ASA Lay Language Papers
165th Acoustical Society of America Meeting

Scientists Use Acoustic Energy to Make Foods Sound

Hao Feng –
Bin Zhou
Hyoungill Lee
Hee Kyung Park
Sindy Palma
Department of Food Science and Human Nutrition
University of Illinois at Urbana-Champaign

Yanfang Li
Department of Agricultural and Biological Engineering
University of Illinois at Urbana-Champaign

Arne J. Pearlstein
Department of Mechanical Science and Engineering
University of Illinois at Urbana-Champaign

Popular version of paper 5aPA6
Presented Friday morning, June 7, 2013
ICA 2013 Montreal

Ultrasound at frequencies beyond human hearing can kill harmful bacteria in food.  The secret behind this bactericidal action is that when ultrasound travels through a liquid, microbubbles form and undergo expansion and contraction, with many ending their short lives in violent implosion, when they catastrophically collapse, releasing energy that can rupture bacterial cell walls. 

University of Illinois scientists have shown how to use ultrasound to improve food safety, providing an alternative to traditional thermal processing, and avoiding potential heat damage.  One example is ultrasonic treatment of juice.  Following a fatal 1996 hemorrhagic colitis outbreak traced to unpasteurized apple juice, the US FDA required juice processors to adopt processes to achieve 100,000-fold reduction in the number of pathogenic microorganisms, a goal that most processors meet by heating.  But thermally treated juices do not taste like fresh juice, and are less nutritious. Experiments in our lab show that ultrasound in combination with mild heat and low pressure, reduce the number of E. coli in apple cider 100,000-fold in less than 90 seconds.  Ultrasound-treated samples have aroma profiles similar to fresh cider, while those in commercial, thermally pasteurized samples were significantly lower.

Microbial safety of fresh produce is a challenge to the produce industry.  To reduce foodborne disease outbreaks, industry usually washes produce with chlorinated water.  However, industrial-scale chemical sanitization is capable of, at best, tenfold reduction (90% of bacteria killed).  To improve on this, we developed a pilot-scale, continuous-flow ultrasonic washer for treating leafy green vegetables.  Uniform spatial distribution of ultrasound in the channel is important, and was verified with simple aluminum foil tests.  Microbial inactivation was tested with single spinach leaves, or batches of leaves.  Single-leaf washing revealed the maximum inactivation capacity when no other leaves in the tank block ultrasound.  In batch-leaf testing, leaves block transmission of ultrasound that would otherwise treat other leaves.  Therefore, in practice, it is essential that good mixing allows each side of each leaf to receive equal ultrasound exposure.  The Illinois facility was specially designed for such mixing.  In single-leaf experiments, each leaf was spot-inoculated with nonpathogenic E. coli on its upper (smooth) surface.  In batch-wash tests, 10% of leaves in a one-pound sample were spot-inoculated.  Post-treatment bacterial survival counts were determined. Compared to treatment with chlorine alone, combined treatment with chlorine and ultrasound achieved additional reductions of 90% and 78% for E. coli inoculated on spinach, for washing in single-leaf and batch-leaf modes, respectively.