My father-in-law is very fond of the sarcastic suggestion ‘how about a glass of water and a quince?’ to any grandchildren having a whinge about being hungry.

It’s quite apt, actually, as we have the most wonderful old quince tree in our back yard. At a guess I’d say it could be 50 years old, perhaps more. After a spring featuring delightful tea-rose-like blossoms (similar examples can be seen here in an unrelated blog), each summer the tree fills with an enormous crop of beautiful quinces. Initially pale green, they ripen to a dull yellow, and are covered in a fine layer of fuzz which can be rubbed off. When cut, the flesh is creamy light green, with deep red seeds.

My favourite way to cook quinces is a la Stephanie Alexander: peel, chop, throw into an enamelled cast-iron pot filled with light sugar solution/vanilla/lemon juice/lid on and then cook in a slow oven (130 degrees centigrade) for about 8 hours. At which point they emerge as glistening segments of garnet surrounded by blood-like syrup.

Complete colour change. But why? There’s gotta be science in that. So I did some research.

Turns out the red colour emerges as a class of chemical compounds called anthocyanins are converted from their colourless, astringent precursor cousins the leucoanthocyanins during cooking.

Harold McGee in On Food and Cooking: The Science and Lore of the Kitchen says it best,

The combination of heat and acidity causes the subunits [of the precursor molecules] to break off one by one; and then oxygen from the air reacts with the subunits to form true anthrocyanins: so the tannic, pale fruits become more gentle-tasting and anything from pale pink to deep red.

The final colour depends on the pH (acidity) of the cooking fluid.

Anthocyanins are also what make blueberries blue, red cabbage red, blackberries black, eggplants purple and blood oranges bloody. According to some experts, we may get cardiovascular benefits or even cancer protection from eating more anthocyanins.

I’m onto it. Quince tart anyone? Here are some I prepared earlier.