Canning Hints

Bubbles often appear in the jar after it is removed from the canner because food is still boiling in the jar. Ordinarily bubbles do not appear once the product has been allowed to thoroughly cool.

The loss of liquid from jars during processing may be unattractive but it will not interfere with the keeping qualities of the food as long as the jar was processed correctly and is sealed.

It is better to overprocess food than underprocess as overprocessing will do little harm, but underprocessing may result in spoilage and unsafe food.

Flat sour, a type of food spoilage, is caused by canning overripe food or allowing precooked foods to stand in the jar too long before processing. It may be prevented by using fresh products and properly processing, cooling, and storing. Flat sour shows no indication of spoilage until the jar is opened.

Mold can form only in the presence of air. Therefore, jars are not sealed if mold is present.

The black deposit sometimes found on the underside of a lid is caused by tannins in the food or hydrogen sulfide which is liberated from the food by the heat of processing. This does not indicate spoilage.

If a jar does not seal, use the food at once, freeze, or repack using different lids. Reprocess for the full recommended processing time.

Two-piece vacuum caps seal by the cooling of the contents of the jar, not through pressure of the screw band on the lid. Therefore, although the screw band is firmly tight, the jar is not sealed until cooled. During processing, the flexible metal lid permits air to be exhausted from the jar.

Adjust two-piece vacuum caps by screwing bands down evenly and firmly until a point of resistance is met -- fingertip tight. Do not use undue exertion.

It is not necessary for the liquid on canned meats to congeal. The liquid will congeal only when there is a large amount of gelatin from cartilage or connective tissue present.

The loss of color from beets during canning is usually due to the variety of beets used or beets that are too old. If possible, can young, tender, very dark red beets which are freshly gathered. Precook beets with 2 inches of the stem and all of the root on, as this helps to retain the juices.

Discoloration of peaches and pears on the top of the jar is often due to enzyme activity, which means that the processing time was not long enough or the temperature not high enough to render the enzyme inactive.

Fruit which has been canned without sugar will often turn brown when exposed to air just as fresh fruit does.

Storing Canned Food

The diameter of Mason jars may vary from one manufacturer to another. Before filling Mason jars, test load your canner. It may be necessary to double-deck pint or 1/2-pint jars to reach the maximum capacity of your canner. It is recommended that you stagger the jars by placing one jar on top of two. The canning rack which accompanied your Pressure Canner/Cooker must be placed on the bottom of the canner to prevent jar breakage. Although it is not necessary to use a rack between layers of jars, if you wish to do so, a rack can be ordered by calling 1-800-877-0441, ordering online at, or writing to National Presto Industries, Inc., Consumer Service Department, 3925 North Hastings Way, Eau Claire, WI, 54703-3703.