Headspace

Preserving Headspace Tool
Alternative: "kitchen only" ruler
What is headspace?    
Headspace is the distance you measure between the top of the liquid (i.e. jam) in the jar and the jar rim. You can measure this with a headspace tool (pictured, left) or you could use a clean plastic ruler instead.
It's important to fill jars, remove bubbles using the other end of the headspace tool (or using a non-metallic utensil) then measure the headspace of every jar precisely before putting the lids on to ensure you are preserving safely.
Headspace does vary depending on what you are preserving, and also which method you are using (boiling water bath or pressure canning).
    
Why do we need headspace?
Leaving this space between the jar contents and the jar lid allows sufficient room for the contents to expand during boiling (processing). The hot air in the jar rises to the top of the jar below the lid, and this air is forced out the jar. This results in a high quality vacuum seal when the air cools and pulls the lid down tightly after processing (usually within a few minutes after processing you will see this happen, but it is important to cool your jars overnight before removing bands from Mason jars and storing your preserves).
   
An incorrect headspace could result in a poor seal.
If there is too much headspace, this will result in a "false seal". The hot air cools and pulls the lid downwards, but there is still air in the jar. Over time, jars may become unsealed, harbour mould growth and/or discolour the surface of your jar's contents.
If there is too little headspace, this could result in a bubbling mess as the jar contents are pressed out of the jar when boiling during processing. Some of the contents may become wedged between the lid and the jar - so the jar will not seal properly and may unseal over time and/or allow contamination.
Pressure canning is at a higher temperature than boiling water canning, which is why the headspace is more (1 inch) for pressure canning. During pressure canning, the air heats up more and needs more room than water bath canning headspace.
   
Which headspace do I use when canning?
ALWAYS follow the headspace specified in your preserving recipe, however if the headspace is not listed, I have added a general guide below:
  • 1/4 inch (0.5cm) Headspace
Chutneys, Fruit Butters, Jams, Jellies, Marmalades & Spreads [Boiling Water Bath Canning]
  • 1/2 inch (1cm) Headspace
Fruits, Pickles & Tomatoes [Boiling Water Bath Canning]
  • 1 inch (2.5cm) Headspace
Meat, Stock, Soup, Non-Pickled Vegetables, etc [Pressure Canning]
   
Still unsure? Contact Food Preserving

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