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Canning -- Equipment

What causes canning jars to break?

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There are several types of breaks that can occur during canning. Each break looks different and can be used to identify the problem. Thermal shock is characterized by a crack running around the base of the jar that may extend up the side of the jar slightly. This could be described as the bottom falling out of the jar. To prevent this from happening, avoid sudden temperature changes, such as putting a hot jar on a cool or wet surface or putting hot food or liquid in a room-temperature jar, or plunging raw packed jars into boiling water. Also use a rack in the canner. Avoid using metal knives or spatulas to remove air bubbles or steel wool pads to clean jars as this might weaken the jar and make it susceptible to thermal shock. Internal pressure breaks are characterized by a vertical crack that divides and forks into two fissures. This is usually on the side of the jar away from the name imprint. In order to prevent pressure breaks, be sure to use the correct amount of headspace, keep heat steady when processing, and do not reduce the pressure artificially by running canner under cool water or jiggling the pressure control. Impact breaks originate at the point of impact and form a spider web network around the impact. To prevent this, handle jars carefully without bumping them into each other. Avoid the use of metal utensils to remove air bubbles. PREPARED BY: Angela M. Fraser, Ph.D., Associate Professor/Food Safety Specialist, and Carolyn J. Lackey, Ph.D., R.D., L.D.N., Professor/Food and Nutrition Specialist, North Carolina State University (August 2004)

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