The following question was written in response to my post “Heat transfer in sugar syrup.” It’s a great question but a complex subject. The following is a vastly simplified explanation, but I hope it begins to explain why the bees can eat cold sugar but not cold syrup. The comment follows:
The alternative [to sugar syrup], fondant, relies on the bee liquidizing the sugar with saliva, so presumably the substance is then at the correct temperature automatically—is that correct?
The answer lies in the different ways heat moves through liquid sugar and solid sugar. In fluids (both liquids and gases) convection plays a large part in heat transfer. Convection is the collective movement of molecules within a fluid. As molecules of fluid become warmer, they jiggle more, and the substance becomes less dense. The less dense portions rise to the top and the colder (denser) portions fall to the bottom. All this rising and falling of molecules causes the fluid to mix.
The syrup in your bee feeder receives warmth from the sun (the warmed atmosphere) and warmth from the bee bodies—both of which cause convective heat currents in the syrup. But since there is usually little heat and a lot of syrup, the heating process is slow. More convective currents will occur in a pail feeder than in a flat baggie feeder simply because there is more room for rising and falling of the molecules in a deep container than a shallow one. At night when the air temperature dips, any excess warmth will leave through the top of the feeders. More heat will be lost through a large surface area (baggie feeder) than a small surface area (pail feeder.)
Here’s a great irony about syrup feeders: Since the bees (a heat source) are directly beneath the feeders, the syrup may be warmer at the bottom of a shallow baggie feeder than at the top (because of little mixing and lots of heat loss) but the bees have to drink it from the top. Conversely, the syrup may be warmer at the top of a pail feeder than at the bottom (because of lots of mixing and little heat loss) but the bees have to drink it from the bottom.
Heat transfer is totally different in your fondant, sugar cakes, or candy boards which are solids. Instead of the warm molecules moving throughout the substance, the molecules stay fixed in one position and just the heat moves from molecule to molecule in a process called conduction.
Although it would take a long time for bee heat to warm the entire sugar cake, an interesting thing happens—since the material doesn’t mix, the surface of the sugar cake, especially that surface nearest the bees, becomes quite warm . . . warm enough to eat!
Mind you, it’s only the thin surface layer that is very warm, but the bees eat slowly and they eat from the surface. The rate of eating is slow enough, in fact, that the newly exposed surfaces have time to warm up before the bees eat them. Whereas in-hive convection warms your entire container of syrup slightly, in-hive conduction warms the surface layer of solid sugar substantially. What a system.