Sugar, water and temperature
Candy making is the art of boiling a sugar syrup until exactly the right amount of water has evaporated. As water leaves, the sugar concentration rises and so does the boiling point of the syrup. Each candy stage, from soft ball to hard crack, corresponds to a concentration, and therefore to a temperature on a candy thermometer. That temperature, however, depends on the air pressure, which falls as you go up in altitude.
Candy stages at sea level
| Stage | Temperature | Used for |
|---|---|---|
| Thread | 223 to 235 °F | syrups, glazes |
| Soft ball | 235 to 240 °F | fudge, fondant, pralines |
| Firm ball | 245 to 250 °F | caramels |
| Hard ball | 250 to 265 °F | nougat, marshmallows, divinity |
| Soft crack | 270 to 290 °F | taffy, butterscotch |
| Hard crack | 300 to 310 °F | lollipops, brittle, toffee |
| Caramel | 320 to 350 °F | caramel sauce, spun sugar |
The altitude adjustment
At higher altitude, water boils at a lower temperature, and sugar syrups reach each stage at a correspondingly lower temperature. The usual rule is to subtract the drop in the boiling point of water from every target. That is about 1 °F for every 500 feet of elevation. The calculator computes the boiling point of water from the standard atmosphere and the heat of vaporization of water.
Worked example
You want to make fudge, which needs the soft ball stage, in Denver at 5,000 feet:
- Water boils at about 202.7 °F there, 9.3 °F below sea level.
- Soft ball at sea level: 235 to 240 °F.
- Adjusted target: 226 to 231 °F.
If you cooked to 238 °F as the recipe says, the fudge would be too firm and might turn crumbly.
Boiling point of water by altitude
| Altitude | Water boils at | Subtract |
|---|---|---|
| Sea level | 212.0 °F | 0 °F |
| 1,000 ft | 210.1 °F | 1.9 °F |
| 2,500 ft | 207.4 °F | 4.6 °F |
| 5,000 ft | 202.7 °F | 9.3 °F |
| 7,500 ft | 198.0 °F | 14.0 °F |
| 10,000 ft | 193.3 °F | 18.7 °F |
Test your thermometer
Thermometers drift. Before you start, clip yours in a pot of boiling water for a few minutes. If it reads 2 °F above the expected boiling point for your altitude, aim 2 °F higher than the adjusted target, and the reverse if it reads low. Weather matters too: a stormy, low-pressure day lowers the boiling point slightly.
Tips for success
- Use a heavy pan large enough for the syrup to triple in volume as it boils.
- Stir until the sugar dissolves, then stop stirring unless the recipe says otherwise, to prevent crystals.
- Wash down the sides of the pan with a wet pastry brush to dissolve stray crystals.
- Make candy on a dry day: humidity makes hard candies sticky and fudge slow to set.
- Confirm the stage with the cold water test if you are unsure of your thermometer.