Why Allow Headspace when Canning and Freezing Food?

Picture this scenario--you are filling jars with hot, crushed tomatoes and have just a little left. You evenly distribute the extra tomatoes between each jar, filling nearly to the top, thinking it is good economics. Unfortunately, if jars are filled too full, food may boil out during processing, and solids or seeds may catch under the sealing compound and prevent the jar from sealing. Without that good seal, the food may end up spoiling, and all your work is for naught.

Next scenario—you are filling jelly jars and need just a little more jelly to make another jar. If you take just a little from each of the other jars, you will have that extra jar—Whoa! What is the problem with that? The processing time may not be long enough to drive all that extra air out of the jar, resulting in a poorly formed vacuum seal. Additionally, the extra air left inside the jar could cause the food to discolor.

Headspace in Canned Foods

• Headspace is the space in the jar between the inside of the lid and the top of the food or its liquid. 
• Headspace will vary from ¼ inch for jams and jelly, ½ inch for tomatoes, to 1 to 1¼ inches for meat and poultry. Always follow recipe guidelines.
• Occasionally use a ruler to check the accuracy of the headspace. Not all jars are shaped the same, so you cannot count on a certain groove or the neck of the jar being a set distance from the rim.
• The headspace you start with may not be the same as when you finish. Foods that are packed into jars hot may shrink when cooled. Air spaces in raw-packed foods rise to the top of the jar when heat processed and may increase headspace. 
• Siphoning may cause the loss of liquid in jars. Raw packing food and using heavy sugar syrups increases siphoning. Loss of liquid can be reduced by allowing the jars to sit in the canner for 5 minutes after processing is completed before removing the jars from the canner. 

Note: If the liquid is lost during processing and the jar is sealed, Do Not open the jar to replace the liquid. Liquid loss is not a sign of spoilage.

Headspace in Frozen Foods

Headspace is needed for frozen foods with a high moisture content. Liquid expands when frozen, causing syrup to overflow. Headspace varies by the type of pack and the size of the container. 

• Fruit packed in juice, sugar, syrup, or water, as well as crushed or pureed fruit, should have ½-inch headspace in wide-top pint containers and 1-inch headspace in wide-top quart containers. 
• The same products should have ¾-inch headspace in pint containers with a narrow opening or 1½-inches in a quart container with a narrow opening. 
• Juice needs 1½-inches headspace in all narrow jars. 
• Fruits and vegetables packed without added sugar or liquid only need ½-inch headspace in either pint or quart containers, regardless of opening size. 
• Vegetables that pack loosely have space to expand between the food pieces and don't need extra headspace. This includes asparagus, broccoli, cauliflower, Brussels sprouts, cauliflower, peppers, and foods that are individually quick frozen (tray packed).

Sources:

LaBorde, L.; Zepp, M.; and Hirneisen, A. (2023, March 1). "Let's Preserve: Basics of Home Canning". Penn State University.

LaBorde, L.; Zepp, M.; and Hirneisen, A. (2023, May 22). "Let's Preserve: Freezing Fruits". Penn State University. 

LaBorde, L.; Zepp, M.; and Hirneisen, A. (2023, June 8). "Let's Preserve: Freezing Vegetables". Penn State University.