University of Missouri Extension

GH1501, Reviewed June 2003

Quality for Keeps: Freezing Basics

Barbara J. Willenberg
Associate State Food and Nutrition Specialist
Department of Food Science and Human Nutrition

Freezing is one of the easiest and least time-consuming methods of food preservation. Most foods retain their natural color, flavor and texture better than when other methods of food preservation are used. In addition, the kitchen remains cool and comfortable during the process.

Disadvantages of freezing include the initial investment for equipment — it costs a great deal to purchase and maintain a freezer. In addition, the size of the freezer limits the amount of storage space, and the freezing process gives some foods an undesirable texture.

Growth of spoilage organisms

Freezing does not destroy spoilage organisms; it merely stops their growth temporarily. During the freezing process, microbial growth can occur under the following circumstances:

Keep the freezer temperature at or below 0 degrees Fahrenheit to prevent the growth of spoilage organisms and to minimize changes in flavor, texture and nutritive value of food.

To prevent contamination of all foods by spoilage organisms, always keep your equipment, work surfaces and hands clean. Washing produce thoroughly before freezing removes garden soil, a source of spoilage organisms.

Freezing does not destroy Clostridium botulinum, the spoilage organism that causes the greatest problem in canning low acid foods like vegetables and animal products. However, Clostridium botulinum will not grow and produce toxin (poison) at correct freezer temperatures (0 degrees Fahrenheit or below). Therefore, freezing is a safe and easy alternative to pressure canning low-acid foods.

When thawing food, remember that freezing did not destroy any spoilage organisms that might have been present in the food. As the temperature of food rises during thawing, growth of spoilage organisms begins. The faster the food warms up, especially on the surface, the faster the growth of spoilage organisms occurs. If you store raw or precooked frozen foods long enough at a high enough temperature after thawing, spores of Clostridium botulinum can grow and produce toxin. For this reason, we recommend that food be thawed in the refrigerator where spoilage organisms will not grow as quickly.

Chemical changes during freezing

Enzymes are proteins present in plants and animals. While the plant or animal lives, enzymes help speed up the ripening and maturing processes. Even after we harvest plants or slaughter animals, enzyme reactions can continue and result in undesirable color, flavor and texture changes in the food. Freezing slows down, but does not destroy, enzymes in fruits and vegetables. That is why it is important to stop enzyme activity before freezing. The two methods you can use are blanching and adding chemical compounds such as ascorbic acid.

Enzymes in vegetables

Enzymes in vegetables are inactivated by blanching. To blanch vegetables, place them in boiling water or steam for a brief period of time. Next, cool the vegetables rapidly in ice water (this prevents further cooking). Although some publications state that blanching is unnecessary, it is essential for top-quality, frozen vegetables.

Blanching also helps to destroy microorganisms on the surface of the vegetables. It makes vegetables like broccoli and spinach more compact, and as a result, they take up less room in the freezer. Follow the recommended time for blanching each vegetable carefully. Over-blanching results in a cooked product and a loss of flavor, color and nutrients. Under-blanching speeds up enzyme activity and is worse than no blanching at all.

Enzymes in fruits

Enzymes in fruits can cause browning and loss of vitamin C. People generally serve fruits raw, so instead of blanching fruits to control enzyme activity, they are treated with a chemical compound. The most common additive is ascorbic acid (vitamin C). Ascorbic acid may be used in its pure form or in commercial mixtures of ascorbic acid and other ingredients such as sugar.

Some publications suggest using an acid solution (citric acid or lemon juice) to control browning for a short time. These publications instruct you to place fruit in the acid solution while preparing it for the freezer. Then place the fruit in freezer containers and either freeze it dry or cover the fruit with unsweetened fruit juice. Acid solutions work well as holding treatments for fruit but do not prevent browning as effectively as treatment with ascorbic acid.

An alternative to holding fruit in an acid solution is to prepare a syrup (light, medium or heavy) that has ascorbic acid added to it. After preparing fruit for the freezer, it is then added to the syrup and packaged for freezer storage.

Rancidity in foods

Fats in meat, fish and poultry become rancid during freezer storage. This is caused by contact with air left in the package or air that enters the package because proper storage materials were not used. However, even with proper packaging materials, rancidity will occur over time. Off flavors are the result of this chemical change. Control rancidity by trimming excess fat from meat before freezing, using a wrapping material that prevents air from reaching the product, and by storing foods for the recommended length of time.

Changes in food texture during freezing

Freezing involves the change of water contained in the food from a liquid to a solid (ice). When water freezes it expands, and the ice crystals formed cause cell walls of food to rupture. As a result, the texture of the product will be much softer when it thaws.

These textural changes are most noticeable in fruits and vegetables that have a high water content. For example, when frozen lettuce thaws, it turns limp and wilted. This is the reason vegetables with a high water content, such as celery and salad greens, are not usually frozen (Table 1). It is also the reason why many frozen fruits are best served while they still contain a few ice crystals. The effect of freezing on fruit tissue is less noticeable when fruit is still partially frozen.

Textural changes due to freezing are not as apparent in products that are cooked before eating because cooking also softens cell walls. Textural changes are also less noticeable in high-starch vegetables, such as peas, corn and lima beans.

Table 1
Foods that do not freeze well

Cabbage, celery, cres, cucumbers, endive, lettuce, parsley, radishes

Cucumbers and cabbage can be frozen as marinated products such as "freezer slaw" or "freezer pickles." These do not have the same texture as regular slaw or pickles.

Irish potatoes, baked or boiled

Cooked macaroni, spaghetti or rice

Egg whites, cooked

Meringue

Icings made from egg whites

Cream or custard fillings

Milk sauces

Sour cream

Cheese or crumb toppings

Mayonnaise or salad dressing

Gelatin

Fruit jelly

Fried foods

Rate of freezing

The amount of damage to cell walls can be controlled by freezing products as quickly as possible. In rapid freezing, a large number of small ice crystals are formed. These small ice crystals cause less damage to cell walls than slow freezing, which produces larger ice crystals.

For best results, freeze foods at 0 degrees Fahrenheit or lower as soon as they are packaged and sealed. Set the temperature control at -10 degrees Fahrenheit or lower about 24 hours in advance to help food freeze rapidly. Do not overload your freezer with unfrozen food. Add only the amount that will freeze within 24 hours, which is usually two to three pounds of food per cubic foot of freezer space. Overloading results in a long, slow freeze and a poor quality product.

Place packages in contact with refrigerated surfaces in the coldest part of the freezer. Leave a little space between packages so that air can circulate freely. When the food is frozen, packages can be restacked close together.

To maintain top quality, store frozen foods at 0 degrees Fahrenheit or lower. This temperature can be maintained in separate freezer units and in some combination refrigerator-freezers. A freezer thermometer can help determine the actual temperature of your freezer.

Spoilage occurs more quickly and shelf life is shorter when you store frozen foods at a temperature higher than 0 degrees Fahrenheit. For example, the same loss of quality in frozen beans stored at 0 degrees Fahrenheit for one year will occur in three months at 10 degrees Fahrenheit, in three weeks at 20 degrees Fahrenheit and in five days at 30 degrees Fahrenheit. Do not attempt to save energy in your home by raising the temperature of frozen food storage above 0 degrees Fahrenheit.

Temperatures that fluctuate up and down cause the ice in foods to thaw slightly and then refreeze. Each time this happens, smaller ice crystals become larger, further damaging cells and creating a mushier product. Frequent changes in temperature also cause water to move out of the product resulting in a less juicy product that is generally lower in quality and nutritive value.

For highest quality and nutritive value, use home frozen foods within the recommended storage times given in Table 2. Food will still be safe to eat after the recommended time period is past if the freezer has been kept at 0 degrees Fahrenheit or lower. However, the quality and nutritive value will be lower.

Table 2
Storage times for home-frozen foods

For more extensive information on the recommended storage time for freezer foods, see MU publication GH 1505, Freezing Home Prepared Foods.

Butter

Margarine

Cheese

Cream

Eggs

Fish

Fruit and fruit juice

Ice cream or sherbet

Meat

Milk

Poultry

Home prepared foods

Vegetables

Yogurt

Moisture loss (freezer burn)

Moisture loss, or ice crystals evaporating from the surface area of a product, produces freezer burn. Freezer burn appears as a fuzzy, grayish white spot on the food surface. Freezer burn is not harmful, but it causes off-flavors and dries out and toughens food. Packaging food in moisture/vapor-proof containers or wrapping and storing food for the recommended length of time will help prevent freezer burn. Covering fruit with syrup and cooked meat with gravy or sauce helps prevent freezer burn in these products.

Tip
It is no bargain to recycle plastic vegetable or bread bags and cottage cheese, ice cream or milk containers. They are not moisture/vapor-proof and food frozen in them will lose moisture, flavor and nutrients.

Containers for freezing

Proper packaging material protects the flavor, color, moisture content and nutritive value of frozen foods from the dry climate of the freezer. Selection of containers depends on the type of food to be frozen, personal preference and types that are readily available. Foods in larger containers freeze too slowly to result in a satisfactory product. For example, do not freeze fruits and vegetables in containers larger than a half-gallon.

In general, packaging materials for the freezer must have the following characteristics:

There are two types of packaging materials for use in home freezing: rigid containers and flexible bags or wrap made for freezer use. If labels of packaging material do not state that the material is for freezer use, it probably isnt.

Rigid containers
Rigid containers made of plastic or glass are suitable for all packs and are especially good for liquid packs as well as fragile or easily broken food. The straight sides on rigid containers make the frozen food much easier to get out and make it easier to stack foods in the freezer. Most rigid containers can be reused.

Regular glass jars, including jars made specifically for canning, break easily at freezer temperatures. If using glass jars, choose wide mouth, dual purpose jars made for freezing and canning. These jars have been made to withstand extremes in temperatures. The wide mouth and straight sides allow easy removal of partially thawed foods. Covers for rigid containers should fit tightly. If they do not, reinforce the seal with freezer tape. Freezer tape is especially designed to stick at freezing temperatures. Do not use masking tape because it will not stick at low temperatures.

Flexible bags or wraps
Bags and sheets of moisture/vapor-proof materials (labeled freezer bags or freezer wrap) and heavy-duty aluminum foil are suitable for dry-packed vegetables and fruits, meats, fish or poultry. Protective cardboard cartons may be used to protect bags and sheets against tearing and to make stacking easier.

Packaging and labeling foods

Effect of freezing on spices and seasonings

When preparing food for freezing, especially dishes that contain several ingredients, it is more convenient to add all ingredients before placing the food in the freezer. However, some spices and seasonings change during freezer storage. To avoid undesirable changes in the food product during freezer storage, note the following:

Tip
Season foods lightly before freezing, and add additional seasonings when reheating or serving.

A freezer plan

You get maximum convenience and economy through carefully planning the use of your freezer. The secret lies in an easy management plan.

Plan the freezer contents. Freeze foods you need and use often. Freeze only the foods your family likes and in amounts you will use within the recommended storage period.

Budget freezer space first for basic foods such as meats, vegetables and fruits. Then, if there is room, plan to freeze baked goods, main dishes, snacks or desserts.

Keep contents organized. An orderly freezer holds many more packages than a disorderly one. Stack similar foods together. Packages you will use first should be the easiest to get to. Use baskets, shelves, or dividers that are furnished with the freezer to help organize the contents.

Keep an inventory. You will always know what is on hand if you keep an inventory of the foods that are in the freezer. A current inventory can help you plan meals and remind you to use old packages of food within their recommended storage times.

Keep the inventory in a handy place so food can be added and subtracted without much trouble. Remember, the simpler it is to use, the more you will use it. You might want to use an index card system with a separate card for each food item. This makes it easy to add items to the inventory and to determine the total amount of a specific food on hand. A sample freezer inventory is shown below.

Table 3
Sample freezer inventory

Food Number of packages Date frozen Number of packages removed
Green beans Pints: 10 8/03 IIIII
Carrots Quarts: 5 7/03 III
Corn Pints: 20 7/03 IIIII I

 

GH1501 Quality for Keeps: Freezing Basics | University of Missouri Extension

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