Caramelization occurs at the melting point of sugar. When a sugar molecule hits the appropriate temperature, it melts. This is similar to ice turning to water above 32 F (0 C). It will take some time for all of a given amount of sugar to melt, but this is relatively insignificant compared to ice melting due primarily to the vast amount of heat involved to melt sugar. Once melted there is no need to maintain it at a specific temperature.
However, time is a factor in achieving the desired temperature for your melted sugar.
- At 356 F (180 C) you have light caramel - it's a pale amber to golden brown in hue
- From 356 F to 370 F (180 C to 188 C) you have medium caramel - this is a golden brown to chestnut brown hue.
- From 370 F to 400 F (188 C to 204 C) you have dark caramel - this is very dark, bitter, and smells a little burned - this is used for coloring only
- At 410 F (210 C) you have monkey's blood - it tastes like burning and the sugar breaks down to carbon
Another thing to be aware of is that caramelization is often mistakenly attributed to the browning of meats, nuts, or bread crust. This is actually an entirely different process called the maillard reaction which requires specific enzymes to be present, and occurs at different temperatures than caramelization.