By Joy Gaze - 28 February 2014

Surrogate microorganisms are harmless microorganisms which have similar resistance properties to pathogenic or spoilage organisms and can be used as substitutes for testing (e.g. food process trials, effects of preservatives).

When developing a process to reduce or eliminate microbiological contamination in a food product, it is often developed and semi optimised in a laboratory using the actual target organism(s) of concern. However, there comes a point in any process development, where the efficacy of the factory process in the factory production area with 'actual' process equipment has to be evaluated. This test of actual equipment is the real 'process validation' and verifies that the process used is effective at reducing by a defined level the target microorganism(s) of concern. However, it is often inappropriate to use the real target organism within the manufacturing environment due to fears of potential risks of safety or spoilage. Surrogate organisms are used in such situations.

As the death kinetics of any microorganism are affected by the physical properties of the particular food matrix under consideration (e.g. pH, Aw, fat content, salt content, moisture level) and the type of process to which the organism will be exposed (e.g. dry heat, moist heat, steam, etc.) it is necessary to make careful comparison of D– and z–value information obtained for target and potential surrogate organisms in order to ensure the suitability of the latter for use in a particular set of circumstances.

We reviewed published literature to establish the extent of knowledge which is publically available on surrogates for use in different categories of foods. This information was used to construct a matrix of potential surrogate microorganisms, foods categories and process types. Our next task is to screen these against their respective target organisms in each food type/process combination, allowing identification of the most suitable surrogates in each case.

We will then determine D– and z–values for these organisms in relevant food types and processes, providing data which will be used to correlate the thermal resistance of these surrogates against those of the target organism in order to assess their suitability for use in validation of similar processes.

The target organisms that we have investigated include Salmonella, Listeria monocytogenes, Bacillus cereus and Byssochlamys species. We have identified 18 potential surrogates for these, in a variety of products and packaging, and for a number of different processes, including steam sterilisation and pasteurisation, roasting, hot fill and sous vide.

The project continues to the end of next year, by when we will have a very valuable matrix of surrogate organism applications for members to utilise.