For anyone who has ever tried to make it at home, perfect fruit jelly can seem like a truly elusive substance. When it works, you end up with a quivering solid of beautiful, translucent color and great fruit flavor. When it doesn’t, you get a runny (though still flavorful) syrup.
To make jelly, you cook crushed fruit with water until it’s soft and starting to lose its color, strain out the solids, and simmer the juice, adding sugar. Then you boil until the liquid reaches 220° to 222°F, or until it thickens enough to fall in a sheet off the side of a spoon, and pour into sterilized jars. (Another doneness test is to put a small amount of the hot jelly or jam on a chilled saucer, freeze for a minute or so, and then push the cooled liquid with your finger. If it wrinkles, it’s done.) Jams and preserves are less work; you just boil the fruit with sugar until the mixture thickens or reaches temperature.
Sounds easy (and it is); however, sometimes fruit jelly gels and sometimes it doesn’t. The key is to have enough pectin, sugar, and acid. You need all three elements in balance to get fruit jellies, jams, preserves, and marmalades to set. (To distinguish your jams from your preserves, check out a lexicon of these terms.)
Pectin is the thickener
The most crucial ingredient in all jellies and jams is pectin, which is made up of huge molecules that occur naturally in all fruit. The goal (and the challenge) of jellyand jam-making is to get these big pectin molecules to connect in a gel network, trapping and immobilizing the sweetened fruit juices within it.
Some fruit—tart apples, blackberries, cranberries—have plenty of pectin. Other fruit, such as peaches and apricots, don’t have nearly enough to gel on their own.
So the first step is knowing whether you have a high-pectin or low-pectin fruit. If it’s the latter, you’ve got two choices. You can supplement it with a commercial pectin, or you can add a high-pectin ingredient like lemon rind (be sure to include the pectinrich white pith) when you boil the fruit.
For all fruit, pectin levels are highest when the fruit is mature but still slightly underripe; pectin amounts start to drop off as the fruit continues to ripen. Therefore, you’ll get a better gel from almost ripe rather than from fully ripe fruit. Or you might want to use some of both, getting the higher pectin from the less ripe fruit and the more intense flavor from riper ones.
Acid is the matchmaker
Even better than adding lemon rind and pith to the fruit is to toss in a whole slice of lemon to a small batch of fruit. The lemon juice in the pulp provides acid, which is the second essential component of any fruit jelly or jam.
Without acidity, pectin molecules won’t build that crucial gel network. Pectin molecules are charged; they repel one another just like the same ends of magnets do. Acids neutralize the charge, so the pectin molecules no longer repel one another and can join.
Fruits that are high in acid and in pectin will gel on their own, while those with lower acid levels won’t. To compensate for low-acid fruits, try adding 1-1/2-teaspoons to 2-tablespoons fresh lemon juice to a small batch of jelly or-jam.
Fruits become less acidic as they ripen, so again, from a gelling standpoint, it’s best to choose those that are not fully ripe.
Some fruits gel better than others
Pectin content and acidity will vary depending on the ripeness, growing conditions, and variety of the fruit—berries are especially hard to pin down. The list below can help you figure out whether a batch of fruit jam or jelly might need a pectin or acid boost, but keep in mind that it’s an approximate guide.
Fruits with high pectin and high acid gel more; fruits with low pectin and low acid gel less.
High pectin, high acid
Tart apples, crabapples, cranberries, blackberries, gooseberries, lemons, red currants, Eastern Concord grapes
High pectin, low acid
Sweet oranges, tangerines, sweet apples, ripe quince
Low pectin, high acid
Apricots, pomegranates, strawberries, sour cherries, pineapples, raspberries, rhubarb
Low pectin, low acid
Peaches, nectarines, pears, blueberries, ripe mangos, sweet cherries, any overripe fruit
Sugar ties up the water
We have one more problem to solve before we can expect a perfectly set jam or jelly. Pectin molecules would rather join with water molecules than with one another. This is where the sugar comes in. Sugar is hygroscopic—it ties up the water, making it unavailable for the pectin molecules, which now have no choice but to connect with one another.
Most jam and jelly recipes don’t skimp on the sugar, so it’s unlikely that you’ll fall short in this area. I usually add 3/4 to 1 cup sugar for every 4 cups unreduced juice for jelly. If you add too much sugar, there’s a risk that it will crystallize out of the mixture or that the jelly will become too stiff. Occasionally, even when you think you have enough pectin, acid, and sugar in the jelly or jam, it still won’t set. You could try reboiling it; it may gel better the second time, and eventually, it will start to thicken through evaporation. But be aware that extended cooking (or excessive heat) can damage the pectin and prevent gelling altogether, though having enough sugar in the mixture should help.
One more tip for gelling jellies: Calcium increases gel strength and can help ensure that a jelly sets. The easiest kitchen source of calcium that I know of is molasses, so if you expect trouble with a particular batch of fruit, you might try adding a half teaspoon of molasses to a small batch of jelly.