Freezing Fruits and Vegetables
Freezing is a quick and convenient way to preserve fruits and vegetables at home. It is a popular method of home food preservation. 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:
- the chemical and physical reactions which take place during the freezing process;
- scientific knowledge of the effect of freezing on the tissues of fruits and vegetables; and
- food microbiology.
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
County extension offices frequently receive questions about whether certain fruits, vegetables, or mixtures of either may be successfully frozen. Such questions can be answered by knowing the effect of freezing on various plant tissues.
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, lettuce, and tomatoes 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 0Â° 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 0Â° 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 0Â° 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, 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 in heavyweight, moistureproof wrap will prevent freezer burn. Freezer wraps will be discussed later.
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 leaflet for pretreatment of the vegetables, to store the frozen product at 0Â° 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 0Â° 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 wrap should be both moisture and vapor proof so that moisture can be kept in the product and air kept away from it.
Many moisture- and vapor-resistant wraps, such as heavyweight aluminum foil, plastic coated freezer paper, saran, and other plastic films, are effective at excluding oxygen. They should be strong, pliable, and adhere to the shape of the food item. These can be sealed easily with heat or freezer tape. Be sure to use only tape that is designated for the freezer because other household tapes lose adhesive quality in the extremely cold freezer temperatures. These wraps are not as convenient for fruits and vegetables as plastic bags or rigid freezer containers.
Plastic film bags made especially for freezing are readily available. They seal with twist and tie tops. Collapsible cardboard freezer boxes are frequently used as an outer covering for plastic bags to protect them against tearing, and for easy stacking in the freezer. Plastic sandwich bags and bread wrappers are not suitable for freezing.
“Freeze-and-cook” bags withstand temperatures from below 0Â° F to above the boiling point and are suitable for both freezing and cooking the product. These come in 1Â½ pint and quart sizes and also as large rolls of plastic so that they can be made the size desired. A heat sealer is necessary for closing these bags. These products are more expensive but allow greater convenience.
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.
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
* plastic freezer bags or other containers
* a timer or a clock with a second hand
* hot pads
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.
Carefully follow the blanching instructions in the included table for each vegetable. Count the blanching time from when the vegetable is immersed in the vigorously boiling water.
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 table). Begin counting time as soon as you place the vegetable in water. 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 as listed on table. * Complete as for boiling water blanching.
Microwave Oven 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 a shorter storage time in the freezer.
Lack of standardization of power levels among various microwave ovens make it impossible to publish a blanching timetable that can be used with all microwave ovens. Follow the instructions in the manufacturer’s microwave cookbook.
Vegetables blanched in the microwave should be chilled in ice water and processed as regular frozen vegetables.
To Freeze Fruits
Wash and sort fruits carefully and discard parts that are of poor quality.
Prepare fruits as you will use them.
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.
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.
Pack into good plastic bags, freezer containers or freezer jars. Allow Â½-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.
To Use Home Frozen Produce
Fruits?Thaw fruit at room temperature in its original package to preserve quality and nutritive value. If faster defrosting is required, submerge (if watertight) in cool or lukewarm water or follow microwave defrosting instructions. Serve as soon as defrosted, preferably while a few ice crystals remain.
Vegetables?All vegetables may be cooked from the frozen state except corn-on-the-cob, which should be partially defrosted. Cook frozen vegetables in a small amount of salted water (about Â½ cup or less). Cook only until tender?about half as long as if the same vegetable were fresh. You can use a pressure saucepan for cooking frozen vegetables. Follow manufacturer’s directions for cooking time. A pack should be thawed enough to break it up before pressure cooking.