The Chemistry of Quince Jelly
Look, new quince jelly labels!
I’ve been looking forward to using my new quince jelly labels on these cute little jars, and here they are. I made 5 jars (keen students of arithmetic may notice that there are only 4 jars in the photograph above, but that’s because we’re already eating one). The 5 jars came from 6 quinces, whereas last time I got only 2 jars from about 15 quinces…but the latter were home grown and smaller and I had to cut out some rotten parts. For the current recipe the quinces came from Abel & Cole rather than from our tree.
Cut up quinces (whole fruit; not peeled or cored)
Boil in pan of water till soft
Mash in pan
Pour hot quince mash into sieve and leave to filter overnight into a bowl
Return filtered liquid to pan and add lots of crystallised honey
Heat it up while mixing to dissolve the honey
Bring to the boil then leave to simmer
Wait for it to start turning red and setting
Wait even longer
Go and look it up on the internet (see “The Chemistry”, below)
Squeeze in some lemon juice, keep stirring
Add citric acid, keep stirring
Finally! It’s starting to set
Hooray it’s setting!
Pour into jars and leave to cool.
Add lids and labels.
The ingredients are: quince, honey, water, lemon juice, citric acid.
The recipe that I followed called for quinces, sugar and water. I made several changes, especially substituting honey for refined sugar, and I wondered whether this led to its slowness in setting. But from some hasty internet reading, that wasn’t the problem. Apparently any kind of sugar will work, provided you stir the sugar and the pectin together in an acidic environment. So I added a squeeze of lemon juice, and then some citric acid, and lo and behold it did reach setting point. Maybe it would have set eventually anyway, but the acid probably accelerated the process, and it also gives the quince jelly a nice tang.
Further investigation into the magic of pectin has told me the following, which I’ll record here since I’ll probably have forgotten it again by next quince season.
Pectin is a polysaccharide found in fruit - it’s part of the plant cell wall and is released when the fruit is boiled in water. Different fruits have different levels of pectin. Quinces are high in pectin, as are some other sour fruits like unripe apples and citrus peel.
The internet led me to the rather brilliant website Compound Interest, which turns out to be the creation of a local chemistry writer here in Cambridge. It shows this informative poster:
The way I picture it, you have your long pectin chains dissolved in the boiling water, and the pectins have branches which need to join up to hold neighbouring pectin chains together and form the 3D molecular network which makes the gel. But the branches attract water molecules, which stop them bonding together, and they are also slightly negatively charged so they repel each other. Sugar addresses the first problem, by drawing the water molecules away from the pectin. And acid fixes the second problem, by protonating the negatively charged groups so that the strands of pectin no longer repel each other, allowing them to bind together as a springy web. Spaces between the pectin strands hold pockets of dissolved honey and quince molecules, which give you a sweet aromatic hit when you eat the jelly.
And so the flavours of summer (the flowers found by the bees) and autumn (ripe quinces) are captured together in these little jars of glowing red.