Frequently Asked Questions
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Canning -- Miscellaneous
Why do air bubbles appear in canned food?
Air bubbles may appear on the surface or throughout canned foods. Some air bubbles are inevitable. However, processing by the hot pack method helps minimize them since air is driven out of the plant tissue during heating. With less air present, there is less chance of air-related problems: floating fruit, liquid loss, and discoloration.
Air bubbles or pockets create safety problems because they can provide the aerobic conditions necessary for some bacteria to survive. The metabolism of these organisms can result in lowered product acidity (pH increase). This is not a big safety concern for an acidic product like applesauce but is a potential problem for a borderline product like tomatoes. In addition to hot packing products, consumers should be advised that "knifing out air bubbles" is an important step in helping to reduce the amount of air in the jar prior to processing. Exhausting during processing will then remove remaining air and insure a tight seal. There are other reasons for this: 1) The enzyme, polyphenol oxidase, exists in many fruits and vegetables. In the presence of oxygen (air) this enzyme reacts with foods to produce darkening of the pigments (browning). The temperature of water bath canning is not sufficient to completely inactivate this enzyme. 2) There is a difference in heat conductivity of air and food. Large air bubbles in home canned food result in uneven heating which might allow some areas in the jar to be underprocessed.