What happens when you heat sugar?

Answer

Best,

if you heat sugar it will caramelize. This is a group of chemical reactions in which the sugar is actually broken down.

If you heat regular table sugar, it will first split into glucose and fructose (dextrose, the sugars in honey). These sugars are more reactive than regular sucrose (beet or cane sugar). Initially, these sugars will lose water (under the influence of heating). This results in degradation products that are even more reactive (ie more unstable) than the original sugars. So they are very sensitive to further reaction. As mentioned, these are quite complex. With some simplification, you could say that actually two things happen to those reactive intermediates:

(1) they actually react with themselves – a bit like a cat would bite its own tail…this creates cyclic molecules that will lose further water. Because the molecules lose water again and again, their polar character decreases (sugars are very polar, water-mining, water-soluble). This makes the molecules more volatile, thus creating components that evaporate faster – so they have a better chance of reaching our noses and indeed many of those molecules have a typical caramel aroma…

(2) the reactive intermediates can also react with other reactive intermediates and the resulting products also react further with other intermediates – the cat now bothers another cat’s tail and that other cat again another and so on… – this creates actually larger molecules; due to loss of water from these molecules, these molecules will be converted into substances that can absorb visible light and therefore acquire a color (brown). The latter may be a bit more difficult to understand, but you could compare it (from afar) with the browning of your meat when you put it in the pan (the color that arises here comes mainly from reactions between sugars and protein): you form large complex molecules that are able to capture visible light, giving them a color.

You can speed up the caramelization or push it in a certain direction (more colour, more aroma…) by eg adding some acid (lemon juice) or making it alkaline (adding carbonate). With the first you get color faster (try it: fry a piece of apple with and without lemon juice!), with the second fairly typical aroma components are formed.

Not easy, but it’s also quite complex chemistry…

Regards

Bruno De Meulenaer