Microwave E-coli eradication process intervention – Stage 1

Summary

Introduction
Escherichia coli (E. coli) are harmless gram-negative, rod-shaped bacteria that form part of the normal human gut flora. Shiga-toxin producing E. coli (STEC) may be especially harmful to humans with E. coli O157:H7 being the most common. Ruminants, particularly cattle and sheep, are reservoirs for STEC especially during processing where it is possible for meat to be exposed to faecal matter.
Current methods target on-farm practices, pre-slaughter holding practices, within slaughter processing practices, strict chilling regimes, hygienic boning room and finished goods handling procedures, and control points mechanisms such as carcase wash systems. However, current methods do not guarantee eradication of bacteria from meat products.
The project aims to ascertain whether principles behind the Gyrotron can be applied to the meat processing sector to eradicate microorganisms, with the initial focus on E. coli. The Gyrotron delivers microwaves to the surface of meat in a non-contact method. The project determined whether the technology can eradicate E. coli without deteriorating meat quality.
Method
The trials were carried out at the Plasma Physic Institute (IFP), which had access to a 28GHz, 15kW Gyrotron, and E. coli inoculation and cell count verification was provided by the Institute of Sciences of Food Production (ISPA). Both institutes are part of the National Research Council of Italy.
A combination of meat samples and inoculated agar plates were exposed to 149 various settings. Meat samples which showed E. coli reduction without surface damage, were further analysed by the Department of Food, Environmental and Nutritional Sciences (DeFENS) for any effects the Gyrotron may have had on meat colour and tenderness.
Results
Initially, there were three settings that could reduce E.coli cell numbers without deteriorating meat quality (refer to Table 1). However, further analysis by DeFENS found a significant difference in colour for Sample A compared to the control. In saying that, Sample A managed a 4.7 log reduction (refer to Table 2).
Table  1.  Treatment  cycles  of  three  settings  using  the  Gyrotron  (GYCOM 28GHZ/15kW/cw)  that  deactivated  E. coli  without  damaging  meat  surface  (prior  to  DeFENS  analysis)SampleABCPower (kW)1098Pulse length (sec)0.030.020.03Number of pulses101013Wait time between pulses (sec)555Total treatment cycle (sec)464661
 
Table  2.  Log  reduction  of  the  three  samples  which  had  reduced  E. coli cell  number without  visual  meat  impact  (prior  to  DeFENS  analysis)SampleCFUA/gCFUA/plateLog ReductionControl4.5x1051.9x106-A<10><364.7b9.5x1034.7x1041.6c1.4x1046.0x1041.5>
AColony Forming Units
Although results were promising, there was uncertainty around progressing with the 28GHz Gyrotron. Continued discussions between Scott Technology and IFP led to a simulation report, which used the results from the microwave trials to determine whether higher frequency microwaves could deactivate E. coli in 20 seconds or less.
On a 100GHz Gyrotron, one setting was found to deactivate 90% of E. coli cells within 20 seconds. Similarly, one setting on the 28GHz Gyrotron could achieve the same level of deactivation within 20 seconds. Theoretically, the 100GHz Gyrotron would be more ideal as it delivers higher frequency microwaves and can be adjusted to deliver higher amounts of energy to the meat surface unlike the 28GHz. This is because a higher frequency source will not penetrate beyond the surface of meat and will not cause any damage to meat surfaces.