The applications of nitrogen
Freezing food
- Liquid nitrogen is used for freezing soft fruit. It also prevents oxygen from reaching the food's surface, helping to stop the growth of bacteria.
The food industry is a major consumer of liquid nitrogen for freezing meat, fish, poultry, dairy and bakery products, and many other items such as ready meals, fruit and vegetables. Liquid nitrogen freezes the food much more quickly than a freezer. This stops water leaking from the cells in the food and forming ice crystals that damage the texture of the food. The nitrogen in the food also stops harmful bacteria getting the oxygen they need to grow and multiply.
Shrink fitting
- Liquid nitrogen is used in shrink fitting, a technique that allows metal parts to be joined together without bolts or welds.
Liquid nitrogen is extremely cold, –196 °C. At room temperature, the metal part to be fitted is too large to fit inside the hole intended for it. Liquid nitrogen cools the metal part, making it contract enough to fit into the hole. As the chilled metal part warms back up to room temperature, it expands inside the hole to make an extremely tight fit.
Producing chemicals safely
- Nitrogen does not allow combustion, so it is used to stop industrial chemicals from igniting inside storage tanks and pipelines.
Many industrial chemicals are flammable. They will catch fire and burn if there is enough oxygen in the air. Chemical storage tanks have the air above them replaced by nitrogen gas. Because nitrogen is unreactive, it stops the chemicals inside igniting, even at high temperatures. This is called nitrogen blanketing.
The chemical industry also uses nitrogen blanketing for other pieces of equipment such as reactors and pipelines. Nitrogen may be used to control the pressure in columns and the oxygen content in oxidation reactions.
Food packaging
- Nitrogen keeps oxygen away from stored foods. This stops fats and other nutrients in the food oxidising to make unpleasant smells and tastes.
Modified Atmosphere Packaging (MAP) is an industrial technique to preserve chilled foods. The air inside the food packet is replaced by one of a range of protective gases, including nitrogen, argon and carbon dioxide. This stops nutrients in the food oxidising to form products with rancid smells and flavours.
MAP increases the shelf life of food without the need for vacuum packaging, artificial preservatives or freezing. It does this while maintaining the appearance, quality and safety of the food.
