To make TikTok fruits 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 105°C, 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.
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.