Preserving foods for another day has been a primary focus of human ingenuity for many thousands of years. Even in the modern age of refrigeration and rapid transportation, with fresh produce available year-round, many people prefer to can or freeze local produce when it's available. Most vegetables benefit from blanching, or immersion in boiling water, before they're frozen. Some cooks add either vinegar or baking soda to the blanching water, but neither is usually necessary or desirable.
What Blanching Does
Fresh green beans contain a number of enzymes intended to break down their flesh and help it decompose, providing a food source for the seeds inside their pods. For humans intending to eat those beans, that's an undesirable outcome. Freezing slows the action of the enzymes but doesn't stop it entirely. That's why they're usually blanched in boiling water before freezing. Boiling the beans for just three minutes deactivates the enzymes, improving the storage life of your beans. It also gives them a vividly green color, much brighter than the beans appeared when they were uncooked.
Beans and Color
Like other green vegetables, green beans derive their color from a pigment called chlorophyll, which plays a key role in their ability to convert sunlight into nutrition. Ordinarily the chlorophyll's color is muted by air contained inside the beans' cell walls, as if you were looking at them through a sheet of bubble-wrap packaging. When the beans are immersed in boiling water, that air heats, expands, and forces its way out of the cells. This leaves the green chlorophyll much more visible, and the color correspondingly more vivid. Beans are usually only blanched for three minutes and then plunged into ice water, so the chlorophyll's color won't be muted by extended cooking.
Vinegar in Blanching
A number of fruits and vegetables, from apples and artichokes to potatoes and bananas, are prone to discoloring when they're cut. Soaking them in water made acidic with lemon juice or vinegar prevents this, and some cooks reason that adding vinegar to blanching water also helps prevent discoloration. In the case of beans and other green vegetables, acidity causes a chemical reaction that quickly robs the chlorophyll of its color and leaves the vegetables dull, drab and unappealing.
Soda in Blanching
A better-founded piece of folklore calls for adding a few pinches of baking soda or a shiny new penny to the blanching water. Both prevent the chemical reaction that dims the color of the beans, but at a cost. Over a period of years, copper can accumulate in your body and become toxic. Baking soda has no such ill effect, and gives the beans a brilliant and durable green hue. Unfortunately it also ruins their texture, softening the cell walls and giving the beans a mushy texture.
For frozen green beans that display both a vivid green color and good texture, it's best to rely on good technique rather than folklore. Blanch your beans in a very large quantity of water -- a gallon or more per pound -- so the water remains at or near a boil when you add your beans. That means less time in the water to ensure three minutes of boiling, and protects the chlorophyll from overcooking. If a litmus strip shows that your tap water is acidic, with a pH below 7, you might need to add a small pinch of soda just to bring it back to the pH of normal tap water.
- On Food and Cooking: The Science and Lore of the Kitchen; Harold McGee
- Clemson Cooperative Extension: Freezing Fruits & Vegetables
- Photo Credit Hemera Technologies/AbleStock.com/Getty Images