Freezing Fruits and Vegetables Module- 13 Lec- 13 Dr. ShishirSinha Dept. of Chemical Engineering IIT Roorkee
Freezing Fruits and Vegetables Advantages of Freezing Many foods can be frozen. Good natural color, flavor and nutritive value retained. Texture usually better. Takes less time Easy to do Foods can be frozen in any quantity How Freezing Affects Food Textural Changes The water in food freezes and expands. Ice crystals cause the cell walls of fruits and vegetables to rupture, making them softer when thawed. Some vegetables with very high water content do not freeze well: celery, lettuce, some tomatoes. How Freezing Affects Food To prevent color and flavor changes, as well as loss of some nutrients, enzymes should be controlled Enzymes are small proteins in foods that start or help with reactions, such as those that cause browning, off-flavors, ripening, mushiness, etc Are slowed down but not destroyed during freezing. How Freezing Affects Food Enzymes in Vegetables Are destroyed by blanching which is a quick heat and quick cooling Enzymes in Fruits Usually controlled the addition of sugar and antioxidants such as ascorbic acid ( vitamin C) or ascorbic acid mixtures
How Freezing Affects Food Fluctuating Freezer Temperatures Ice in food thaws a little and then refreezes Ice crystals get bigger each time. Food becomes mushy as large ice crystal growth damages cells Moisture is pulled out of the food Other quality losses speeded up due to higher temperatures. Moisture Loss Freezer burn tough and dry, but safe. General Freezing Guidelines o 1. Freeze foods at 0 F or lower. o 24 hours in advance of freezing large quantities of food, set freezer at -10 F or lower 2. Keep work areas, containers, utensils clean 3. Follow established directions. 4. Freeze foods immediately after prep. 5. Do not overload freezer Freeze amount that will freeze in 24 hours (2 to 3 pounds of food per cubic foot). 6. Pack already frozen foods together to prevent thawing 7. Place unfrozen foods in contact with surfaces and in coldest parts of freezer 8. Leave space around packages so cold air can circulate. 9. Arrange frozen foods so that the foods frozen longer can be used first 10. Check thermometer 11. Time of storage- 1 year at 0 F Packaging Moisture-vapor resistant
Prevents transfer of moisture and air in and out of the package Durable and leak-proof Does not become brittle and crack at low temperatures Resistant to oil, grease or water Protects foods from absorption of off flavors or odors. Easy to seal and label. Packaging Rigid Containers Plastic freezer containers. Wide-mouth canning/freezing jars. Good for liquids or soft, juicy, or liquid-packed foods. May be reusable. Hold their shape and can be stored upright. Figure 1 Rigid Containers packaging Packaging Bags Wraps» plastic (such as polyethylene)» heavy-duty aluminum foil» laminated paper- freezer paper
Figure 2 Bags & wraps packagingg Packaging Vacuum Sealers - Always defrost in the refrigerator - Cut several holes in the plastic so environment in the bag is not air-free Figure 3 vacuum packaging
Packing Foods Food must be cool Pack in serving size quantities. Figure 4 packaging food Packing Foods to be Frozen Pack foods tightly Avoid trapped air (oxygen) Allow for headspace as food may expand except: uneven vegetables like broccoli and asparagus Figure 5 Packing Foods to be Frozen
Packing Foods- Sealing Press all air from bagged foods. Except for headspace. Use tight lid on rigid containers Keep sealing edges clean and dry. Use freezer tape over seams of looser- fitting covers. Trapped food or liquids in sealing area will freeze, expand, and loosen seal. Always label with date and contents Freezing Fruits Frozen in many forms Whole, sliced, crushed, juiced. Best quality Optimum maturity and freshness. Immature or overripe both produce lower quality when frozen. Wash and work with small amounts at a time to preserve best quality. Figure 6 Freezing Fruits Preventing Fruit Darkening Ascorbic acid ( Vitamin C)
Heating the fruit Ascorbic Acid Mixtures - Fruit Fresh and others Sweetened Packs for Fruit Sugar Syrup Pack Better texture. Not needed for safety. Fruits should be covered with syrup. Place crumpled water-resistant paper in top of container. - Have some other added ingredients. -Follow package directions to obtain correct strength Do not work as well: Citric acid Lemon juice Sugar syrup Salt/vinegar solution Figure 7 Preventing Fruit Darkening
Preparing Peaches in Syrup Figure 8 Preparing Peaches in Syrup Sweetened Packs for Fruit Sugar Pack Sliced soft fruits (strawberries, peaches, etc.) make their own syrup when mixed with the right proportion of sugar. Layer fruit and sugar in bowl or pan. Allow mixture to stand 15 minutes to make juice or syrup before packaging. Unsweetened Packs for Fruit Dry Pack Good for small whole fruits
such as berries that don t need sugar. Simply pack into containers and freeze. Or may be frozen individually, in single layer, on a tray first. Tray pack next slide Figure 9 Unsweetened Packs for Fruit Dry Tray Pack for Fruit Fruit pieces may be frozen individually, in single layer, on a tray first. Freezee until firm then package in rigid container or bag. Will pour out of container easily when frozen. Figure 10 Dry Tray Pack for Fruit
Dry Tray Pack for Fruit Can remove only the amount needed at one time. Fruit pieces retain shapes. Fruit pieces do not clump as when packed directly into containers or with sugar syrup. Figure 11 Dry Tray Pack for Fruit Unsweetened Packs for Fruit Pectin Syrup Good for strawberries and peaches. Mix 1 package powdered pectin and 1 cup water. Bring to boil, boil 1 minute. Remove from heat, cool and add 1-3/4 cups more water. Water or Unsweetened Juice Packs
Texture will be mushier. Color poorer. Freezes harder, takes longer to thaw. Sugar Substitutes May be used in the pectin syrup, juice or water packs. Or could be added just before serving. These do not help with color retention or texture, like sugar does. Use amounts on product labels or to taste. Freezing Vegetables Select young, tender, high-quality vegetables. Sort for size and ripeness. Wash and drain before removing skins or shells. Wash small lots at a time, lifting out of water. DO NOT SOAK. Work in small quantities, preparing as directed. Preventing Flavor and Color Changes in Vegetables Blanching Primary method to destroy enzymes for vegetables. Will also soften hard veggies to make packaging easier. Will also remove some microorganisms. Under-blanching can be harmful; it will stimulate enzymes and not destroy them. Check required blanching times for each food. How to Blanch Vegetables Use specific directions. Work in small quantities.
Figure 12 Blanch Vegetables How to Blanch Vegetables In Boiling Water Use blancher with lid or a kettle with basket and lid. Have 1 gallon water per 1 lb. of vegetables. Place vegetables in blanching basket. Lower vegetable into vigorously boiling water. Put lid on. Water should hardly stop boiling or return to a boil within a minute. If water keeps boiling, begin timing immediately. Otherwise, wait for water to come back to a boil. How to Blanch Vegetables Steam Blanching Use kettle with tight lid and basket. 1 to 2 of boiling water in bottom of pan. Vegetable should be in a single layer in basket. Start timing when covered.
Takes 1-1/2 times longer than water blanching. Check times, however, for each food. How to Blanch Vegetables Microwave Blanching Not widely recommended at this time. May not be effective enzymes not inactivated completely by uneven heating. Usually does not save time. Have to do very small quantities. If you have directions from a source you trust, try small quantities at first and see if you like the quality after a period of frozen storage. This is not a safety issue, as long as frozen food is always stored frozen, but improper blanching will affect quality. How to Blanch Vegetables After blanching in water or steam, cool immediately in cold water. Change water frequently or use running water or iced water (1 lb. ice per 1 lb. vegetable). Cooling time should be the same as the blanching time. Drain thoroughly. Types of Pack for Vegetables Dry Pack Pack after the vegetables are blanched, cooled, and drained. Pack quickly, pushing air out of package as you work towards top. Types of Pack for Vegetables Tray Pack After draining, spread pieces in a single layer on a shallow pan. Freeze firm.
After first hour, check often. Package quickly, pushing air out as you work.