Why should you leave an empty space at the top of your preserve?

Why should you leave an empty space at the top of your preserve?

Why should you leave an empty space at the top of your preserve?

We explain the reasons why it is essential that in your gourmet preserves there is an empty space at the top

If you are already making gourmet preserves and/or prepared dishes, probably you already have this in hand, although you may not know why you do it.

However, if you are planning to start canning soon, we can tell you that this topic is an essential requirement in the pasteurization or sterilization of any canned food.

Did you know that the empty space at the top is technically known as headspace?

Not all preserves require headspace, this depends on the type of packaging you use and its lid.

There are many types of containers on the market, made of different materials and with different closures. For example:

  • Glass vessels
  • Cans or metal containers
  • Plastic containers: semi-rigid trays, bags or pouches

Of all of them, the only ones in which a twist off or screw cap is used are the glass ones. It is in these that we must necessarily leave a headspace.

Remember that…

Only jars with twist-off or screw-on lids require you to leave an empty space at the top.

Reasons why you should leave an empty space at the top of your preserves

Producer making preserves in glass bottles and leaving empty space for subsequent sterilization.

We let you know the reasons. The headspace in twist-off or screw cap containers has two basic functions:

1st and most important: To achieve hermetic sealing.

This means that during pasteurization or sterilization a vacuum is created inside the container that keeps the lid tightly closed throughout its lifetime until it reopens and makes the typical ¡blop! sound.

2nd: Allow the ingredients to expand when heated.

Keep in mind that if you overfill the canister, it could happen that when the food expands, it will come out through the lid gasket, so that the canister will lose part of its contents and the autoclave and the rest of the canisters will get dirty. This is a very common problem in preserves made with oil in which the space is not left empty.

Why only on twist-off or screw top containers?

The reason is its closure and not its packaging. And it is because of their roughness, they are the only covers with a gasket, which, if you look closely, coincides with the area where it comes into contact with the glass. When heated, this roughness allows the expanded air to escape from the canister and when it cools down, it seals the canister hermetically and no longer allows air to enter or escape.

As the container heats up, the air in the headspace expands. This air will expand more or less depending on:

  • The space we leave, the more space, the more expansion.
  • The temperature we apply in pasteurization and / or sterilization, the more heat, the more expansion.

As the gasket heats up, it allows air to escape due to the pressure caused by expansion. When the vessel starts to cool down, the air that has not escaped contracts and the gasket closes hermetically, creating a partial vacuum and preventing anything from entering or leaving. If this were not the case, water from the autoclave or, again, non-sterile air could enter.

The vacuum will be greater the more headspace you leave and the higher temperature you reach with your autoclave.

How much empty or headspace should I leave?

Producer making preserves in a glass jar and leaving empty space for subsequent sterilization.

It all depends on what you are packing, but in general, as a minimum, you should leave 1-2 centimeters above.

Specifying a little more we can tell you that, according to a study carried out on The hermetic seal and headspace:

  • For low-acidity foods, such as vegetables and meats, you should leave 2.5 centimeters (1 inch).
  • For highly acidic foods such as tomatoes and fruits, 1.30 cm (1/2 inch).
  • or jams, jellies, pickles and condiments, a little less than 1 centimeter (1/4 inch).

Remember that…

An inch is 2.54 centimeters.

And what about containers that do not use a twist off or screw caps?

Containers that do not have a seal, such as metal and plastic containers, do not make a hermetic seal in the autoclave. This is unnecessary since the way it closes is much safer than in a glass jar.

Cans, semi-rigid trays, bags or pouches should be filled to the top or vacuumed before sealing (this is the case for bags).

Unlike glass jars with twist-off or screw-on lids, it is not advisable to leave too much air between the food and the closure because expansion and increased pressure can cause the bag or can to deform.

Do some foods expand more than others?

We must make it clear that the expansion of food is usually not much. The backpressure and stiffness of the container is usually sufficient to counteract the possible expansion of the ingredients. What does dilate are air and oil.

You should pay special attention to this if you work with glass jars with twists off or screw caps and the covering liquid is oil. As we have already explained, the oil tends to expand, so it can end up coming out through the lid gasket and stain everything around it. That is why, in this type of canning, you must be very careful to leave enough headspace.

The expansion of the food, air and oil will be greater or lesser depending on the temperature applied in the process. The more heat, the more expansion, which means that it is during sterilization that these elements expand the most.

Twist off threaded canning containers that need empty space

More Recommendations

Now that we’ve answered your question and you know what kind of containers require empty space or headspace, you’re probably wondering how to make your gourmet preserves and/or prepared dish look as tasty and healthy as if it were prepared on the spot. In the following article we will tell you how to prevent your gourmet preserves or prepared dishes from undergoing changes when sterilized.

And if you have more questions about canning, do not hesitate to browse the articles on our blog and or ask us.

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