Is it Possible to Keep Blanched Green Beans Really Green?

green bean salad

If you regularly follow this blog (hi mom!) you know that I posted a while back about whipping together a green bean salad because I had gotten my hands on some really fresh green beans. In the process of making that recipe, I noticed that the beans got really bright green after they were blanched, but ended up a drab olive color by the time I had dressed them and we were ready to eat.

I wondered: was there any way I could keep green beans green?

Why Blanch?

First off, why bother cooking green beans – or any green vegetable for that matter – in the first place?

Cooked vegetables can be better than raw because:

  • Breaks down cell walls, releasing sugars and aromatics that greatly improve flavor.  Ever tried chewing on a raw green bean?  Yuck.
  • Softens texture by breaking down cellulose, hemicellulose, and pectin in cell walls.
  • Can assist in the absorption of nutrients.  Although many vitamins are broken down with even a small amount of cooking, the activity of some nutrients can actually increase.  Nutrient availability depends on the vegetable and nutrient in question, the cooking method, and cooking time.

Blanching can be superior to other cooking methods for several reasons:

  • Boiling water distributes heat more evenly than steam, making it easier to control cooking time.
  • When cell walls in plants break, acid escapes and begins to turn green vegetables olive-drab.  Cooking water dilutes the acids, slowing discoloration.
  • It’s possible to season your cooking water with desirable flavors.

How to Make Cooked Vegetables Stay Green: the Big Takeaways

To make keep vegetables green,

  • Test the pH of your water.  If in doubt, boil a few peppers and watch them for color change.  If you like the way they look, you’re good.  If not, try adding a tiny pinch of baking soda (I calculate something like a tenth of a gram/L) and work up from there.  Aim for a pH of about 7.5, which is all you need to preserve green color.
  • Limit blanching time.  I found I preferred a cook time of just 2 to 3 minutes.  Immediately evacuate to an ice bath or rinse under cool water until cool to the touch.
  • Hold off dressing vegetables with acids for as long as possible, especially if they will be served heated.

To keep texture firm,

  • Timing is everything.  Natural crispness degrades quickly and can never be restored.
  • A pre-soak in calcium makes a noticeable difference, but in the case of peppers, I wouldn’t call the effect “desirable”.
  • Cooking in acid helps too, but the structure will eventually breakdown.  Also, peppers cooked in acid taste terrible.

To preserve nutrient content:

  • Cooking vegetables breaks down chlorophyll. But chlorophyll breaks down to pheophytin, which has many of the same health benefits.  So don’t worry about vitamin content based on color alone.
  • An alkaline environment will degrade the availability of ascorbic acid (duh!) and B vitamins.  However, the vitamins in the vegetable itself should not be affected, especially if you cool the blanched vegetables immediately.
  • Cooking sous vide seals vitamins inside vegetables.

And now for that science stuff…

What Makes Vegetables Green?

3D render of chlorophyll molecule

via wikipedia

Plants make energy through photosynthesis – the conversion of light into energy. The molecule chlorophyll absorbs light most efficiently in the red and blue portions of the visible light spectrum, but reflects most green light, which is why we see plants as green.

Chorophyll a and chlorophyll b are the types of chlorophyll most often found in plants. There are many other variants, but all chlorophylls share a common structure, as illustrated above. A magnesium ion (the yellow thing) nestles in the center of a chlorin ring. The molecule wags a long tail of carbons and hydrogens that resembles a fat molecule. This tail makes chlorophyll soluble in oil, but not in water.

What Breaks Down the Green Color in Vegetables?

chorophyl to pheophytin a illustration

adapted from wikipedia

Chlorophyll can break down through three mechanisms:

  • Heat. Heat shakes everything up and can knock a magnesium ion right out of its cozy chlorin hug. Heat accelerates the action of acids. However, heat also deactivates the enzyme chlorophyllase (see below).
  • Acids. By definition, H+ ions swim around freely in acidic environments. Since Hydrogen is more active than Magnesium, it can displace the magnesium found at the heart of chlorophyll and create pheophytin, a yellowish-green or olive drab molecule, as illustrated above.
  • Chorophyllase. Chlorophyllase is an enzyme that breaks down chlorophyll, but only over a long period of time. If vegetables are going to be stored for a long time, it helps to blanch them, which deactivates chlorophyllase. Chlorophyllase doesn’t play much of a role in cooking, however.

References: On Food and Cooking; Koca et al 2006, Effect of pH on chlorophyll degradation and colour loss in blanched green peas, Food Chemistry.

What are Some Ways to Preserve Green Color (and do they work)?

test setup

For each of my tests, I used about a quart of heavily salted water. I heated the water over high until it reached a rolling boil. I removed the product with a slotted spoon to a bowl of ice water and stirred until the peppers had cooled.  I used green peppers because they’re green and cheap.

Approach #1: Limit Cooking Time.

Heat breaks down chlorophyll and the amount of heat transferred depends greatly on how much time a vegetable stays exposed to hot water. The below test was done with salted tap water, no additives.

plain blanched peppers

My Notes:

  • 0 min: raw, crispy
  • 1 min: still raw taste, sweeter
  • 1:30: sweet, nice, good for salad
  • 3:00: rawness gone, but sweetness reduced. more of a cooked taste
  • 5:00: sorta unpleasant cooked pepper taste, soft and squishy
  • 10:00 texture like gelatin, little taste, gross.

Conclusion: For peppers, a cook time of between 2 and 3 minutes is ideal for both taste and color.

Approach #2: Try Ion Replacement

Harold McGee writes in On Food and Cooking that zinc or copper added to cooking water can replace the magnesium ion in chlorophyll and resist displacement by hydrogen. Unfortunately, both zinc and copper can be toxic in high amounts. I had read that cream of tartar can help preserve the color of blanched foods.

I wondered if the potassium in cream of tartar (its chemical name is potassium bitartrate) might be serving a function similar to copper or zinc. The below test was done with a very large pinch of cream of tartar added to the cooking water.

saturated cream of tartar

I measured in 30 second intervals for this test. The top left pepper was taken out at 30 seconds and the bottom right came out at 4:30. Unfortunately, the peppers started changing colors around the three minute mark, so it looked like this approach wouldn’t work.

After further research, it turns out that zinc and copper are special in their ability to form chlorophyll complexes that stay green in color. The potassium in cream of tartar has nothing to do with its ability to “preserve color in vegetables”. Cream of tartar is a weak acid; its presence in cooking water helps to keep red vegetables red and cauliflower white!

Reference: Y. H. Hui (ed.) 2006, Handbook of Food Science, Technology, and Engineering.

Approach #3: Tweak Acidity

peppers cooked in lemon juice

First, here’s a picture of what happens if you cook peppers in acidified water. I squeezed a lemon into the cooking water and tested.

I measured in 30 second intervals for this test. The top left pepper was fished out at 30 seconds and the bottom right came out at 5:30. I did this test hoping the acidity might prevent mushiness, but the peppers started getting noticeably mushy at 2:30. Also, most of these samples tasted pretty awful.

peppers cooked in saturated CaOH

In the next test, I used a saturated solution of calcium hydroxide to make the cooking water very alkaline. I was also hoping the calcium in the calcium hydroxide would keep the peppers firm, but the alkalinity of the solution overwhelmed it and the test became of demonstration of how mushy vegetables get when you make the cooking water too basic.

peppers get way mushy in alkaline cooking water

Like I said, the peppers got really mushy, really fast.  Yuck.

peppers cooked in slightly alkaline water

In this final test, I used a pH test kit to measure the pH of my tap water. The kit wasn’t very precise, but it showed that my water straight out of the tap was just barely acidic. I needed water that was just barely alkaline.

I tried a few different combinations of using just a tiny amount baking soda, using lots of baking soda with lots of citric acid, and using a small amount of baking soda along with a small amount of cream of tartar. I found it easiest to add a very small amount of baking soda and correct the pH using cream of tartar if necessary.

As the above image shows, the peppers came out a brilliant green and were not noticeably more mushy than peppers cooked in plain tap water. These peppers were removed at 30 seconds-3 minutes, with the bottom right one coming out at 5 minutes (just to see what would happen).

Approach #4: Keep Away from Vinaigrette

Just to see whether cooking in baking soda could possibly make the peppers more resilient to acidic marinades or vinaigrette, I soaked two batches of cooked peppers in dijon mustard (which is acidic) and water.  As is pretty clear from the above image, both samples had browned significantly within the course of an hour.  Looking back at my test pictures, I’d say the color first began to change at about the 20-minute mark.

Doesn’t Baking Soda Make Stuff Mushy?

The holy grail of cooking green vegetables is a product that is both vibrant green and crisp, like a fresh vegetable. Dave Arnold over at cooking issues had previously recommended adding calcium hydroxide to cooking water instead of baking soda to produce a result that was both green and firm.

Calcium firms texture by cross-linking with the pectin found in vegetables’ cell walls.

plain blanched peppers vs peppers presoaked in CaOH

The above image is a side-by-side comparison of plain blanched green peppers against peppers that were soaked for 30 minutes in a saturated solution of calcium hydroxide prior to blanching. The soaked peppers are on the bottom.

Here are my notes (leftmost calcium hyrdoxide test starts at 1:30, rightmost is 10 minutes).

  • 1:30: clear cement taste firm, but not crisp. More… resilient
  • 3:00: pretty much the same as 1:30
  • 5:00: softer, still somewhat firm. similar to 5:00 untreated. less taste/sweetness
  • 10:00: a little more firm than untreated, less cooked-veggie taste

Judging by the picture, above, I would say that the calcium-hydroxide-treated pepper is noticeably more green at the 10 minute mark. Unfortunately, I didn’t think the firming qualities were worth the losses in flavor.

I next did a test with a huge pinch of CaOH, but as I detailed above, the peppers came out really mushy. I suspect there wasn’t enough calcium dissolved into the cooking water to offset the mushiness caused by high alkalinity.

peppers cooked with pinch of CaOH

The above image shows peppers cooked with a small pinch of calcium hydroxide from 30 seconds up to 5 minutes (I skipped 4:30). The peppers were more green than those cooked in plain tap water, but the calcium didn’t seem to make a big difference to texture. The peppers were similar in texture to those cooked in plain tap water.

I also did tests using calcium chloride, a calcium salt that is much more soluble than calcium hydroxide. I was hoping the greater availability of calcium would help to firm the peppers, but I didn’t see a significant improvement over the baseline.

I think what happened with these tests was that the breakdown of cell walls in pepper simply overcame the strength of the gels formed between calcium and pectin.  As mentioned above, peppers soaked in calcium hydroxide for 30 minutes before cooking definitely get more firm, but not nearly as firm as a raw pepper.  Perhaps better results would be possible with a different vegetable.

How Much Baking Soda is Enough?

pH refers to the negative log of the molarity (moles/liter) of a solution, that is:

pH = -log10[H+]

Let’s assume our tap water has a starting pH of 6.5 (slightly acidic) and we want to get the water up to a pH of 7.5, slightly alkaline.  Let’s say we’re working with 1 liter of water.

starting pH = -log10[H+]
6.5 = -log10[H+]
10^-6.5 = [H+] =3.16×10^-7 moles/L

Using the same logic, we can calculate that and pH of 7.5 corresponds with [H+] =3.16×10^-8 moles/L, a tenfold decrease.

In order to end up with a pH of 7.5, we need to neutralize 3.16×10^-7 – 3.16×10^-8 moles = 2.844 × 10^-7 moles.  This is also the amount of OH- ions we are hoping to add to the solution using baking soda.

Baking soda has an acid dissociation constant (pKa) of 10.33 and a corresponding Ka of 10^-pKa = 4.67^10^-11

Then you’ll want to set up this equation: x^2 + Ka*x – [NaHCO3)*Ka = 0
where x is the amount of OH you hope to end up with and [NaHCO3] is initial amount of baking soda you need to add, which is what you’re solving for.  I used this yahoo answer to figure out this math and solved with wolfram alpha to find that an initial concentration of 0.0017 moles of baking soda are required.

Multiply the moles by the molar mass of baking soda (84g) to find that only .14 grams of baking soda would be required to increase the pH of the water a sufficient amount!

Won’t Baking Soda Destroy Vitamins?

stability of water-soluble vitamins

Adapted from the Handbook of Food Science, Technology and Engineering by Hui and others.

I’ve often seen the trick of adding baking soda to cooking water given out alongside a stern warning to never, EVER do it, because “baking soda will leach all the vitamins out from vegetables and destroy them”.  Waahh–huh?

Here’s the truth.  The above table lays it out plainly.  Ascorbic acid (vitamin C) and most of the B complex of vitamins are not stable in alkaline water.  This makes sense – most acids aren’t stable in basic solutions.  However, cooking water doesn’t have to be very alkaline in order to keep colors green.  It just has to not be acidic.

I recommend getting your hands on a few pH testing strips to better gauge the alkalinity of your cooking water.  Add a pinch of baking soda or calcium hydroxide to the water and test the pH.  If it’s too alkaline, dial it back with a pinch of cream of tartar or a dash of vinegar.

Unfortunately, water-soluble vitamins will escape into cooking water due to osmosis.  The only way to reduce vitamin loss is to keep the vegetables submerged as briefly as possible.  Its feasible to think that perhaps blanching in vegetable stock instead of tap water might reduce vitamin loss (assuming the stock would be saturated with water soluble vitamins already, thereby reducing osmotic pressure), but I haven’t found any resources to back that idea up.

I have read that Alex and Aki of Ideas in Food have experimented with sous vide (in a vaccum) blanching, which helps to lock in both the flavors and vitamins of blanched vegetables.  I haven’t tested how feasible this is yet.

Further reading: research which nutrients are most prevalent in vegetables at World’s Healthiest Foods.

So, you made it to the bottom of the post!  Congratulations :-)  Would you use baking soda in your cooking water?

About Kevin Liu

Kevin Liu is a hopeless food geek obsessed with bacon, kale, and cocktails (but not usually all at once). You can follow him at @kevinkliu. Kevin is an editor at Science Fare.

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2 Responses to Is it Possible to Keep Blanched Green Beans Really Green?

  1. Andres Torrubia October 3, 2011 at 5:39 am #

    Hi guys! I Love your blog. Any thoughts or practical tips to blanch basil? I’ve tried it a few times and noticed that if using boiling water timing is everything so maybe you could use a lower temp to deactivate browning enzymes (as you heat the water for cooking pasta anyway).

    • Kevin Liu October 3, 2011 at 4:38 pm #

      Andres, basil is a really interesting subject! I am assuming you are interested in making pesto and keeping that pesto green. Here are a few links you might be interested in: egullet discussion and chowhound post .

      To sum up the points presented, blanching helps to deactivate enzymes, which is one of the reasons pesto turns brown. Unfortunately, most enzymes only break down at 100C, the boiling point of water. This includes chorophyllase, the enzyme that specifically breaks down chlorophyll. I would put the basil into a strainer, dip the strainer in the hot water for a few seconds, then put the strainer in an ice and water bath immediately. You will also want to squeeze out as much water as possible from the basil, because water accelerates browning.

      Alternatively, you could try
      -freezing the basil, then squeeze the water out once it thaws (google cryo-blanching)
      -add some citric acid or ascorbic acid (vitamin C) to the pesto to prevent oxidation
      -cook pasta with a squeeze of lemon juice to prevent a particular type of browning that occurs when pesto touches hot pasta
      -make fresh pasta (no blanching), vaccuum pack it, and freeze it!

      hope that helps.

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