When blanching green vegetables, should you cover the pot?
Blanching involves briefly dunking fruits or vegetables in boiling water to set color, flavor, and texture. Some sources say that to successfully blanch green vegetables, the lid must be kept off of the pot to allow the acids they contain to evaporate, rather than trapping them in the cooking water and turning the vegetables brown.
To examine the validity of this claim, we blanched batches of broccoli, green beans, and broccoli rabe in both covered and uncovered pots. After blanching, the pH (acid) level of the water in both pots was identical for each vegetable. Moreover, all of the vegetables were bright green, and the covered and uncovered batches tasted exactly the same. The bottom line: When it comes to blanching, it makes not a whit of difference whether you cover the pot or not.