Freezing is an excellent easy and non-chemical method of preservation of fruits and vegetables at home before the next year's crop is ready. Crops such as small berries, corn, peas, beans, asparagus, broccoli, peppers and leafy vegetables can preserve by this method. Harvested fresh fruits and vegetables continue to undergo chemical changes that 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. Enzymes in the fruits and vegetables must be inactivated to prevent the loss of nutrients and color and flavor changes that will occur. The freezing process is a combination of the beneficial effects of low temperatures at which microorganisms cannot grow.
During freezing water makes up over 90 percent of the weight of most produce and is held within the fairly rigid cell walls that give support, structure and texture to the fruit or vegetable. To maintain top quality, frozen fruits and vegetables should be stored at 0°F 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 0°F. A freezer thermometer can help determine the actual temperature of the freezer. 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.
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 will maintain high quality for 12 to 18 months at 0°F or lower. To maintain top nutritional quality in frozen fruits and vegetables, it is freezing essential to select fresh, firm-ripe produce, blanch vegetables, store the frozen product at 0°F.
The tray pack and the dry pack are the two basic methods for packing vegetables for freezing. Tray Pack is a 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 bag or container as soon as they are frozen. Long exposure will result in loss of moisture. Do not leave headspace for these packs. Dry Pack 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. When packing broccoli, alternate the heads and stems. Allow 1.0-1.5 cm headspace (except for loose packing vegetables such as asparagus and broccoli that do not require headspace).
Steps for freezing vegetables
Choose vegetables for freezing that are at their peak of flavor and texture. Harvest the vegetables in a cool morning and process immediately or refrigerate the vegetables until processed to preserve quality and nutrients. Carefully follow the blanching instructions. Count the blanching time from when the vegetable is immersed in the vigorously boiling water. Enzymes are inactivated by the blanching process, which is the exposure of the vegetables to boiling water or steam for a brief period of time. 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.
Steps for freezing fruits
Wash fruit in cold water before it is hulled or peeled, gently lifting fruit from the water. Wash 2 to3 times, or until water is clean. Don't leave fruit soaking in water. Light-colored fruits- Apples, peaches, bananas, etc. quickly turn brown when cut because of the reaction of oxygen in the air and natural enzymes in the fruit. 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 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. 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. Most fruit have a better color, flavor and texture if packed with sugar or syrup.
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I am working as a scientist at NBPGR, New Delhi