Measuring Headspace in Canning Jars
Headspace is the space in the jar between the top of the food or liquid and the inside of the lid. Care must be taken in filling jars to the correct headspace.
Leaving too little headspace in canning jars restricts food from expanding as it boils. Inadequate space for expanding the food may force some of it out of the jar and under the lid, leaving particles of food between the seal and the jar rim. If this occurs, the jar won't produce a vacuum seal.
Leaving too much headspace may cause a discoloration in the top portion of the food. Excess headspace can keep your jar from producing a vacuum seal if the processing time isn't long enough to exhaust the excess air in the jar.
Always use the headspace stated in the recipe. If the recipe doesn't give you a headspace allowance, use these guidelines:
- 1-inch headspace for low-acid foods -- vegetables, meats, poultry and seafood
- 1/2-inch headspace for high-acid foods -- fruits and tomatoes
- 1/4-inch headspace for juices, jams, jellies, pickles and relish, chutney, sauces and condiments
There are two methods of determining headspace: Use a ruler to locate the 1-inch, 1/2-inch and 1/4-inch position on the jar (the ridges on the top portion of the jar normally indicate the various headspace positions) or use a plastic headspace measuring tool found in some canning utensil kits.