DAN SOUZA: Pesto is a simple, versatile sauce that's
easy to make in a food processor.
But many home cooks who try and make it are
frustrated when their bright green sauce quickly
turns black, like this pesto here.
Why does basil darken this way?
Well, when basil is cut, oxygen mixes with chemical compounds
called polyphenols, which are on the surface cells of the leaf.
This reaction causes the dark color and bruised appearance.
The more you chop the basal, the more polyphenols
you are exposing to the oxygen, and the darker it will get.
And this is why pesto can turn almost black.
However, basil is so prone to this enzymatic reaction
that even folding or rolling the leaves can
break enough vascular tissue and fibers on the surface of the leaves
to cause discoloration.
And I've got two leaves here to show the point.
This leaf here was folded up, kind of rolled up a bit.
And this one was not.
As you can see, there's lots of dark lines
where that oxidation has already taken place.
Enzymes are biological agents that increase the rate of chemical change,
and not just in basil.
Unlike many foods with strong flavors, garlic and onion
have almost no aroma in this state.
No one cries when they're shopping for an onion.
But slice into that onion, and the aroma can be overpowering.
That's when the tears can flow.
So what's happening?
An enzymatic reaction, similar to the process that causes
sliced basil to darken.
Before slicing, enzymes and amino acids are
stored in separate sections of the garlic and onion's cells.
When the cell walls are ruptured by a knife, the enzymes and amino acids
react with each other to create new compounds.
These compounds are responsible for the characteristic flavor
of garlic and onions.
In garlic, odorless sulfur-containing amino acids come into contact
with an enzyme called alliinase.
The reaction of the two eventually produces
a new compound called allicin, which is responsible for garlic's
familiar scent and flavor.
The more the garlic is broken down, the more flavor you get.
So for example, minced garlic, or garlic pressed through a mincer like this,
is going to have a lot more flavor than a piece of garlic
that I just slice very thin.
In this garlic, I'm breaking a lot of those cells
and I'm allowing a lot of that reaction to take place.
Here, I'm doing much less cellular damage.
So in onion, the process is the same, but the compounds
created are called thiosulfinates.
Unlike garlic, onions also contain an enzyme
called LF synthase, which is responsible for producing the tear-causing compound
propanethial-S-oxide.
In the test kitchen, we've tried more than a dozen purported home recipes
for keeping tears at bay, everything from freezing onions
to lighting a match while we're slicing them.
But the best solution is to wear contact lenses,
or don a pair of ski or swim goggles.
What you need is a physical barrier to protect your eye.
If you don't wear contact lenses, your best bet is a pair of goggles.
Looks a little bit funny, but it works.
Also, using a sharp knife with a really good edge
will inflict less cellular damage and can actually
limit production of propanethial-S-oxide,
so you want to make sure your knife is sharp.
So we understand how enzymatic reactions start.
But what about stopping them?
One way to halt the enzymatic reactions is with the application of heat.
Heat deactivates enzymes, while at the same time
transforms these potent flavor and aroma compounds into more pleasant forms.
This is what happens when you cook sliced garlic and onions.
You can also add another ingredient to stop the reaction.
In the test kitchen, we ran a simple test with minced garlic
for a basic vinaigrette, with three parts olive oil
and one part lemon juice.
In one batch, we added one teaspoon of minced garlic
immediately to the dressing.
That's this batch here.
In a second batch, we soak the minced garlic in lemon juice for 10 minutes.
Then we made the dressing with the garlic and lemon juice.
Finally, in a third batch, we let the grated garlic sit for 10 minutes
while we prepared the other ingredients, then finally
mixed it in with the other ingredients.
So while these seem like small adjustments to the order of operations,
the flavor differences are huge.
The citric acid in the lemon juice speeds the conversion
of that harsh-tasting allicin to some mellower compounds, the same ones
that are created when garlic is heated.
Soaking the garlic in lemon juice was a bit more effective
than putting the garlic directly in the dressing with all
of the other ingredients.
But either approach is far better than letting the minced garlic hang out
for 10 minutes.
In that sample, the garlic flavor was overpowering,
and it made for a pretty bad dressing.
So this is a really simple experiment.
I encourage you to try it at home.
You'd really be able to taste the differences yourself.
Now, what does all this have to do with pesto?
Well, you can use heat or acidity to keep sliced basil from changing color.
Now, you can find a lot of recipes on the internet that
suggest throwing a vitamin C tablet, otherwise known as ascorbic acid,
into the food processor with the other ingredients.
I have two bowls of pesto here that I made two hours ago.
The only difference is that this one has 1,000 milligrams of vitamin C in it.
As you can see, it's bright green, versus the one that doesn't have any.
Now unfortunately, you can really taste the vitamin C
and it makes the pesto kind of unpleasantly tart,
so this isn't really a great option.
But I have a third bowl of pesto here that I also made two hours ago.
Now as you can see, it's pretty much the same color as the vitamin C batch.
Now, I can tell you the big difference is that it tastes exactly
like the darkened pesto.
And that's because it's made with the identical ingredients
as the dark sample, with just one change.
I blanched the basil leaves in simmering water for 20 seconds
before processing them with the remaining ingredients.
And as I mentioned earlier, heat deactivates
the enzymes that react with oxygen to cause
that dark color in the first place.
And the best part is it's completely effective.
When we ran this test and refrigerated the pesto for a week,
it was still brilliant green.
The only trouble with this technique is that you need to bring a pot of water
to a boil, then need to shock and blanch the basil in a bowl of ice water.
So this method makes great sense if you're turning a bumper crop of basil
from your garden into pesto that you plan to freeze for many months.
But it's a lot of work for pesto you want to toss with pasta tonight,
so I'm going to show you a simpler way with the test kitchen's favorite pesto
recipe.