How do Invert Sugars deter crystallization in Sucrose solutions? Invert Sugars, by themselves, are notorious for their reputation to crystallize fast (think honey), but their addition to solutions of Sucrose (think simple syrup) deters the crystallization process. Take Caramel, Sucrose that has already decomposed into Fructose and Glucose and subsequently into the numerous products of caramelization: It crystallizes or, at least, turns grainy without the addition of Invert Sugars or a weak acid (Citric, Tartaric) (although the crystallization process is retarded, taking days even at low temperatures). Why do Invert Sugars deter crystallization when added in small quantities in solutions such as Caramel, which, I think, do already contain Invert Sugars as they are already largely decomposed? I would like to know of how Invert Sugars physically interact with the other sugar molecules and prevent them from coming together, even though they're not really resistant to crystallization (as pointed out above).

An additional (and optional) question: Why does the rate of cooling of a sugar solution affect its crystallization? So, while a slowly cooled solution crystallizes more, a faster cooled solution is more likely to crystallize less spontaneously.
I would like to know of how Invert Sugars physically interact with the other sugar molecules and prevent them from coming together


Glucose and fructose molecules (being essentially parts of sucrose) can merge into sucrose crystals during their formation, slowing or even completely stopping their growth in certain directions (if their concentration is high enough).


  even though they're not really resistant to crystallization (as pointed out above)


Glucose and fructose crystallization process is much slower than that of sucrose. From confectionery process's point of view, once the fast growing sucrose crystal structure is disrupted and its growth hindered no crystallization occurs.


  Why does the rate of cooling of a sugar solution affect its crystallization?


Because crystals' growth takes some time. If cooled fast, neither existing crystals will have a chance to grow, nor new seeds will appear. If cooled really fast, saturated sucrose syrup can even turn into glass with no crystals at all.

[literature]: Sgualdino, G., Aquilano, D., Vaccari, G., Mantovani, G., &amp; Salamone, A. (1998). Growth morphology of sucrose crystals. Journal of Crystal Growth, 192(1-2), 290–299.