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Extension > Food > Food Safety > Preserving and Preparing > Canning > Canning Basics 3: Ensuring High Quality Canned Foods

Canning

Canning Basics 3: Ensuring High Quality Canned Foods

William Schafer, Food Technologist — Department of Food Science and Nutrition

Reviewed 2012 by Suzanne Driessen, Extension Educator — Food Safety.

Begin with high-quality, fresh foods suitable for canning. Quality varies among different fruits and vegetables. Examine food carefully for freshness and wholesomeness. Discard diseased and moldy food. Trim small diseased or discolored areas from food.

Can fruits and vegetables picked from your garden or purchased from nearby producers when the products are at their peak of quality. This is 6 to 12 hours after harvest for most vegetables. For best quality, apricots, nectarines, peaches, pears, and plums should be ripened one or more days between harvest and canning. If you must delay the canning of other fresh produce, keep it in a shady, cool place.

Fresh, home-slaughtered red meats and poultry should be chilled and canned without delay. Do not can meat from sickly or diseased animals. Put fish and seafoods on ice after catching or buying. Clean (gut, devein, etc.) immediately and can within two days.

Maintaining Color and Flavor in Canned Food

To maintain good natural color and flavor in stored, canned food, you must:

Follow these guidelines to ensure that your canned foods retain optimum colors and flavors during processing and storage:

Advantages of Hot Packing Over Raw Packing

Many fresh foods contain from 10 percent to over 30 percent air. The length of time canned food retains high quality depends on how much air is removed from food before jars are sealed.

Raw-packing is the practice of filling jars tightly with freshly prepared, but unheated food. Such foods, especially fruit, will float in the jars. The entrapped air in and around the food may cause discoloration within two to three months of storage. Raw-packing is more suitable for vegetables processed in a pressure canner.

Hot-packing is the practice of heating freshly prepared food to boiling, simmering it 3 to 5 minutes, and promptly filling jars loosely with the boiled food. Whether food has been hot-packed or raw-packed, the juice, syrup, or water to be added to the foods should also be heated to boiling before adding it to the jars. This practice helps to remove air from food tissues, shrinks food, helps keep the food from floating in the jars, increases vacuum in sealed jars, and improves shelf life. Pre-shrinking food permits filling more food into each jar.

Hot-packing is the best way to remove air and is the preferred pack style for foods processed in a boiling water canner. At first, the color of hot-packed foods may appear no better than that of raw-packed foods, but within a short storage period, both color and flavor of hot-packed foods will be superior.

Controlling Headspace

The unfilled space above the food in a jar and below its lid is termed headspace. Directions for canning specify leaving ¼ inch for jams and jellies, ½ inch for fruits and tomatoes to be processed in boiling water and from 1 to 1¼ inches in low-acid foods to be processed in a pressure canner. This space is needed for expansion of food as jars are processed, and for forming vacuums in cooled jars. The extent of expansion is determined by the air content in the food and by the processing temperature. Air expands greatly when heated to high temperatures; the higher the temperature, the greater the expansion. Foods expand less than air when heated.

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