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A cool choice: cryogenic freezing of meat

The colour of meat is largely determined by the state of the myoglobin molecules in the muscle, which becomes bright red when myglobin is oxygenated.
Liquid nitrogen: found in the study to be as effective as CO2 for the preservation of meat products.
The quality of minced meat is improved by using the liquid nitrogen injection system.

The race for the best refrigerant is on. In today’s fast-moving meat-processing and packaging industry, high capacity mixers are used to meet the demand for formed meat products such as burgers. To achieve high product quality, temperature control inside the mixer is vital. Cryogenic technology offers benefits over traditional mechanical freezing systems by ensuring rapid temperature reduction. Both carbon dioxide (CO2) and liquid nitrogen (LIN) are common cryogenic refrigerants, suitable for chilling a variety of meat products. How can processors be certain however, of the quality of the products when using either refridgerant?
There have been assumptions amongst food manufacturers that the overall product quality of chilled meat will differ depending on whether CO2 or LIN has been used in the chilling process. This theory can be attributed to CO2’s ability to inhibit bacterial growth in Modified Atmosphere Packaging (MAP). But is it really advantageous to use CO2 instead of LIN in cryogenic freezing and chilling? To investigate this, leading gas, equipment and services supplier, Air Products, commissioned a study to compare the chilling of meats with these refrigerants.


The Kennedy study
Conducted by Dr Chris Kennedy*, the study analyses meat products, their interaction with CO2 and how it compares to chilling with LIN [1] . There have been many assumptions over the years that compared with LIN, the use of CO2 as a refrigerant can have certain positive effects on microbial shelf-life, colouration and dehydration. The Kennedy study looks at these three aspects in turn.

Microbiology and shelf-life
CO2 is commonly used as a component in the MAP of foods to slow the growth of spoilage organisms. For this reason, chilling with CO2 is often considered in order to leave meat products with some degree of protection against spoilage and a potential increase in shelf-life. But is this the case?
The study reviewed measurements where a muscle sample was used to measure the rate of uptake of CO2 and the rate of desorption of CO2 after removal from the atmosphere [2]. The sample takes around 24 hours to become fully saturated at 4°C. The initial rise in concentration is fairly rapid, reaching 50% of the saturated values within one hour. Similarly, the rate of desorption is also very rapid. In these experiments, the desorption rate takes around 30 minutes for the concentration of CO2 in solution to reduce by 50%. For this reason, it seems highly unlikely that that a chill process using CO2 on its own, without subsequent MAP packing, would have any effect on the shelf-life of meat products. Where subsequent MAP packaging is used, a careful calculation is required of the amount of CO2 absorbed in the meat at the time of packaging in order to determine the ultimate gas balance in the pack.

Colour is key
The colour of meat is largely determined by the state of the myoglobin molecules in the muscle. Myoglobin is a water-soluble protein, which stores oxygen in the muscle. Immediately after cutting, meat is a deep purplish-red colour because of the myoglobin pigment. As the meat is exposed to oxygen in the atmosphere, the myoglobin becomes oxygenated and converts to oxymyoglobin. The result is a bright red colour, which is generally associated by consumers with fresh meat.

Placing meat in a CO2, or nitrogen, atmosphere could lead to the loss of oxygen on the surface, thereby reducing oxymyoglobin to myoglobin – but any colour changes would be reversed on return to a normal atmosphere. Chilling in a carbon dioxide atmosphere would lower the pH of the meat and lead to some lightening of the colour. This is unlikely to be significant and the meat will again revert to the pre-chilling condition as carbon dioxide is released after chilling. Meat processors can therefore be confident that neither LIN nor CO2 will have any direct effect on their product’s colour when used in their chilling process.


Dehydration rates
Comparing dehydration rates between the two refrigerants is difficult because the processes employed are not identical. However, based on the rate of change of the surface temperature of the product alone, similar chilling equipment will produce less dehydration with a LIN refrigerant because of the faster reduction of the surface temperature [1]. The refrigerant gas employed would not affect the amount of vapour that can be carried away from the surface.

Higher quality through fast freezing
Results from the Kennedy study have shown that many of the positive effects of using CO2 as an MAP gas for meat do not directly translate to the use of CO2 for meat chilling without subsequent MAP packaging. This means that meat processors have a choice between CO2 and LIN when considering a cryogenic refrigerant for their chilling process.

However, although CO2 and LIN can both be used in chilling processes, the difference in their thermal properties mean that they are not interchangeable and the equipment design for the two refrigerants is often quite different. For example, when CO2 is used for chilling applications, it is often in the form of a solid snow which is mixed in with the product.

For chilling with liquid nitrogen, Air Products has developed the Freshline LIN-IS (LIN Injection System). This innovative injection equipment is controlled by unique software that automatically determines the optimum quantity of liquid nitrogen that is injected directly into the product. The tailor-made system can be retrofitted to new or existing mixers, buffers and any type of stainless steel vessel.

Meat processors looking to optimise production line efficiency can benefit from this cutting-edge technology. In recent times, new high capacity meat mincing machines have been developed to meet an increasing demand for burgers and other formed products. The heat generated by these machines often leads to fat smears and high temperatures, which provide an ideal breeding ground for bacteria – subsequently affecting the quality of the meat.

Currently, the addition of frozen meat into the mixture is used to control the temperature inside the mincing machine. The results are not always reliable and the quality of the minced meat is considerably reduced because the frozen meat added is not sufficient to chill the entire batch. The LIN–IS solves this problem, as it affords a high level of temperature control that cannot be replicated by using a frozen product. By being able to control the temperature of the meat itself, the nitrogen injection is regulated, providing the cold that is required at all times. This results in improvements in the quality of the minced meat; reduced fat break-up and regular cuts, with zero risk of contamination.

Supply and demand
A key benefit of the Kennedy study is that meat processors can be confident that there is a real choice available between CO2 and liquid nitrogen for injection and temperature control in their mixing applications. These findings has been backed up by work that Air Products has carried out with customers on the direct comparison of LIN and CO2 injection in their meat mixing applications. When customers changed from CO2 injection to the new Freshline LIN-IS system for chilling of meat in the mixing process, no differences in product quality could be found. These findings mean the customer has the flexibility to choose the products that make the most sense to them – with the Freshline LIN-IS system offering a particularly high level of temperature control.

Conclusion
CO2 is often perceived by food manufacturers as having positive effects, compared to liquid nitrogen, on shelf-life, colour and the dehydration rate during cryogenic freezing of meat products. The Kennedy study dispels this myth, by analysing the interaction of meat with CO2 compared to LIN. The study finds that meat producers can choose between both refrigerants without fear of affecting product quality.

*Dr Kennedy is the founder of NutriFreeze, a professional scientific and engineering advisory service to the food and food-refrigeration industries. Prior to founding NutriFreeze he was a senior lecturer in Food Process Technology at the University of Leeds. Dr Kennedy is a member of the Institute of Refrigeration, The Institute of Food Science and Technology and the Institute
of Physics.

References

1. Kennedy CJ. Comparison of Chilling Meats by Carbon Dioxide and Nitrogen. To receive a copy of the paper, please contact Emma Guthrie at Air Products – email: guthriej@airproducts.com
2. Jakobsen M and Bertelsen G. Solubility of Carbon Dioxide in Fat and Muscle Tissue. Journal of Muscle Foods 2005; 17: 9-19.

Air Products serves customers in industrial, energy, technology and healthcare markets worldwide with a unique portfolio of atmospheric gases, process and specialty gases, performance materials, and equipment
and services.
For more information,
visit: www.airproducts.com

For further information on cryogenic
freezing, contact:
Hugh Ashworth
Air Products
Tel.: +44 (0)1270 614314
E-mail: ashworh2@airproducts.com


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