How to Thicken Jam: 5 Ways to Fix Runny Jam

Canning jam at home is a method of food preservation. Canning extends the shelf life of jams, jellies (made with fruit juice), and marmalades (made with fruit peels), but the process can be confusing for first-time canners, especially since there are no actual cans involved; instead, the jam goes in sterilized jars.

Sterilize the jars and lids in a boiling water bath, then fill the jars with the jam, leaving some headspace. Next, tighten the lid, and place the sterile jars back into the boiling water bath for ten minutes of processing. These jam jars will keep in a cool, dark place for many months and up to a few years. Learn more with this guide to canning for beginners.

Jam making is a trial-and-error process, even when using a specific jam recipe, and there are a few reasons for this. Here are some factors that can cause runny jam:

• Lack of acidity: Acids like lemon juice help lower the jam mixture’s pH, which reacts with the natural or added pectin to help thicken the jam. If a jam recipe doesn’t call for lemon juice or lime juice, it could rely on another ingredient, like added pectin, to set the jam. Adjust the recipe to cut back on added sugar or other thickening agents and add lemon juice. Acidity is still necessary for strawberry jam and apple jelly, even though those fruits are naturally high in pectin.
• Low cooking temperature: The temperature to which you cook jam significantly impacts its thickness. If the jam doesn’t reach a high enough temperature, the pectin or other gelling agents won’t set. If the temperature is too high, it will be too sticky and gummy. Ideal jam sets when taken to 220 degrees Fahrenheit. To test if your jam has set, place a small amount onto a plate and let it come to room temperature. If it keeps its shape, the batch of jam is thick enough to start canning.
• Not enough pectin: Some fruits have more natural pectin than others. So-called pectin fruits include apples, carrots, oranges, and grapefruits. Fruits very low in natural pectin include apricots, plums, blueberries, peaches, raspberries, and strawberries. The natural pectin in fruits and vegetables thickens jams. If there isn’t enough pectin in the fruit itself and you don’t add extra pectin, the result is runny jelly or jam. Additionally, if the fruit is overripe, its pectin levels are lower. Added pectin comes in a couple of forms. You can find powdered pectin and liquid pectin at grocery stores and online.
• Too little sugar: Much like pectin, some fruits are higher in natural sugar than others. Jams made from fruits with higher sugar content thicken on their own more easily than those with less sugar. A no-sugar or low-sugar recipe might not call for enough sugar to naturally thicken the jam, even at a rolling boil.

Whether you want to ensure a thick jam from the beginning or fix a runny jam, these techniques can help:

1. 1. Add chia seeds to the recipe. Chia seed jam is a method of making jam that requires no cooking. The chia seeds soak up the juice and moisture from fruit purée and plump up, giving a thickening effect to the jam without using heat or added sugar or pectin. Think of it as a chia seed pudding made with fruit rather than milk.
2. 2. Use cornstarch. Cornstarch is a common thickening agent for jams, sauces, soups, and glazes. It’s also a viable option for jam recipes with less added sugar or ones that feature fruits with naturally less sugar or pectin. One thing to keep in mind with cornstarch as a thickening agent is that it makes the jam a little bit cloudy or milky-looking. Mix cornstarch with water to create a slurry, then add it to the jam mixture. Bring it to a boil, and the jam should thicken almost immediately.
3. 3. Try commercial pectin. Commercial pectin helps thicken jam made with low-pectin fruits, like blackberry jam, or vegetable jelly recipes, like pepper jelly. It comes in powder or liquid form, but the former is the most common. Add powdered pectin to the fruit juice, chopped fruit, or strained purée before heating the jam mixture. Bring the jam to a rolling or hard boil (meaning you can’t stir down the bubbles) over high heat, and then add the additional sugar. For liquid pectin, bring the fruit juice or purée and sugar to a boil first, then add the liquid pectin.
4. 4. Use gelatin sheets or powder. While it can be tricky to work with, incorporating gelatin into the recipe is an excellent way to rescue runny jams. There are two ways to thicken jam with gelatin, but the easiest method is to soften sheets of gelatin in hot water and then add them to the jam as it cooks. Alternatively, dissolve powdered gelatin in warm water and add it to the still-cooking jam.
5. 5. Reduce it on the stovetop. If you’ve made jam and waited for it to cool, but it is still runny, pour the jam back into a saucepan and bring it back to a boil. Boil until the jam starts to reduce and therefore thickens.

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