About 15 years ago as a student I used to work at a fudge making store. I remember it was not quite as straight forward as boil, cool, sell.

There was a certain calculation to determine the correct temperature to stop boiling the fudge mixture. I know the general idea is 234°F or 112°C, but I can remember we had a humidity sensor reading, and that had to be used in a calculation to get to the correct or precise temperature to pour out the fudge mixture onto the huge marble table to start cooling and shaping the logs of fudge.

Is there someone out there who knows how this calculation works?

Here is a link with some good advice

2 Answers 2


I think you have it backwards. They weren't measuring humidity to calculate temperature. Rather, for home cooks, temperature is used to determine humidity.

In fudge making, the goal of cooking is simply to reduce the water in the sugar syrup to a specific concentration. Sugar syrups produce different hardness of candy at different concentrations.

When cooking other things- such as meat- the temperature is important because bad bugs are killed or meat proteins denatured at specific temperatures. Sugar, on the other hand, isn't changed at the temperatures used in candy making. Water is just boiling away. It's usually a bad thing when the temps get hot enough to start changing the sugar, ie caramelization.

Sugar syrups happen to boil at higher temperatures as they become more concentrated. Therefore the temperature can be used to determine the concentration. Temperature is much easier to measure than water concentration. Unfortunately, the boiling temperature of water changes dramatically depending on elevation. I moved from sea level to the mountains and the boiling point of water changed by 9 degrees F. That is a significant difference in candy concentration and I have to reduce the cooking time for my recipes.

In a commercial setting, it is worth the cost and effort to measure the syrup humidity directly. It's more foolproof. However, for home cooks, thermometers are cheap and easy to use. Therefore, most recipes are written in terms of temperature and not sugar concentration.

  • okay, taken. but just one other thing. say if the elevation between the Navada desert and Amazon rain forest where the same, surely the consistency of the fudge will be quite different because of the dramatic atmospheric humidity differences?
    – morne
    Commented Oct 25, 2016 at 9:15
  • As I understand it, the temperature of the syrup at a given concentration, and the thickness of the candy at that concentration would not change with atmospheric humidity. The fudge would soften quickly as it absorbed water from the air. Commented Oct 25, 2016 at 14:40

When making any old fashion candies the humidity controls if the candy sets up. No MATTER how long you cook. Humidity has to be fairly low when making old fashion candy. I just don't know exactly how low. I wait until it's down to 45% . And I have no problem.

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