protein-stabilised emulsions - ozscientific · 2018. 7. 14. · effect of homogenisation on fat...
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Protein-stabilised emulsions
Ranjan Sharma
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Emulsion - definition
• An emulsion consists of two immiscible liquids (generally oil and water) with one liquid forming the continueous phase while the other the dispersed phase.• Oil-in-water (O/W)• Water-in-oil (W/O)• Water-in-oil-in-water (double emulsion, W/O/W)• Oil-in-water-in-oil (double emulsion, O/W/O)
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Examples
• O/W – milk, cream, mayonnaise, soups and sauces
• W/O – butter and margarine
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Oil-in-water emulsions
Saladdressing
Mayonnaise
Raw milk Homogenised/Recombined
milkUHT
beverage Canned/Retortedbeverage
Demanding more physical stability
Coarse Fine
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O/W - Chocolate milk and infant formulae
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O/W - Complete nutritional formula
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O/W - Mayonnaise
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W/O – butter and margarine
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Two & three-phase emulsions
o
w
water
oil
w
o
water
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Emulsion formation
Oil Water
Oil-soluble/dispersibleingredient
Water-soluble/dispersibleingredients
Blend tank
Homogenisation Packaging Further heating
Additives
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Emulsion formation
Oil Water
Oil-soluble/dispersibleingredient
Water-soluble/dispersibleingredients
Blend tank
Homogenisation Packaging Further heating
Additives
Important variables
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Sequence of events
• During homogenisation, fat globules with sub-micron size are formed
• Milk proteins migrate to the newly formed fat globule surfaces
• Capability to form a stable emulsion is determined by the ability of the protein to unfold at the fat-water interface
• Protein load affects the stability of emulsion towards heating and storage
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Setup for reconstitution of protein ingredient
Powder-waterBlending unit
Recirculation line
Reconstitution tank
Agitator
PowderWater
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Homogenising devices
• High-speed blender• Colloid mills• High-pressure valve homogeniser• Ultrasonic homogeniser• Microfludiser• Membrane-based homogeniser
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Homogenising devices
• High-speed blender• Colloid mills• High-pressure valve homogeniser• Ultrasonic homogeniser• Microfludiser• Membrane-based homogeniser
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Homogenisation efficiency
EH = ------------ x 100ΔEmin
ΔEtotal
EH – homogenisation efficiency
ΔEmin – Minimum amount of energy theoretically required toform emulsion = ΔAγ (interfacial area and interfacial tension)
ΔEtotal – Actual amount of enegy expended during homogenisation
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Homogenising valve
Homogenisedproduct
Unhomogenisedproduct
Valve pressure
Seat
Impact ring
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Two-stage homogeniser
Unhomogenisedproduct
Homogenised product
Stage 1
Stage 2
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Effect of homogenisation on fat globules in milk
Natural milk Homogenized milk
10 mm 10 mm
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Casein micelles and whey proteins at oil-water interface
Casein micelle
Fat globule
200 nm
Sharma (1993)
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TEM of an oil-water emulsion
Casein micelle
Fat globule
Sharma (1993)
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Particle size distribution – A stable emulsion
0 2 4 6 8
10 12 14 16 18 20
Freq
uenc
y (%
)
0.01 0.1 1 10 100 Particle size (um)
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0
4
8
12
16
20Vo
lum
e fr
eque
ncy
(%)
0.01 0.1 1 10 100Particle size (µm)
Particle size distribution – unstable emulsion
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Protein adsorption
Spherical interface(emulsion or foam)
Flat interface(soild surface, e.g.Stainless steel tank)
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PHYSICO-CHEMICAL PROPERTIES OF MILK PROTEINS
•Strong hydrophobic regions•Low cysteine•High ester phosphates•Little or no secondary structure•Unstable in acidic conditions•Micelles in native form•Random coil in dissociated form
CASEIN
•Balance in hydrophobic and hydrophilic residues
•Contains cysteine and cystine•Globular, much helical•No ester phosphate•Easily heat denatured•Stable in mild acidic conditions•Present as soluble aggregates(<10 nm)
WHEY PROTEIN
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Selective Chemical Composition and Physico-Chemical Attributes of Milk Protein Products
Attribute Milk protein Sodium caseinate Whey protein concentrate concentrate
Moisture (%) 4.0 4.0 4.0Total protein (N*6.38, %) 82.5 92 83.5Casein (%) 66.0 92.0 0Whey protein (%) 16.5 0 83.5Calcium (%) 2.20 0.01 0.06Potassium (%) 0.01 0.005 0.05Phosphorus (%) 1.40 0.80 0.18Protein state Casein micelles, Soluble aggregates Soluble whey
soluble whey protein of casein proteins
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Kinetics of protein adsorption
Diffusion-controlled (Ward & Tordai, 1946)
dΓdt = C0 (D/Πt)1/2
Γ – surface protein concentration, t – timeD – diffusion coefficient
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Kinetics of protein adsorption
Diffusion-controlled (Ward & Tordai, 1946)
Γ = 2C0(Dt/Π)1/2
Total adsorbed protein
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Kinetics of protein adsorption
Convection-controlled (Walstra, 1983)
Γ (t) = KC0dg
(1+dp/dg)3
t
C0 is the bulk protein concentration, dg and dp arethe fat-globule and protein-particle sizes, respectively,and K is a constant
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Protein load
Γ = --------------------------------------------Protein at the oil droplet surface (mg)
Total droplet surface area (m2)
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Protein load at oil-water interface
Protein Protein load (mg/m2)αs-Casein 3-4.2β-Casein 1-1.75κ-Casein 4.2Casein micelle 20Sodium caseinate 2.2-2.6Skim milk powder 10-23β-Lactoglobulin 1.7
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Factors affecting protein load
• Volume of oil• Protein concentration• Homogenisation temperature• Homogenisation pressure• Aggregation state of protein• Pre-treatment of protein, i.e. Hydrolysis or cross-
linking
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Types of protein adsorption
• Reversible and irreversible adsorption• Competitive adsorption
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Basic emulsion characteristics
• Thermodynamically unstable • Possible to make kinetically stable
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Emulsioninstability
Emulsion
Creaming
Flocculation
Inversion
Coalescence Oil separation
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Colloidal interactions
• Van der Waals Interactions• Electrostatic interactions• Polymeric steric interactions• Depletion interactions• Hydrophobic interactions• Hydration intrations• Thermal fluctuation interactions• Total interaction potential
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van der Waals interactions
δ+δ− δ+δ−
δ+δ−
δ+δ+
δ+δ−
δ−
δ−
Induction
Rotation
Dispersion
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Electrostatic double-layer forces
++
++
+++ +
++ +++
+
-
-
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-
-
-
--
--
-
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-- -
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Polymeric steric interactions
Oil
Water
Surfactant Flexible polymer Globular polymer
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Depletion interactions
Particle excludedFrom distance rc
Droplet 2
Colloidalparticle
Droplet 1
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Hydrophobic interactions
Water
Non-polar group
Clathrate-like structure
oil
Adsorbed protein
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Other interactions
• Hydration interactions• Thermal fluctuation interaction• Bridging flocculation• Hydrodynamic interactions
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DLVO Theory for colloidal stability
+
-Distance of separation (nm) 250
Inte
ract
ion
ener
gy Total interaction
van der Waals’ attraction
Electrostatic repulsion
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Colloidal forces important for emulsion stability
Type of force Character Origin Influenced byvan der Waals Attraction Permanent &
fluctuating dipolesRefractive indexDielectric constant
Electrostatic Repulsion Surface charge Ionic strength, pHSteric Repulsion Adsorbed
polymersPolymer coverage& solubility
Bridging Attraction Adsorbedpolymers
Polymer coverage
Depletion Attraction Non-adsorbedpolymersMicelles
Molecular weightPolymerpolydispersity
Polyelectrolytes
Repulsionor attraction
Adsorbedpolyelectrolytes
Ionic strength,polyelectrolytecoverage
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Colloidal forces important for emulsion stability
Type of force Character Origin Influenced by
Hydrophobic Attraction Water-wateraffinity
Solvent properties,surfacehydrophobicity
Hydration Repulsion Dehydration ofpolar group
Emulsifier headgroup, crystallinity
Protrusion Repulsion Reduction inmovement ofemulsifiers normalto the interface
Fluidity of thelayer, head-groupsize, Oil/waterinterfacial tension
Bergenstahl A & Claesson PM (1997) Surface forces in emulsion. In Food Emulsions (Friberg S & Larsson K, eds), pp 57-109, Marcel Dekker, NY
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Structural organisation of molecules in liquids
• Thermodynamics of mixing• Potential energy change on mixing• Entropy change on mixing• Free energy change on mixing
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Structural organisation of molecules in liquids
• Thermodynamics of mixing
Immiscible liquid Miscible liquid
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Overall interactions
Dispersedphase
rh
Continuous phase
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Overall interaction
• Attractive interactions dominate at all separations• Repulsive interactions dominate at all separations• Attractive interactions dominate at larger
separations, but repulsive interactions dominate at short separations
• Repulsive interactions dominate at large separations, but attractive interactions dominate at short separation
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Inter-particle pair potential
w (h) = wattractive (h) + wrepulsive (h)
Energy required to bring two emulsion droplets from an infinite distance apart
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Inter-particle pair potential
• Energy required to bring two emulsion droplets from an infinite distance apart to a surface-to-surface separation of ’h’
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Attractive interactions dominate at all separations
0 5 10
0
-50
50
100
-100h (nm)
wR
wA
wTotal
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Repulsive interactions dominate at all separations
0 5 10
0
-50
50
100
-100h (nm)
wR
wA
wTotal
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Attractive interactions dominate at large separations, but repulsive interactions dominate at short separations
0 5 10
0
-50
50
100
-100h (nm)
wR
wA
wTotal
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Repulsive interactions dominate at large separations, but attractive interactions dominate at short separations
0 5 10
0
-50
50
100
-100h (nm)
wR
wA
wTotal
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Characterisation of emulsions
• Emulsifying properties of proteins• Emulsifying activity• Emulsion capacity• Surface hydrophobicity
• Emulsion stability• Emulsion droplet size• Protein load• Creaming and oil separation• Heat stability
• Emulsion rheology• Emulsion microstructure
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Recommeded reading
• Food Emulsions: Principles, practice and techniques by D.J. McClements, CRC Press, Boca Raton, USA, 1999
• Food Emulsions edited by Friberg, S.E. And Larsson, L, Marvel Dekker, Inc, New York, 1997
• Emulsions and Emulsion stability, edited by Sjöblom, J, Marvel Dekker, Inc, New York, 1996