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.
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.
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.