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Why do red onions sometimes turn blue when you cook them?

Richard Cornish
Richard Cornish

The red colour in red onions is caused by natural pigments called anthocyanins.
The red colour in red onions is caused by natural pigments called anthocyanins. iStock

​My red onions turn blue when I cook them. L. Craig

Red onions are a cultivar of the everyday onion Allium cepa and have been developed independently in Romania, Italy and the US. Red onions are generally used raw because they are sweeter, about 11.5 brix (a measurement of sugar) than regular onions. By comparison, a ripe brown onion is about 9.5 brix, about the same as a ripe watermelon. Red onions can contain more of the compounds that make them sharp on the palate, which is why some chefs soak them in water for 20 minutes to leach out some of those harsher flavours. Another trick is to gently massage salt and sugar into the sliced red onions, allow them to sit for 20 minutes, then rinse off the salt and sugar, and pat the onions dry with a clean dishcloth. The red colour is caused by natural pigments called anthocyanins. The same stuff makes shiraz grapes that lovely deep purple colour and blueberries blue. Anthocyanins have been associated with many health benefits. They are also sensitive to changes in pH. Under alkaline conditions, the red hue changes to a bluer tinge. If you cook red onion with alkaline foods, such as nuts, lentils, and some vegetables, those anthocyanins revert to their blue hue. The same is true of red cabbage. To maintain the red hue, add a little acid, such as a splash of vinegar or a squeeze of lemon juice.

I wrapped a cut onion in plastic wrap, and the cheese in my fridge then smelled of onions. V Schulz

Many of the plastic films we use in the kitchen are permeable to gas. This means air, and atomised particles, will pass through. The effect is amplified if there is a temperature or pressure differential. So, if food is hot or is fermenting and producing carbon dioxide, there will be more gas escaping through the plastic film. You will notice that chip packets are made with a fine layer of aluminium sandwiched between layers of plastic. Aluminium is not permeable, so it stops moist air from entering the bag, causing the chips to go soft. Therefore, if you want to stop your cut onions from stinking out the fridge, wrap them in aluminium foil, beeswax cloth or in a thick-walled plastic container or glass jar, as my grandmother did.

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Letters

We don't get a lot of anti-vegetable correspondence at the Brain Food Institute, but this one dropped into our mailbag this week. "I don't understand this obsession with kale," writes N. Moncrief. "It is horrible stuff. As an experienced home cook, I would recommend ignoring it completely in favour of the old-fashioned Aussie standby silverbeet (or slightly more fancy rainbow chard). It requires far less cooking, can be eaten raw when young, can be substituted for spinach in nearly every recipe and IT TASTES BETTER."

Send your vexing culinary conundrums to brainfood@richardcornish.com.au or tweet to @foodcornish

Richard CornishRichard Cornish writes about food, drinks and producers for Good Food.

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