Suitable surrogates for Aspergillus species for the microbiological validation of UV-based sterilization methods


UV system with low pressure lamp for sterilization of surfaces

Surface sterilization protects consumers and products

The sterilization of surfaces is vital in many areas of public life and industry in order to safeguard the health of people and to ensure hygienic and safe production processes. Hygienic surfaces are absolutely essential, for example, in the food and pharmaceutical industries and also in public means of transport and public facilities such as washrooms, to name but a few areas. A wide variety of surfaces on objects as diverse as packaging materials, pipes, aseptic packaging machinery, conveyor belts, food products, surgical instruments, walls, door handles, handholds, and touchpads are sterilized using a range of methods on different scales. For example, each year more than one hundred thousand square kilometers of food packaging materials are sterilized in order to protect products from spoiling and to safeguard the health of consumers. This demonstrates the importance and widespread use of surface sterilization.

UV-based sterilization methods are effective and favorable in cost

The methods that are used for surface sterilization are as varied as the applications. Fundamentally, the methods can be differentiated into thermal, chemical, and physical processes. UV radiation based methods are an important and widely used approach for sterilization using relatively simple and in some cases inexpensive equipment without the need for wet chemicals. The group of physical surface sterilization methods includes a number of established methods such as low pressure and medium pressure lamps as well as recent developments such as flash technologies, UV LEDs, and others.

Quantification and validation using Challenge Tests

In order to quantify the antimicrobial effect of different sterilization methods one uses so-called Challenge Tests. Test microorganisms are used here to measure the efficiency of the methods. Challenge Tests are relatively complex procedures but are necessary for a variety of reasons. Indeed, UV radiation is difficult to quantitatively measure in industrial situations and external boundary conditions such as reflection, scatter, shadowing, humidity, etc. and the effect of such on the sterilization efficiency cannot be estimated. Additionally, the geometries of the objects being sterilized and the materials they are made of have a major effect on the microbiological effect of the radiation.

Such Challenge Tests are, for example, also recommended by the VDMA for the microbiological validation of hygienic filling plants for food products (e.g. VDMA FS NuV no. 10; 2016).

The results of such validations are naturally dependent on the resistance of the test organisms and so the latter must be precisely defined. For UV based methods, spores of mold fungi of Aspergillus niger (DSM 1957), and in particular Aspergillus brasiliensis (DSM 1988), have been used in Europe as test organisms for a long time.

Research on new test germs for industrial plants

In 2016, however, both these organisms, which were recommended by the VDMA, were upgraded from risk level 1 to risk level 2 in the "Technical Regulations for Working with Biological Materials: TRBA 460“. As such, it is no longer permissible to work with these organisms except in approved laboratories, meaning it is also impossible to use these germs on-site in production locations for Challenge Tests.

This objective of this project is to find a suitable surrogate germ for the aforementioned Aspergillus strains.

Identification of surrogates for Aspergillus species

The high resistance of Aspergillus brasiliensis represents a particular challenge. Any new test germ must have similarly high resistance. Varying the culture conditions to set the resistance is being investigated.

The project aims to map the resistance of the new test germs. Various types of lamps (low pressure, medium pressure) are being used for this. The dose-effect relationship, which does not purely obey pure physical laws, is also being studied in order to define optimum irradiation parameters. For practical application of the test germs, the following key parameters need to be studied and defined for new validation recommendations:

  • Storage properties
  • Change in resistance as a function of time
  • Optimal method for sterilizing given surfaces

Project term: 2020 to 2022
Project management/ funding: Arbeitsgemeinschaft industrieller Forschung AiF (via Industrievereinigung für Lebensmitteltechnologie und Verpackung e. V. - IVLV) / Federal Ministry of Economics and Energy BMWi