The five factors that are responsible for most of the quality losses of frozen foods are enzymes, air, microorganisms, large ice crystals, and evaporation of moisture.
Enzymes and Blanching
Enzymes are naturally occurring substances in plants that control the ripening process. Freezing only slows enzyme activity. Most frozen vegetables will lose quality in the freezer unless they have been blanched.
Why Blanch Vegetables?
- To improve flavor, color, texture, and nutrient retention
- To slow or stop the action of enzymes in the ripening process
- To cleanse the surface of dirt and organisms
- To brighten the color of green vegetables
- To wilt or soften vegetables, making it easier to fill containers
Correct blanching time is critical in having a quality product. Refer to the blanching directions in this fact sheet. Most vegetables are blanched in boiling water. Steam blanching normally takes 50 percent more time than water blanching and is ideal for delicate vegetables.
Exclusion of air from the food prevents enzyme reactions and oxidation, which causes surface browning. This problem is more common in fruits, but some vegetables, such as potatoes, are also affected.
Bacteria, molds, and yeast are present on all fresh foods and multiply rapidly when the temperature is between 40°F and 140°F. Unlike canning, freezing does not kill most microorganisms in food, but it does prevent their growth if the food is held at 0°F or lower. When thawed, the surviving organisms can grow again. This is why proper handling and preparation techniques are essential.
Ice Crystals—Freeze Quickly
Small ice crystals are desirable in frozen food to preserve its texture. Large ice crystals rupture food cells and cause a soft, mushy texture. Small crystals are formed when food is frozen quickly and kept at a constant storage temperature of 0°F or lower. Avoid adding more than 2 pounds of frozen food per square foot of freezer space because a larger volume of food will slow the freezing process and may raise the temperature of already frozen food.
Evaporation of Moisture—Packaging Materials
Poor packaging that leaves food unprotected in the freezer allows foods to lose moisture, which will cause a loss of color, flavor, and texture. Long-term exposure to air causes drying of plant fibers, known as freezer burn. Proper packaging materials aid in preventing freezer burn.
Good Packing Materials
- Moisture-vapor resistant
- Durable and leak proof
- Resistant to cracking and brittleness at low temperatures
- Resistant to oil, grease, and water
- Able to protect foods from absorption of off-flavors and odors
- Easy to seal
- Easy to label
Waxed paper, paper cartons, cottage cheese cartons, ice cream and juice cartons, or any rigid carton with cracks or a poorly fitting lid are not suitable for long-term storage. They do not adequately prevent the loss of moisture or the drying out of food from exposure to air.
Tips for Packing Vegetables
- Cool or chill foods before filling them into packages.
- Package foods in quantities that will be used for a single mealsized serving.
- Allow ½ inch of headspace for all types of containers. Vegetables
- that pack loosely, such as asparagus and broccoli, require no headspace.
- When vegetables are packaged in bags, press the air from the bag.
- Label packages with the name of product, added ingredients (such as salt), date packaged and date to use by, number of servings or quantity, and type of pack (such as whole, sliced, or diced).
- Freeze foods as soon as they are packaged and sealed.
- Do not overload the freezer with unfrozen food.
- Spread unfrozen foods out in the freezer so that they will freeze more rapidly.
Individually Quick Freezing or Tray Freezing
Foods such as cut green beans, peas, whole-kernel corn, and small mixed vegetables are suitable for freezing quickly before being packaged. This allows a partial amount of the food to be poured from containers without being thawed. After vegetables have been blanched, cooled, and drained, place them one layer deep on cookie sheets or shallow trays and freeze uncovered justuntil solid (4 to 6 hours), then quickly package and seal.
- Bring 1 gallon of water to an active boil. Lower 1 pound of vegetables into the water. Cover. Return to a boil. Start counting the blanching time when the water returns to a boil.
- As soon as blanching is complete, vegetables should be cooled quickly in 3 to 4 gallons of cold water.
- Chill at least as long as vegetables were blanched.
- Asparagus, small spears: 2 minutes
- Asparagus, large spears: 4 minutes
- Green, wax, or Italian beans, small: 2 minutes; large: 3 minutes
- Broccoli, 1½-inch pieces: 3 minutes
- Brussels sprouts, small heads: 3 minutes; large heads: 5 minutes
- Cabbage, quarters: 4 minutes; wedges: 2 minutes; shredded: 1½ minutes
- Carrots, sliced or diced: 2 minutes; whole: 5 minutes
- Cauliflower, small pieces: 3 minutes; large pieces: 5 minutes
- Corn, whole cut kernel or cream style: 4 minutes
- Corn on the cob, small ears: 7 minutes; medium ears:
- 9 minutes; large ears: 11 minutes
- Okra, small pods: 3 minutes; large pods: 5 minutes
- Mushrooms: Steam whole 5 minutes, buttons or quarters 3 1/2 minutes, slices 3 minutes.
- Peas, black-eyed and green, small: 1½ minutes; large: 2½ minutes
- Sugar peas, small: 2 minutes; large: 3 minutes
- Zucchini or summer squash, ¼- to ½-inch slices: 3 minutes
- Chopped onions and peppers usually don’t need blanching.
Sweet potatoes, pumpkin, spaghetti squash, and tomatoes should be cooked before freezing. Steam blanching normally requires 50 percent more time than water blanching. Steam blanching is less likely to cause water-soaked vegetables and is ideal for broccoli and other delicate vegetables.
If you don’t have a blancher, substitute a colander, sieve, or deep-fryer basket.
Did You Know?
If you have problems with frozen cauliflower turning dark, try blanching it in boiling water that contains 1 Tablespoon lemon juice per quart of water. Cooked tomatoes may be frozen successfully. Raw, whole tomatoes do not freeze well and may become watery and develop an off-flavor after a month in the freezer.
Prepared by Luke LaBorde, associate professor of food science and Martha Zepp, extension project assistant
SeriesLet's Preserve: Freezing Vegetables
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