Lemon juice and its oxidation properties

[piracer]

So i was watching a cooking show on making chimichurri and they were saying how you should only add the lemon juice at the very end to prevent the herbs from oxidizing and turning black.

But with artichokes or apple slices, one would add lemon juice at the start so it doesnt turn black.

Anyone know the chemistry behind these two things? Because the two of them are both oxidizing something.

[nickrey]

My thought would be that the lemon juice in chimmichurri is a vital part of the flavour profile as well as preventing oxidation. By putting the lemon juice in at the end for chimmichurri you add sufficient to balance the taste, which can only be done when all the other ingredients are added. For artichokes or apples it is simply a way of stopping the brown and therefore can be added earlier.

[piracer]

The lemon juice is there as a major flavoring component, but their saying you should add it at the last minute, meaning, dont make a batch of chimichurri and let it sit. If you let the sauce sit, it will turn brown because of the lemon juice. But again, for apples and artichokes, its added to PREVENT browning, which is where the confusion lies.

I wonder if its some way chlorophyll reacts to acid...

[Mjx]

"Treatment of cholorophyll-a with acid removes the magnesium ion replacing it with two hydrogen atoms giving an olive-brown solid, phaeophytin-a. Hydrolysis of this (reverse of esterification) splits off phytol and gives phaeophorbide-a. Similar compounds are obtained if chlorophyll-b is used." (http://www.ch.ic.ac.uk/local/projects/steer/chloro.htm)

[piracer]

Ah, that makes a lot more sense now. Thanks!