Emulsion Droplets: Characteristics and Importance

Transcription

1 Emulsion Droplets: Characteristics and Importance Jochen Weiss *Food Structure and Functionality Laboratories Department of Food Science & Biotechnology University of Hohenheim Garbenstrasse 21, 7599 Stuttgart, Germany Emulsion Workshop November 1314 th, 28, Amherst, MA 1

2 Influence of Droplet Characteristics on Emulsion Properties Droplet Characteristics Food Structure Quality Attributes Concentration Particle size distribution Electrical charge Interfacial properties Physical state Organization Interactions Shelflife Texture Appearance Flavor Nutrition Need to know how to control, measure and report droplet characteristics! 2

3 Droplet Concentration: Influence on Emulsion Properties 3 FLUIDLIKE SOLIDLIKE Dilute Concentrated % 2% 4% 6% 8% 1% f f C Mayonnaise Dressing (Regular) Heavy Cream Dressing (Light) Light Cream Infant Formula Milk Skim Milk Beverage

4 Droplet Concentration: Influence on Emulsion Properties Texture Appearance Stability Relative Viscosity Lightness R elative Relative C ream Creamin ing V elocity Vel Droplet Concentration [%] Droplet Concentration [%] Droplet Concentration [%] 4

5 Droplet Concentration: Specification Disperse phase volume fraction: φ = V V Droplets Emulsion Disperse phase mass fraction: Volume% = 1 φ φ m = M M Droplets Emulsion Mass% = 1 φ m 5

6 Droplet Concentration: Number of Droplets per Unit Volume N = 3 φ / 4 π r m 3 2 Of major importance to: Fat crystallization in O/W emulsions (impurities/droplet) Distribution of reactants in emulsions (prooxidants/droplet) Tm [ºC] (ºC) Het. Nucl Hom. Nucl 5 1 Radius [µm] 6

7 Droplet Concentration: Droplet Surface Area per Unit Volume S = 3 φ / r m 2 m 3 Of major importance to: Amount of emulsifier needed to cover surface Rate of surface mediated reactions e.g., lipid oxidation, lipid digestion FA Release Large S Small S Time [min] 7

8 Droplet Size: Influence on Emulsion Properties Small Particles Large Particles U [mm/day] Radius [µm] µ Shelflife: Creaming, Droplet Aggregation Quality: Appearance, Flavor, Texture

9 Particle Size Distribution Specifications Volume Fraction [%] Problem Droplets Particle Diameter [µm] µ A manufacturer should establish a particle size distribution specification based on known product performance (e.g., stability, optics, texture, flavor, mouthfeel): Mean particle diameter < a critical diameter Median particle diameter < a critical diameter Percentage of droplets < a critical diameter

10 Representing Droplet Size Data Prepare d N = 3.4 µm Average Measure Particle Size Distribution Number % Number % µm Particle Diameter µ [µm] Tabulate Graph Size Range (µm) d i (µm) N i

11 Representing Droplet Size Data: What to Present? Monodisperse Droplets all same size Report d or r Polydisperse Droplets different sizes Report PSD Report mean d or r Report width 11

12 Particle Size Distributions Graphical Representation Histogram Frequency Cumulative Number % Particle Diameter [µm] Histogram: Amount in specific size categories is reported F [µm 1 ] F (µm ) V% i = F i d i µ Particle Diameter [µm] Frequency: Amount is equal to the area under the curve C % C i = % < d i µ Particle Diameter [µm] Cumulative: Amount below a specific size is reported 12

13 Representing Droplet Size Data: Specifying Size and Frequency 1. Size Parameter. A measurement of the size of the particles in the sizeclass: radius (r) or diameter (d) 2. Frequency Parameter. A measurement of the amount of particles present in the sizeclass: number (N), surface (S) or volume (V) Size Range d i N i (µm) (µm) Amount can be absolute, fraction or percentage. d 13

14 Particle Size Distributions: Use Appropriate Frequency Parameter! 12 1 mm NaCl 15 mm NaCl 12 1 mm NaCl 15 mm NaCl Number % Volume% % Particle Diameter [µm] µ Particle Diameter (µm) Particle Diameter [µm] V i = (π/6) N i d i 3 mm 15 mm 14

15 Representing Droplet Size Data: Using Mean Particle Sizes Volume % Volume % h 24 h 7 h h µ Particle Diameter [µm] Average d 32 [µm] 32 µ Time [h] Average values simplify presentation, but information is lost! Which mean is the best to use? 15

16 Representing Droplet Size Data: Defining Mean and Width x Number % d N = 5 µm σ N =.3 µm d N = 5 µm σ N = 1 µm σ 5 1 µ Particle Diameter [µm] Mean: Width: x = φ i i x i i ( x) 2 σ = φ i Here: x i = size parameter f i = frequency weighting parameter x i Mean = Measure of Central Tendency Width = Measure of Spread of Data Around Mean (x/s) 16

17 Mean Representing Droplet Size Data: Defining Mean and Width Width Name of Mean Frequency Weighting Parameter Size Parameter Definition of Mean Name of Width Definition of SD d N φ N,i = N i /ΣN i d i Σφ Ν,i d i d S φ S,i = S i /ΣS i d i Σφ S,i d i d V φ V,i = V i /ΣV i d i Σφ V,i d i σ N [Σφ N,i (d i d N ) 2 ] σ S [Σφ S,i (d i d S ) 2 ] σ V [Σφ V,i (d i d V ) 2 ] 17

18 Defining Mean Particle Sizes: Equivalent Ways of Expressing Means Name of Mean Definition Name of Mean Definition d N Σ(N i /ΣN i d i ) d 1 ΣN i d i /ΣN i d S Σ(S i /ΣS i d i ) d V Σ(V i /ΣV i d i ) d 32 ΣN i d i3 /ΣN i d i 2 d 43 ΣN i d i4 /ΣN i d i 3 S i N i d i 2 V i N i d i 3 18

19 Representing Droplet Size Data: Use Appropriate Mean Size! Mean Particle Diameter [µm] (µm) ph d Stable? Effect of ph on Emulsion Stability: Protein stabilized emulsion containing cationic polysaccharide 19

20 Representing Droplet Size Data: Use Appropriate Mean Size! Mean Particle Diameter [µm] (µm) ph d43 d Using appropriate means can help Identify issues with particular particles size classes! Effect of ph on Emulsion Stability: Protein stabilized emulsion containing cationic polysaccharide 2

21 Representing Droplet Size Data: Choose Appropriate Mean Size! Volume % Vol% 5 d 1 d 32 d µ Particle Diameter [µm] d 1 =.9 mm d 32 =.21 mm d 43 = 1.2 mm Note: d 1 and d 32 are sensitive to the majority of particles d 43 is sensitive to the presence of a few large aggregates 21

22 Representing Droplet Size Data: Always Examine Full PSD! Number% % Monomodal Bimodal 5 1 µ Particle Diameter [µm] d 1 = 5 µm σ 1 = 2 µm These distributions have the same mean diameter and the same distribution width!!! 22

23 Representing Droplet Size Data: Some Factors to Consider Using and Reporting Droplet Size Data: Size Parameter Diameter or Radius? Frequency Parameter Number or Volume? Always Examine PSD Mean Size & Width d 1, d 32, d 43? Mean Particle Diameter [µm] (µm) Volume% % d43 d ph mm NaCl 15 mm NaCl µ Particle Diameter [µm] 23

24 Representing Droplet Size Data: Choice of Appropriate Mean Size Factors Affecting Choice of Appropriate Mean Size: Instrument: The sensitivity of an instrument to the particles in a particular size category depends on its underlying physical principles Feature of PSD of Interest: Majority of particles or presence of large aggregates? Utilization of Information: Sometimes a particular mean size is required in a calculation, e.g., d 32 is used to calculate the surface area of an emulsion. 24

25 Droplet Interactions and Organization Nonflocculated Flocculated Low Viscosity Stable to Creaming High Viscosity Unstable to Creaming Droplets interactions have a large impact on emulsion microstructure and overall properties 25

26 Aggregation Depends on Balance of Colloidal Interactions h w(h) ATTRACTIVE van der Waals Hydrophobic Depletion REPULSIVE Electrostatic Steric Aggregation occurs when attractive interactions outweigh repulsive interactions 26

27 Predicting Colloidal Interactions w(h)/kt Attractive Repulsive Overall h h Sign Magnitude Range 27

28 Colloidal Interactions Knowledge of colloidal interactions used to: Predict stability to droplet aggregation Identify origin of emulsion instability Develop strategies to prevent instability Predict emulsion rheology Stable Unstable 28

29 Influences droplet stability to aggregation, and therefore 1 physicochemical properties Influences droplet interactions with other charged species biopolymers, mineral ions, vitamins, antioxidants, etc Fe 2 Droplet Charge: Importance Ca 2 Diam eteter ph SO

30 Adsorption of charged species to droplet surfaces pk a Emulsifiers, biopolymers, small ions (e.g. OH ) PO 4 CO 2 SO 4 NH 3 Droplet Charge: Origin OH Protein Hydrocolloid Surfactant 3

31 Droplet Charge: Measurement of ζpotential Electrode Electrode Measure Direction and Velocity gives Sign and Magnitude Charge measured by electrokinetic techniques Effective electrical potential on droplet Depends on charge density and ionic strength 31

32 Droplet Charge: Utilization in Emulsion Science Knowledge of droplet charge can be used to: Predict emulsion stability to droplet aggregation Characterize interactions between droplets and other charged species 6 ζpotential ph ζpotential Chitosan (wt%)

33 Interfacial Characteristics Influence emulsion stability to flocculation, coalescence, partial coalescence, Ostwald ripening & creaming Influence partitioning of food ingredients antioxidants, flavors, colors, minerals etc Influence release characteristics Characteristics: Composition Thickness Rheology Charge 33

34 Droplet Characteristics: Practical Significance A better understanding of the role of droplet characteristics on emulsion properties would enable manufacturers to improve product shelflife, appearance, flavor, texture and develop reduced fat products 34