Creating the right atmosphere

Materials World magazine
13 Oct 2014

Leon Arkesteijn, Product Manager at Bosch Packaging Technology, explains the advances made by modified atmosphere packaging.

As consumers become increasingly health-conscious, food preservation technologies have had to shy away from relying on chemicals. One of the most effective ways to keep food without the use of such preservatives is to incorporate modified atmosphere packaging (MAP) technologies. For food manufacturers, this technology can help to prevent food spoilage during shipping, decrease product waste and enhance a brand’s reputation for freshness, building consumer loyalty and greater sales potential.

By controlling the packaging environment, MAP makes it possible to reduce or eliminate many growth factors for micro-organisms that lead to food spoilage. It offers an alternative method of preserving foods such as heat sterilisation or freezing, and is able to extend product freshness without affecting the taste, quality, texture or nutrition of foods. One of MAP’s greatest assets is that it can be applied to a wide range of products – from cheese and coffee to meat and produce.

Food, meet gas
It is estimated that the right packaging, combined with optimal temperatures, can delay food spoilage by as much as 800%. MAP introduces and regulates gas mixtures within a package to enable this. Fresh foods have varying rates of spoilage, and the science behind MAP is based on blending the right percentages of gases into a packaging environment. The three gases most commonly used for MAP are O2, CO2 and N2. When these gases are introduced into a closed packaging environment (for example, one with a high-barrier film), they substitute the atmosphere inside a package and hinder the chemical reactions that cause food spoilage. Since fresh foods are living products, they consume O2 and expel CO2, water and heat. In the case of fruit and vegetables, ethylene is expelled as part of the ripening process. CO2 inhibits the growth of many micro-organisms, while O2 helps fruit and vegetables breathe, and N2 serves as an inert ‘filler’ gas because it does not react with foods. Maintaining the right mixture of gases and regulating moisture is critical to extending shelf life.

The blend of gases is unique and depends on the specific food being packaged. For instance, depending on the gas mixture, the shelf life of meats can be extended by two to five days, fresh fish by three to 10 days, soft cheese by eight to 21 days, fruits and vegetables by three to eight days and pre-baked bread by five to 20 days.

The perfect recipe

The longevity of products packaged with MAP technology starts with the gas mixture. A gas mixer is incorporated into the packaging process when more than two gases are required. Manufacturers have a choice of manual or digital gas mixing – digital mixing is more expensive but offers greater precision, and mixtures can be pre-programmed. It also enables product changeovers to be made quickly, enhancing productivity for manufacturers.

Depending on the product features, the right packaging materials are necessary to ensure protection against O2, water and/or light to avoid spoilage. For example, a lightproof packaging material creates a UV barrier and coloured glass helps with light-sensitive goods. For water-sensitive products, steam barriers contained within the packaging material are recommended, such as Al-metallisation, polyethylene or polypropylene. Packaging material such as aluminum protects oxygen-sensitive goods from air penetrating the packaging.

As the desired gas mixture is flushed into the packaged environment and unwanted gas is flushed out, hermetic packaging is required to ensure gas does not escape from or enter the pack. If there is a leak, it will compromise the protective gas mixture and the shelf life of the product. Over the past five years, R&D at Bosch has taken place to develop ultrasonic sealing technology for flow wrappers, which is one way to create hermetic seals. An alternative to ultrasonic sealing that would create airtight seals could be wrappers that feature a long-dwell sealing system, preventing gases from flowing in or out. The long dwell sealing jaws run along with the film, at identical speeds. This allows for longer sealing times compared to rotary sealing jaws, which is the more common way of sealing. This system allows for sealing times of up to one second and ensures hermetically sealed packaging, which is an absolute requirement for MAP. In summary, the product characteristics determine which technology is the right one.  

To each according to its need

The ideal blend of gases depends on the product characteristics. For instance, fresh meat usually requires a large amount of O2 to stay fresh. That is why an increasing number of retailers sell meat in high O2 (70%) MAP with CO2 (30%). However, meat that contains a high amount of fat needs a minimal amount of O2, as it will spoil colour and taste.

Fresh fish needs a careful balance of CO2 and O2 because both aerobic and anaerobic microbes are present. This means that O2 promotes the growth of some microbes while hindering the growth of others.

Hard cheese such as cheddar has a low water content and is susceptible to mould growth. An environment of 100% CO2 can extend shelf life by up to 10 weeks.

The water in soft cheese can react with CO2 to reduce the volume of gas in a pack, causing it to collapse. By incorporating N2 into the pack, it functions as a filler gas to prevent this collapse.
Fresh fruit and vegetables such as lettuce consume O2 and produce CO2 and water. To extend shelf life, storage temperatures and O2 levels should be kept low. Some O2 is necessary to prevent anaerobic processes that produce ethanol and promote spoilage. Some fruits and vegetables should not be packed in a tight package and need to have O2 venting through holes in the pack. Bread spoilage in the form of mould is due to low water content. Because moulds are aerobic and thrive on O2, MAP replaces O2 with CO2 to delay growth. Common applications include baked bread loaves, tortillas, baguettes and pittas.  

Alternative techniques
Besides MAP by means of gas flushing, other common techniques for packaging fresh food include perforation and O2 absorbers. These alternatives are better suited to some specific products:

• Perforation – some products benefit from coming into contact with O2, and perforating a pack is one technique that enables this. Either the packaging machinery is equipped with a perforation system, or manufacturers use perforated film. A good example of a product that benefits from this approach is tomatoes.

• O2 absorbers – another option in the packaging toolbox, O2-absorbing packets are added to enclosed packaging. Packs are filled with food-grade formulations that hinder rancidity, smells and other signs of spoilage. Typical uses include breads, cheeses, cakes, pastries, nuts and processed meats.