Thanks, Slow Cooked Pittsburgh. I was in a bit of a rush when I answered this question, so I skipped over a some details. One is that the purple pigments do not change to green. Instead, once they break down, the only thing left to see is the chlorophyll that is still intact. The acids are unlikely to save the purple, at least not in the form with the color we'd like.
For anyone whose interested in the science of pigment in general, here's a related factoid: Lobsters typically have two pigments, a blue one and a red one. When lobsters are alive, we see the mix of pigments, which appears to be green and brown. The blue pigment breaks down with heat, so voila! In cooked lobsters, we just see them as red. Sometimes, in the wild, you can find a lobster with only one or with neither pigment. Public aquariums love to display these animals.
Another one that's not a cooking example, but important in places with seasons, Autumn leaves: those yellows, oranges, and reds are around year round, but it's only when autumn comes, and the chlorophyll starts to break down that we see those colors.
You might have better luck if you blanche the asperagus in the acidified water, rather than steaming. I do agree with Greenstuff, no hope of purple, but you can probably get the reddish purple.
Ah, pigments! Green comes from chlorophyll. It's hard to have a plant without chlorophyll, because that's the molecule that captures energy from sunlight. Chlorophyll can be damaged by heat and water. That's why bright green vegetables turn muddier. But their location in the cells is a little more protected than the purple pigments.
Purple pigments generally located in parts of the cell that are readily affected by heat and water. The cells break open, and the pigments break down. Some purple pigments, called anthocyanins, are very sensitive to changes in acidity, turning different shades of blue and reddish purple. I don't think there's any hope of preserving the purple color you want and also have cooked the asparagus. Sad.
Purple put to the test - steamed two to three minutes, lemon juice, ice bath = muddy green asparagus!
Panfusine, you know your stuff! If anyone else knows a different technique, bring it on.
Adding acid is the only way to maintain any of the color (depending on what you are making, you can add vinegar or citrus juice, especially lemon). You also want to keep your cook time to a minimum. If, for example, you are using them in a fritatta or salad and the pieces will be cut, you should cut them before cooking to reduce cook time further. If they are tender enough, you might even consider serving them raw--purple varieties tend to have extra sugar and may be a bit nutty. I'm just imagining lovely raw purple asperagus with some golden yellow farm egg hollandaise or bernaise, wow, that would be a great appetizer or side dish.
I thought the same thing. The heat reacts and turns the purple green while cooking. To maintain any of the purple, you probably want to keep cooking time to a minimum (2-3 minutes) and plunge the asparagus into an ice water bath immediately after removing from the heat. Steaming would be better than blanching for this. Lemon juice may also assist, and if not, it will at least make the asparagus tasty!
I always thought that the purple pigmentation in asparagus was the same as that in purple bell peppers in that It turns green when cooked, am looking forward to the answers for this pickle!
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For anyone whose interested in the science of pigment in general, here's a related factoid: Lobsters typically have two pigments, a blue one and a red one. When lobsters are alive, we see the mix of pigments, which appears to be green and brown. The blue pigment breaks down with heat, so voila! In cooked lobsters, we just see them as red. Sometimes, in the wild, you can find a lobster with only one or with neither pigment. Public aquariums love to display these animals.
Another one that's not a cooking example, but important in places with seasons, Autumn leaves: those yellows, oranges, and reds are around year round, but it's only when autumn comes, and the chlorophyll starts to break down that we see those colors.
Purple pigments generally located in parts of the cell that are readily affected by heat and water. The cells break open, and the pigments break down. Some purple pigments, called anthocyanins, are very sensitive to changes in acidity, turning different shades of blue and reddish purple. I don't think there's any hope of preserving the purple color you want and also have cooked the asparagus. Sad.
Panfusine, you know your stuff! If anyone else knows a different technique, bring it on.