When you throw what you've boiled into an ice bath to halt the cooking process and preserve the color and texture, that's called blanching.
Blanching is a cooking process in which a food, usually a vegetable or fruit, is scalded in boiling water, removed after a brief, timed interval, and finally plunged into iced water or placed under cold running water (known as shocking or refreshing) to halt the cooking process.
Blanching is a must for most vegetables to be frozen. It slows or stops the enzyme action which can cause loss of flavor, color and texture. The blanching time is very important and varies with the vegetable and size. Underblanching stimulates the activity of enzymes and is worse than no blanching.
Blanching causes cell death and physical and metabolic changes within food cells. Heat damages cytoplasmic and other membranes, which become permeable and result in loss of cell turgor (Fig. 11.4).
Lack of nutrients in cooking water, high energetic costs, high water consumption and recycling are some drawbacks of vegetable blanching.
Blanching stops enzyme actions which otherwise cause loss of flavor, color and texture. In addition, blanching removes some surface dirt and microorganisms, brightens color and helps slow vitamin losses. It also wilts greens and softens some vegetables (broccoli, asparagus) and makes them easier to pack.
Blanching helps to stabilize the color, especially of peas and other green vegetables, and protects flavor and texture. Blanching also helps to cleanse the surface of vegetables, destroying microorganisms on the surface, and it wilts or softens vegetables and makes them easier to pack.
You may blanch with steam, hot water (scalding), or in a microwave oven. Steaming is preferable to scalding because some of the nutrients that are water-soluble can be lost in the blanching water. Steaming retains these nutrients to a greater degree.
Most vegetables take between 2-5 minutes. When the vegetables are done, quickly remove them from the boiling water with a slotted spoon and plunge them into the ice bath to stop the cooking process.
There are two types of blanching—water and steam. Water blanching is typically the best for home freezing, although steam blanching is best for broccoli, sweet potatoes, winter squash, pumpkin. Steam blanching takes about 1.5 times longer than water blanching. Timing is critical and varies based on veggies and size.
The general wisdom for blanching vegetables is to boil them in a large volume of water, always with salt, and then shock them in ice water.
Blanching helps vegetables keep their vibrant colors and retain nutrients, and stops the enzymes that would otherwise lead to spoilage. Freezing vegetables without blanching them first results in faded or dulled coloring, as well as off flavors and textures.
Blanching as a unit operation is a short time heating in water at temperatures of 100° C or below.
Shocking, a step that typically follows blanching, involves plunging just-blanched vegetables into ice water to immediately stop the cooking process. Doing so keeps the vegetables' color bright and their texture crisp-tender.
In order to stop the enzyme action that breaks vegetables down, the food must reach 180 degrees Fahrenheit. Blanching will take up to a minute longer at high altitudes, since water boils at a lower temperature.
Think of the blanch water as a brine. You want about 1½ cups of kosher salt per gallon of water.
Vegetables that do not require blanching before drying are onions, green peppers, and mushrooms. Water blanching: Bring a large pot of water (two-thirds full) to a rolling boil.
Drop bag in very cold water to cool (same time as blanched). Drain on paper towel or cloth. Arrange pretreated vegetables on drying trays in single or thin layers, 1/2 inch deep or less. Dry in dehydrator or oven as described below.
This is a clue that blanching is more of a prep technique than a cooking technique. Blanching does not, and should not, cook the food. If the food is going to be cooked, that will happen later. Otherwise, blanched food is still considered raw.
To blanch broccoli, start by bringing a large pot of water to a boil. Once boiling, submerge chopped broccoli florets into the pot. It does not take long to blanch broccoli – only about 30-60 seconds, so keep an eye out for when the color changes from dull green to bright green.
Blanching can help clean the surface of vegetables and can kill bacteria, molds and other organisms existed on the surface of vegetables. Blanching stops the enzyme action. This will help preserve the taste, color, and texture of the vegetable. Blanching helps get the vegetables prepared for long-term freezer storage.