Quite simply it’s a flowing mixture of sugar and usually at least a little water. That water is partly responsible for the fact that most syrups, despite their sugar concentration, don’t crystallize easily. But there’s usually something else in there that inhibits crystallization, a little something called invert sugar. Invert sugar is a term that gets tossed around quite a bit in cooking and baking circles, about as much as “caramelization” and “Maillard reaction.” But what is it exactly?
Basically, invert sugar is a mixture of sucrose (25%) and its two component sugars, glucose and fructose (75%). Invert sugar exists in nature but is usually made by humans for various culinary and scientific applications. You get it by making a mixture of sucrose (table sugar) and water, then heating it and adding an acid. In the kitchen that acid can be lemon juice, tartaric acid, vinegar or any number of others.
The effect of heat and acid on sucrose is that it splits the molecule into the above mentioned mixture, and once that’s done it not only won’t re-form as sucrose, it will remain a liquid. As for why, this is the point at which my own understanding starts to break down. All I know is that for some reason fructose won’t form crystals when it’s in the presence of both glucose and sucrose. Organic chemists in the audience, please feel free to weigh in here.
As for why invert sugar is called invert sugar, I do know that. Put on your physics hat and join me here.