Electron spin resonance for measuring radicals in food and medical products

Gamma sterilization is increasingly being used to preserve food or sterilize heat-sensitive pharmaceutical products. For pharmaceuticals and medical devices, the World Health Organization (WHO) specifically recommends this sterilization, in which products are irradiated with high-energy gamma rays from a cobalt-60 beam source (Figure 1). As a result of the irradiation, the genomes of germs and pathogenic microorganisms are destroyed and the organisms are killed, significantly extending the shelf life of the products. Compared to sterilization with ethylene oxide or steam sterilization, the treatment is very gentle. Another advantage of treatment with gamma rays is that products can be sterilized or sterilized in their packaging - without any significant increase in temperature or the use of chemicals.

However, irradiation also breaks chemical bonds in the products themselves, creating free radicals. Radicals are atoms with an unpaired electron and are highly reactive, so they can react uncontrollably with their environment. As a result, new compounds may be formed, some of them toxic, which could later cause undesirable side effects in the application of the product, e.g. a drug. Recent research results at Fraunhofer IGB show that the radicals generated by gamma sterilization can be extremely stable. Figure 2 shows the amount of radicals decaying over time in a gamma-irradiated antibiotic. Even several hours after exposure of the product to gamma radiation, most of the radicals are still detectable. Ongoing measurements also show that even after weeks, the amount of radicals does not significantly decay. Knowing whether and how many radicals are generated by the sterilization process makes it possible to stay below limits for toxic compounds.

At Fraunhofer IGB, we have been using ESR spectroscopy for quite some time to detect radicals, for example to detect the decay curves of the radical density at material surfaces after plasma treatment.

Due to unpaired electrons, radicals exhibit a quantum mechanical spin, which in turn is associated with a magnetic moment. Electron spin resonance (ESR) spectroscopy takes advantage of this: By applying a directed magnetic field to a sample containing radicals, the energy levels of unpaired electrons are split (Zeeman effect). When the sample is exposed to microwave radiation whose quantum energy corresponds to the Zeeman splitting, resonant absorption occurs. Sensitive microwave absorption measurements can be used to determine the spin number, the radical number and also the type of radical.

Only a few milligrams of solids or powders or a few milliliters of liquids are required to determine the radical quantities. The measurement is time-, irradiation dose- and/or temperature-dependent after irradiation. In addition to free radicals, we can also quantify reactive oxygen species (ROS) and nitric oxide (NO) in biological systems, e.g. cells or blood, using ESR spectroscopy.

In addition, we use ESR spectroscopy to determine the antioxidant effect of antioxidants. Ascorbic acid (vitamin C), for example, is a radical scavenger. Due to its antioxidant effect, it protects cells from damage. We detect ascorbic acid radicals, which are formed by the trapping of unwanted radicals, by means of ESR spectroscopy.

Using ESR spectroscopy, we can quickly and reliably investigate how high the maximum gamma ray dose for sterilization must be in order to kill germs and pathogens on the one hand and keep the radical load in the product as low as possible on the other.

In the following products, we quantify the amount of radicals and, if necessary, also detect the type of radicals:

• Food (coffee, malt, cereals, fruits, vegetables, herbs and spices, etc.)
• Food packaging
• cosmetics and toiletries
• medical disposables
• implants and pharmaceuticals
• pharmaceutical precursors and packaging materials