Freezing Summer's Bounty
By Angela Treadaway
Freezing is a quick and convenient way to preserve fruits and vegetables at home. It is the method of food preservation that preserves the greatest quantity of nutrients. To maintain top nutritional quality in frozen fruits and vegetables, it is essential to:
• Select fresh, firm-ripe produce.
• Blanch vegetables as directed. All vegetables need to be blanched for longer storage life. If you do not blanch, foods will need to be eaten in a couple of months.
• Store the frozen product at zero degrees. Use within suggested storage times.
Harvested fresh, fruits and vegetables continue to undergo chemical changes that can cause spoilage and deterioration of the product. Therefore these products should be frozen as soon after harvest as possible and at their peak degree of ripeness. Enzymes in the fruits and vegetables must be inactivated to prevent the loss of nutrients and color and flavor changes that will occur.
Enzymes in Vegetables: Enzymes are inactivated by the blanching process, the exposure of the vegetables to boiling water or steam for a brief period. The vegetable must then be cooled rapidly 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 top-quality frozen vegetables. Blanching also helps to destroy microorganisms on the surface of the vegetable, brightens the color, helps retard loss of vitamins and helps make some vegetables such as broccoli and spinach more compact.
Enzymes in Fruits: The major problems associated with enzymes in fruits are the development of brown colors and loss of vitamin C. Because fruits are usually served raw, they are not blanched like vegetables. Instead, ascorbic acid (vitamin C) is used either in its pure form or in commercial mixtures such as Fruit Fresh to control the activity of the enzymes.
Other methods to control browning include soaking the fruit in diluted 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.
Preventing Rancid Flavors: Rancid oxidative flavors may develop through contact of the frozen product with air. This problem can be prevented by using a wrapping material that does not permit air to pass into the product and by removing as much air as possible from the freezer bag or container before freezing.
Cause of Textural Changes: Water makes up over 90 percent of the weight of most produce and is held within the rigid cell walls that give support, structure and texture to the fruit or vegetable. Freezing fruits and vegetables consists of freezing the water contained in the plant cell. When the water freezes, it expands, and the ice crystals cause the cell walls to rupture. The texture of the thawed produce will be much softer than it was when raw.
For example, when a frozen tomato is thawed, it turns into a pile of mush and liquid. This explains why celery, lettuce and tomatoes are not usually frozen, and why it is recommended raw frozen fruits be eaten before they have completely thawed. Textural changes due to freezing are not as apparent in products that 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.
Prevention: When produce is frozen quickly, it forms a large number of small ice crystals. These small ice crystals produce less cell wall rupture than slow freezing, which produces only a few large ice crystals. This is why it is sometimes recommended that the temperature of a freezer be set at the coldest setting several hours before foods will be placed in the freezer. The maximum amount of unfrozen product that can be frozen at one time is 2-3 pounds to each cubic foot of freezer space per 24 hours.
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 heavy-weight, moisture-proof wrap will prevent freezer burn.
MICROBIAL GROWTH IN THE FREEZER
The freezing process does not actually destroy the microorganisms that may be present on fruits and vegetables. While blanching destroys some microorganisms and there is a gradual decline in the number during freezer storage, sufficient populations are still present to multiply and cause spoilage of the product when it thaws. For this reason, it is necessary to inspect carefully any frozen products that have accidentally thawed if the power goes off or the freezer door is left open.
RECOMMENDED STORAGE TIMES
Longer storage of fruits and vegetables than what is recommended will not make the food unsafe for use but will decrease its quality.
Fruits: Most frozen fruits maintain high quality for eight to 12 months. Unsweetened fruits lose quality faster than fruits packed in sugar or sugar syrups.
Vegetables: Most vegetables will maintain high quality for 12-18 months at zero degree or lower. However, it is a good idea to use your home-frozen vegetables before the next year’s crop is ready for freezing.
SELECTING FREEZER CONTAINERS
Use good-quality freezer containers that are both moisture- and vapor-proof so moisture can be kept in the product and air kept away from it.
Moisture- and Vapor-Resistant Wraps: Heavy-weight aluminum foil, plastic-coated freezer paper and Saran are effective at excluding oxygen. They should be strong, pliable and adhere to the shape of the food item. Seal these only with tape that is designed for the freezer because other household tapes lose adhesive quality in the extremely cold freezer temperatures. Wraps are not as convenient for fruits and vegetables as plastic bags or rigid freezer containers.
Plastic Film Bags: These 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: These bags withstand temperatures from below zero degrees to above the boiling point and are suitable for both freezing and cooking the product. They come in 1½-pint and quart sizes and also as large rolls of plastic so they can be made the size desired. A heat sealer is necessary for closing these bags. These products are more expensive, but convenient because you can cook in them.
To maintain top quality, frozen fruits and vegetables should be stored at zero degrees or lower. Higher temperatures increase the rate at which deterioration can take place and can shorten the shelf life of frozen foods. Do not attempt to save energy by raising the temperature of frozen food storage above zero degrees. A freezer thermometer can help determine the actual temperature of the freezer. If the freezer has number temperature settings such as from one to nine, check the manual to see what settings are recommended for different uses. Changing temperatures in the freezer can cause the migration of water vapors from the product to the surface of the container as may be seen in improperly handled commercially frozen foods.
METHODS OF PACKING VEGETABLES
There are two basic methods for packing vegetables for freezing: dry pack and tray pack.
Dry Pack: This is the term 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 to prevent freezer burn. 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. Pack the frozen pieces into a freezer bag or container as soon as they are frozen. Long exposure will result in loss of moisture.
For more questions on preserving fruits and vegetables, contact your local county Extension agent or call Regional Extension Agent Angela Treadaway at 205-410-3696.