A lot of web pages claim that sugar caramelizes at ~160C. But also a lot of recipes caramelize sugar in a simmering water. The boiling point of water is only 100C. So how can these recipes succeed so well in practice?

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    see this excellent video by Minutephysics on why there's no single melting point for sugar – Jorn May 31 at 10:40
  • Are you truly trying to caramelize the sugar or are you trying to melt the sugar to make caramel? – Joe May 31 at 15:07

a lot of recipes caramelize sugar in a simmering water.

Calling the solution "simmering water" isn't a good characterization. The boiling point of pure water is 100C. But the boiling point rises as the concentration of sugar in the solution increases. Once you're above 75% or so, the boiling point increases significantly. For 90% sugar (still 10% water remaining), it's up to around 120C.

As the water evaporates, the sugar concentration, the boiling point, and therefore the temperature all increase. When you reach 160C, there's probably less than 1% water and the decomposition of the sugar (carmelization) rate starts to increase significantly.

There are some charts/tables for different concentrations here, but they only goes up to 90%

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    Incidentally I started from solid sugar. It boils just fine. – Joshua May 31 at 16:51

Recipes that call for water to be added to the sugar for making a caramel do so to help all the sugar melt, by dissolving some or all of it in the water. This prevents premature crystallization of the sugar. When you keep heating the mixture, all the water will evaporate, at which point the temperature of the (now pure) molten sugar will rise above 100 °C and will go through its different cooking stages. The sugar will caramelize when it reaches a temperature of 160 °C.

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    A process sometimes described as “wet caramel”. “Dry caramel” heats the sugar crystals directly in the pot or pan. – Stephie May 30 at 17:21
  • But from my personal observation, the caramelization (coloring) starts before the water is entirely evaporated, which can be told from the volume change? – Fermat's Little Student May 30 at 23:09
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    @Fermat'sLittleStudent, why would you assume anything has a uniform temperature, especially when it has a heat source on one side (stove) and a heat sink on the other (atmosphere)? Here is a exercise to prove that a temperature gradient exists in your pan: Heat a greased pan on high until water will dance, add frozen meat, cook until lightly burnt. Notice that the meat is still raw in the middle and probably frozen. Same thing. – hildred May 31 at 0:16
  • @hildred Not quite the same thing, as in the sugar-water mixture convection helps to equilibriate the temperature, especially as the stuff is boiling. There will still be some effect, though. – LSchoon May 31 at 8:00
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    @Fermat'sLittleStudent for the same reason, I am a little sceptical of determining whether there is water left by change in volume, as the boiling mixture will expand some due to the bubbles. A much more accurate way to tell if the water has evaporated is to look at the size of the bubbles, which will change when the water is gone. – LSchoon May 31 at 8:01

While the other answers are right, it should be pointed out that caramelisation does not happen at 160°C - not only at that temperature, at least. Thermal decomposition happens as both a factor of temperature and time. In fact, it is possible for sugar to undergo thermal decomposition well under 160°C: Stella Parks over at Serious Eats does it at 150°C, and Harold McGee has done it at 125°C

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