Please save my pesto!
July 13, 2011 7:29 AM   Subscribe

Why does my pesto sometimes stay a gorgeous green color and sometimes turn this horrible brown?

One of the ways I've been thoroughly enjoying summer is by making pesto pretty often--like a couple of times a week. I make it the same way every time (my super-lazy version of pesto). I throw basil leaves (from my garden or the farmer's market), garlic, parmesan, pine nuts, and olive oil into a Magic Bullet-type mixer thing, grind it all up, pour it on hot pasta. Super super easy and delicious.

I don't measure anything so obviously it comes out slightly different every time. But last week I made a batch that stayed this amazing, beautiful bright green color, and it was the best pesto I've made yet. Yesterday, though, I made more and it immediately turned this dark brown color. It tasted okay, not great, but it left a bad taste in my mouth all evening.

I've read about blanching basil to keep it green and things like that. But I guess my question is--what did I do the time before this (accidentally) that made it stay so green? And why does it turn so dark other times?
posted by devotion+doubt to Food & Drink (20 answers total) 10 users marked this as a favorite
My immediate guess is air got to your pesto causing an ingredient to brown. I would suggest what I do with my guacamole - when you make your pesto cover with plastic wrap directly on the surface and refrigerate until you're ready to serve. This slows down the browning process. The surface protects better than an air tight container alone.
posted by BuffaloChickenWing at 7:39 AM on July 13, 2011

I don't know why it's turning brown immediately, but when I store my pesto in the fridge I top it off with a thin layer of olive oil to seal it from the air.
posted by hydrophonic at 7:40 AM on July 13, 2011 [1 favorite]

Part of it might have to do with how hot the pasta is when you mix in the pesto.
posted by Jon_Evil at 7:40 AM on July 13, 2011

It goes brown because of oxidisation. Pour a thin layer of olive oil over the top of it in the container to keep the air away from it until you're ready to serve.
posted by col at 7:45 AM on July 13, 2011 [1 favorite]

Yesterday, though, I made more and it immediately turned this dark brown color.

Do you mean the pesto immediately turned brown when you made it? Or were you using some pesto you'd made the day before? If it's immediately turning brown then it's not down to storage (keeping under a layer of oil, etc) as other have said, and must be something about the ingredients or preparation.

Are you using the exact same ingredients each time?
posted by EndsOfInvention at 7:52 AM on July 13, 2011

Response by poster: EndsOfInvention--yes, I mean that it turned very dark as soon as I put it on the pasta. It was a fresh batch, so no storage issues--and it went straight from the Bullet to the pan.

I used exactly the same ingredients--only difference was the source of the basil. I didn't cut the basil for either batch--just tore it into the Bullet. Although maybe I tore the brown batch more? Would (slightly) bruising the basil by tearing it have this effect? Should I just throw whole leaves in and chop 'em up?
posted by devotion+doubt at 8:08 AM on July 13, 2011

Best answer: hydrophonic nails it: the oil in pesto not only emulsifies it, it helps preserve it as well. A good pesto should be practically pourable. My guess is you didn't use enough olive oil.
posted by Gilbert at 8:10 AM on July 13, 2011 [1 favorite]

Response by poster: Oh! It was definitely thicker than the previous batch. Not enough olive oil totally makes sense! Thanks so much everyone!
posted by devotion+doubt at 8:13 AM on July 13, 2011 [1 favorite]

I have had the same problem occasionally. I have solved by adding 1 part of flat parsley to 3 parts of basil: the basil always stay green and its flavor is not noticed because the basil overwhelms it.
posted by francesca too at 8:14 AM on July 13, 2011 [1 favorite]

At work we make ours with basil and spinach (roughly 1/2 and 1/2). Everyone loves it.

Although we also make it with way too much oil so it's more oil flavored with pesto.
posted by theichibun at 8:19 AM on July 13, 2011 [1 favorite]

A small bit of lemon juice in the processing will help retain the bright green of your pesto, and not adversely affect taste.

Alternately (and way more complexly) quickly blanching the basil leaves before processing will keep them green, too. You need to make sure they are completely dry before processing, though, so this approach turns a couple of minutes into most of the afternoon.

And, no matter what you do, always add a thin cover of olive oil to your pesto when you store it.
posted by Thorzdad at 8:20 AM on July 13, 2011

Yes, the more oil the better when it comes to keeping it green.
posted by fivesavagepalms at 8:34 AM on July 13, 2011

Not sure what you did the first time to keep it green, but I've seen people add a small bit of powdered vitamin C to preserve color also.
posted by mu at 8:46 AM on July 13, 2011

Harold McGee's book The Curious Cook devotes most of a chapter to why pesto darkens more quickly some times than others. I don't have my copy in front of me here at work, but I'll be sure to look it up and summarize it this evening.
posted by Johnny Assay at 9:23 AM on July 13, 2011

Response by poster: Thanks, Johnny Assay--I'd be interested to hear what he says!
posted by devotion+doubt at 10:11 AM on July 13, 2011

I make a thicker pesto, but I put a layer of plastic wrap right on the surface to keep it from oxidizing. Then I freeze it and have yummy pesto all year.
posted by miyabo at 12:12 PM on July 13, 2011

Best answer: You might also have too much water (from washing ingredients) in there--use a salad spinner, a towel, or both to make sure the basil, etc., is pretty dry.
posted by Riverine at 2:22 PM on July 13, 2011

Response by poster: These are great suggestions. I definitely had not enough olive oil and too much water left on the basil. As always, Metafilter rocks.
posted by devotion+doubt at 2:28 PM on July 13, 2011

Best answer: So here's what Mr. McGee has to say about the browning of pesto, and techniques for slowing same:
  • The browning is caused primarily by a set of enzymes in the leaves called polyphenol oxidases, which work to attach oxygen atoms to various phenol compounds in the cells. In a living cell, the enzymes' interaction with the phenols is strictly regulated, but when the cell is damaged, the enzymes go hog wild.
  • Ascorbic acid (vitamin C) can undo the action of these enyzmes, being used up itself in the process. However, the amount required to keep pesto green for more than half an hour or so makes the pesto unacceptably sour.
  • Adding parsley or spinach to the pesto doesn't seem to help.
  • Blanching the basil (for about 5 seconds) before making the pesto inactivates the enyzmes that turn the pesto brown. The net result is that the pesto does stay green for much longer, though not indefinitely (the phenol compounds are apparently capable of capturing oxygen atoms on their own, just more slowly.) The blanching also adversely affects the pesto's flavour.
  • The phenol compounds are concentrated in the stems & veins of the pesto, so be sure to remove the stems & leaf bases from the basil leaves before processing.
  • The enzymes & phenols in the nuts make a difference too. Pesto made with walnuts browns more quickly than pesto made with pine nuts. Pesto made with sunflower seeds browns even more quickly than that.
  • If your pesto is going on pasta, the wheat also contains phenols that can brown, accelerating the discoloration. These can be largely neutralized by lowering the pH of the pasta's cooking water; at least 1/4 cup of lemon juice, or 1 1/3 tablespoons of cream of tartar, are needed per quart of pasta water. McGee notes that this changes the texture & flavour of the noodles, but is of the personal opinion that the improved appearance can be worth it (he says he does it for company, but not for day-to-day use.)
  • The Roman naturalist Pliny claimed that "pounded basil, if covered with a stone, breeds a scorpion."
OK, that last one has nothing to do with keeping pesto green, but I couldn't not include it.
posted by Johnny Assay at 3:01 PM on July 13, 2011 [2 favorites]

Response by poster: OK, in case anyone is following up, I made a fabulous, green, beautiful batch of pesto last night. I made sure the basil leaves were dry, made sure not to get any stems in there, and--I think this was the key--put in WAY more olive oil to basil than I had been. Made sure all the leaves were coated with olive oil before I started chopping them in the Magic Bullet.

I thought it would be just this crazy oily mess, but apparently I'd been using too much basil and too little olive oil in the previous batches, because this came out great!

Thanks for all the advice, MeFites!
posted by devotion+doubt at 8:30 AM on July 23, 2011

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