The World's Healthiest Foods

What is blanching?

Blanching is a term that usually refers to the very brief cooking of a fresh vegetable prior to freezing. Blanching typically involves vegetables rather than other types of food (like fruit or meat) because vegetables are particularly susceptible to enzyme-triggered changes that can result in lost flavor, lost color, and lost texture during freezing.

The prevention of unwanted changes in flavor, color, and texture is not the only purpose of blanching, however. Other purposes include:

It is worth remembering that the freezing of a food does not destroy bacteria, yeast, or molds that might be present on the food. Freezing (down to a temperature of 0°F/-18°C) will inactivate these micro-organisms, but when the food is eventually thawed, these micro-organisms may become active once again. Given these factors, the blanching of fresh vegetables prior to freezing makes sense to lower potential risk of activity on the part of unwanted micro-organisms.

Experts agree that blanching must be carried out very carefully in terms of time. If you "overblanch" your vegetables, you will end up destroying a wide variety of nutrients as well as flavor, color, and texture. If you "underblanch," you may end up increasing the activity of enzymes you were seeking to de-activate.

Types of Blanching

There are two basic types of blanching. The first type uses boiling water and involves direct placement of the fresh vegetable into the boiling water for a brief period of time. Direct immersion in boiling water is sometimes referred to as "water blanching." While this process is usually carried out on the stovetop, it can also be done in a microwave in a microwave-safe container. (If you decide to water blanch in a microwave, we recommend Pyrex(TM) glass as a container because of the potential risk of plastic residues migrating from the container into the food during the microwaving process.)

The second type uses steam. It requires the use of a steamer basket or double boiler that avoids immersion of the vegetable directly in the boiling water and instead surrounds the vegetable only with steam.

Both water blanching and steam blanching are followed by quick and brief immersion of the vegetable into cold water. As a guideline for cooling in ice cold water, simply keep the vegetable immersed in the cold water for the same amount of time as was used for steaming. For example, if you steamed for 3 minutes, immerse in cold water for 3 minutes as well.

We recommend steam blanching versus water blanching to minimize nutrient loss. Steam cooking helps to minimize nutrient loss by minimizing water contact with the surface of the food. Research on vegetable steamingâ€"especially steaming for very short periods of timeâ€"provides consistent evidence of nutritional benefits from this method of heating in comparison to boiling.

Practical Tip

In terms of time, optimal steaming of a food usually brings out its most vibrant color. Once those greenest greens or most yellow yellows shine forth in a vegetable being steamed, it's usually time to stop the steaming process and move on to the cold water immersion step in blanching. In addition to watching closely for these color changes, however, it can be critical to make use of a kitchen timer that can be set for the precise lengths of time that correspond to the amount of steaming needed.

References

Andress EL and Harrison JA. (2006) So easy to preserve. 5th Edition. Bulletin 989. Cooperative Extension Service, University of Georgia, Athens, GA.

Food Safety and Inspection Service (2010). Freezing and Food Safety. The U.S. Department of Agriculture Food Safety and Inspection Service (FSIS), Washington, D.C. Available online at: http://www.fsis.usda.gov/factsheets/

National Center for Home Food Preservation. (2011). How do I freeze? University of Georgia, Athens, GA. Available online at: http://www.uga.edu/nchfp/how/freeze/blanching.html