Shock Waves (1999) 9: 49 55
Repeated application of shock waves as a possible method
for food preservation
A.M. Loske", F.E. Prieto", M.L. Zavala, A.D. Santana, E. Armenta
Instituto de Física, UNAM, A.P. 20-364, 01000 México D.F., Mexico. E-mail: loske@fenix.ifisicacu.unam.mx
Received 8 April 1998 / Accepted 17 September 1998
Abstract. In order to study the possibility of using underwater shock waves to cause death in non desired
microorganisms found in certain foods, Escherichia coli in suspension was exposed to hundreds of shock
waves on an experimental electrohydraulic shock wave generator. Using a parabolic reflector it was possible
to produce a plane shock front and expose many test tubes to the action of the shock waves at the same
time and under the same conditions. The amount of surviving bacteria was determined by plate counting
for different numbers of applied shock waves. Pressure measurements using needle hydrophones are also
reported. Experimental results indicate that electrohydraulically generated shock waves are capable of
producing a significant reduction in an E. coli population. An increase in the applied shock wave number
produced a nearly exponential reduction in the E. coli population.
Key words: Food preservation, Effect of shock waves, Escherichia coli
1 Introduction The destructive effects of ultrasonic waves on bacterial
cells, known for many years (Davies 1959), and the dam-
ages on living cells observed during ESWL (Delius et al.
Since its introduction in 1980, extracorporeal shock wave
1988), lead to the idea of using underwater shock waves
lithotripsy (ESWL) has become the standard treatment
as a possible method for food preservation.
for the majority of patients with renal and ureteral calculi
(Chaussy et al. 1980; Loske and Prieto 1998) and an alter- In the food industry, heat treatments are commonly
native in the treatment of gallbladder stones (Nahrwold
used to inactivate pathogenic microorganisms. Neverthe-
1993), pancreatic concrements (van der Hul et al. 1993),
less, because heat may affect the organoleptic and nu-
and stones of the salivary gland (Hessling et al. 1993).
tritional characteristics of food, there is great interest in
New clinical applications of shock waves are the treat- non thermal processes like ionizing irradiation (i.e. Å‚, ²,
ment of non-union fractures (Haupt et al. 1992), as well as
and X rays), addition of preservatives, cold storage, pulsed
the management of pseudarthrosis (Schleberger and Senge
electric fields, oscillating magnetic fields, high hydrostatic
1992), tendinopathy and other orthopedic diseases (Haupt
pressure and intense light pulses (Downing 1989; Mertens
1997). The treatment of tumors with shock waves is an- 1994; Pothakamury et al. 1993; Barbosa-Cánovas et al.
other experimental approach (Oosterhof et al. 1991). It
1994; Qin et al. 1995; Russell 1982). Some of these tech-
has been shown that colony growth of tumor cells de- niques are still being explored as possible alternatives.
creases as shock wave number increases (Berens et al.
It is the purpose of our investigation to evaluate the
1989). Unfortunately the tumor growth suppression ob-
possibility of using underwater shock waves in order to
served in vivo is temporary.
cause death in non desired microorganisms found in cer-
Because of the successful applications of shock waves
tain foods, preventing them from carrying out the biolog-
to medicine (Loske and Prieto 1995), low intensity under-
ical processes necessary for their existence and prolifera-
water shock waves and the behavior of materials under the
tion. This article reports our first results of the effects on
influence of low pressure shock waves received increased
Escherichia coli (E. coli) ATCC (American Type Culture
attention in the last fifteen years. For the same reason, the
Collections) 10536 under the action of weak underwater
effects of shock waves on living cells have also been the
shock waves, generated with an experimental electrohy-
subject of many investigations (Delius 1994; Loske and
draulic shock wave generator.
Prieto 1995).
Although in the device here described an electric dis-
"
charge in water is used to produce the shock waves, this
Present address: Laboratorio de Choques Débiles, Física Apli-
does not mean that the method is similar to the use of high
cada y Tecnología Avanzada, Universidad Nacional Autónoma
de México, A.P. 1-1010, Santiago de Querétaro, Querétaro voltage pulsed electric fields to preserve foods. In our case,
76000, México microorganisms are not exposed to an electric field.
50 A.M. Loske et al.: Shock waves as a mechanism for food preservation
As often in this kind of experiments, E. coli ATCC
10536 was chosen as the first microorganism to study the
effect of electrohydraulically generated shock waves, be-
cause it is a well known and easy to handle bacteria. Addi-
tionally, the comparison with the results obtained by Ker-
foot et al. (1992) and by Teshima et al. (1995) promised
to be interesting.
2 Material and methods
2.1 The experimental shock wave generator
A new experimental underwater shock wave generator,
Fig. 1. Sketch of the parabolic stainless steel reflector with
named MEXILIT II was designed and constructed. The
the spark gap electrode assembly at its focal point and the
MEXILIT II, similar to its former version (Prieto et al.
rack holding the test tubes
1991), consists of a pulsed power circuit, operating be-
tween 10 and 1000 Joules, providing multiple pulses to a
spark gap immersed in water. The spark gap electrode as-
sembly is at the focal point of a parabolic stainless steel re-
flector, with a focal distance of 20.0 mm, a latus rectum of
80.0 mm, a maximum internal diameter of 172.0 mm and
a depth of 92.5 mm (see Fig. 1), mounted on the bottom
of a 1200×800×600 mm Fiberglass water tank (see Fig. 2).
Application of high voltage (up to 30 kV) across a pair of
electrodes induces a spark, creating the sudden ionization
of the water. The fast expansion of the gas bubble gener-
ates a shock wave, propagating into the surrounding wa-
ter and reflecting off the reflector, creating a plane shock
front. The MEXILIT II, shown in Fig. 2, has the Fiberglass
water tank mounted on an iron frame. A three dimensional
computer controlled position system is placed on top of the
device, in order to fasten and move any probe, pressure
transducer, test tube or sample to any desirable position
within the tank. Basically the electric circuit consists of a
Fig. 2. Sketch of the MEXILIT II electrohydraulic shock wave
computer controlled capacitor charging system and a dis- generator. For clarity the front side of the water tank was not
charge device. In this work, capacitance and voltage were drawn
set to 80 nF and 20Ä…0.1 kV respectively. The spark gap
was set to one millimeter. Electrodes, with the shape of a
truncated cone (Loske and Prieto 1993), were allowed to
burn in for 400 discharges at 18Ä…0.1 kV. Tap water having
The first was only to confirm the culture growth, and the
a conductivity of 960Ä…5 microsiemens/cm and a temper-
second to dilute the sample with saline solution until the
ature of 27ą0.1ć%C was used. Basically, the MEXILIT II is
reading corresponding to the desired bacteria concentra-
similar to electrohydraulic shock wave lithotripters used
tion was obtained.
in ESWL, except for having a parabolic reflector instead
A total of 56 disposable test tubes (Elkay Products
of the ellipsoidal reflector used for clinical applications.
Inc., model 127-P507-STR) were filled and heat sealed.
Half of the pipettes were placed inside the water tank of
the MEXILIT II. The other 28 test tubes remained as con-
2.2 Sample preparation
trol samples in a separate water bath at the same tempera-
A24 hr at 37ć%C culture of E. coli ATCC 10536 in nutritive ture, for the same time as each of the treated test tubes.
broad (Merck V552243-448) was used. After cultivation, As can be seen in Figs. 1 and 2, the pipettes were placed on
the cells were collected by centrifugation and resuspended a specially designed plane circular Lucite rack, capable of
in a 0.9% NaCl solution. After three washes using the same holding 28 tubes at the same time. The rack was fastened
procedure, a suspension containing 105 107 CFU/mL was with an ordinary laboratory clamp to the position con-
prepared. trol system of the MEXILIT II at an arbitrary distance of
The concentration of bacteria before cultivation and 122.5Ä…0.5 mm from the focus of the reflector, which corre-
after the washes was registered and adjusted with a pho- sponds to a 50 mm separation between the upper border of
tometer (Lakeside Mannheim Boehringer model 4010) at the reflector and the Lucite rack (Fig. 1). All samples were
623 nm, using media and 0.9% NaCl solution as a blank. positioned so that their center was 107.5Ä…0.5 mm from a
A.M. Loske et al.: Shock waves as a mechanism for food preservation 51
horizontal line (latus rectum) going trough the focus of
the reflector. Shock waves were generated at a frequency
of 0.4 Hz. The water level was 45 mm over the border of
the reflector.
2.3 Experimental procedure
The experiment was repeated five times. Each time a to-
tal of 2000 shock waves were applied to the rack hold-
ing the test tubes. After every 500 shock waves, four test
tubes were randomly taken out of the shock wave gener-
ator, mixed in a flask and identified. The same procedure
was applied simultaneously to the 28 control tubes, not
exposed to the shock waves.
The following biochemical analysis were performed in Fig. 3. Pressure record obtained using a needle hydrophone at
about 107 mm from the latus rectum of the parabolic reflector
order to detect possible changes in the E. coli metabolism:
shown in Fig. 1 and at about 18 mm from its axis of symmetry
Kligler, H2S, citrate, mobility, indole and urea.
Samples were serial-diluted (1:10) and the amount of
surviving bacteria determined by plate counting (agar
plate count: Merck V877063708). The number of colony-
forming units per milliliter (CFU/mL) obtained to fill the
it crosses the baseline again. This should not be confused
pipettes before treatment, was used as a sample for zero
with some reported data, using ellipsoidal reflectors, where
discharges.
the width is defined as the time over which the pressure is
greater than one half of the peak compressional pressure
pulse. The implications of using these definitions of the
2.4 Pressure measurements
pulse rise time and widths are explained in the Discussion
section.
The pressure applied was recorded using a needle hydro-
phone (Imotec, GmbH, Würselen, Germany) with a 20 ns
rise time. Signals coming from the gauge were sent to
3 Results
the input channel of a Tektronix 2430A digital oscillo-
scope (Tektronix, Inc., Beaverton, Oregon), placing the Figure 3 shows a typical pressure record obtained at 107Ä…
hydrophone at the axis of symmetry of the parabolic re- 0.5 mm from the focus of the reflector and at 18Ä…0.5 mm
flector, at 107Ä…0.5 mm from the focus, and also at ten from its axis of symmetry. The signal was obtained using
other positions, moving the transducer horizontally away a 50 µs/div time base. Each vertical division corresponds
from the axis in 9 mm steps. Two hundred measurements to about 10 MPa. The electromagnetic signal of the high
were recorded at each position. A new set of electrodes voltage discharge can be seen at the beginning of the trace
was used for each position and allowed to burn in for at the instant (T) when the oscilloscope was triggered. The
400 discharges at 18 kV. In order to measure the pres- direct shock wave arrives after about 84 µs and is followed
sure drop due to the test tubes, the Imotec pressure gauge approximately 22 µs later by the reflected pressure wave.
was placed inside the water tank at 100Ä…0.5 mm from the All pressure variations, recorded at the other positions,
spark gap. After burning in the electrodes, 50 pressure showed a similar behavior.
profiles were recorded without covering the gauge. After The average peak positive pressure of the reflected
that, the gauge was immersed in an inactivated E. coli pulse, corresponding to the first ten transducer positions
suspension inside a test tube and placed at the same po- was 44Ä…7 MPa, having a width and rise time of 4Ä…0.5 µs
sition in order to take another set of 50 measurements. and 2.8Ä…0.1 µs, respectively, followed by a negative pres-
All measurements were done at the voltage, capacitance, sure pulse of 6Ä…3 MPa. A statistical analysis revealed no
water temperature and conductivity already mentioned in significant difference between pressure measurements at
2.1, using the cursors of the digital oscilloscope, and fed the different positions, except for the last two at 81 and
into a personal computer for carrying out the statistical 90 mm from the axis of symmetry of the reflector, were the
analysis. positive pressure dropped about 25 and 40%, respectively.
In order to save time while measuring with the cursors This is probably due to diffraction of the pressure wave at
of the oscilloscope, all rise times were defined as the time the borders of the reflector. Because of this, the mentioned
required for the wave to rise from the baseline to the max- average pressure values refer only to the pressure at the
imum amplitude and not in the conventional way, as the axis of symmetry and the first nine consecutive positions.
time required to rise from 10% to 90% of the maximum No test tubes were located at more than 65 mm from this
amplitude. For the same practical reasons, the widths were axis.
measured at the baseline and defined as the time from The measured pressure drop due to test tubes filled
the instant where the pulse rises, to the instant where with cell suspension was about 20%.
52 A.M. Loske et al.: Shock waves as a mechanism for food preservation
where t stands for time in minutes, N0 for initial number
of microorganisms and N for number of microorganisms
which survived after t minutes. In this study, results are
given in dose or applied shock waves , instead of time.
This is due to the fact that the shock wave generation
frequency is a parameter which can be set and modified
depending on the selected voltage and capacitance of the
shock wave generator. In this experiments the mean value
of K was 0.0018, with a coefficient of variation (standard
deviation divided by the average) of only 0.13.
The average dose D = t/(log N0-log N), needed to re-
duce the initial amount of microorganisms 90% was about
569 shock waves, having a coefficient of variation of 0.14.
Figure 5 is a graph of the logarithm of the survival
population vs. the applied shock wave number, showing
the expected behavior continued to 6 D. The straight line
Fig. 4. Graphs of the logarithm of survival E. coli population
has a slope K = 0.0018. This means that the reduction
vs. the number of applied shock waves for five experiments.
seems to follow an exponential behavior.
Least squares linear fits to the experimental results are shown
Since in this case a frequency of 0.4 Hz was used, it
would take about 24 minutes to generate 569 shock waves.
This means that it would be necessary to apply electrohy-
draulic generated shock waves (at the already mentioned
voltage, capacitance and frequency) for about 24 minutes
to reduce the E.coli population from 106 to 105 CFU/mL.
In order to inactivate the initial population, 6 D or about
143 minutes are needed (Block 1994).
4 Discussion
The cell container and environment around and within
the cell tube are important because they will directly in-
fluence on the transmission of the shock wave to the cells.
Polypropylene was chosen for the test tubes because its
acoustic impedance approximates that of water. Neverthe-
Fig. 5. Expected behavior of E.coli growth after shock wave
less, pressure measurements revealed that the shock wave
exposure
lost about 20% of its value when passing through the test
tube. Pipettes with thinner walls or made out of a different
material, could reduce this pressure attenuation.
Spark gaps in water generate broad band pressure pul-
ses with very short rise times and high pressures which
Biochemical analysis did not reveal any change in the
depend on several parameters, some of which can be con-
metabolism of the E. coli microorganisms.
trolled and some can not. The reported variations in pres-
Results indicate a nearly logarithmic reduction in the
sure measurements are typical of electrohydraulic shock
microorganism population after shock wave exposure. In
wave generators (Coleman and Saunders 1989; Prieto et
order to determine the mortality index of the exposed
al. 1994). These variations did not affect our results be-
E. coli ATCC 10536 bacteria, an initial count between
cause microorganisms were exposed to hundreds of shock
105 and 107 CFU/mL was used. This value is compara-
waves.
ble to the concentration reported by the Association of
The electrode tips of the shock wave generator wear off
Official Analytical Chemists for some contaminated food
due to the high temperatures and forces acting on them
products (Analytical Chemists, vol. I, 15th edition, Wash-
during each electric discharge. As a result of this erosion,
ington DC, Association of Official Analytical Chemists,
the electrodes have a limited lifetime. In order to reduce
Inc. (1990) pp 435 436, 803 805).
time between voltage application and spark gap genera-
Figure 4 are the graphs of the logarithm of the survival
tion, the electrode gap should not exceed 3 mm (Loske and
population vs. the number of applied shock waves for the
Prieto 1993). Furthermore, as the electrode gap becomes
five experiments, showing a similar slope K. Generally, K,
larger, the pressure of the shock wave increases. Addition-
referred to as velocity constant or mortality index (Block
ally, in general the electric spark gap does not link the
1994) is obtained using the formula
two electrodes by the shortest path. Therefore, the electric
discharge is rarely located at the focus and leads to dis-
N = N0e-Kt , persed pressure peaks around the second focus of ESWL
A.M. Loske et al.: Shock waves as a mechanism for food preservation 53
lithotripters. This can be improved by axial positioning Ohshima et al. (1991) found that the intact cells of E. coli
of the electrodes in the reflector, as in the MEXILIT II. JM 109/pKPDH2 are difficult to be destroyed by shock
Considering a 400 discharge burn in at 18 kV, the practical waves using a shock tube which does not generate UV
lifetime of the electrodes was estimated to be about 2400 light, indicates a possible influence of the spark-generated
shock waves at 20 kV, using an 80 nF capacitance. Pres- electromagnetic radiation.
sure measurements have shown that between 400 and 2400
Even if it is known that E. coli can grow at static pres-
discharges, the pressure profile is fairly constant. Beyond
sures up to 55 MPa, the response to dynamic pressures is
2400 shock waves, the pressure amplitude variation, as
expected to be different, since in this case there is not
well as the number of misfires, increase significantly. Due
an even distribution of pressure in the cell suspension.
to this, the maximum number of applied shock waves was
Furthermore, static pressures do not produce cavitation
2000. The extrapolation of our experimental data (Fig. 5)
in the suspension. Cavitation is generated whenever there
revealed that a total of about 3420 shock waves are needed
is a rapid transformation of positive pressure into ten-
to completely inactivate the bacteria. In order to replace
sile stress. In the MEXILIT II, the pressure wave initially
the worn-off electrode with a new one, it is necessary to
produces a high positive pressure, which is rapidly trans-
empty the water tank of the MEXILIT II. It takes about
formed into tensile stress within microseconds, resulting
30 minutes to empty the tub, replace the electrode, fill
in the formation of vapor-filled cavities. These cavities im-
the tank again, adjust the water temperature and con-
plode, creating very high energy densities.
ductivity to the desired values and burn in the new elec-
In general, microorganisms can be killed by static pres-
trode. During this process, the pipettes would have to be
sure of about 100 MPa, but the complete sterilization is
taken out of the shock wave generator and placed in a
often difficult because of so called persisters . These are
separate water bath having the same temperature. After
many reasons why simple compression and decompression
that, the experiment could be continued until 3420 shock
does not harm microorganisms in the same way as the re-
waves have been administered. Since a 30 minute waiting
peated administration of a very short high pressure pulse
time would significantly alter the results, the electrode
followed by a negative pulse. Cavitation depends on the
was not changed and the experiment was stopped after
pressure of the medium, the presence of microbubbles in
2000 shock waves. In the future, this shortcoming could
the sample and the existence of a liquid-air interface. The
be solved using a different type of electrodes or using an
mechanism by which cavitation may cause biological dam-
ellipsoidal reflector in order to increase the pressure and
age are high localized temperature and pressure gradients.
reduce the number of shock waves needed to perform an
The bactericidal effect of ultrasound has been attribu-
experiment with a D6. This could reveal the existence of
ted to cavitation (García et al. 1989). It is interesting to
bacteria that were originally resistant to shock waves or
point out that the increase in human renal cell carcinoma
became so during shock wave treatment. As already ex-
xenografts loss in tubes containing air was reported to
plained, the disadvantage of using an ellipsoidal, instead
be 40% higher as compared to sample tubes without air
of a parabolic reflector, is that only one pipette should be
(Steinbach et al. 1992). This might be explained by an
placed at the second focus and exposed to the shock waves
increased occurrence of transient cavitation, caused by re-
at a time. This significantly increases the experimentation
flection of the pressure wave at the liquid-air interface. The
time. If shock wave application reveals to be a convenient
interface results in perturbation in the shock front with re-
method to be used in the food or pharmaceutical industry,
sultant surface shear and cavitation within the suspension.
other shock wave generation mechanisms will have to be
It is for these reasons that the microorganism death is ex-
developed.
pected to reduce in the absence of a liquid-air interface. In
Shock waves from electrohydraulic generators are con-
our case, the test tubes were only filled up to about 75%.
sidered weak. Nonlinear effects appear only in the prox-
As far as we know, Kerfoot et al. (1992) did the first
imity of regions where the energy is concentrated. This is
experiments designed to isolate the effects of shock waves
the case in extracorporeal lithotripters, using ellipsoidal
on bacterial cells (Pseudomonas aeruginosa, Streptococ-
reflectors, but not in this study, where a parabolic reflec-
cus faecalis, Staphylococcus aureus and Escherichia coli)
tor was used.
and determine whether bactericidal activity exists. In this
It is important to point out that the radiant output of study, the suspension received 200 shock waves at 20 kV
the underwater spark has a continuum in the ultraviolet and a rate of 100 shocks per min on a HM3 Dornier elec-
(UV), having a peak at approximately 55 to 150 nm. This trohydraulic lithotripter (Dornier Medizintechnik GmbH,
ultraviolet radiation could contribute to microorganism Germering, Germany). The experiment was repeated de-
death. Nevertheless, the intensity of this radiation is re- livering 4000 shock waves at the same energy and rate.
duced significantly during its path through the water and Aliquots of bacterial suspensions of each of the four bac-
the test tube. The influence of this UV radiation on the terial strains were also exposed to 4000 shock waves gen-
reduction of microorganism population is currently been erated by a Wolf Piezolith 2200 piezoelectric lithotripter,
studied. Experiments on human tumor cells, exposed to which does not generate UV radiation, at energy level 4
electrohydraulically generated shock waves using opaque and a rate of 120 shock waves per min. Contrary to our
polypropylene pipettes, have shown no evidence of cell results, the authors concluded that shock waves do not
death due to UV light (Berens et al. 1989). Obviously this possess significant bactericidal activity. It is important to
result could be different when using E. coli. The fact that notice that, even if the MEXILIT II shock wave genera-
54 A.M. Loske et al.: Shock waves as a mechanism for food preservation
tor is capable of reproducing the pressure field generated death is not associated with an increase in temperature
by a HM3 lithotripter, in our study, at 20 kV the cells due to the positive pressure pulse. Nevertheless, as already
received less energy due to the fact that a parabolic reflec- mentioned, localized temperature rise due to collapse of
tor was used instead of the conventional ellipsoidal reflec- cavitation bubbles may produce microorganism death.
tor of the HM3. Since Kerfoot et al. filled each test tube An experiment, that could help to understand the
completely in order to exclude air bubbles, we conclude mechanisms involved in microorganism death would be
that the bactericidal effect observed in our study is due to partially immobilize the microorganisms in gelatin, in-
to shock wave reflection and cavitation at the air-fluid in- stead of suspending them in a liquid. This would reduce
terface. Cavitation may also explain reports of decreases cavitation and microorganism collisions almost completely,
in both persistent urinary tract infection and bacteriuria even if it is well known that E. coli is capable of moving
after ESWL of infection stones (Michaels et al. 1988; Pode through gelatin.
et al. 1988). Experiments using aerated fluids in order to For biological tests, the reported variation of K is low.
enhance cavitation in the cell suspensions, could show a
strong bactericidal effect of shock waves.
Since the resistance of some microorganisms to heat is
5 Conclusions
reduced by previous treatment with ultrasound (Burgos et
al. 1972) a combination of shock wave and heat treatment Our results indicate that electrohydraulically generated
should also be investigated in the future. shock waves are capable of producing a significant reduc-
tion in an E. coli population. An increase in the applied
As already mentioned, Ohshima et al. (1991) found
shock wave number between 500 and 2000 shock waves,
that the intact cells of E. coli JM 109/pKPDH2 are dif-
generated using an 80 nF capacitor and a voltage of 20 kV,
ficult to be destroyed by shock waves. They confirmed
produces a nearly exponential reduction in the E. coli pop-
that these cells of E. coli were killed predominantly when
ulation.
small bubbles were introduced into the cell solution. Nev-
A detailed knowledge of microorganism shock wave
ertheless it has to be noticed that they used a shock tube,
killing could have many practical applications to the food
which generated a positive pressure of about 0.1 MPa, hav-
industry, specially considering that pressures as high as
ing a pulse duration of approximately 900 µsec, which is
100 MPa do not denature proteins, which means that this
weak and slow, compared to pressure pulses of 44 MPa
process may be selective. Furthermore, the treatment of
and pulse durations of about 4 µsec, generated with our
pharmaceutical solutions and suspensions as well as bio-
device, which additionally generates a negative pressure
materials, could also be possible. This would be of spe-
pulse. Using the same shock tube, Teshima et al. (1995)
cial interest in those cases where microorganism inactiva-
showed that the destruction of spheroplast of recombinant
tion using radiation, chemical substances or thermal pro-
cells of E. coli JM 109/pKPDH2 can be monitored by mea-
cedures deteriorates important properties or is restricted
suring phenylalanine dehydrogenase activity leaked from
by law. In terms of food preservation, shock waves give
the cells. Electron microscopic analysis of the cells after
solids a better chance to bacterial survival than liquid or
100 shock waves at 14 MPa showed cell rupture.
foamy foods.
It is to be expected that the effectiveness of the shock
wave depends on the maximum pressure amplitude (peak
compressional and rarefactional), the rise time, the dura-
Acknowledgements. The authors would like to thank Marisol
tion of the pulse, and the repetition rate.
Saldańa for assistance during the experiments and Arturo
Méndez for performing pressure measurements and for his im-
The exact mechanism of the induced microorganism
portant participation in the construction, testing and mainte-
death is still unknown. Cavitation, micro jets, accelera-
nance of the shock wave generator. Salomón Tacher L., Marco
tion, shearing forces, and formation of free radicals may
Veytia and Francisco Mercado contributed to the design of the
cause the observed effect. These mechanisms will be af-
MEXILIT II. The help of Francisco Fernández in the design
fected on the suspension media used.
and repair of electronic equipment is gratefully acknowledged.
Another possible mechanism of cell death are reso-
Acknowledgments are also due to Abel Gutiérrez R. for aid
nance effects and collisions between the microorganisms.
and expertise in microbiology and to Jorge García for per-
Experiments using higher microorganism concentrations
forming some UV-radiation transmission measurements. This
possibly could help to determine the importance of colli-
study was partially supported by the DGAPA of the National
sions for microorganism death.
University of Mexico (UNAM).
The compression of the suspension causes a transient
increase in temperature. Nevertheless in this case the tem-
perature rise is very small. Berens et al. (1989) estimated
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