Emerging Technologies, WS 2011/12

Ultrasound –Fundamentals

Katharina Schössler

2

The sound spectrum

Echolocation(Sonars)

Material testing

Medicaldiagnostics

Infrasound Audible sound Ultrasound Hypersound

3

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

4

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

5

Generation of ultrasound

Hielscher Ultrasound Technology, 2006

1 Transducer

2 Generator

3 Sonotrode

4 Horn

• Generation and transmission of high frequency sound waves

6

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

7

Transducers II

Piezoelectric transducers

Applied mechanical force electric dipole moment electrical charge

Piezoelectric materials: Quartz, ceramics, zinc oxide, polyvinylidene fluoride

8

Emitters

Baths

• Emitters radiate the ultrasonic wave from the transducer to the medium

JTT Ultraschall, 2009JTT Ultraschall, 2009

Probes

Transducers

TransducerHorn(amplification)

Sonotrode

9

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…

10

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]

11

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

12

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]

13

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

14

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

15

Mechanisms of action in the soundfield

1. Acoustic streaming

2. Cavitation

3. Radical formation

4. “Sponge effect”

16

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)

17

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

18

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

19

• 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

20

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

21

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

22

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

�� → � ∙ ��� ∙

� ∙ �� ∙→��

�� ∙ ��� ∙→ ����

� ∙ ��� ∙→ ���

23

„Sponge Effect“

• Contraction and extension of the treated medium

• Vibration

• Improved mass transfer

• Generation of micro-channels

• Pressure fluctuation � increased evaporation rate

US

24

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

25

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

��

26

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]

27

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

28

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• …

Emerging Technologies, WS 2011/12

Ultrasound –Applications in Food Technology

30

Ultrasound in Food Technology

Knorr et al. (2010)

31

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� � /�

32

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)

33

Flow measurement

• Transit-time flow meter

• Doppler flow meter

Hauptmann et al. (2002)

34

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

35

Foreign bodies and product defects

Knorr et al. (2004)

Detection of a foreign body in a yoghurt beaker

36

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)

37

Gelation of tofu curd

Leemans & Destain (2009)

38

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

39

High power US

• Frequency: kHz range• Intensity > 10 W/cm2

• Leads to cavitation• Material alteration and effect on food constituents

Hielscher Ultrasound Technology, 2006

40

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

41

High power ultrasound - Application in liquids

Patist & Bates (2008)

US – Application in liquids, K. Schössler

42

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

43

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)

44

Influence of sound amplitude

E. coli Lb. acidophilus

D-v

alue

45

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

46

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 ·

47

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

48

Inactivation of enzymes

J. Kuldiloke, 2002

Manothermosonicationsynergistic effect of� ultra-sound� high pressure and� heat treatments

Polyphenoloxidase

Peroxidase

Pectinesterase

Polygalacturonase

49

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)

50

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

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

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)

53

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

54

Crystallization

• Improved heat transfer• Cavitation bubbles act as crystallization nuclei• Cavitation disrupts large crystals

Li & Sun (2002)

US – Optimzation of thermal processes, K. Schössler

55

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

56

Extraction

Li et al. (2004)

US – Influencing mass transfer, K. Schössler

Oil extraction from soybeans

• Extraction medium: hexane

• Temperature 25°C

57

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

58

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

59

1 Transducer; 2 Flange; 3 Stand plate;4 Stand; 5 Booster; 6 Sonotrode

Ultrasonic pre-treatments

R.Sevenich (2011), J. Salimi (2011)

60

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)

61

Influence of the water content

J. Salimi (2011)

62

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)

63

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)

64

Texture apple

US effects > 1 mm

J. Salimi (2011)

65

Texture potato

Micro-effects in deeper tissue layers with effect on textural characteristics

65%

J. Salimi (2011)

66

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

67

Air-facing side

Sonicated side

Analysis in layers with d= 0.6 mm

Ultrasound

Haver & Boecker, Germany

Contact ultrasound

T. Thomas (2010)

68

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)

69

Ultrasound

Influencing mass transport

T. Thomas (2010)

70

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)

71

Crust formation

P. Apicella (2011)

72

Browning

4

3

2

1

0

00

000000 00 0 1 2 3 4

USDA Color Standard for frozen French fries

P. Apicella (2011)

73

Fat uptake

P. Apicella (2011)

74

Oil and water content profiles

P. Apicella (2011)

75

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

76

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)

77

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

78

Cutting of porous products

Schneider et al., 2009

Non-US

US

79

Cutting compact and porous foods

Non-US US

Schneider et al., 2009

80

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)

81

Quality aspects

TextureYoghurtVegetable juices

Vegetable and fruit productsMacromolecular solutions

Schössler et al. (2011)

FlavorCheese ripeningWine Aging

LipoxydationOff-flavor

US – Quality aspects, K. Schössler

82

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)

83

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

-

+

84

Quality of pasteurized milk

Bermúdez-Aguirre et al. (2009)

Slightly reduced protein content

Improved bioavailability of milk fatIncreased lightnessHomogenization

-

+

Thank you for your attention!katharina.schoessler@tu-berlin.de