Use carbon dioxide as a hurdle in MAP systems

When designing modified atmosphere packaging (MAP) systems for products, it's necessary to consider several factors so that you develop a system that maintains high quality and safety. The primary gases for MAP applications are carbon dioxide, nitrogen and oxygen. Carbon dioxide is the key gas when you are concerned with food safety. The degree of bacterial growth inhibition offered by carbon dioxide will vary depending on the target microorganism and the product itself. When applying MAP, a food processor must consider the capacity of a product to take up additional carbon dioxide. For nonrespiring foods, you have to correctly design the gas mixture and the ratio of headspace-to-produce volume.

Carbon dioxide inhibits microbial activity by dissolving into the liquid phase of food, reducing pH. It's thought to penetrate the membrane of the microorganism, causing changes in permeability and functionality. The ability of carbon dioxide to inhibit microbial growth will vary depending on the organism. Some bacteria may resist the gas and pose a risk if present in foods intended for MAP.

Scientists at the Swedish Institute for Food Research (SIK, PO Box 5401, S-402 29 Goteborg, Sweden) tell us that for low-fat, high-moisture foods, the goal of MAP should be to inhibit the growth of microbes. For high-fat, low-moisture products, the goal should be protection from oxidation. Generally, the initial numbers of microorganisms on the packaged product should be as low as possible. Well-controlled chilling and good hygiene during handling and packaging are required to achieve optimal shelf life.

Carbon dioxide levels should be about 20% when MAP is used with meat products. The gas retards bacterial growth, especially the growth of some spoilage organisms. On another front, fresh fish has a very short shelflife. But the proper implementation of MAP will enable you to extend shelflife by up to 50% in some instances as compared with air storage. The main concern with this product is growth and toxin production by nonproteolytic strains of C. botulinum that are able to grow at 3.3 C. Approximately 20% carbon dioxide should inactivate all sporeformers in well-cooked fish, we're told.

Cheese is very susceptible to mold growth. But the growth on hard cheese can be inhibited using carbon dioixde levels from 80% to 100%. Levels of 20% to 40% are needed for soft cheese characterized by high water activity. When it comes to baked goods, mold growth and chemical breakdown result in staling. Some products are sensitive to oxidation reactions that cause rancidity and water uptake, creating a soggy product. MAP storage benefits these products because of the low oxygen atmosphere possible. Carbon dioxide concentration is recommended at 20% to 40% levels.

Researchers also are exploring the use of argon and nitrogen as hurdle techniques.

Further information. Ulf Ronner; phone: [phone]; fax: [phone].