Science of Cooking: Ask the Inquisitive Cooks!

" What difference does altitude make in jam-making? "

Dear Anne and Sue,

When I make my jam, I cook it to 219° F (104° C), but when my friend in Calgary uses the same recipe, the jam fails. Why?

—Submitted by Brian in Victoria

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Hi Brian,
You've already discovered that temperature is a good test of gel point, but gel point also varies according to altitude. When a fruit/sugar mixture boils, water evaporates and the sugar becomes more and more concentrated in the syrup. Since sugar has a high affinity for water , it does a great job of attracting water away from pectin, so the pectin is more inclined to bond with itself. Eventually pectin molecules form the network that sets the jam.

If you aren't using commercial pectin, it's important to test that the jam or jelly has reached its gel point. The concentration of sugar required to form a gel is usually in the range of 60 to 65 percent. Since sugar concentration and temperature are related, the easiest way to determine if a jam or jelly has reached the gel point is to check the temperature with a candy thermometer.[JANET: Link to candy section?] A 60-percent solution boils at 217° F (103° C). So in Victoria (British Columbia), which is at sea level, gelling takes place when the boiling syrup reaches 217° F-221° F (103° C-105° C).

At higher altitudes, however, atmospheric pressure is less, so water boils at lower temperatures. This means liquids evaporate more quickly. The syrup becomes concentrated and reaches its gel point at a lower temperature. In Calgary, which is roughly 3000 feet (915 m) above sea level, gelling happens at about 214° F (101° C). So when your friend in Calgary boils jam until it reaches 219° F (104° C), it is overcooked. Too much water has boiled away, leaving a sugar concentration that's too high. The jam is likely to be gummy and sticky. Colors may be darker, and the texture quite tough.

For others cooking at high altitudes, note the temperature at which water boils at your altitude, and add 8° F (4° C) to determine the gelling temperatures. Or use the following as a guide:

Altitude		Gel Point
3000 feet (915 m)		214° F (101° C)
4000 feet (1219 m)		212° F (100° C)
5000 feet (1524 m		211° F (99° C)
6000 feet (1829 m)		209° F (98° C)
7000 feet (2134 m)		207° F (97° C)

Good luck!
Anne and Sue


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