Date: March 1989 (Revised April 1995)
Source: NDSU Extension Service Nutrition Specialists
To assure good quality frozen vegetables, most need a pretreatment called blanching. When you blanch, you briefly heat vegetables in boiling water or steam to inactivate naturally-occurring enzymes in the plant. These enzymes cause undesirable changes during frozen storage. The changes include faster nutrient loss, vegetable toughening, flavor and color loss.
The brief heating also reduces the number of microorganisms on food and enhances the color of green vegetables. Blanching sufficiently wilts vegetables like spinach and chard so they pack better. Without blanching, the flavor in vegetables changes noticeably.
Blanching is simple. To blanch in water, simply place water in a large kettle or vegetable blancher and bring it to a rolling boil. You need a gallon of water for a pound of vegetables. Clean and cut vegetables as needed. Place them in a wire basket or the perforated blancher--insert and immerse in boiling water.
Start timing as soon as you put vegetables in water. The time required for blanching varies among vegetables. Call your county extension office for information on specific blenching times. Keep the kettle covered during blanching. If you have a steamer, you can use it for blanching, but it will usually take longer to adequately heat-treat the food. Nutrient losses from blanching are slightly less when you steam-blanch, but are relatively small in either case.
As soon as the time is up, remove the vegetables and immediately put them in ice cold water. Chill the vegetables until they are completely cold, about the same amount of time as blanching, then drain well and package for freezing.
You can use your microwave oven to blanch vegetables, but blanching times are longer than when you use a kettle of water on the range. You also have to work with only one or two cups of vegetables at a time, so the microwave is not as efficient to use if you are preparing a large quantity of vegetables for the freezer.
If you have further questions, contact your county office of the NDSU Extension Service.
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For More Information Contact your North Dakota County Extension Office of the NDSU Extension Service for additional information or see our main NDSU Web Page for publications and articles on Agriculture, Horticulture, Youth and Family, Business and Community and Food and Nutrition at http://www.ag.ndsu.nodak.edu/