It’s a 50-cent word that we commonly associate with food packaging, but what is a “hermetic seal,” or what does “hermetically sealed” really mean?

The simple definition is that it means the container is airtight. Originally, the term applied to glass jars and other rigid containers, but now its use has been extended to plastics that do not let air pass through them.

Most of us are conditioned to check the lids of products in glass jars to confirm that there is a slight curvature present. This indicates that there is a vacuum in the container that is formed when the product is hot-filled into the container. Once the lid is put on the jar and the contents begin to cool, the air in the headspace at the top begins to contract. It is the shrinking of this volume of air that provides the vacuum and draws the centre of the lid downwards to create the dimple effect. When you open the container, the sudden inward rush of air releases the vacuum and causes the lid to make the distinctive “pop” that we like to hear.

Non-rigid containers made of composite plastic materials can also be airtight, but there is not the noticeable curvature to the lid, nor the popping sound when the container is first opened.

Hermetic seals are important in providing safe food for us as consumers. Most spoilage microorganisms require oxygen from the air to survive. Let’s use the example of cooked strawberry jam to explain things. When it is made, the jam is heated to its boiling point, which creates the rich flavour, dissolves the sugar, and kills off the microorganisms that may have been present on the strawberries. While it is still hot, the jam is poured into clean glass jars and the lids are applied. The sealed jars are inverted in some processes to ensure that the underside of the lids are heated sufficiently to destroy any potentially lingering microorganisms.

After cooling, the jam jars can be stored at room temperature for a prolonged period of time, while still remaining safe to consume. However, once the container is opened and the airtight seal is broken, it’s a whole new ball game.

In our example, when you open the jar of jam, air from your surroundings can enter it. Now, the clock starts ticking and it’s all downhill from there in terms of quality and food safety. Airborne microorganisms can enter the jar, so you need to take steps to prevent them from growing. Of course, the simplest thing to do is keep the opened jar of jam in the refrigerator, which did not need to be done before you broke the hermetic seal.

Jam is a very interesting product since it has a certain level of resistance to microbial growth. This is due to the sugar in the jam, as well as the pectin, binding the water that is present so tightly that it is not available for microorganisms to use to support their growth. Unfortunately, all that can change.

As more and more jam is used, the volume of air in the jar increases and undesirable things start to happen. Each time you open the jar, you allow moist warm air to flow into it. When you put the lid back on the jar and place it back in the refrigerator, or even leave it at room temperature, the moisture in the trapped headspace air can condense and cause water droplets to form on the surface of the jam. There is also what is called a “dynamic equilibrium” set up between the jam and the trapped air above it. With time, some of the moisture from the jam may escape into the headspace air as water vapour. Just like the water vapour in the air that entered the jar when you opened it, this vapour will also condense.

Read more: What the heck does hermetically sealed really mean?