The team from the University of Illinois said combining technologies is key to bridging the gap between current capacity and desired outcomes of decontamination.
Hao Feng et al. said their pilot-scale flow-washing system can reduce E.coli O157:H7 by 99.99%.
While the technique has potential to enhance microbial safety, screening/blockage of produce leaves must be minimised as well as the variance in the residence-time distribution.
Feng, a University of Illinois professor of food science and human nutrition, said the US Department of Agriculture (USDA) is looking to achieve a 4 to 6 log reduction in pathogen cells (a 6 log reduction would achieve a million-fold reduction in pathogenic bacteria).
The food processing industry can now achieve a 1 log or tenfold reduction, while the described technique yields a 4 log reduction.
“Combining technologies is the key to bridging the gap between our current capacity and what USDA would like to see,” Feng added.
"Previous work with ultrasound used a tank or a medical-style probe, which doesn't provide consistent and even distribution.”
The system uses three pairs of large-area ultrasonic transducer boxes operating at 25, 40 and 75kHz to form a channel through which ultrasound is provided to spinach leaves that are undergoing a continuous-flow chlorine wash.
Feng said the placement of the produce through the channel turns was also important.
“We had to find ways to make sure that leaves received similar exposure to ultrasound, taking care to minimize the chance that one leaf would block a nearby leaf's exposure to the sound waves."
The efficacy of the new wash system was assessed both by cavitation damage to aluminium foil, and by inactivation of E. coli O157:H7 inoculated on spinach leaves.
“The inactivation efficacy of an ultrasound-assisted produce wash system was shown to be affected by a number of factors, including uniformity of the ultrasound field, degassing, residence time, distance to the ultrasound-emitting surface, and blockage of ultrasound by other produce leaves,” said the researchers.
Screening or blockage by produce leaves were identified as critical factors determining the inactivation efficacy because if even part of a leaf escaped the full ultrasonic treatment, it could contaminate the rest of the produce, Feng said.
Blockage reduces the exposure of screened leaves to ultrasound resulting in significantly lower microbial count reduction.
The technique can also be used on iceberg and romaine lettuce with similar results.
Published online, ahead of print, doi: 10.1016/j.ifset.2012.09.007
“Continuous-flow ultrasonic washing system for fresh produce surface decontamination”
Authors: Bin Zhou, Hao Feng,