Instruments, ultrasound, and oils

Frank PoddProcter Department of Food Science

University of Leeds

16/11/2002

Content• Background

• Ultrasound Velocity

• Ultrasound Spectrometry

• New Developments Single particle scattering theory

Multiple particle scattering theory

New electronics

New cell design

Leeds Food Science 

Eric Dickinson Emulsions

Brent Murray

Interfaces

Malcolm Povey

Ultrasound

Colloids

Bronek Wedzicha

Small molecule interaction

Food Processing

David Borrill

Biochemistry

Mike Morgan

Instruments within the colloids group• Rheometers,

• Brewster angle microscope, Langmuir trough, surface shear rheometers, bubble expansion chambers,

• Surface layers,

• Simulations,

• Confocal microscopy,

• Atomic force microscopy,

• Acoustic microscopy,

• Ultrasound creaming rig,

• Ultrasound velocity,

• Ultrasound spectroscopy.

1 m

micellesurfactantprotein

Liquid oil particle coated with surfactant Overall ultrasound

property depends on:

• Continuous phase• Dispersed phase• Surfactant• Droplet shape & its size distribution

Thermal propertyViscosityCompressibility

Ultrasound & Food Emulsions

v B

1

Bulk modulus

DensityAdiabatic compressibility

Ultrasound VelocityThe Wood equation

v j jj

j jj

1

, ,

2 1 2 11 1( ) , ( )

Phase volume of jth phase

Ultrasound Velocity Urick equation

1 1 1212

2

v v

a a

a

2 1

1

2 1

1

a a

a

2 1

1

2 1

1

2 12

12

23

RC C

C

p p

p

2

2 2

1

1 1

1

1 1

( )1 2 2

1 1

2CC Rp

p

Modified Urick Equation

1400

1450

1500

1550

1600

1650

-10 0 10 20 30 40

Temperature / °C

Vel

ocity

/ m

s-1

I II III

Sound velocity in margarine

1400

1450

1500

1550

1600

1650

1700

-20 -10 0 10 20 30 40 50

Temperature (°C)

1000

1200

1400

1600

1800

-20 -10 0 10 20 30 40 50 60

Temperature (°C)

The velocity profile during crystallisation for virgin olive oil shows a smooth curve.

This adulterated virgin olive oil displays a spikier

velocity curve

Detecting adulteration in olive oil?

Figure 4: Plot of solids against temperature for 20.75% (v/v) WACB-in- w ater emulsions cooled at 5°C/hour (0.8% Tw een 20 & 1.0% sodium caseinate).

0

0.2

0.4

0.6

0.8

1

5 10 15Temperature (°C)

Sol

ids

SodiumcaseinateTween 20

Figure 7: Plot of solids against time for 20.75% (v/v) WACB-in-water emulsions (0.8% Tween 20) crystallised isothermally at 14.2, 15.0, 15.5 and 15.8°C. Heterogeneous volume particle size

distribution models are fitted.

0

0.05

0.1

0.15

0.2

0.25

0.3

0 2 4 6 8 10

Time (minutes)

Sol

ids

14.2°CHet vol psd model15.0°CHet vol psd model15.5°CHet vol psd model15.8°CHet vol psd model

Crystallization in cocoa butter emulsions

1455

1460

1465

1470

1475

1480

1485

1490

1495

1500

0 5 10 15 20 25Temperature (°C)

Ultr

ason

ic v

eloc

ity (m

/s)

Sodium caseinate

Tween 20

Do surfactants affect crystallisation?Plot for 20% v/v WACB oil-in-water emulsion cooled at 5°C / hour. A three

stage process occurs with sodium caseinate during the crystallisation:

1. Bulk volume crystallisation initially,

2. Surface crystallisation (the sodium caseinate macromolecule protects the droplets more than Tween 20),

3. Instability stops the sodium caseinate from preventing droplet collisions, thus the crystallisation rate increases.

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20Time (days)

Sol

ids

(n-h

exad

ecan

e fra

ctio

n)

Mixing of emulsion droplets

Plot of solid content for an 32% v/v n-hexadecane oil-in-water emulsion crystallised at 6°C. In the first 7 days a dialysis tube was used as a barrier to prevent collisions between supercooled liquid and solid droplets. Thereafter, the contents of the dialysis tube were mixed with the liquid.

Does crystallisation occur due to micelle transport?

Ultrasound spectroscopy has opened a new dimension in food emulsion study

• Rheology• Component analysing• Stability monitoring (flocculation, creaming, coalescences, etc.)• Particle sizing ( particle size distribution, PSD)

US spectroscopyParticles scatter ultrasound…

The effect of scattering can be a frequency dependence in the ultrasonic velocity and attenuation

New Developments @ Leeds(in the ultrasound group)

New stable scattering theory with known error bounds.

Multi-particle theory enabling an estimation of particle spacing.

New US instrumentation New US sensors

Later Epstein and Carhart Later Epstein and Carhart (J. Acous. Soc. Am. 1953)(J. Acous. Soc. Am. 1953) and Allegra and Allegra and Hawley and Hawley (J. Acous. Soc. Am. 1972)(J. Acous. Soc. Am. 1972) developed a model for the developed a model for the attenuation of sound through a suspension of attenuation of sound through a suspension of isolated spheres due to thermal and viscous effects.isolated spheres due to thermal and viscous effects.

Ultrasound propagation was first formulated by Ultrasound propagation was first formulated by Lord Rayleigh Lord Rayleigh (The Theory of Sound 1892) .(The Theory of Sound 1892) .

Although the theory is exact it is prone to numerical Although the theory is exact it is prone to numerical difficulties and so an alternative solution technique difficulties and so an alternative solution technique is required.is required.

Scattering background

Magnitude of error knownMagnitude of error knownWell conditioned numericallyWell conditioned numericallyNot constrained to geometryNot constrained to geometry

Results of new single particle Results of new single particle scattering theoryscattering theory

Single Particle SystemSingle Particle SystemIncident plane waveIncident plane wave

Reflected waveReflected wave

Thermal fields.Thermal fields.11M particle at M particle at 11MHz generates a MHz generates a thermal field of thermal field of 11M depthM depth

Single oil droplet suspended in mediumSingle oil droplet suspended in medium

Transmitted waveTransmitted wave

The Multiple Scattering Problem

Oil particle(1 m diameter)in water

Thermal field ( 1 m thick in water at 1 MHz) generated by particle pulsation in the presence of the excitation field

Multiple scattering of the thermal field is different to multiple scattering of the acoustic field. If the particles stay together for the period of the wave

thermal fields will scatter coherently. If the particles move in less than this time then the thermal scattering will be incoherent.

Enables the determination Enables the determination of inter-particle spacing?of inter-particle spacing?

Results of new multiple particle Results of new multiple particle scattering theoryscattering theory

New Cell Design

• Small sample volume (2ml)

• Low coefficient of thermal expansion

• Small heat capacity

• High thermal conductivity

• Cell designed for high pressure experiments

• Choice of transducers - 1MHz to 30MHz frequency range

Designed for crystallisation experiments

New Electronics

• Measure the pulse amplitude in addition to the group velocity

• Velocity and attenuation spectrometry

• Accurate temperature measurement – detect heat from crystallisation?

• Aiming for inline use

• Low cost!

The Acoustiscan builds up a profile of property differences along the cell height. It uses both pitch catch and pulse-echo techniques

Colloidal stability can be quantifiably determined using the Acoustiscan. A major factor in colloidal stability is the particle size distribution. This can also be determined ultrasonically, by using the FSUPER for example.

Monitoring stability and creaming

The FSUPER has several advantages, such as:- Rapid and accurate measurement Wide frequency range (1-15MHz) A small amount of sample required ( ~ 15ml)

FSUPERThis type of characterisation can be peformed by the Frequency Scanning Ultrasound Pulse Reflectometer (FSUPER)

The particle size distribution can be estimated from the analysis of the frequency dependent ultrasonic velocity and attenuation data. The system can also monitor emulsion stability, measure the amount of surfactant covering the emulsion droplets and identify substances spectroscopically.

The method has the potential to characterise emulsions on-line, and in real time.

Many thanks go to Malcolm Povey, Scott Hindle, and Toni Crosthwaite for supplying

data and providing help and advice.

Acknowledgements