Unexpected blue color when cooking mustard greens
March 1, 2022 8:02 PM   Subscribe

I sauteed some mustard greens in olive oil in a cast iron pan, and the liquid that was left in the pan had a distinct dull blue color. I am assuming something in the greens reacted with the pan - chemists of MeFi, what did I make?
posted by each day we work to Food & Drink (14 answers total) 1 user marked this as a favorite
Did you cook with garlic? Any acids (vinegar, citrus)?

Garlic has sulfur-containing alliin and allinase; allinase catalyzes alliin into allicin. The process is sped up with heat/ acid. Iron can also speed up this process.

Allicin can react with amino acids to produce pyrroles, heat can polymerize pyrroles into polypyrolles - which are a bright blue.

It might be dull from dilution from released chlorophyll components/ homogenized with oil.

Looking it up, mustard greens might/ can also contain pyrroles, which can be similarly polymerized by heat.

Harmless. Commonly seen in pickled garlic.
posted by porpoise at 8:21 PM on March 1, 2022 [9 favorites]

Just greens and oil? Was there any garlic involved?
posted by Don Pepino at 8:21 PM on March 1, 2022 [1 favorite]

Oops - missed that you were cooking with olive oil.

Olive oil, when heated too much, can oxide and turn a distressing blue-ish (this is from experience, I don't know what the chemical components/ pathways are).

If cooking with olive oil, keep the temperatures low.

If you want to cook at high(er) temps, go with something like grapeseed, or better yet, hempseed (there are no cannabinoids in "real" hempseed oil) - very high smoke point, tasty (slightly nutty), and has an amazing omega 3:6 profile.
posted by porpoise at 8:26 PM on March 1, 2022 [4 favorites]

Response by poster: The heat was not too high, also no garlic or acid. The exact cooking process was to heat up the oil to medium heat - definitely not excessive, no smoking - add the greens and saute for some minutes then add some salt. I cook with olive oil all the time, and have never seen it turn a shade of blue!
posted by each day we work at 8:48 PM on March 1, 2022

Best answer: I get this when I cook purple/red leafed mustard greens, any chance that's what you used? I doesn't seem to happen when I use the bright green leafed ones.
posted by mandymanwasregistered at 8:56 PM on March 1, 2022 [4 favorites]

When I blanch purple mustard greens, the pot of water turns that striking blue, so I don't think it's the garlic or oil.
posted by mandymanwasregistered at 8:57 PM on March 1, 2022 [3 favorites]

Just a guess but maybe mustard greens contain both blue and yellow (ie mustard seed) within them, which looks green as combined in the leaves but which could distill out into its primary components.
On the other hand I have no idea if this is right.
posted by Tim Bucktooth at 9:37 PM on March 1, 2022 [1 favorite]

have never seen it turn a shade of blue!

New batch of olive oil? Counterfeit "extra virgin" olive oil is a thing.

"Mustard greens" could potentially be any number of related species/ subspecies-cultivars (CULTIvated VARiety).

Brassica are the same genus and they range in morphology from cabbage to gai lan, not to mention variety in phytochemical profile within the same species/ phenotype (general looks).

Could be that what you got "naturally"* happened to produce a small molecule that got released upon cooking and/ or reacted upon heat (and iron) to produced another small molecule that reflected only blue wavelength light.

*by random mutation and kept getting propagated, or a deliberate breeding scheme that subsequently got selected for
posted by porpoise at 11:35 PM on March 1, 2022

Response by poster: I get this when I cook purple/red leafed mustard greens, any chance that's what you used?

Yes, exactly that.

New batch of olive oil? Counterfeit "extra virgin" olive oil is a thing.

The batch I use all the time, from my 25L can that I got from friends some months ago when my previous 25L can ran out. They press it from olives they grow on their trees, I am *very* confident this is olive juice with nothing added to it.
posted by each day we work at 12:52 AM on March 2, 2022 [2 favorites]

Best answer: tl;dr You have produced indigo or something similar.
Where did you source your mustard greens? Isatis tinctoria = Dyer's Woad is another brassica with yellow flowers which will turn blue in a process involving heat and alkalinity. Blue lurks below the surface in a lot of brassicas [red cabbage, beetroot] which are red because of a local acid environment. Litmus [paper] makes a similar red - blue change as the acidity decreases aka the pH goes up. Sometimes heat alone will drive off / degrade the acid; but bases like baking soda, or better washing soda, will neutralize it quicker. You can probably make the blue go away with a squirt of lemon juice. If you like the blue Cody shows you how [10m] to keep it - very old pee involved.
posted by BobTheScientist at 12:54 AM on March 2, 2022 [8 favorites]

Best answer: Am pretty sure it's the pigment from the mustard greens. When I wash red kale for salads, the water is reddish but it turns blue overnight. No heat or oil involved. I think I read somewhere that it's due to a ph change.
posted by whitelotus at 6:20 AM on March 2, 2022 [1 favorite]

Best answer: Not sure about your mustard greens, but many plants use anthocyanins to achieve reddish/purplish hue, this is the compound in grape skins that causes wine to be red for example. These compounds are generally pH sensitive and are red in acidic conditions but then turn blue in neutral or basic conditions.

I don't know much about the indigo mentioned above, but I bet if you add vinegar and it turns red you have extracted some anthocyanins. From my cursory google indigo is blue while under pH~12 and yellow above. So adding some sodium carbonate (not bicarbonate) to raise the pH and turn it yellow might indicate indigo. If you're raising the pH that much though, lots of colorful compounds can be destroyed, so I don't know if that test would be conclusive.
posted by crossswords at 10:09 AM on March 2, 2022

Best answer: Red cabbage juice is a well-known pH indicator, I'm guessing your mustard greens contain the same compounds. A fun demonstration/experiment is to boil up some red cabbage, get the bright red/magenta juice, and drop a little baking soda into it. As the baking soda dissolves and the pH rises, the juice turns blue-green. Drop a little vinegar in and it will fizz with the acid-base reaction, and as the pH drops again it turns back to red/magenta. You can repeat this indefinitely. Fun for kids and adults.
posted by biogeo at 11:27 AM on March 2, 2022 [2 favorites]

Response by poster: So it turns out there was a little bit of the greens left over. I squeezed some of the juice out and added some lemon - it promptly turned pink, so pH-sensitive pigment is the right answer; iron in the pan was a red herring I guess.
posted by each day we work at 11:58 PM on March 2, 2022 [2 favorites]

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