When Should Gas Flushing or Steam Flushing Be Used During Sealing?

When sealing a bucket, cup or container, some air always remains between the product and the sealing film or lid. This space is known as the headspace. The air in the headspace contains oxygen, which can affect the quality and shelf life of both food and non-food products.

By applying gas flushing or steam flushing immediately before sealing, a large proportion of this air can be displaced. As a result, less oxygen remains inside the package. The process can also help prevent the sealing film or lid from bulging after the package has been closed.

Why Reduce Oxygen?

Oxygen can cause oxidation, discolouration, changes in flavour and loss of quality. Exposure to oxygen can also affect the stability or shelf life of certain non-food products.

Reducing the amount of oxygen in the headspace can therefore contribute to:

• reduced oxidation and quality loss;
• a longer or more stable shelf life;
• a lower risk of a bulging sealing film or lid;
• a more consistent package appearance.

The most suitable method depends on the product, filling temperature, packaging, sealing film and required shelf life.

When Should Gas Flushing Be Used?

During gas flushing, the air in the headspace is displaced by a suitable gas immediately before sealing. This reduces the amount of oxygen remaining in the closed package.

Gas flushing can be beneficial when a product is sensitive to oxygen but additional heat or moisture is undesirable. This may be the case, for example, with products that are filled cold or at a lukewarm temperature.

Important factors include the type of gas used, the volume of gas, the size of the headspace and the time between gas flushing and sealing. The package must be closed as quickly as possible after gas flushing. Otherwise, ambient air may re-enter the package.

When Should Steam Flushing Be Used?

During steam flushing, steam is introduced into the headspace immediately before sealing. The steam displaces part of the air and therefore also part of the oxygen.

After sealing, the steam cools and condenses into a small amount of water. This reduces the gas volume inside the package and may create a slight vacuum. The sealing film or lid is then drawn slightly inwards rather than bulging outwards.

This principle is frequently used for hot-filled products such as sauces, soups, apple sauce and fruit preparations. The combination of hot filling and steam injection is also known as steam flow closing.

The Effect of Filling Temperature

The amount of air in the headspace is not the only factor that determines whether a package will bulge. The filling temperature also plays an important role.

At high filling temperatures, the air and vapour inside the package may expand. This can create pressure against the sealing film or lid. Lowering the filling temperature can therefore make a noticeable difference. In certain applications, a reduction of approximately 10 degrees can significantly reduce bulging.

The temperature must, of course, remain suitable for the product, the production process and the applicable food-safety and shelf-life requirements.

What If Gas Develops Later?

A bulging seal is not always caused by trapped air or a high filling temperature. If a package only begins to bulge at a later stage, gas may be developing within the product.

For applications in which gas must be able to escape from the package, Unilid Solutions can incorporate a pressure-release valve into the seal. This valve allows excess gas to escape, limiting the pressure beneath the sealing film or lid.

The cause of the gas formation must always be investigated as well. The valve prevents pressure from building up, but it does not resolve a potential issue with the product or production process.

A Domed Insert as an Alternative

Gas flushing or steam flushing is not suitable or desirable for every product or production process. In some cases, additional moisture is not permitted, gas cannot be used or the technology is not compatible with the production line.

In these situations, Unilid can install a domed insert in the sealing head. During sealing, this insert presses the sealing film slightly inwards. If pressure develops inside the package at a later stage, the result is a flatter seal rather than visibly bulging film.

This solution does not reduce the amount of oxygen, but it can help minimise visible bulging.

Which Solution Is Right for Your Process?

Gas flushing and steam flushing are primarily intended to reduce the amount of air and oxygen in the headspace. A pressure-release valve is designed to discharge gas that develops at a later stage. A domed insert helps keep the final seal surface flatter.

The selected solution must always be matched to:

• the product and filling temperature;
• the packaging and sealing film;
• the size of the headspace;
• the sealing temperature, sealing time and pressure;
• the production-line capacity;
• the required shelf life and package appearance.

Practical testing using the complete product and packaging combination is therefore essential.

The sealing machines and format sets supplied by Unilid Solutions can optionally be equipped for gas flushing or steam flushing. Depending on the application, a pressure-release valve or domed insert can also be incorporated into the solution.