Food Freezing Guide – North Dakota State University

Introduction

Freezing is one of the easiest, quickest, most versatile and most convenient methods of preserving foods. Properly frozen foods maintain more of their original color, flavor and texture and generally more of their nutrients than foods preserved by other methods.

Good freezer management is important. The following tips will help you get the most of your freezer dollar.

The condition of the food at the time of freezing will determine the final quality of the frozen food. Frozen food can be no better than the food was before it was frozen. Freezing does not sterilize foods as canning does. It simply retards the growth of microorganisms and slows down chemical changes that affect quality or cause food spoilage.

Freezing, heating and chemical compounds can control enzyme actions. Freezing slows enzyme activity so that many frozen foods, such as meats and many fruits, will keep satisfactorily with little or no further treatment.

Enzymes in vegetables are inactivated by heat during the recommended blanching process. Enzymes in fruits, causing browning and loss of vitamin C, are controlled by chemical compounds (antioxidants).

Oxygen in the air may cause flavor and color changes if the food is improperly packaged.

Microorganisms do not grow at freezer temperature, but most are not destroyed and will multiply as quickly as ever when the frozen food is thawed and allowed to stand at room temperature.

The formation of small ice crystals during freezing is desirable. Fast freezing is the most practical way to form small ice crystals. Large ice crystals associated with slow freezing tend to rupture the cells, causing an undesirable texture change.

Maintain temperature of 0 degrees Fahrenheit or less to keep frozen foods at top quality. The storage life of foods is shortened as the temperature rises. For example, the same loss of quality in frozen beans stored at 0 F for one year will occur in three months at 10 F, in three weeks at 20 F, and in five days at 30 F.

Fluctuating temperatures result in growth in the size of ice crystals, further damaging cells and creating a mushier product. Changes in temperature can also cause water to migrate from the product.

Improperly protected food will lose moisture, color, flavor and texture. Ice crystal evaporation from an area at the surface results in freezer burn, which is a dry, grainy, brownish area that becomes tough. Freezer burn does not render a food unsafe, only less desirable.

The prime purpose of packaging is to keep food from drying out and to preserve nutritive value, flavor, texture and color. Labels on packages will say if the product is suitable for freezer storage. A good packaging material should have the following characteristics:

The packaging you select will depend on the type of food to be frozen, personal preference and availability. For satisfactory results, do not freeze fruits and vegetables in containers larger than one-half gallon.

Packaging not sufficiently moisture/vapor-resistant for long-time freezer storage includes ordinary waxed paper and paper cartons from ice cream and milk.

Rigid containers are made of plastic, glass, aluminum and heavily waxed cardboard and are suitable for all packs. These are often reusable. Straight or tapered sides on rigid containers make it much easier to remove frozen foods.

Glass jars used for freezing should be made for the purpose. Regular glass jars may not withstand the extremes in temperature. Do not use regular, narrow-mouth canning jars for freezing foods packed in liquid. Expansion of the liquid could cause the jar to break at the neck.

Cans, such as shortening and coffee cans, are good for packaging delicate foods. Line the can with a food-storage bag and seal the lid with freezer tape because they are not airtight.

Baking dishes can be used for freezing, heating and serving. Dishes may be covered with a heavy aluminum foil taped with freezer tape. To reuse the baking dish after the food is frozen, wrap the food in casserole-wrap fashion.

Ice cube trays are good for freezing foods in small amounts. Freeze food until firm and then transfer to freezer bags.

Bags and sheets of moisture/vapor-resistant materials and heavy-duty foil are suitable for dry packed vegetables and fruits, meat, fish or poultry. Bags can also be used for liquid packs. Protective cardboard cartons may be used to protect bags and sheets from tearing and to make stacking easier.

Laminated papers made of various combinations of paper, metal foil and/or cellophane are suitable for dry packed vegetables and fruits, meats, fish and poultry. Laminated papers are also used as protective overwrap.

Keep a list of all the foods in the freezer. Update the list each time food you put food in or take it out of the freezer. Use of an inventory can prevent overstorage of foods and loss of quality.

See sample.

Most of the changes that appear during thawing are a result of freezing and storage. When food is thawed the ice crystals melt, the liquid is either absorbed back into the food or leaks out from the food. Slow, well-controlled thawing usually results in better return of moisture to the food and results in a food more like the original food than rapid thawing.

Thawing in the refrigerator is the safest thawing method. Food standing at room temperature gives microorganisms the opportunity for growth and activity.

See Thawing Fruits.

The basis for safety in refreezing foods is the temperature at which thawed foods have been held and the length of time they were held after thawing. You can safely refreeze foods that still contain ice crystals or if they are still cold, i.e. below 40 F, and have been held no longer than one or two days at this temperature after thawing. In general, if it is safe to eat it is safe to refreeze.

Unfortunately, you often dont know the time and temperature. In these cases, you need to consider the following points.

If the freezer will not be operational within a day or two:

Use dry ice if available. Twenty-five pounds of dry ice in a 10 cubic foot freezer should hold the temperature below freezing for two to three days with less than half a load and three to four days in a fully loaded cabinet if you obtain dry ice quickly following interruption of freezer operation.

Place dry ice on boards or heavy cardboard on top of packages. Open freezer only when necessary. Dont handle dry ice with bare hands as it will cause burns. When using dry ice be sure the room is ventilated.

If dry ice is not available, other options are to:

- Cover the freezer with layers of newspaper and blankets. Pin the blankets away from the air vent. The air vent must be open because the freezer needs air when electricity comes on. A blanket cover will help even when using dry ice.

- Find other freezer storage at a locker plant or with friends and neighbors. Transfer foods in insulated boxes or well-wrapped in layers of newspapers.

Refreezing needs to be done quickly. Clean the freezer before refilling. If the freezer has an adjustable temperature control, turn it to the coldest position.

Check each package or container of food. You often can check nonrigid containers without opening by squeezing to feel for ice crystals. If they need to be opened they should be carefully rewrapped.

Place the warmer packages against the refrigerated surface when possible, but leave space between packages for air circulation.

The quality of refrozen foods is diminished.

Label and use refrozen foods as soon as possible.

Foods that have defrosted have no remaining ice crystals. If defrosted foods have warmed above refrigerator temperature (40 F) they should not be refrozen, except for very high acid foods, such as fruits.

Many thawed foods, i.e. those still containing many ice crystals or a firm-to-hard core of ice in the center, may be safely refrozen.

Any signs of spoilage, off-odors or color in any food indicate you should dispose of the food without tasting.

Remember, however, that you cant rely on appearance and odor. Some foods may look and smell fine, but if theyve been at room temperature too long, food poisoning bacteria may have multiplied enough to cause illness.

Meats, such as beef, pork, veal, lamb and poultry can be refrozen when they are still firm with ice crystals. Meat still safe to eat can be cooked and refrozen. Discard meats if they have any signs of spoilage such as an off-color or off-odor.

Fruits usually ferment when they start to spoil, which will not make them dangerous to eat but will spoil the flavor. Defrosted fruits that smell and taste good can be refrozen.

Vegetables should be refrozen only if they contain plenty of ice crystals.

Shellfish, prepared foods or leftovers should not be refrozen if defrosted. If the condition of the food is poor or even questionable, get rid of it. It may be dangerous.

Never refreeze melted ice cream, cream pies, eclairs or similar foods.

Unfrosted cakes, uncooked fruit pies, breads and rolls can be refrozen.

The investment in the foods in the freezer may be significant, but so are the benefits of serving safe foods.

Recent studies have shown the nutrient content of frozen, fresh ready-to-eat and canned foods to be nearly comparable. Nutrient content is the highest when foods are preserved or eaten as soon after harvest as possible.

Fresh, tender vegetables right from the garden are best for freezing. If vegetables cannot be frozen immediately after harvesting, store them in the refrigerator to preserve freshness until they can be prepared and frozen.

See table 1, for approximate yield of frozen vegetables from fresh.

Not all vegetables freeze well. Be sure to contact your county extension office for information on recommended varieties of vegetables for North Dakota to ensure a good crop to fill the freezer.

Blanching is scalding the vegetables in water or in steam for a short time. It is a very important step in freezing vegetables because it slows or stops the action of enzymes. These enzymes are essential for growth and ripening. If the enzyme action is not stopped before freezing, the vegetables may continue maturing, develop off-flavors, discolor, or toughen so they may be unappetizing in a short time.

This heating process also wilts or softens vegetables and makes them easier to pack. Some microorganisms are killed and the color is brightened.

Blanching times vary with the size and kind of vegetable. The times recommended are just long enough to stop or destroy the enzymes. Be sure to follow the recommended blanching times.

Underblanching can stimulate the activity of some en-zymes and is worse than not blanching at all. Overblanching results in loss of vitamins, minerals, flavor and color.

Use a blancher that has a blanching basket and cover or fit a wire basket into a large kettle with a cover. A cheesecloth bag is another option. Use at least 1 gallon of water for each pound of vegetables. Put the vegetables into the basket or bag and lower the basket into the boiling water. Cover and start counting time immediately. Keep heat high and continue boiling for the time specified for the vegetable you are freezing. If boiling stops, you are blanching too large a quantity at one time.

The same water can be reused several times for blanching; just be sure to bring it back to a vigorous boil before adding vegetables.

Put 1 to 2 inches of water in a kettle and bring to a rolling boil. Suspend a thin layer of vegetables in a wire basket or cheesecloth over the rapidly boiling water. Cover, keep the heat on high, and steam blanch vegetables the time recommended in the table.

Microwave-blanched vegetables are not as acceptable as either water- or steam-blanched vegetables. Using the microwave for vegetable blanching does not save significant time. This is because the time required when using either water or steam is so short.

If you choose to use the microwave for blanching vegetables, check the oven manufacturers recommendations.

After vegetables are heated they should be cooled quickly and thoroughly to stop the cooking. After heating, plunge the basket of vegetables immediately into a large quantity of cold water. Change water frequently or use cold running water or ice water. If ice is used youll need about 1 pound of ice for each pound of vegetables. It takes about as long to cool the vegetables as it does to heat them. When vegetables are cool, remove from water and drain thoroughly.

When using rigid containers leave head space (see below). Food expands when it freezes, and the top may pop off the container if it is too full.

When using freezer bags, press air out of unfilled part of the bag, seal and freeze. If the bag has a twist closure, be sure to twist the bag and fold the bag back in a gooseneck before securing the twist. If this is not done, air will rush back into the bag.

Vegetables can be packed either solid or loose.

Solid pack: Put drained vegetables into freezer containers. Pack tightly to cut down on the amount of air in the container or bag. Seal and freeze.

Loose or tray pack: This enables you to pour out as much as you want from a large package. Spread the drained vegetables in a single layer on a tray and freeze. As soon as they are frozen, pour into rigid freezer containers or bags, seal and return to freezer. Remove as you need them.

Most frozen vegetables should be cooked without thawing them first. Corn on the cob is an exception it should be partially thawed so the cob is heated through by the time the corn is cooked. This takes about 10 minutes.

Do not overcook vegetables. Cook only until tender. Most vegetables have been partially cooked when blanched, so less cooking time is required than for fresh vegetables. Cook in a small amount of water; about cup is sufficient.

Note: The microwave oven does an excellent job of cooking frozen vegetables. Follow the manufacturers recommendations for times and power selections.

Most fruits can be frozen satisfactorily, but the quality of the frozen product will vary with the kind of fruit, stage of maturity and type of pack.

Select a variety suitable for freezing. Select fruits with a firm texture and well-developed flavor and treat them with respect. Wash small lots (2 to 3 quarts) at a time to avoid bruising. Wash through several changes of cold water, lifting produce out of the water so dirt will not settle back on the fruit. Do not let produce soak.

Fruits are prepared for freezing in about the same way as for serving. It is best to prepare enough fruit for only 2 to 3 quarts at one time.

If fruit is to be crushed, suit the method to the fruit. A wire potato masher, pastry fork or slotted spoon may be used to crush soft fruits. Firm fruits may be crushed more easily with a food chopper. A colander, food press or strainer is useful for making purees. Blenders and food processors tend to liquify the fruit too much.

Do not use galvanized ware with fruit or fruit juices because the acid in fruit dissolves the zinc, which is poisonous. Metallic off-flavors may result from the use of iron utensils, chipped enameled ware or tinware that is not well tinned.

Some fruits such as peaches, apples, pears and apricots darken quickly when exposed to air and during freezing.

While preparing, keep peeled, halved, quartered, sliced or diced fruits that turn dark in a solution of ascorbic acid. This procedure is also useful for preventing stem-end discoloration in cherries and grapes. You can get ascorbic acid in several forms:

Pure powdered form seasonally available among canners supplies in supermarkets. One level teaspoon of pure powder weighs about three grams. Use one teaspoon per gallon of water as a treatment solution.

Vitamin C tablets economical and available year round in many stores. Buy 500-milligram tablets; crush and dissolve six tablets per gallon of water as a treatment solution.

Commercially prepared mixes of ascorbic and citric acid seasonally available among canners supplies in supermarkets. Citric acid powder is sold in supermarkets, but it is less effective in controlling discoloration. If you choose to use these products, follow the manufacturers directions.

There are several types of fruit packs suitable for freezing: syrup pack, sugar pack, unsweetened pack, tray pack or sugar replacement pack.

Most fruits have a better texture and flavor if packed in sugar or syrup. Some can be satisfactorily packed without sweetening, but vitamin C losses are greatest when fruits are packed without sugar. The type of pack will depend on the intended use. Fruits packed in syrup are generally best for serving uncooked; those packed in dry sugar or unsweetened are best for most cooking purposes because there is less liquid in the product. Unsweetened packs and sugar replacement packs are often used by people on special diets.

The proportion of sugar to water used depends on the sweetness of the fruit and the taste preference of the user. A 40 percent syrup is recommended for most fruits. Lighter syrups are desirable for mild flavored fruits to prevent masking the flavor. Heavier syrups may be used for very sour fruits.

To make syrup, dissolve the sugar in lukewarm water until the solution is clear. Cool syrup before using.

Use just enough cold syrup to cover the prepared fruit after it has been settled by jarring the container (about 1/2 to 3/4 cup of syrup per pint). To keep the fruit under the syrup, place a small piece of crumpled waxed paper or other water resistant wrapping material on the top and press the fruit down into the syrup before closing, sealing and freezing.

Fruit can be packed dry. Place the clean fruit in the carton, cover and freeze, or cover with water containing ascorbic acid or packed in unsweetened juice. For fruit packed in water or juice, submerge fruit by using a small piece of crumpled water-resistant material as for syrup and sugar packs. Seal tightly and freeze.

Unsweetened packs generally yield a lower quality product than packs with sugar. However, some fruits such as raspberries, blueberries, scalded apples, gooseberries, currants, cranberries and rhubarb give a good quality without sugar.

Put fruit in a bowl or shallow pan. Sprinkle sugar over the fruit and mix gently until the juice is drawn out and the sugar is dissolved. Use a large spoon or pancake turner for mixing.

Soft sliced fruits, such as peaches, strawberries, plums and cherries will yield sufficient syrup for covering if the fruit is layered with sugar and allowed to stand for 15 minutes. Some whole fruits may be coated with sugar and frozen.

Pack fruit and juice into container. Place a small piece of crumpled water-resistant paper on top to hold fruit down in the juice. Seal and freeze.

This is a good pack for the fruits mentioned above in unsweetened pack section. Spread a single layer of prepared fruit on shallow trays. Freeze and package in freezer bags promptly. The fruit pieces remain loose. Later the bag may be opened, the needed amount taken out, and the bag reclosed and returned to the freezer.

Sugar substitutes can be used when freezing fruits. They will give a sweet taste but do not furnish the beneficial effects of sugar like color protection and thick syrup. Fruits frozen with sugar replacements will freeze harder and thaw more slowly than those preserved with sugar.

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Food Freezing Guide - North Dakota State University