy. I think the "sulphite ammonia caramel" that's used in acidic soft drinks is much the same, energy-wise, as plain burned-sugar caramel.
If it were just the caramel, though, you'd think that the mould would have grown in the unsterilised bottle quite soon after I'd stoppered it up and left it where it could soak up the morning sunlight every day.
The sunlight may have something to do with why the cola is the colour that it is, too. It's much paler than it was when I first bottled it, and I noticed the colour change long before I noticed any mould.
I suppose the acidity of the cola could have retarded mould growth. Perhaps the breakdown of aspartame into its constituent amino acids (due to the action of the acid, and possibly the sunlight again) had something to do with it.
Or maybe Aristotle was right.