Food Science: Why Blanched Vegetables (Sometimes) Turn Brown

A few weeks ago, we talked about blanching as a great way to prepare vegetables for salads and crudite platters.

Cooking them quickly in boiling water brings out both their flavor and their bright colors.

That is to say, except for when they turn brown! Why does this happen? Read on!

The most likely reason our veggies turned brown is because we covered the pot with a lid after adding the vegetables.

Yes, covering the pot certainly seems like the logical thing to do! After all, a covered pot conserves energy, brings the water back up to a boil, and keeps the steam inside the pot–all things that would theoretically help our veggies cook more quickly.

Here’s what is really going on in that pot:

As the veggies hit the boiling water, volatile acids are released into the water and are carried away in the steam.

When the pot is covered, the steam and the acids it contains are forced back into the water. Once there, the acids react with the chlorophyll in the vegetables, turning them an unsightly shade of brown.

A similar reaction will take place if there’s too little water in the pot (thus concentrating the acids in the water) or if you overcook the vegetables (thus prolonging the exposure time to the acids).

Solutions? Use a large amount of boiling water for blanching, test the vegetables frequently to check their doneness, and leave the pot uncovered!

Got a kitchen question tickling your noggin? Ask away!

Related: Why Sliced Fruit Turns Brown

(Image: Flickr member darwinbell, modified from original by Emma Christensen under Creative Commons license)