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Emerging Technologies, WS 2011/12
Ultrasound –Fundamentals
Katharina Schössler
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The sound spectrum
Echolocation(Sonars)
Material testing
Medicaldiagnostics
Infrasound Audible sound Ultrasound Hypersound
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Ultrasonic waves
• In fluids : Longitudinal waves (pressure waves)
• Direction of oscillation of medium particles parallel to direction of travel of the sound wave
• Transversal waves may form in elastic materials and firm tissue
Sound:
Oscillation of pressure transmitted through a solid, liquid or gas
Direction of travelDirection of oscillation
a) Longitudinal wave b) Transversal wave
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Historical Overview
1917: Paul Langevin: Echolocation of icebergs
1912: Foundering of the Titanic due to a collision with an iceberg
1925: George W. Pierce: First ultrasound-Interferometer
1830: First Ultrasound Generation
From 1915: ASDIC (Anti Submarine Detection Investigation Committee) + SONAR (Sound Navigation and Ranging)
1927: Wood & Loomis: Characterization of typical ultrasound effechts
1937: Brothers Dussik: Beginning of medical diagnostics
1950: first industrial ultrasonic tester
1958: Bommel & Dransfeld:Exploitation of hypersound
1964: Gutfeld & Nethercot:quantenacoustic ultrasound generation
1974: Lemons & Quate:Scanning Acoustic Microscope
1900 19501925 1975
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Generation of ultrasound
Hielscher Ultrasound Technology, 2006
1 Transducer
2 Generator
3 Sonotrode
4 Horn
• Generation and transmission of high frequency sound waves
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Transducers I
Magnetostrictive transducer
• Deformation of ferromagnetic materials by an external magnetic field
• Elastic change in length(µm/m – mm/m)
Ferromagnetic materials:
Iron, cobalt, nickel and associated alloys
Magnetic field
Magnetic field
∆l
First sonars
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Transducers II
Piezoelectric transducers
Applied mechanical force electric dipole moment electrical charge
Piezoelectric materials: Quartz, ceramics, zinc oxide, polyvinylidene fluoride
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Emitters
Baths
• Emitters radiate the ultrasonic wave from the transducer to the medium
JTT Ultraschall, 2009JTT Ultraschall, 2009
Probes
Transducers
TransducerHorn(amplification)
Sonotrode
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Two approaches in the application of US
Medical ImagingNon-destructive Testing
Diagnostic Ultrasound Power Ultrasound
High FrequenciesLow Energy
Low FrequenciesHigh Energy
1 -10 MHz 20 -100 kHz
non-destructive material altering
Food ProcessingSonochemistrySoldering…
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Process parameters
)2sin(max, ftPP aa π=
+
-
Aco
ust
ic p
ress
ure
Time (s)
Pa,max
Ultrasound
• Amplitude, Pa [bar, µm]
• Frequency, f [kHz]
• Wavelength, λ [m]
• Velocity, c [m/s]
� ��
�
Others
• Temperature [K]
• Pressure [bar]
• Viscosity of treated medium [Pa·s]
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Effect of medium characteristics
• Sound wave: oscillation of medium particles
• Transmission of sound waves depends on a medium
• Strong influence of medium characteristics on sound transmission
1. Deceleration
2. Dampening
3. Reflection
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Ultrasonic velocity
• Elastic modulus and density depend on structure, composition and physical state of the medium
• Deceleration of US waves by highly viscous solutions or firm tissues �minimized sonication effect
• Observable effects strongly depend on the initial process parameters
• E.g. impact of US on meat tenderness
ρE
c =2c: Ultrasonic velocity [m/s]
E: Elastic modulus [N/m²]
ρ: Density [kg/m³]
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xeAA α−⋅= 0
Sound wave amplitude
A: Amplitude [µm]
A0: Amplitude (x=0) [µm]
α: Attenuation coefficient
• Dampening of the sound wave amplitude
• Two major causes: adsorption & scattering
• Associated with physicochemical properties of the medium (concentration, viscosity, molecular relaxation, microstructure); e.g. droplet size and quantity in emulsions
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Reflection
21
21
ZZ
ZZ
A
AR
i
r
+−==
R: Reflection coefficient
Z1,2: Impedances of materials 1 & 2 [N·s/m³]
Ar: Amplitude reflected wave [µm]
Ai : Amplitude incident wave [µm]
• � ��
�∙�; p: sound pressure, v: particle velocity, S: surface area
• At interfaces between two materials sound waves are partially transmitted and partially reflected
• Principle of US imaging
• Similar acoustic impedances mean only little reflection and large transmission � deep penetration e.g. of a food product
• Zair,20°C: 413 Ns/m³ Zwater,20°C:1.48·106 Ns/m³
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Mechanisms of action in the soundfield
1. Acoustic streaming
2. Cavitation
3. Radical formation
4. “Sponge effect”
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Acoustic streaming
Image: Courtesy NASA/JPL-Caltech
• Steady current in a fluid driven by the adsorption of high amplitude oscillations
• Major US effect at amplitudes below the cavitation level
• Effects on boundary layers (heat & mass transfer)
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Cavitation
• Sufficiently high sound pressure amplitudes:Local pressure < vapor pressure of the liquid (cavitation threshold; increases with frequency)
• Formation and growth of gas bubbles; “activation” of gas inclusions (cavitation nuclei)
• Hydrodynamic shear-forces and microstreaming
• Bubble growth due to rectified diffusion
Stablecavitation
Inertialcavitation
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Stable cavitation
Thresholds for inertial cavitation as a function of initial bubble radius
Apfel & Holland, 1991
• Acoustic streaming
• Shear forces
• Degassing
• Bubbles pulsate about an equilibrium radius
• Level for change to intertial cavitation depends on frequency an initial bubble
size
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• Sudden expansion and rapid collapse of bubbles; or high amplitude pulsation
• Collapse: Compression heating of gas
• Gas shocks, electrical discharges, sonoluminescence
Inertial cavitation
Intense pressure, shear and temperature gradients
12 000 K4 400 bar
Hot Spots(local effects)
• Mixing
• Cleaning effect
• Emulsification/
Dispersion
• Improved
extraction
• Disruption of
boundary layers
� increased heat
transfer
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Cavitation close to solid surfaces
• During collapse cavitation bubbles involute
• Formation of a liquid jet towards the surface
• Strong mechanical effects
1 2 3 4
Increasing static pressure
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Influence of process parameters
• Pressure– High ambient pressure impedes bubble growth;
increase of the violence of each collapse; increasedradical formation
• Temperature– High temperature � increased water vapor pressure
inside cavitation bubbles � cushioning effect duringcollapse; less radical formation
• Frequency– Low frequencies favor inertial cavitation
• Amplitude– Negative sound pressures must be sufficiently high
to induce cavitation
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Radical formation
• Radical formation due to bubble implosions• Depends mainly on local temperature peaks• Increased radical formation per bubble collapse with decreasing frequencies
due to higher final bubble size; but: larger amount of collapsing bubbles at higher frequencies
� Oxidation
• Reduced antioxidant activity
• Off-flavour due to pyroylsis and lipid oxidation
Radical formation at water filled cavities
�� → � ∙ ��� ∙
� ∙ �� ∙→��
�� ∙ ��� ∙→ ����
� ∙ ��� ∙→ ���
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„Sponge Effect“
• Contraction and extension of the treated medium
• Vibration
• Improved mass transfer
• Generation of micro-channels
• Pressure fluctuation � increased evaporation rate
US
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Heating
US induced heating• Heat generated from motion of medium
particles• Thermal and viscous damping of
bubble movement• Hot-Spots associated with bubble
collapse
US + Temperature• US + T: Thermosonication• Synergistic effects in microorganisms and enzyme inactivation• US effects render cells and proteins more susceptible to thermal stress
Heating
Cooling
T Process Control
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Energy of an ultrasound treatment
Patist, 2008
Specific energy input E: Energy / Volume of tre ated medium (kJ/kg)
Intensity I: Energy emitted at sonotrode’s surface ( W/m²)
M
tPE
⋅=
P Power (kW)t Treatment time (s)M Treated mass (kg)f Frequency [kHz]
� ∝1
��
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Measurement of ultrasonic energy
• Hydrophones:Determination of the acoustic pressure(reversed transducer, microphone)
• Calorimetry: Measurement of acoustic energy converted into heat
� � ⋅ �� ⋅ Δ Q: Energy input [W]m: Sample mass [kg]Cp: Heat capacity of the sample [J/K]∆T: Change in temperature [K/s]
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Further characterizations of the sound field
CHEMICAL PHYSICAL
Local Chemical Methods
Global Chemical Methods
Erosion Methods
Optical methods
Calorimetrical Methods
⇒ Aluminium Foil⇒ Weissler Reaction⇒ Chemoluminescence⇒ Electrochemical Sensor
⇒ Model Reactions (Dosimetry)
⇒ Sonoluminescence(SBSL, MBSL)
⇒ Laser-Doppler-Anemometry⇒ Radiation Pressure Scale⇒ Optical Sound Pressure and
Velocity Sensors
⇒ Calorimetry⇒ Thermoacustic Sensor⇒ Elastic sphere radiometry
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Ultrasound – Fields of application
Medical
• US-Thermotherapie• US-Lithotropsie• Diagnostics• …
Process Engineering
• US-Cleaning• Sewage processing• US-Welding• US-Cutting• …
Sonars
• Detektion of shoals• Determination of water depth• …
Sonochemistry
• Initiation and increase of chemicalreactions
• Homogenizing• Degassing• …
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Emerging Technologies, WS 2011/12
Ultrasound –Applications in Food Technology
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Ultrasound in Food Technology
Knorr et al. (2010)
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Low intensity diagnositc US
• Frequency: MHz range• Intensity < 1 W/cm2
• Principle: measurement of US velocity (c), attenuation (α) and/or phase (frequency and/or time dependent)
• Change of c, α and phase: molecular interactions, phase transitions, molecular rearrangements etc.
Pitch and Catch -/ Pulse echo time of flight measurement
Pitch Catch� � /�
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Level measurement
a
b
• Pulse-echo-technique• Time of flight measurement• Measurement sensitive to bubbles (strong
attenuation)
a. Determination of c from time-of-flight over a known distance
b. Determination of liquid level
Hauptmann et al. (2002)
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Flow measurement
• Transit-time flow meter
• Doppler flow meter
Hauptmann et al. (2002)
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Composition determination
Hauptmann et al. (2002)
• Speed of sound sensors• Ultrasonic attenuation sensors• Acoustic impedance sensors
(don’t require liquids transparent to US)
� Monitoring of fermentation processes
� Particle size determination� Concentration measurements� Crystallization measurements
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Foreign bodies and product defects
Knorr et al. (2004)
Detection of a foreign body in a yoghurt beaker
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Determination of food material properties
• Measurement of materials characteristics
• Monitoring of textural changes (gelation of milk and tofu, mechanical characteristics of cheese)
• Conventional methods: microscopy, texture analysis, rheology � require laboratory practice, time consuming, invasive, unsuitable for real-time applications
Leemans & Destain (2009)
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Gelation of tofu curd
Leemans & Destain (2009)
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US-testing: Advantages and challenges
Advantages
• Non-invasive• In-line measurement• Rapid response (< 1s)• Low energy consumption• Long-term stability• High resolution• High accuracy
Challenges
• Exact knowledge of acoustic properties of the treated medium
• Measurements highly disturbed by gas bubbles
• Complex signal processing• Only integral information along the
entire sound path• Attenuation of sound increases
with frequency
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High power US
• Frequency: kHz range• Intensity > 10 W/cm2
• Leads to cavitation• Material alteration and effect on food constituents
Hielscher Ultrasound Technology, 2006
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Ultrasonic cleaning
• Acoustic streaming (due to particle movement) & micro-streaming (effect near gas bubbles) � acceleration of dissolution of soluble contaminants; enhancement of mass transport
• Mechanical effects of cavitation: pitting, cell disruption, shock waves, bubble collision near surfaces, microjetting, shear stress
1 2 3 4
Increasing static pressure
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High power ultrasound - Application in liquids
Patist & Bates (2008)
US – Application in liquids, K. Schössler
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Pasteurization
Cell interior
Cell wall/-membrane
freeing of the cytoplasma-
membrane (Alliger, 1975)
DNA, radicals
(Hughes and Nyborg,
1962)
surface rubbing,
fracture, leak
(Kinsloe et al, 1954)
displacement of weakly
bound ATPase moieties
from cellmembrane
removing of surface
particles, demage of cell
wall structures (Schuett-
Abraham et al.,1992)
Cavitationshear disruption (microstreaming)localized heatingfree radical formation
(Hughes and Nyborg, 1962)
Ultrasound-induced cell damage
US – Application in liquids, K. Schössler
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Inactivation of microorganisms
Ultrasound + Heat Thermosonication
Ultrasound + Pressure Manosonication
Ultrasound + Heat + Pressure Manothermosonication
TS
Bacillus subtilis spores
MTS
US – Application in liquids, K. Schössler
Zenker et. al (2003)
Heat
MTS20kHz, 117 µm, 300kPa
Raso et. al (1998)
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Influence of sound amplitude
E. coli Lb. acidophilus
D-v
alue
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Specific energy requirement
25
50
75
100
0
25
50
75
100
0 2 4 6 2 4 6
ultrasonic energyheat energy
Spe
cific
Ene
rgy
Req
uire
men
t [kJ
/kg]
Log N/No [-]
A
B
CONVENTIONAL HEATING US - COMBINED HEATING
M. Zenker, V. Heinz
& D. Knorr 2004
A: E. coli
B: Lb. acidophilus
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Enzyme inactivation - mechanism of action
• Mechanical and chemical effects of cavitation
• Breakdown of hydrogen bondingsand van der Waals interactions� changes in secondary and tertiary structure� loss of biological activity
• Radical effects (oxidation, interaction with amino-acid residues)
• Effects depend on chemical structure OH ·
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Inactivation of enzymes
Manothermosonication (MTS)
US – Application in liquids, K. Schössler
Vercet et. al (1999)
Inactivation of heat-resistant PME from orange
Thermal, 70 °°°°C
MTS, 200kPa, 117 µm, 33°°°°C
MTS, 200kPa, 117 µm, 70°°°°C
MTS, 70°°°°C, theo
Raviyan et. al (2005)
Thermosonication (TS)
Inactivation of pectinmethylesterase(PME) from tomato
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Inactivation of enzymes
J. Kuldiloke, 2002
Manothermosonicationsynergistic effect of� ultra-sound� high pressure and� heat treatments
Polyphenoloxidase
Peroxidase
Pectinesterase
Polygalacturonase
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Enzyme activation
• Breaking up flocks of aggregated enzymes � improved enzyme/substrate contact
• Improved transport of substrate to enzyme due to micro-jets
• Reduction of boundary layers (immobilized enzymes)
• Stimulation of biological reactions leading to enzyme production
• Improved enzyme extraction from cells
Process control:• Temperature• Radical effects (esp. enzymes in
free solution associating at bubble surfaces)
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Emulsification/Homogenization
Fields of application:
• Chemical Industry
• Polymeric Industry
• Cosmetics
• Developments in Food Processing (Juice, Mayonnaise, Dairy Products…)
High-pressurehomogenization (122.4 bar, 60°C)
Native
Ultrasound(450 W, 10 min)
Wu et al. (2001)
5 µm
Milk• Cavitation
• Shear forces
• Influence on boundary layers
US – Application in liquids, K. Schössler
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51
Separation
US – Application in liquids, K. Schössler
Masudo & Okada (2001)
Formation of a standing wave
� Aggregations of particles and droplets
Riera-Franco de Sarabia et al. (2000)
Solid-liquid separation
� Dewatering of filter cake
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52
Viscosity alteration
US – Application in liquids, K. Schössler
Viscosity decrease
• Acoustic streaming � Hydrodynamic forces
• Cavitation � High local pressures
� Depolymerization of macromolecules
Initial viscosity of CMC solutionsGrönroos et al. (2008)
Viscosity increase
• Increased contact of sonicated material and water
• Improved water binding capacity
• Increase in viscosity and stability of food systems
Flow behavior of yoghurt sonicated before and after inoculation
Tim
e (s
)
Wu et al. (2008)
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Optimization of thermal processes
Influence on boundary layers
0.01-1 mm boundary layer
SW TTq−
=α••
wall
Ultrasound
US – Optimzation of thermal processes, K. Schössler
Bubble formation, cavitation, degassing
Kim et al. (2004)
• Improved heat transfer
• Reduced fouling
Fields of application:
• Milk processing
• Concentration of fruit juices
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Crystallization
• Improved heat transfer• Cavitation bubbles act as crystallization nuclei• Cavitation disrupts large crystals
Li & Sun (2002)
US – Optimzation of thermal processes, K. Schössler
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Influencing mass transfer
US – Influencing mass transfer, K. Schössler
Vilkhu et al. (2011)
• Various processes in food production governed by mass transfer
• Mass transfer resistances limit yield and production rates
Ultrasound:
• Acoustic streaming
• Cavitation
• Interparticle collisions
• Particle breakdown
Processes:
• Extraction
• Drying
• Brining
• Osmotic dehydration
• Enzyme activation
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Extraction
Li et al. (2004)
US – Influencing mass transfer, K. Schössler
Oil extraction from soybeans
• Extraction medium: hexane
• Temperature 25°C
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Drying
Drying Process
Heat Transfer
(Evaporation)
Mass Transfer
(Removal of Vapor)
Surface Evaporation
Drying front inside the product � Resistances against heat
and mass transfer
Ultrasound
Cavitation Cyclic Compression and Rarefactions
Shear effects
Cell disintegration
Influence on boundary layers
Influence on internal resistances
Improved mass transfer to surrounding media
Reduced heat and mass transfer
resistances
US – Influencing mass transfer, K. Schössler
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US assisted drying concepts
Hot-air drying
Drying of fruits and vegetables(z.B. Gallego-Juarez, 1999, 2007)• Air-borne ultrasound• Contact ultrasound
Freeze-Drying
• Air-borne ultrasound, drying of coffee and tea concentrates(Moy & DiMarco 1970)
Pre-treatments
• For lasting changes improving mass transfer, e.g. cell disruption and formation of microchannels(z. B. Fernandes et al., 2006, 2007)
Osmotic drying
• Drying of fruits
Air-borne ultrasound for freeze-drying processesMoy & DiMarco, 1970
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1 Transducer; 2 Flange; 3 Stand plate;4 Stand; 5 Booster; 6 Sonotrode
Ultrasonic pre-treatments
R.Sevenich (2011), J. Salimi (2011)
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Pre-treatments: Leakage
300 600 900 12000,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
Treatment Intensity (J/cm²)
Amplitude 25 µmAmplitude 100 µm
∆ pH
5 s
20 s
10 s
40 s
15 s
60 s
20 s
80 s
R.Sevenich (2011)
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Influence of the water content
J. Salimi (2011)
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Control Surface 400 µm 800 µm
Visible cell damage < 1 mm
75 % relative moisture
400 µmControl Surface 800 µm
100 % relative moisture
Microstructure apple
J. Salimi (2011)
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Control Surface 400 µm 800 µm
100 % relative moisture
90 % relative moisture
400 µmControl Surface 800 µm
Visible cell damage < 1 mm
Micro-structure potato
J. Salimi (2011)
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Texture apple
US effects > 1 mm
J. Salimi (2011)
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Texture potato
Micro-effects in deeper tissue layers with effect on textural characteristics
65%
J. Salimi (2011)
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Contact US-assisted drying
Schössler et al. (2012)
• Screen as sample supporting and sound transmitting surface
• Temperature: 70°C
Hot-air drying of apple cubes
US – Influencing mass transfer, K. Schössler
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Air-facing side
Sonicated side
Analysis in layers with d= 0.6 mm
Ultrasound
Haver & Boecker, Germany
Contact ultrasound
T. Thomas (2010)
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900 µm
unbehandelt 300 µm
1500 µm
• After 5 h contact US treatment
• Visible cell damage in 1-2 mm depth
Effects at micro-structural level
T. Thomas (2010)
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Ultrasound
Influencing mass transport
T. Thomas (2010)
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Ultrasound-assisted frying
Storage
Conditioning
Cutting
Blanching
Drying
Par-Frying
Freezing
Finish-Frying
Dark, 6°°°°C
30 min, room temp.
strips, 4 x 0.8cm
60s 80°°°°C, 20s 80°°°°C
30 min. 80°°°°C (75%)
150s, 177°°°°C + US(6 kJ/kg, 1.8 µm Amplitude)
2h +, -20°°°°C
4 min, 180°°°°C
P. Apicella (2011)
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Crust formation
P. Apicella (2011)
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Browning
4
3
2
1
0
00
000000 00 0 1 2 3 4
USDA Color Standard for frozen French fries
P. Apicella (2011)
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Fat uptake
P. Apicella (2011)
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Oil and water content profiles
P. Apicella (2011)
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Filtration/Screening
Lamminen et al. (2004)
US – Influencing mass transfer, K. Schössler
• Vibrational energy moves particles and liquids
• Friction at screen or filter is reduced
• Improved flow characteristics• Research dealing with membrane
filtration
Hielscher Ultrasonics, Germany Haver & Boecker, Germany
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Extrusion
US – Influencing mass transfer, K. Schössler
Knorr et al. (2004)
• Reduced drag resistance• Improved flow behaviour• Modification of product structure
Material flow stress
Non-US
US
Akbari et al. (2007)
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Ultrasonic cutting
Schneider et al., 2009
• Ultrasonic vibrations increase stiffness at microscopic scale; create a brittle structure� reduced product deformation and damage
• Faster initiation of fracture• Reduced friction between knife
and product• Reduced cutting force
Process control• Temperature increase due to
absorption of acoustic energy• Cavitation effects in products
containing large amounts of free liquids
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Cutting of porous products
Schneider et al., 2009
Non-US
US
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Cutting compact and porous foods
Non-US US
Schneider et al., 2009
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Defoaming
• Partial vacuum on foam bubble surface produced by high acoustic pressure
• Radiation pressure on bubble surface
• Bubble resonance leading to friction and coalescence
• Cavitation• Atomizing from liquid film surface• Acoustic streaming
Riera et al. (2006)
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Quality aspects
TextureYoghurtVegetable juices
Vegetable and fruit productsMacromolecular solutions
Schössler et al. (2011)
FlavorCheese ripeningWine Aging
LipoxydationOff-flavor
US – Quality aspects, K. Schössler
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220
240
260
L (
+)
Asc
orb
ic a
cid
[m
g/l]
0 7 14 21 28 35
Storage Time [Days]
ControlTUST
Qualität
Storage stability of citrus juice(storage time 18 days)
Control MTS Zenker et al. (2004)
Color and stabilityBrowningLightnessStorage stability
Browning
Nährstoffe
• Numerous mechanical and chemical US effects
• Positive and negative effects strongly depend on process and product
Kuldiloke (2002)
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Quality parameters of apple cider
Ugarte-Romero et al. (2006)
Reduced turbidity
No effect on titrable acidity, pH, °BrixImproved lightness in comparison to the control
-
+
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Quality of pasteurized milk
Bermúdez-Aguirre et al. (2009)
Slightly reduced protein content
Improved bioavailability of milk fatIncreased lightnessHomogenization
-
+
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