快猫短视频

MAKING jam is a great way to enjoy a glut of summer fruit. Sugar preserves work rather like a salt cure, with the high concentration of sugar drawing water out of microbial cells. Because sugar molecules are heavier than sodium and chloride ions, you need a lot more sugar to do the job. That is why jams are often made with a roughly equal weight of sugar to fruit.

The moist yet solid consistency of jams and jellies comes from the physical structure of a gel, a sponge-like network that traps water molecules in separate pockets. The network is made from pectin, a polymer made of long chains of sugar molecules, found in plant cell walls.

Fruits such as quinces, apples and citrus fruits are high in pectin. For other fruits, you can add pectin to ensure that the jam sets 鈥� there is even a special jam sugar containing pectin for this purpose, though adding an apple to a batch of low-pectin fruit also works.

To make the jam, start by heating the fruit gently with a little water to soften it. As it is heated, the pectin chains come loose from the cell walls and dissolve in the fluid released. In water, pectin molecules become negatively charged and repel each other, so they need help to join together into a gel network. Adding sugar when the fruit is cooked aids this: it helps the gel to form by attracting water molecules to itself, so the pectin molecules are more exposed to each other.

Acid released when the fruit is heated neutralises the negative charges, allowing the pectin chains to bond. Bringing the mixture to the boil to evaporate water also helps bring the pectin molecules closer together. Some recipes call for lemon juice to be added at this stage, which provides additional pectin and acidity.

The most challenging part of jam-making is knowing when to stop cooking the mixture and pour it into jars. Too early and the mixture won鈥檛 have 鈥渞educed鈥� enough for the jam to set; boil it for too long and the pectin breaks up too much, stopping it from becoming jam.

One way to tell is to use a thermometer: the more water that is evaporated, the hotter the mixture gets. When it reaches 105掳C, this indicates a sugar concentration of about 65 per cent, normally about right for the pectin molecules to join together (though that does depend on acidity and other factors). Another way is to chill a saucer in the freezer, then place a blob of jam on it. If the surface of the jam mixture 鈥渨rinkles鈥� when you poke it with your finger, the jam will set and you should stop cooking.

If it fails to set, it may be because there wasn鈥檛 enough good-quality pectin in it, or that the pectin was damaged by prolonged heating. Or perhaps the mixture lacked enough acid. Reboiling it and adding liquid pectin or more acid may help.

For other projects visit聽newscientist.com/maker.