Canning 101: How to Use a Thermometer to Achieve Set

February 18, 2014(updated on October 3, 2018)

three thermometers

We are currently smack dab in the middle of marmalade season. Though citrus is available all year round, it is both at its peak and most affordable during January, February, and March. Because of this, I’ve been getting a number of questions about marmalade making, in particular, the art of using a thermometer to determine when a batch of marmalade has reached its set point.

The reason this comes up more during marmalade season than other times of the year is that citrus is naturally high in pectin and so many marmalades can be made without the addition of any commercial pectin. The trick then becomes cooking the fruit and sugar combination to around 220 or 221 degrees F, which is known as sugar’s gel point.

When the sugar reaches that gel point, it undergoes a physical transformation and thickens. That increased thickness gives it the ability to bond with the natural pectin in the citrus and create a thick, spreadable marmalade.

thermometer probes

The issue that people are having is that they are finding a mismatch between the temperature that their thermometer is displaying and the consistency of the cooking marmalade. Typically, the marmalade appears far more cooked than the temperature on the thermometer read-out would indicate. The result is a burnt, overset preserve that is deeply frustrating, given how much work is involved in prepping a batch of marm.

There are two reasons that this can occur. One is that the thermometer is giving a faulty reading. The way you can test to determine whether your thermometer is reading accurately is to bring a saucepan of water to a boil. Once it starts rolling, insert the thermometer into the water. If you’re at sea level, it should read 212 degrees F. If you’re at higher elevations, that rolling boil will be achieved at lower temperatures. If the reading is wildly different from that which your elevation would indicate, get yourself a new thermometer.

thermometer probes with notes

The other reason that your thermometer might not be reading accurately is that is may not be be sufficiently covered with the cooking preserve. Every thermometer has a mark indicating how much the probe must be submerged in order to give a true reading. As you can see in the picture above, the three thermometers in my kitchen all need to be submerged to different depths in order to perform accurately.

If you’re making a small batch of marmalade, you sometimes run into a situation where there’s just not enough volume in the pot to fully submerge a traditional candy or deep frying thermometer (I often run into that problem with the left and center thermometers). In my case, I deal with that situation by using the Thermapen on the right or by using other methods to check my set.

Try the plate/saucer test or if it’s a truly small batch, use your eyes and ears. As it reaches the set point, marmalade will simmer more vigorously. As you stir, watch to see if it is leaving an open space for a moment after you pull your spoon through. That’s a sign of thickening as well.

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12 thoughts on "Canning 101: How to Use a Thermometer to Achieve Set"

  • Gah! I was *just* struggling with this an hour or so ago while making a batch of kumquat marmalade. My digital thermometer definitely worked better than the traditional candy one, and I’m getting better at spotting set by eye. Thanks for the incredibly timely post!

  • Interesting! I’d made orange marmalade last week and it was more orange marmalade syrup. I was impatient and only got the temperature up to 214. I’ll have to try again!

  • I talked about that issue in my last blog post! Thanks for the heads up, will definitely be testing the thermometer I used.

  • I’ve had very uneven results with and without added pectin until I started using Christine Ferber’s method. She never uses pectin and cooks all kinds of low and no-pectin fruits to 221˚. It takes time and patience to wait until that temp but it works every-single-time!

    I also have to recommend her method of simmering a maceration and letting it develop in the fridge overnight before continuing to boil it up to full temp.

    I love my Thermopen too!

  • I had no idea there was a point on the thermometer to submerge to get a true reading! I feel so stupid. That may be the reason my vanilla-pear jam came out as cement

  • When reading a recipe that gives a set-point temp that needs to be reached, how does one know if that needs to be adjusted for elevation? Or does it need to be?

    1. The set point doesn’t need to be adjusted, it can just be a lot harder to reach the set point at higher elevation.

  • My daughter made marmalade using preserving sugar with added pectin rather than jam sugar. The marmalade is very tasty but very thick and impossible to spread. Can I rescue this in any way by re-boiling it and adding extra liquid? I’d really like to know please if this would be possible as it is such a waste – she made eight jars! Thank you.

  • I make marmalade every year at this time, my son gave me a new recipe which involves boiling the oranges whole.

    I did this and followed the receipy to the letter.

    The result is that the marmalade has gone rock hard in the half litre Kilner jars, is there anything I can do to soften it up? i managed to get some out of a jar and it tastes quite superb !! TG