Oxygen in living color

Sept. 15, 2005
An oxygen-barrier test system helps designers ensure plastic packaging will keep food and beverages fresh for their intended shelf life.

Senior Editor

The chemical in the Oxygen Indicator Solution (OIS) is colorless in the reduced state. When exposed to oxygen, it turns blue, deepening in intensity with increased ingress. The rate of the color change is proportional to the rate of oxygen entering the package and can be correlated to conventional oxygen transmission rates.

For most packages, oxygen ingress is not evenly distributed. The Oxygen Indicator Gel (OIG) system shows precisely where oxygen enters a bottle. Common failure points for rigid plastics include the closure and base areas, hot-fill panels, and lesscrystalline regions in the sidewalls.

Beer brewers have recently joined the ranks of food and beverage processors that have switched from conventional glass and metal containers to plastic. Successful conversion, however, has meant overcoming barrier limitations of polymers such as PET (polyethylene terephthalate), PP (polypropylene), and PE (polyethylene) over the planned shelf life.

A polymer's barrier property (i.e., its ability to slow down or stop the passage of a gas through its structure) is of primary importance for packaging designers. Especially so for those working with food products that will sit on a shelf for an extended time.

Barrier is most commonly quantified by measuring polymer permeability — the rate at which a given gas passes through the polymer under a given set of conditions. Low permeability means that gases (O2 and volatile flavor/odor components) and moisture have a difficult time passing through the packaging.

Shelf-life stability is often closely linked to packaging permeability. Shelf life is defined differently depending on the food or beverage packaged. Each food category has its own ingredients and processes, as well as packaging and storage/handling requirements that can reduce life on the shelf.

Shelf life is the time period between the packaging of a product and its use, for which the quality of the product remains acceptable to the product user. The end of shelf life could be determined by different factors such as microbiological activity, color change, texture or viscosity change, nutrient loss, rancidity, etc. With the conversion of glass and metal to plastic packaging, oxygen is of great concern.

Fruit juices lacking adequate oxygen barriers, for example, eventually lose vitamin C. Ketchup and other tomato-based products gradually change color and baby food will get more bland and may become rancid. In the presence of oxygen, polyunsaturated animal and vegetable fats degrade more easily than monounsaturated fats. The reaction produces unstable peroxides that go on to create reaction products that are responsible for rancid flavors.

The way to prevent oxidation is to use material that serves as an oxygen barrier. Plastic barrier materials don't all completely block oxygen. They are classified in a range from low barrier, designed for packaged produce that needs to "breathe," to high barrier for products such as beer and coffee.

Ultralow permeability materials help keep oxygen out, so there's less chance beer will develop a skunky flavor and that the coffee oils will turn rancid. Conversely, good barrier materials on the inside of a coffee pouch can help keep volatile components from escaping.

Thus it's important to not only quantify how much oxygen enters a package over time, but to also understand how oxygen ingress impacts the food within. The impact of oxygen on shelf life may not be a big deal for products consumed within days or weeks of issue. But for those with storage life of months or years, inadequate barrier properties and/or package design flaws can significantly influence how stable the product will be one year down the road.

Although published barrierproperty data for polymers is a good place to start during the material-selection process, designers won't be able to predict product shelf life with 100% certainty. The only way to know for sure is to do testing, which can be time consuming.

It is unrealistic to assume that oxygen-sensitive products will stay fresh for years when stored in warehouses or on shelves. The only way to gauge how products are likely to hold up is with long-term storage stability tests under various environmental conditions.

Unfortunately, package designers rarely have the luxury of longterm testing. Instead, the usual route entails combining results from accelerated stability evaluations with historical data. This data is typically gleaned from previous long-term stability tests of similar packaging materials and products.

The gas-flush method is the conventional way of testing oxygen-barrier properties. It tests packages under specific conditions of oxygen, temperature, and humidity or RH.

For example, consider a typical oxygen-permeability measurement for flat-film packaging. The film sample clamps between two chambers. One chamber holds an inert gas while the other is filled with oxygen. The test takes place for a given time period, at a known temperature, and under one atmosphere of pressure. Permeability is expressed as millimeters of gas permeating one square meter of film in 24 hr for a specified thickness, or as cubic centimeters (cc) of gas permeating 100 in. 2 of 0.001-in.-thick film in 24 hr.

Bottles and other packages are not clamped between two chambers but are exposed to ambient RH, temperature, and oxygen concentration.

Current ASTM standards for permeability such as D3985 ( Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using a Coulometric Sensor) and F 1307 (Standard Test Method For Oxygen Transmission Rate Through Dry Packages Using a Coulometric Sensory) measure gas-transmission rates of packaged goods in the dry state. But most foods are not dry.

A testing system has been devised to measure oxygen-transmission rates (OTR). The Oxygen Indicator Solution (OIS) system by Guelph Food Technology Centre (GFTC), Guelph, Ont., has several advantages over conventional testing methods says GFTC Senior Research Scientist Carol Zweep. It can provide a means to accurately predict the oxidative shelf life of a liquid-based product. This can save valuable time that would be required to perform actual shelf-life testing of foods stored in new packaging.

The system quantifies oxygen ingress of packaging in its intended state and actual storage environments. It does not use unrealistic temperature increases that may create deteriorative pathways that would not exist under normal handling and storage. "Measurements are representative of the entire packaging system, including closures. And it can evaluate the performance of oxygen-scavenging packaging materials," says Zweep.

The system can give oxygensensitive products an accelerated shelf-life test by exposing them to higher oxygen levels. "We store products in an 80% oxygen level which provides approximately four times the accelerated shelf life. Ambient air contains about 21% oxygen," says Zweep.

OIS uses a proprietary aqueous solution that turns from clear to blue, deepening in intensity with increased oxygen ingress. The solution goes inside transparent packaging under an inert nitrogen atmosphere. The whole package then sits in the beam of a spectrophotometer that monitors gradual color change of the solution by measuring the absorbance of the blue peak. The rate of the color change is proportional to the rate of oxygen entering the package. Use of stoichiometry makes it possible to translate the rate of color change into conventional OTR measurements. OIS oxygen permeability units are cubic centimeter/package/day or 24 hr at ambient RH, temperature, and oxygen concentration.

The chemical used in OIS is sensitive to light so test samples should be stored in the dark. Heat also tends to accelerate the chemical reaction and permeation through the packaging material.

The system does have limitations determining OTR for large bottles, says Zweep. Large bottles will not physically fit in the spectrophotometer. Also large bottles experience a large influx of oxygen and the blue color change can be too rapid to quantify well. And "odd" shaped bottles don't work if they lack straight-walled sections that give light a straight path to the spectrophotometer, says Zweep. Dark beer bottles have also been tested but, in general, clear and colorless bottles work best for the spectrophotometer (OIS) analysis.

"The OIS is a patented proprietary method that is currently not an ASTM method," says Zweep. "But we have been approached by ASTM to make the technique a standard method."

A second method dubbed Oxygen Indicator Gel gives visual evidence of where packages are most susceptible to oxygen ingress. OIG is based on the same chemical used in OIS. The OIG method fixes the chemical in a gel. The gel color change (as seen by visual inspection) pinpoints localized areas inside the package where there is the greatest oxygen ingress.

OIG determines common failure points in rigid plastic. These points include areas around closures, the base area, hot-fill panels, and less-crystalline regions of the sidewalls. Sealable flexible packages are often prone to problems at the corners and the sealant layer.

The technique is a qualitative method that helps package designers troubleshoot and improve the overall package OTR. The gel gives visual information absent from conventional OTR testing methods. Traditional gasflush methods only give an indication of overall oxygen ingress and can't identify locations in the packaging where oxygen enters preferentially. In most cases, however, oxygen ingress is unevenly distributed. The OIG gives the precise location where oxygen enters the package.

"OIS systems could also be used in pharmaceutical companies to evaluate flexible packaging for oxygen-sensitive drugs," says Zweep. Additionally, several samples can be tested simultaneously, making the method economical as an alternative OTR analysis. Firms interested in using the technique do so by getting a license from GFTC.

Guelph Food Technology Centre, (519) 821-1246, www.gftc.ca

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