Shocking news for Listeria

Listeria (and other bacteria) in whey protein solutions can be inactivated in low temperatures and acidic environments without destroying nutrients.

Removing bacterial contamination from foodstuffs is a challenge — there are many ways to eliminate harmful bacteria but frequently these methods affect the nutritive value, flavour or texture of the food.

Enter electroporation

Electroporation is a physical transfection method that uses an electrical pulse to create temporary pores in cell membranes. One emerging method relies on what are known as pulsed electric fields (PEF), which destroy bacterial cell membranes but also use considerably lower temperatures than those applied in conventional thermal preservation. Until now, this process has mainly been used with fruit juices and other fluid foodstuffs.

Now, a team of researchers from the University of Natural Resources and Life Sciences, Vienna (BOKU), working in conjunction with a global food company, has used state-of-the-art equipment supplied by partner organisation EQ BOKU to identify conditions under which Listeria in more viscous whey-protein solutions can be safely eliminated by means of pulsed electric fields, while simultaneously preserving valuable nutrients.

Cool and acidic — just right

“Temperatures of 20°C or so and an acidic environment with a pH value of about 4 are ideal conditions for electroporating bacteria in protein solutions like the whey protein we tested,” said Prof Henry Jäger, Deputy Head of the BOKU’s Institute of Food Technology, summarising the study results which have now been published in the Journal of Food Engineering. “In this context, ‘ideal’ means that sufficient bacteria are inactivated while also preserving valuable ingredients to the greatest possible extent.”

Fine-tuning

The study compared two different concentrations of whey proteins (2% and 10%). A Listeria strain that is harmless to humans was then added to these solutions and the degree of inactivation resulting from pulsed electric fields of different intensities was compared. A temperature of between 20 and 40°C, and a pH value of 4 or 7, was selected. After electroporation, the residual level of sensitive nutrients, such as vitamins and immunoglobulins, and the Listeria concentration were measured.

“Temperature plays a decisive part in this process. The energy produced by the pulsed electric field causes the medium to warm quickly, which damages many valuable proteins,” explained Prof Jäger. “This effect increases in line with the strength and duration of the electric field, which in turn kills off the bacteria more efficiently. So it’s all about finding the right balance.” The research team showed that a low starting temperature of about 20°C and a pH value of 4 represents the ideal equilibrium, allowing for greater intensity of the electric pulses and, in turn, effective inactivation of the Listeria.

The study will make an important contribution to the development of more efficient and less damaging methods of removing germs from sensitive products, as well as enhancing overall food safety.

More information: Felix Schottroff et al. Pulsed electric field preservation of liquid whey protein formulations – Influence of process parameters, pH, and protein content on the inactivation of Listeria innocua and the retention of bioactive ingredients, Journal of Food Engineering (2018). DOI: 10.1016/j.jfoodeng.2018.09.003

Image credit: ©stock.adobe.com/au/SFIO CRACHO