Freezing Fruits and Vegetables

William Schafer and Shirley T. Munson

Freezing is a quick and convenient way to preserve fruits and vegetables at home. It is a popular method of home food preservation throughout Minnesota. Home frozen fruits and vegetables of high quality and maximum nutritional value can be produced if the directions below are followed. These directions are based on:

1. the chemical and physical reactions which take place during the freezing process;
2. scientific knowledge of the effect of freezing on the tissues of fruits and vegetables; and
3. food microbiology.

HOW TO PREPARE VEGETABLES FOR FREEZING (Chart)
HOW TO PREPARE FRUITS FOR FREEZING (Chart)

Chemical Changes During Freezing

Fresh fruits and vegetables, when harvested, continue to undergo chemical changes which can cause spoilage and deterioration of the product. This is why these products should be frozen as soon after harvest as possible and at their peak degree of ripeness.

Fresh produce contains chemical compounds called enzymes which cause the loss of color, loss of nutrients, flavor changes, and color changes in frozen fruits and vegetables. These enzymes must be inactivated to prevent such reactions from taking place.

Enzymes in vegetables are inactivated by the blanching process. Blanching is the exposure of the vegetables to boiling water or steam for a brief period of time. The vegetable must then be rapidly cooled in ice water to prevent it from cooking. Contrary to statements in some publications on home freezing, in most cases blanching is absolutely essential for producing quality frozen vegetables. Blanching also helps to destroy microorganisms on the surface of the vegetable and to make some vegetables, such as broccoli and spinach, more compact.

The major problem associated with enzymes in fruits is the development of brown colors and loss of vitamin C. Because fruits are usually served raw, they are not blanched like vegetables. Instead, enzymes in frozen fruit are controlled by using chemical compounds which interfere with deteriorative chemical reactions. The most common control chemical is ascorbic acid (vitamin C). Ascorbic acid may be used in its pure form or in commercial mixtures with sugars.

Some directions for freezing fruits also include temporary measures to control enzyme-activated browning. Such temporary measures include soaking the fruit in dilute vinegar solutions or coating the fruit with sugar and lemon juice. However, these latter methods do not prevent browning as effectively as treatment with ascorbic acid.

Another group of chemical changes that can take place in frozen products is the development of rancid oxidative flavors through contact of the frozen product with air. This problem can be controlled by using a wrapping material which does not permit air to pass into the product. It is also advisable to remove as much air as possible from the freezer bag or container to reduce the amount of air in contact with the product.

Textural Changes During Freezing

Water makes up over 90 percent of the weight of most fruits and vegetables. This water and other chemical substances are held within the fairly rigid cell walls which give support structure, and texture to the fruit or vegetable. Freezing fruits and vegetables actually consists of freezing the water contained in the plant cells.

When the water freezes, it expands and the ice crystals cause the cell walls to rupture. Consequently, the texture of the produce, when thawed, will be much softer than it was when raw. This textural difference is especially noticeable in products which are usually consumed raw. For example, when a frozen tomato is thawed, it becomes mushy and watery. This explains why celery and lettuce are not usually frozen and is the reason for the suggestion that frozen fruits, usually consumed raw, be served before they have completely thawed. In the partially thawed state, the effect of freezing on the fruit tissue is less noticeable.

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

Rate of Freezing

The extent of cell wall rupture can be controlled by freezing produce as quickly as possible. In rapid freezing, a large number of small ice crystals are formed. These small ice crystals produce less cell wall rupture than slow freezing which produces only a few large ice crystals. This is why some home freezer manuals recommend that the temperature of the freezer be set at the coldest setting several hours before foods will be placed in the freezer. Some freezer manuals tell the location of the coldest shelves in the freezer and suggest placing unfrozen products on these shelves.

All freezer manuals give guidelines for the maximum number of cubic feet of unfrozen product which can be frozen at one time. This is usually 2 to 3 pounds of vegetable to each cubic foot of freezer space per 24 hours. Overloading the freezer with unfrozen products will result in a long, slow freeze and a poor quality product.

Changes Caused by Fluctuating Temperature

To maintain top quality, frozen fruits and vegetables should be stored at 0° F or lower. This temperature is attainable in separate freezer units and in some combination refrigerator-freezers. A freezer thermometer can help you determine the actual temperature of your freezer. If your freezer has number temperature settings, such as from 1 to 9, check the manual to see what settings are recommended for different uses.

Storing frozen foods at temperatures higher than 0° F increases the rate at which deteriorative reactions can take place and can shorten the shelf life of frozen foods. Do not attempt to save energy in your home by raising the temperature of frozen food storage above 0° F.

Fluctuating temperatures in the freezer can cause the migration of water vapor from the product to the surface of the container. This defect is sometimes found in commercially frozen foods which have been improperly handled.

Moisture Loss

Moisture loss, or ice crystals evaporating from the surface area of a product, produces freezer burn—a grainy, brownish spot where the tissues become dry and tough. This surface freeze-dried area is very likely to develop off flavors. Packaging designed specifically for freezing foods will prevent freezer burn.

Microbial Growth in the Freezer

The freezing process does not actually destroy the microorganisms which may be present on fruits and vegetables. While blanching destroys some microorganisms and there is a gradual decline in the number of these microorganisms during freezer storage, sufficient populations are still present to multiply in numbers and cause spoilage of the product when it thaws. For this reason it is necessary to carefully inspect any frozen products which have accidentally thawed by the freezer going off or the freezer door being left open.

Nutrient Value of Frozen Foods

Freezing, when properly done, is the method of food preservation which may potentially preserve the greatest quantity of nutrients. To maintain top nutritional quality in frozen fruits and vegetables, it is essential to follow directions contained in this bulletin for pretreatment of the vegetables, to store the frozen product at 0° F and to use it within suggested storage times.

Storage Times for Frozen Foods and Vegetables

Fruits—Most frozen fruits maintain high quality for 8 to 12 months. Unsweetened fruits lose quality faster than those packed in sugar or sugar syrups.

Vegetables—Most vegetables will maintain high quality for 12 to 18 months at 0° F or lower. However, it is a good idea to plan to use your home frozen vegetables before the next year crop is ready for freezing.

Longer storage of fruits and vegetables than those recommended above will not make the food unfit for use, but will decrease its quality.

Selecting Freezer Containers

You must use good quality freezer containers to maintain the quality of frozen fruits and vegetables. A high quality container should be both moisture and vapor proof so that moisture can be kept in the product and air kept away from it.

There are two types of packaging materials for home use: rigid containers and flexible bags or wrappings.

Rigid containers made of plastic are suitable for all packs and are especially good for liquid packs. They are often reuseable and make the stacking of foods in the freezer easier. Cardboard cartons for cottage cheese, ice cream and milk are not sufficiently moisture-vapor resistant to be suitable for long-term freezer storage. Covers for rigid containers should fit tightly.

Flexible freezer bags and moisture-vapor resistant wrapping materials such as plastic freezer wrap, freezer paper and heavy-weight aluminum foil are suitable for dry packed products with little or no liquid. Regardless of type, press to remove as much air as possible before closing.

Methods of Packing Fruits

There are three ways to pack fruits for freezing: sugar pack, syrup pack, and unsweetened pack. Although some fruits may be packed without sweeteners, the flavor of many fruits is retained better with the use of sugar. Gooseberries, currants, cranberries, blueberries, and rhubarb give good quality packs without or with sugar.

To freeze fruits using sugar pack, sprinkle the required amount of sugar over the fruit. Gently stir until the pieces are coated with sugar and juice.

To make sugar syrup, dissolve the needed amount of sugar in cold water. Stir the mixture and let stand until the solution is clear.

Methods of Packing Vegetables

There are two basic methods for packing vegetables for freezing, the tray pack and the dry pack.

Dry pack—This is the method used to describe the packing of blanched and drained vegetables into containers or freezer bags. Pack the vegetables tightly to cut down on the amount of air in the container. If the vegetables are packed in freezer bags, press air out of the unfilled part of the bag. When packing broccoli, alternate the heads and stems.

Tray Pack—This is the method of freezing individual pieces of blanched and drained vegetables on a tray or shallow pan, then packing the frozen pieces into a freezer bag or container. This method produces a product similar to commercially frozen plastic bags of individual vegetable pieces and is particularly good for peas, corn, and beans.

In this method it is most important to pack the individually frozen pieces into a bag or container as soon as they are frozen.

Freezing Vegetables

1. Assemble the necessary equipment for processing vegetables.
   • a large kettle (minimum capacity of 2 gallons)
   • a colander, wire basket, or net bag for blanching
   • large pans for cooling
   • ice cubes or ice blocks for cooling
   • knives
   • plastic freezer bags or other containers
   • a timer or a clock with a second hand
   • hot pads
     
     
2. Choose vegetables for freezing that are at their peak of flavor and texture. If possible, harvest the vegetables in the cool part of the morning and process as quickly as possible. If the freezing process is delayed, immerse the vegetables in very cold water or refrigerate in shallow trays to preserve quality and nutrients.
   
   
3. Carefully follow the blanching instructions on the included chart for each vegetable. Start counting the blanching time as soon as the water returns to a boil.
   The quality of water used to blanch the vegetables can have an effect on the texture of certain vegetables. Very hard water can cause the toughening of vegetables such as green beans. If you have problems with excessively tough green beans, check into the level of hardness in your water supply.

To Blanch in Boiling Water

• Use 1 gallon water for each pound of vegetable except for leafy greens, which need 2 gallons per pound.
• Bring water to rolling boil.
• Immerse wire basket or blanching basket mesh bag containing vegetable.
• Cover kettle and boil at top heat the required length of time (see chart). Begin counting time as soon as the water returns to a boil. You may use the same blanching water 2 or 3 times. Keep it at required level. Change the water if it becomes cloudy.
• Cool immediately in ice water for same time used for blanching. Keep chilling water ice cold.
• Drain the vegetables thoroughly. Extra water will form too many ice crystals.
• Pack using dry or tray pack method.
• Freeze.

To Blanch in Steam

• Put 1 inch of water in kettle, bring to a rolling boil.
• Suspend a thin layer of vegetable in wire basket or loose cheesecloth over rapidly boiling water.
• Cover and steam blanch vegetable required amount of time.
• Complete as for boiling water blanching.

Microwave Blanching

Some directions are available for microwave blanching of vegetables. Incomplete inactivation of enzymes may occur due to the variability of microwave heating resulting in off flavors and loss of texture and color.

Microwave blanching will not save time or energy.

To Freeze Fruits

1. Wash and sort fruits carefully and discard parts that are of poor quality.
2. Prepare fruits as you will use them.
3. Check the chart for fruit being frozen to see if an anti-browning treatment is suggested. Use ascorbic acid preparation as recommended in the chart or in the manufacturer's instructions.
4. Use dry sugar, or sugar syrup in proportions suggested in the chart. Dissolve sugar needed in cold water. Stir. Allow to stand until sugar is completely dissolved. Do not heat. You may hold sugar syrup 2 days in the refrigerator. If you are preparing a sugarless pack of fruits that brown, be sure to treat with ascorbic acid or other anti-browning agents.
5. Pack into good plastic bags, freezer containers or freezer jars. Allow ½-inch headspace for expansion. Pack fruits, such as peaches, that tend to darken, in rigid containers and under the syrup by placing crumpled wax paper between lid and fruit.

Produced by Communication and Educational Technology Services, University of Minnesota Extension.

In accordance with the Americans with Disabilities Act, this material is available in alternative formats upon request. Please contact your University of Minnesota Extension office or the Extension Store at (800) 876-8636.

University of Minnesota Extension is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, color, creed, religion, national origin, sex, age, marital status, disability, public assistance status, veteran status, or sexual orientation.

The information given in this publication is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by University of Minnesota Extension is implied.