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Old 10-14-2006, 05:01 PM   #11
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Hmmmm... as far as I know acids plus chlorophyll equals army green.

If you have McGee's "On Food and Cooking" (you do, don't you? ) refer to the "Cooking Fresh Fruits and Vegetables" section. Knowing a bit of chemistry is good here too (grab a text at Barnes & Noble).

There are two ways green vegetables lose their color or turn nasty. They either lose their chlorophyll to the surrounding water, or undergo a reaction where the chlrophyll is transformed into a molecule with different color attributes. An enzyme active between 150-170F called chlorophyllase can strip the chlorophyll of it's hydrocarbon tail and causes it to become water-soluble rather than fat soluble. It then leaks out into the cooking water. Chlorophyllase is destroyed at the boiling point, so keeping the water at 212 is important. Letting it drop to the 150-170F range by not having enough water to make up for the heat the vegetable is sucking out of the water exacerbates the problem. So use lots of water, or steam, which actually allows you to quickly raise the ambient temperature in the pot/pan above 212 with a heavy lid. Don't pile the veggies though, or they will insulate each other and you will get a slow temp increase where the covered veegetables will be in that 150-170F range a long time.

Chlorophyll molecules have a magnesium atom at their center, which can be displaced by heat and/or natural enzymes in the plant itself. This then allows hydrogen ions to move in and turn chlorophyll-a into pheophytin-a (grayish-green), and chlorophyll-b into pheophytin-b (yellowish). When an acid is added to water it separates and creates a butt-load of hydrogen ions. When it comes to keeping chlorophyll bright green, adding acid is like sticking a fork in your eye. You can actually help prevent this by adding alkaline substances like sodium-bicarbonate (baking soda) - but if you add too much it can make the veggie mushy. Most tap water is slightly alkaline to prevent pipe-corrosion, so this usually isn't a problem - but fruits & veggies alos have natural acids that can affect the chlorophyll in the same way.

The trick is to cook at high heat with lots of water (destroying chlorophyllase), and cook only long enough to collapse the air pockets in the cell wall/membrane (allowing the chlorophyll is be seen) and then quickly stopping the cooking process before the heat displaces too many of the magnesium atoms and too many acids are released to replace the displaced magnesium.

Now I only passed CHEM I in college, so someone here with more smarts/experience can probably do a much better job...

I'm not quite sure how adding another acid later helps to preserve the color. Sounds kinda like it would be counter-productive? I dunno.
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Old 10-14-2006, 05:56 PM   #12
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Ha, I realized this was about pesto using basil which is susceptible to phenolic browning. It's ok, McGee has that too... let me brush-up...
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Old 10-14-2006, 06:02 PM   #13
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Ok, phenolic browning is caused when phenolic compounds in the cell's vacuoles mix with enzymes in the cell's cytoplasm in the presence of oxygen (pick up a basic micro-bio book for this great info!). So it takes three components. You can either remove the oxygen by keeping the pesto covered in oil (and grinding it with oil), or you can deactivate the enzymes in the cytoplasm by blanching the leaves.

Good experiment. I've never tried blanching my basil leaves before making pesto.

Apparently Vitamin C has natural anti-oxidants that prevent the phenolic browning as well. I guess once you process the pesto, the fat soluble chlorophyll is protected by the olive oil from the vitamin C, and then vitamin C goes to work preventing the phenol reaction.

I think thats it.
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Old 10-14-2006, 09:37 PM   #14
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Acid in the greens, Steve? Bring some of those up next time! whoa...the colors!

j/k.



Seems like everyone has posted some great info, I learned something.
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