Protein-stabilised emulsions

Transcription

1 Proteinstabilised emulsions Ranjan Sharma 1

2 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. Oilinwater (O/W) Waterinoil (W/O) Waterinoilinwater (double emulsion, W/O/W) Oilinwaterinoil (double emulsion, O/W/O) 2

3 Examples O/W milk, cream, mayonnaise, soups and sauces W/O butter and margarine 3

4 Oilinwater emulsions Coarse Fine Salad dressing Mayonnaise Raw milk Homogenised/ Recombined milk UHT beverage Canned/ Retorted beverage 4 Demanding more physical stability

5 O/W Chocolate milk and infant formulae 5

6 O/W Complete nutritional formula 6

7 O/W Mayonnaise 7

8 W/O butter and margarine 8

9 Two & threephase emulsions water o water w w oil o 9

10 Emulsion formation Oilsoluble/dispersible ingredient Oil Water Watersoluble/dispersible ingredients Blend tank Homogenisation Packaging Further heating 10 Additives

11 Emulsion formation Oilsoluble/dispersible ingredient Oil Water Watersoluble/dispersible ingredients Important variables Blend tank Homogenisation Packaging Further heating 11 Additives

12 Sequence of events During homogenisation, fat globules with submicron 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 fatwater interface Protein load affects the stability of emulsion towards heating and storage 12

13 Setup for reconstitution of protein ingredient Agitator Recirculation line Water Powder Reconstitution tank Powderwater Blending unit 13

14 Homogenising devices Highspeed blender Colloid mills Highpressure valve homogeniser Ultrasonic homogeniser Microfludiser Membranebased homogeniser 14

15 Homogenising devices Highspeed blender Colloid mills Highpressure valve homogeniser Ultrasonic homogeniser Microfludiser Membranebased homogeniser 15

16 Homogenisation efficiency ΔE min E H = x 100 ΔE total E H homogenisation efficiency ΔE min Minimum amount of energy theoretically required to form emulsion = ΔAγ (interfacial area and interfacial tension) ΔE total Actual amount of enegy expended during homogenisation 16

17 Homogenising valve Valve pressure Impact ring Homogenised product Seat Unhomogenised product 17

18 Twostage homogeniser Stage 2 Homogenised product Unhomogenised product Stage 1 18

19 Effect of homogenisation on fat globules in milk Natural milk Homogenized milk 10 mm 10 mm 19

20 Casein micelles and whey proteins at oilwater interface Fat globule Casein micelle Sharma (1993) nm

21 TEM of an oilwater emulsion Fat globule Casein micelle Sharma (1993) 21

22 Particle size distribution A stable emulsion Frequency (%) Particle size (um) 22

23 Particle size distribution unstable emulsion 20 Volume frequency (%) Particle size (µm) 23

24 Protein adsorption Spherical interface (emulsion or foam) Flat interface (soild surface, e.g. Stainless steel tank) 24

25 PHYSICOCHEMICAL PROPERTIES OF MILK PROTEINS CASEIN WHEY PROTEIN 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 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) 25

26 Selective Chemical Composition and PhysicoChemical Attributes of Milk Protein Products Attribute Milk protein Sodium caseinate Whey protein concentrate concentrate Moisture (%) Total protein (N*6.38, %) Casein (%) Whey protein (%) Calcium (%) Potassium (%) Phosphorus (%) Protein state Casein micelles, Soluble aggregates Soluble whey soluble whey protein of casein proteins 26

27 Kinetics of protein adsorption Diffusioncontrolled (Ward & Tordai, 1946) dγ dt = C 0 (D/Πt) 1/2 Γ surface protein concentration, t time D diffusion coefficient 27

28 Kinetics of protein adsorption Diffusioncontrolled (Ward & Tordai, 1946) Total adsorbed protein Γ = 2C 0 (Dt/Π) 1/2 28

29 Kinetics of protein adsorption Convectioncontrolled (Walstra, 1983) Γ (t) = KC 0 d g (1+d p /d g ) 3 t C 0 is the bulk protein concentration, d g and d p are the fatglobule and proteinparticle sizes, respectively, and K is a constant 29

30 Protein load Γ = Protein at the oil droplet surface (mg) Total droplet surface area (m 2 ) 30

31 Protein load at oilwater interface Protein Protein load (mg/m 2 ) α s Casein 34.2 βcasein κcasein 4.2 Casein micelle 20 Sodium caseinate Skim milk powder 1023 βlactoglobulin

32 Factors affecting protein load Volume of oil Protein concentration Homogenisation temperature Homogenisation pressure Aggregation state of protein Pretreatment of protein, i.e. Hydrolysis or crosslinking 32

33 Types of protein adsorption Reversible and irreversible adsorption Competitive adsorption 33

34 Basic emulsion characteristics Thermodynamically unstable Possible to make kinetically stable 34

35 Creaming Emulsion 35 Flocculation Inversion Coalescence Emulsion instability Oil separation

36 Colloidal interactions Van der Waals Interactions Electrostatic interactions Polymeric steric interactions Depletion interactions Hydrophobic interactions Hydration intrations Thermal fluctuation interactions Total interaction potential 36

37 van der Waals interactions δ δ+ δ δ+ Induction δ δ+ δ δ+ Rotation δ δ+ δ δ+ Dispersion 37

38 38 Electrostatic doublelayer forces

39 Polymeric steric interactions Water Oil Surfactant Flexible polymer Globular polymer 39

40 Depletion interactions Droplet 1 Droplet 2 Particle excluded From distance r c Colloidal particle 40

41 Hydrophobic interactions oil Water Adsorbed protein Nonpolar group Clathratelike structure 41

42 Other interactions Hydration interactions Thermal fluctuation interaction Bridging flocculation Hydrodynamic interactions 42

43 DLVO Theory for colloidal stability + Electrostatic repulsion Interaction energy van der Waals attraction Total interaction 0 Distance of separation (nm) 25 43

44 Colloidal forces important for emulsion stability Type of force Character Origin Influenced by van der Waals Attraction Permanent & fluctuating dipoles Refractive index Dielectric constant Electrostatic Repulsion Surface charge Ionic strength, ph Steric Repulsion Adsorbed polymers Polymer coverage & solubility Bridging Attraction Adsorbed Polymer coverage polymers Depletion Attraction Nonadsorbed polymers Micelles Molecular weight Polymer polydispersity Polyelectrolyt es Repulsion or attraction Adsorbed polyelectrolytes Ionic strength, polyelectrolyte coverage 44

45 Colloidal forces important for emulsion stability Type of force Character Origin Influenced by Hydrophobic Attraction Waterwater affinity Hydration Repulsion Dehydration of polar group Protrusion Repulsion Reduction in movement of emulsifiers normal to the interface Solvent properties, surface hydrophobicity Emulsifier head group, crystallinity Fluidity of the layer, headgroup size, Oil/water interfacial tension Bergenstahl A & Claesson PM (1997) Surface forces in emulsion. In Food Emulsions (Friberg S & Larsson K, eds), pp 57109, Marcel Dekker, NY 45

46 Structural organisation of molecules in liquids Thermodynamics of mixing Potential energy change on mixing Entropy change on mixing Free energy change on mixing 46

47 Structural organisation of molecules in liquids Thermodynamics of mixing Immiscible liquid Miscible liquid 47

48 Overall interactions Continuous phase Dispersed phase h r 48

49 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 49

50 Interparticle pair potential Energy required to bring two emulsion droplets from an infinite distance apart w (h) = w attractive (h) + w repulsive (h) 50

51 Interparticle pair potential Energy required to bring two emulsion droplets from an infinite distance apart to a surfacetosurface separation of h 51

52 Attractive interactions dominate at all separations w R 0 w Total 50 w A h (nm) 10

53 Repulsive interactions dominate at all separations w R 0 w Total 50 w A h (nm) 10

54 Attractive interactions dominate at large separations, but repulsive interactions dominate at short separations w R 0 w Total 50 w A h (nm) 10

55 Repulsive interactions dominate at large separations, but attractive interactions dominate at short separations w R 0 w Total 50 w A h (nm) 10

56 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 56

57 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,