Power ultrasound
Ultrasoniccleaninghas beenaroundfor the last 40±50years in industry utilizing
the concept of welding low power (20 000±40000 kHz)ultrasonic transducers to
the bottomof a steelCOP tank.Thetechnologywas shown to be highly
effectivefor batchcleaningof engineeringcomponentsin combinationwith
chemical detergents. However, because of low powerand energy efficiency
conversion, the technology has always been limited to small-scale batch
applicationsand verylongresidence times.
Recentadvancesin low-frequency,high-powerultrasound(20 kHz to 1 MHz)
make this technology attractive for a large number of food processing
applicationsincludingcleaning and sanitizing foodcontact surfaces.Ultrasound
causes tiny bubbles, naturallypresent in a liquid, to expand(rarefaction) and
contract extremely rapidly to the pointwhere they collapse. This occurs
thousandsof times per second (e.g.40 000 times per second at 40 kHz). The
temperaturein the induced cavitationbubble reachesabout 5000 ÎK and the
pressureabout 2000 atmospheres (Bates,2004), so a verylargeamountof
energy is transferredinto the liquidsystem, whichcan be harnessed to produce
highshearenergy wavesand micro-streaming(high-velocity liquid).The high
shearenergywaves are responsible for producing the ultrasonic cleaning effect.
A synergisticeffectis reported when using ultrasoundin conjunction withan
acceptedgermicidal agent(Bates,2004).There are also reportsin the literature
of ultrasonic activity against protozoa, bacteria, yeast, and molds. The
mechanismof the antimicrobialeffectis not fullyunderstood; however, it is
believed to be due to the mechanical disruptionof the cell wall and a
corresponding leakageof DNA(Bates,2003).
Non-equilibriumatmospheric pressureplasma
Non-equilibriumatmosphericpressureplasma,frequentlyreferredto as cold
plasma,comprisespartlyionizedgases,whichare generatedin high-voltage
electricfields.So-calledcoldplasmais formedat or aboutroomtemperature.
Scientistshaveknownof low-temperatureplasmasinceat leastthe end of the
19thcentury;onlywithinthe pastseveralyearshavetechniquesemergedto
makecoldplasmagenerationpractical.Coldplasmaat atmosphericpressure
can be generatedby various methods,among these, are dielectricbarrier
discharge(DBD), resistive barrier discharge(RBD) andalso atmospheric
pressureplasmajet technology(Laroussi,1996).Thesemethodscan generate
relativelylarge volumesof non-equilibrium,low-temperatureplasmaat or
nearatmosphericpressure.Coldplasmacan be createdusinga numberof
differentgases.Air or othergas mixturescan be usedto generatethe plasma.
Thegasesare sourcesof UV and visiblelightand IR radiation,and free
radicalssuchas atomicoxygenand hydroxylgroups,all of whichplayan
important rolein microbial inactivation. Mounir Laroussihas devised an
apparatusthat createsmini-plasma inside a Plexiglas cubeby passing an
electric current through helium gas via specially calibrated electrodes.
Laroussi'sprocessis scalable;cold-plasmacontainers of virtuallyany size
462 Handbookof hygiene controlin the foodindustry