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I was recently making some mint spheres using reverse spherification, as a bit of background here was my process: I blended around 15g of mint leaves with 250ml of boiling water and left to steep for an hour. I strained the infusion through a 50 micron filter and then added some sugar, lemon juice and 3.5% calcium gluconolacte before thickening with 0.4% of xanthan gum. The spherification went fine but the colour of the liquid went an unappetising shade of brown and what I would like to know is there a way, aside from adding food colouring, to preserve the mint's green colour?

Stefano
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    I would like to know this as well. I make mint jellies and always have to add food coloring because the natural color degrades. – Sobachatina Mar 26 '12 at 16:29

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Plants are green due to the presence of chlorophyll. When chlorophyll is exposed to acids, alkalis, heat or enzymes, it first loses its long hydrocarbon tail, becoming water-soluble instead of fat-soluble. Then, hydrogen ions replace the magnesium atom in the center of the chlorophyll molecule, turning it to pheophythin, which has a dull green-yellow-grey color.

If you insist on continuing the heat treatment, you will lose a lot of the chlorophyll color. You can reduce your loses by using an alkali environment and by adding metal ions. The metal ions part can be as simple as boiling the tea in a copper pot. For alkali, use baking soda. It will turn the leaves into mush (so it is preferable not to use it for vegetables, even though it keeps the color), but it will help with retaining the green. I think that the changed pH shouldn't be a problem, as Lersch's Hydrocolloids says that reverse spherification works at pH levels 2.8-10, but is inhibited by pH below 4. Also, I don't see why you steeped for an hour; normal mint tea is steeped for 5 minutes. If you want stronger tea, use more mint, not longer steeping times.

If you insist on a color coming from mint, but are not particular about steeping, I would suggest getting a mint taste in another way. Juicing the mint will be best, but will require heaps of plant matter, so probably not practicable unless you have access to a mint meadow. It would be much better to rupture the cell walls by freezing, and then leech the mint taste and color in some solvent - water will be OK, alcohol will probably give you a stronger extract. Just steep the defrosted mint for a long time in the water (or alcohol, or mix of both), and then use without adding acids or heating. Again, adding a bit of baking soda may help with the color. But pay attention, if your final product has too high a pH, its taste will be perceived as soapy.

And the easiest solution is, of course, to use food coloring. While some people may consider it unnatural, I don't think it does any bad, especially considering that you are already doing some highly unusual things to your food. It gives you a much easier time with the other aspects of preparation (head and acids).

rumtscho
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  • Yes, I think we're far past the point of worrying about 'unnatural' ingredients. :) Didn't realise that the steeping time was unrelated to the strength of the infusion, all I wanted was maximum extraction. If I had a Vitamix, I'd just put a bunch of mint in and make a puree that way but I don't so that's no use here. The main reason I used lemon juice was for sourness as I was aiming at a sweet/sour contrast and also to mask the calcium gluconolactate because no matter what they might say it does have a discernable taste and it's not very pleasant. – Stefano Mar 26 '12 at 21:27
  • Also, I know acidic ingredients cause discolouration but I've also seen ascorbic acid being touted as a way to keep basil in pesto green and that would solve my sourness problem too. – Stefano Mar 26 '12 at 22:22
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    Now you mention it, there is also the advice to add citrus juice to guacamole. I was speaking mostly from theoretical knowledge in the answer. But the interesting thing is that both pesto and guac are not cooked. Maybe in this case the magnesium dislocation stops being a problem and oxidation becomes the primary concern (although McGee doesn't mention chlorophyll oxidation). You could try if it works with a cold extract and an acid, but maybe it is only OK when the cell walls are whole to protect the chlorophyll - traditional pesto is mashed, not blended. – rumtscho Mar 26 '12 at 23:16
  • I'm wondering if I blanch the mint in boiling water for 10 seconds to deactivate chlorophyllase then shock it in iced water before trying your suggestion of a cold extract would that help. Looks like I've a few experiments to run! – Stefano Mar 27 '12 at 06:15
  • @Stefano I don't know where you read about the c-lase, but McGee says that there are two things which happen to chlorophyll, the first is the loss of the carbon tail, the second is the magnesium displacement. You probably have to lose the tail to get your green color water-solvable. And if not, both acids and bases will remove it. So in theory, it is the second step you want to prevent. Besides, a 10 seconds heating is unlikely to get you above 70°C. This is still theory, I might be wrong. But I would try to think of other ways first, and try blanching as an additional option. – rumtscho Mar 27 '12 at 09:56
  • I got it from here: http://sciencefare.org/2011/09/26/green-vegetables-green but it seems I didn't read the page thoroughly enough: the action of the enzyme is over a long period so the blanching step is useful more so if you want to store the product over a long period of time, say in the freezer, for example. – Stefano Mar 27 '12 at 14:26
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You can try to add some baking soda when you put the mint leaves in the hot water.

Baking soda retains colour in boiling broccoli and other green veggies, so there's a chance this could help you with mint as well.

Mien
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  • Doesn't that work by undoing the effect of adding the lemon juice, by increasing the pH? – Jacek Konieczny Mar 26 '12 at 18:21
  • I'm not entirely sure what you mean. Yes it gives a higher pH, while the lemon juice lowers the pH. But the baking soda and the lemon juice aren't added at the same time. – Mien Mar 26 '12 at 20:59
  • Further to his, according to sciencefare.org (http://sciencefare.org/2011/09/26/green-vegetables-green) you only need to raise the pH of the water you're cooking to 7.5 to get the increased greening effect. As part of the experiments I menitoned in another answer, one of them will be blanching the mint leaves in this slightly alkalinised water. – Stefano Mar 27 '12 at 09:20
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Try grinding the mint with a mortar and pestle, with your lemon juice as a solvent. Since you strain it, the final solution won't have chunks of mint.

Chloe
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