Fun fact: It is impossible to cook an egg on Mount Everest. Because the lower the air pressure, the lower the boiling point of water. Water vaporises at 70 °C on the highest mountain in the world, which is 8,848 metres above sea level. But 83 °C is required for an egg to become hard. No problem for mountaineers. They do without or switch to a steam cooker. But the physical principle presents a challenge to caterers and central kitchens - particularly if they want to use a packaging machine to vacuum pack hot food directly after cooking, so that the shelf life can be extended. Why? Here the so-called vapour pressure curve comes into play. The greater the negative pressure, the lower the boiling point - exactly as on the mountain. If the machine creates a negative pressure of 200 mbar for example, the water content in the product vaporises at 60 degrees centigrade. From this point onwards it is no longer possible to reduce the pressure any further.
That is the problem: When vacuum packing hot food, some residual air remains, which can adversely affect the shelf life
When vacuum packing hot products, some residual air with 21 percent oxygen content always remains in the header space. A residual air content that can reduce the shelf life of the food. “Many caterers and kitchens therefore let the hot food cool down before vacuum packing. Or they use active cooling. But by doing this, they lose either time or energy,” says Dominik Eberhard, Product Manager for Thermoforming Packaging Machines at MULTIVAC. “In order to ease the burden on caterers and kitchens, we have launched a new steam flushing system for small to medium batch sizes onto the market, and this is called SFP Light.” SFP is an abbreviation for Steam Flush Packaging.
The solution: The SFP Light steam flushing system packs hot products without a vacuum source
The thermoforming packaging machine first forms the pack cavities for the product from a plastic film - for packing five kilograms of goulash for example at a temperature of 60 °C. The portions then travel into a hermetically enclosed sealing station. Hot steam at a temperature of 180 °C now flows through the station. Finally the upper web is sealed to the formed film, and the steam remains in the pack. The secret: Gases have a significantly greater volume than liquids. When the steam turns to water again during cooling, the pack automatically contracts. “Thanks to SFP Light, it is possible to pack hot products without any significant residual air pockets. And all this without any vacuum source, which always has a limited effect due to the vapour pressure curve,” explains Dominik Eberhard. “There is also no danger of the products boiling or packs bursting, since there is no negative pressure in the product space.” Another beneficial side effect of the steam flushing system: The hot gas at 180 °C kills bacteria on the surface of the food, so that the shelf life is extended.
And why SFP Light? “MULTIVAC has offered a SFP system for many years now for thermoforming packaging machines with high throughputs on an industrial scale. The new version is designed for smaller batch sizes in kitchens and catering companies. It is less complex as well as being space-saving and more cost-effective.”