Food Preservation: Freezing Basics

Freezing is the easiest, most convenient, and least time-consuming food preservation method. Most foods freeze well with the exception of produce with a high-water content, cream-based items, and cooked starchy foods such as cooked noodles and rice.

All fresh produce contains enzymes, which are compounds that help the plant ripen and mature. During freezing, enzyme action is slowed but not stopped. If not inactivated, these enzymes can cause color and flavor changes, and loss of nutrients during freezer storage.

Freezing slows the growth of microorganisms (bacteria, molds, yeasts) that cause food spoilage or illness. As soon as food thaws, microorganisms continue to grow.

Inactivation of Enzymes in Vegetables

Blanching is essential for obtaining quality frozen vegetables. Blanching vegetables before freezing inactivates the enzymes that impact product quality. Blanching is accomplished when vegetables are exposed to boiling water or steam for a brief period and then rapidly cooled in cold water (60 degrees Fahrenheit or below) for the same amount of time to prevent cooking.

Blanching also helps destroy microorganisms on the surface of the vegetables. When blanched, vegetables such as broccoli and spinach become more compact. Following the recommended times for blanching a specific vegetable is important. Over-blanching results in a cooked product with a potential loss of flavor, color, and nutrients. Under-blanching stimulates enzyme activity and is worse than no blanching at all.

Microwave blanching is not recommended as it produces uneven results because of varied heat patterns within a microwave oven. Microwave blanching requires working with only small quantities at a time—no time is saved when working with large quantities of vegetables.

Prevention of Color Changes in Fruit

Enzymes in fruits can cause browning and loss of vitamin C. Fruits are not usually blanched. Instead, ascorbic acid (vitamin C) is used to control enzymes in frozen fruits. Commercial mixtures of ascorbic acid are available for home use. Citric acid, lemon juice, and packing fruit in sugar or sugar syrup will also control browning, but ascorbic acid is the most effective for controlling browning.

Prevention of Off Flavors

Rancid flavor is another type of change that can occur in frozen products. This occurs when fat, such as found in meat, is exposed to air over a period of time. It can be controlled by using a storage method that does not allow air to reach the product. To prevent the development of rancid flavor, remove as much air as possible from the freezer bag or container used to store the product before freezing it.

Freezer Storage

Store frozen fruits and vegetables at zero degrees Fahrenheit or lower for the best quality. Use a freezer thermometer to ensure the temperature is correct. Storing frozen foods at temperatures higher than zero degrees Fahrenheit increases the rate of deterioration and shortens the shelf life of foods. Fluctuating freezer temperatures can cause the ice in the food to thaw slightly and then refreeze. Every time this happens smaller ice crystals form larger ones, further damaging cells and creating a mushier product.

Moisture loss and the evaporation of ice crystals from the surface of a product both produce freezer burn—a grainy, brownish spot where the tissues become dry and tough. Freezer-burned food is likely to develop an off flavor, but it is not an indicator of food safety. Packaging products in heavyweight, moisture-resistant materials prevents freezer burn.

Containers for Freezing

Foods for the freezer should be packed properly to protect their flavor, color, moisture content, and nutritive value. Select packaging materials that offer all of the following characteristics:

• food grade (intended to be in contact with foods)
• moisture and vapor-resistant, durable, and leak-proof
• resistant to oil, grease, and water
• not susceptible to becoming brittle and cracking at low temperatures
• able to protect foods from absorbing other flavors or odors during storage
• easy to seal
• easy to label

Suitable packaging materials include rigid plastic containers with straight sides made for freezing, glass jars and lids made for freezing, heavy-duty aluminum foil, moisture-vapor-resistant bags, and freezer paper. Disposable containers intended for short-term storage, such as bread bags; yogurt, milk, or ice cream tubs; and regular aluminum foil or waxed paper, do not provide effective protection against flavor and moisture loss, or freezer burn during long-term storage.

Packaging Foods

Cool all food and syrup before packing. Pack food in quantities suitable for your intended use.

Pack cold food tightly into containers. Allow ample space between the food and the closure, called headspace, for food expansion upon freezing. The amount of space needed will vary depending on the food and the size of the containers. When packing food in bags, remove excess air before sealing. Label and date each package. It is also helpful to list the number of servings on the label.

For quick freezing, spread newly packaged food among frozen foods leaving a small space between packages. Add only the amount of unfrozen food to the freezer that will freeze within 24 hours—about 2–3 pounds of food to each cubic foot of freezer capacity.

Additional Resources

Note: This fact sheet uses language, recipes, procedures, and data from reliable sources such as the ones listed below. For the most part, the original text has been preserved because safety around food preservation is especially important. For safe and healthy preserved foods, be sure to carefully follow directions.

Learn more about freezing food:

• Freezing Cooked Foods
  (ohioline.osu.edu/factsheet/HYG-5308)
• Freezing Fruits
  (ohioline.osu.edu/factsheet/HYG-5349)
• Freezing Meat, Poultry, and Game
  (ohioline.osu.edu/factsheet/HYG-5334)
• Freezing Vegetables
  (ohioline.osu.edu/factsheet/HYG-5333)
• Freezing and Canning Venison
  (ohioline.osu.edu/factsheet/hyg-5367)
• Preserving Herbs: Freezing and Drying
  (ohioline.osu.edu/factsheet/hyg-5360)
• Freezer Storage
  (ohioline.osu.edu/factsheet/HYG-5402)
• What To Do When Your Freezer Stops
  (ohioline.osu.edu/factsheet/HYG-5357)
• The National Center for Home Food Preservation
  (nchfp.uga.edu)
• University of Georgia Extension, So Easy to Preserve
  (extension.uga.edu/publications/detail.html?number=B989&title=so-easy-to-preserve)

Adapted from information originally compiled by Ruth Anne Foote, Extension Educator, Family and Consumer Sciences, Ohio State University Extension.

Revised September 30, 2016 by Kate Shumaker, Extension Educator, Family and Consumer Sciences, Ohio State University Extension-Holmes County.