Stability of Food Emulsions (2) David Julian McClements Biopolymers and Colloids Laboratory Department of Food Science
Droplet Coalescence Oiling Off Coalescence Aggregation due to fusing together of two or more individual droplets to form a bigger droplet
Droplet Coalescence Oil A few nms Water Oil Droplet coalescence depends on precise molecular details of droplet interfaces - difficult to predict!
Factors Influencing Droplet Coalescence Relative magnitude of forces between droplets Resistance of interface to disruption Duration of contact between droplets Shearing and tearing of interfaces
Evolution of Coalescence Homogeneous Coalescence Coalescence rate independent of droplet size Heterogeneous Coalescence Coalescence rate increases with droplet size
Factors Influencing Droplet Coalescence: Ingredient Interactions Without chitosan (1-month old emulsion) 0.01 wt. % chitosan (the same magnification)
Factors Influencing Droplet Coalescence: Freezing & Thawing 0 wt% sucrose 20 wt% sucrose Hydrogenated palm oil-in-water emulsions stabilized by WPI (-40 ºC/40ºC) sucrose modifies ice crystal formation
Factors Influencing Droplet Coalescence: Dehydration 0 wt% sucrose 20 wt% sucrose Oil-in-water emulsions stabilized subjected to freeze-drying
Droplet Coalescence: Influence of Interfacial Membranes 1º Highly Unstable 2º Stable to Coalescence Only 3º Stable to Coalescence, Flocculation & Creaming % Coalescence 100 80 60 40 20 0-10º -40º 1º 2º 3º Emulsion 3 Cycles: -10ºC/30ºC
Features of Coalescence φ (%) 35 30 25 20 15 10 5 0 hours 24 hours 0 0.1 1 10 100 Diameter (µm) Bimodal PSD Evolution Oiling off
Strategies to Reduce Coalescence Principle (1). Reduce Attraction (2). Increase Repulsion (3). Decrease droplet contact (4). Increase resistance of membrane to rupture Method Avoid depletion Avoid bridging Avoid hydrophobicity Alter ph or I (E/S) Increase thickness (S) Add thickening or gelling agent Use polymeric emulsifier
Measurement of Coalescence Techniques & Protocols Instrumental Techniques Microscopy Particle Sizing Creaming stability/oiling Off Experimental Protocols Storage Tests Accelerated Storage Tests Environmental Stress Tests
Characterization of Coalescence: Microscopy Methods Particle Size Distribution, Flocculation vs. Coalescence (Image Analysis Software)
Characterization of Coalescence: Particle Size Analysis 14 Volume% 12 10 8 6 4 37 0-20 37 ºC : All liquid -0 ºC: Fat crystallizes -20 ºC: Water crystallizes 2 0 0.01 1 100 Particle Diameter (µm) Potential Problems: Sampling, Distinguishing from flocculation
Characterization of Coalescence: Distinguishing from Flocculation Volume% 4 3.5 3 2.5 2 1.5 1 0.5 0 0.01 0.1 1 10 100 Particle Diameter (µm) Emulsion to Be Tested Volume% 12 10 8 6 4 2 0 0.01 0.1 1 10 100 Particle Diameter (µm) Initially Flocculated Add Deflocculant (e.g., surfactant) Volume% 4 3.5 3 2.5 2 1.5 1 0.5 0 0.01 0.1 1 10 100 Particle Diameter (µm) Initially Coalesced
Measurement of Oiling Off Stable Emulsion Coalescence Oiling Off Extensive droplet coalescence can lead to the formation of a thin layer of oil on top of a product (sometimes with little change in PSD of bulk emulsion)
Oiling Off : Solvent Extraction Method Petroleum ether shake Emulsion Measure amount of oil that can be extracted by an organic solvent
Oiling Off : Dye Dilution Method Add Dye Mix Centrifuge Absorbance Emulsion Cuvette Measure dilution of dye solution by free oil
Characterization of Coalescence: Coalescence/Oiling-off by DSC 0 Fat Crystallization -10 0 10 20 30 40-0.05 0 Water & Fat Crystallization -10 0 10 20 30 40-0.05 Heat Flow -0.1-0.15-0.2 Heat Flow -0.1-0.15-0.2-0.25-0.25-0.3-0.3 Temperature (ºC) Temperature (ºC) Fat crystallization behavior of oil-in-water emulsions
Characterization of Coalescence: Coalescence/Oiling-off by DSC 100 Oiling Off (%) 80 60 40 20 0 Water Crystal n Fat Crystal n Tween 20 Casein -40-20 0 20 40 Temperature (ºC)
Characterization of Coalescence: Centrifugation Methods Centrifuge Time Speed Oil Cream Serum H Released H C P CR OSM g = ρ ( ) V Total V Released A
Partial Coalescence Aggregation Fusion Clumping of partially crystalline droplets due to penetration of fat crystal from one droplet into another droplet
SEM Images of Partial Coalescence 40ºC (Liquid droplets) 0ºC (Partially Crystalline Droplets) O/W Emulsions viewed by SEM (John Coupland, Penn State)
Influence of Droplet SFC on Partial Coalescence E C (%) 100 80 60 40 20 0 0 20 40 60 80 100 SFC (%)
Influence of Interfacial Membrane on Partial Coalescence Thin Membrane -Prone to PC - e.g., Tween 20 Thick Membrane -Resistant to PC - e.g., casein Oiling Off (%) 100 80 60 40 20 0 Tween 20 Casein -10 0 10 20 Temperature (ºC)
Case Study: Ice Cream Manufacture Add Surfactant & Age Fat globules covered with thick milk protein membrane Fat globules covered with thin surfactant membrane
Partial Coalescence in Ice Cream Partially coalesced droplets in continuous phase Air bubble Partially coalesced droplets around air bubble Ice cream viewed by cryo-sem (Douglas Goff. Guelph)
Methods of Controlling Partial Coalescence Aggregation Fusion Control droplet crystallization (SFC) Control thickness & viscoelasticity of membranes Control droplet-droplet interactions Control droplet collision frequency or contact time
Measurement of Partial Coalescence: Techniques & Protocols Instrumental Techniques Microscopy Particle Sizing Creaming stability/oiling Off Solid fat content versus temperature Experimental Protocols Storage Tests Accelerated Storage Tests Environmental Stress Tests
Ostwald Ripening Growth of large droplets at the expense of small droplets due to molecular diffusion of oil molecules through the aqueous phase driven by differences in Laplace pressure
Ostwald Ripening Time Growth of large droplets at the expense of small droplets due to molecular diffusion of oil molecules through the aqueous phase driven by differences in Laplace pressure
Features of Ostwald Ripening φ (%) 35 30 25 20 15 10 5 0 hours 24 hours 48 hours 0 0.1 1 10 100 Diameter (µm) Monomodal PSD Evolution d 3 proportional to time d 3 (µm 3 ) 0.04 0.035 0.03 0.025 0.02 0.015 0.01 0.005 0 0 200 400 600 Time (hours)
Influence of Oil Type on Ostwald Ripening 4 d 3 (µm 3 ) 3 2 1 Decane Hexadecane 0 0 25 50 75 100 Time (hours)
Food Emulsions Susceptible to Ostwald Ripening High Susceptibility Emulsions containing oils with high water solubility, e.g., flavor oils, essential oils, SCFA Emulsions containing alcohol in the aqueous phase, e.g., cream liqueurs Low Susceptibility Emulsions containing oils with low water solubility, e.g., TAGS
Ostwald Ripening δ<r> 3 / δt = 8 γ V m S D / 9 R T Methods of Retarding Ostwald Ripening: Reduce oil solubility in water Reduce interfacial tension Incorporate low solubility oil into droplets Use membrane resistant to deformation
Measurement of Partial Coalescence: Techniques & Protocols Instrumental Techniques Microscopy Particle Sizing Experimental Protocols Storage Tests Accelerated Storage Tests Environmental Stress Tests
Conclusions Many different physicochemical processes contribute to the instability of food emulsions For a particular food product it is necessary to identify the dominant instability mechanism Emulsion science can then be used to improve food emulsion stability