The use of a protective atmosphere means that, before the pack is sealed, all air is extracted and then replaced with a mixture of carbon dioxide (CO2) and nitrogen (N2). These gases are naturally present in air, which normally consists of 20.9% oxygen, 79% nitrogen, 0.03% carbon dioxide and small amounts of inert gases. Many micro-organisms normally need the oxygen in the air to grow. When the oxygen is removed and the volume of CO2 increased beyond what is naturally present in air, the growth of these micro-organisms is inhibited. Nitrogen has little or no inhibiting effect on the growth of micro-organisms but is used as a filling gas so that the pack maintains its shape and does not become deformed during storage.
In an enclosed packaging system, the normal oxygenated atmosphere is replaced with a mixture of carbon dioxide (CO2), nitrogen (N2) and in some cases also other gases. This inhibits bacterial growth and the shelf life of the product is increased by 50 200%, depending on the product. For fresh fish an increased shelf life of 50%, compared with air at the same temperature, is typical.
Soluble gas stabilisation (SGS)
Gas is absorbed into the aqueous flesh of the fish. By saturating the fish with gas (usually CO2) before packing a higher concentration of gas can be achieved. This improves the inhibiting effect on bacteria and allows the use of a smaller gas volume, which also improves the appearance of the pack.
Big gains from using SGS
Work has gone into dissolving CO2 into the product before packing so that the volume of packaging can be reduced, without impairing either shelf life or product quality. The method has proved to be successful and if the industry takes it up the volume of packaging can be reduced by 40 50%, which would give a weight reduction of about 15%.
One way of increasing the CO2 level is to expose the food to increased pressure. Systematic experimentation has optimised pressure, temperature and gas saturations.
The gas is dissolved in the product
Many fresh meat and fish products are now packed in modified atmospheres to extend shelf life. This extended shelf life is provided by the amount of CO2 that is available in the pack for the product to absorb, which makes for a large pack volume. A measurement method has also been developed that can be used to determine the volume of gas in the pack, as well as a mathematical model that calculates the gas composition in the pack and the concentration of CO2 in the product.
Benefits for many types of food
SGS treatment of chicken, peeled prawns, halibut, cooked meat products and salmon has been carried out over recent years. SGS treatment of from 1 to 12 hours and from 1 to 4 atmospheres has been investigated. For all products an equal or better shelf life was noted, compared with traditional modified atmosphere packing, and pack volume could be reduced. SGS treatment must be adapted to the different products, the product geometry and the packaging solution by adjusting time, degree of fill and balancing pack gas.
SGS treatment brings benefits for the industry, dealers, customers and the environment by reducing the volume and amount of packaging.
By using SGS, pack volumes can be reduced by 40-50 %, without impairing shelf life or quality.