Nitrogen vs CO2: Air Products' handy guide

The company, which provides both carbon dioxide and liquid nitrogen to the food industry, commissioned an independent expert, Doctor Chris Kennedy, to review the current scientific evidence about the effectiveness of the two gases in chilling and preserving different types of meat.

Kennedy, a member of the UK's Institute of Refrigeration, the Institute of Food Science and Technology and the Institute of Physics, said that research had shown that while CO2 and liquid nitrogen could be used as cryogenic refrigerants, suitable for chilling meats, the difference in their thermal properties meant that their use was not interchangeable.

"The overall product quality of chilled meat will differ depending on whether carbon dioxide or liquid nitrogen has been used in the chilling process," his report summary states.

Kennedy looked at the different process involved in chilling meat using the two gases, and focused on three key areas: microbiology/shelf life, colour and dehydration.

His assessment of the microbiological impact of the two gases showed a clear difference: as an inert gas, nitrogen does not have any effect on microbiology, but carbon dioxide can help reduce microbial growth in food as the gas is absorbed by water in the food, leading to a drop in pH. "However, this bacteriostatic effect only lasts as long as the food is maintained under a CO2 atmosphere," the report noted.

"Therefore it is unlikely that chilling with CO2 alone would have a significant effect on decreasing microbial growth within the meat," .

As far as colour is concerned, nitrogen again has no significant effect, while using CO2 in modified atmospheric packaging "has a minor positive effect on the quality of the meat colouring," Kennedy notes.

However, this effect only lasts as long as the meat is maintained under a CO2 atmosphere, he adds, stressing that "chilling with CO2 will have no effect on the colour of the meat as compared to using nitrogen".

Kennedy says that "it is difficult to determine a difference in dehydration between a carbon dioxide and nitrogen chilling process" but that based on refrigerant temperature alone, a nitrogen system "will result in around 20 per cent lower dehydration than an equivalent carbon dioxide system".

The results of Kennedy's synthesis of available science "that there is very little difference in the final quality of the chilled meats regardless of whether CO2 or nitrogen is used in the chilling process" .

"Carbon dioxide has demonstrable effects on the quality of meat products when used in modified atmospheric packaging - showing a positive effect on product colour and a reduction in bacterial growth; however it is incorrect to assume that these effects translate to the use of CO2 in chilling."

Air Products said that it had commissioned Kennedy's report "to enable us and our customers to make decisions regarding the choice of gas to use for chilling products in mixing and blending applications".

"It supports the ten-year of experience that Air Products has in providing technologies for this application."

The report was commissioned by the UK arm of US-based Air Products.