Introduction of emulsions Effect of polysaccharides on emulsion stability Use of polysaccharides as emulsifier. Polysaccharides in Food Emulsions

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1 1 Introduction of emulsions Effect of polysaccharides on emulsion stability Use of polysaccharides as emulsifier 2 Basic concepts of emulsions Interfacial tension (): the force that operates on an interface and acts perpendicular from the boundaries to decrease the area of interface. Surface tension is used for the interface when one phase is air Surfactant: a surface active substance that reduces the interfacial tension (and the total free energy) 3 Basic concepts of emulsions Two types of surfactants: macromolecules and small molecules Basic structure of small-molecule surfactants: a hydrophilic part and a hydrophobic part. These molecules like to stay at interface (to reduce the free energy), with hydrophilic part in water and hydrophobic part in oil or air Hydrophile-lipophile balance (HLB): 7 means equal solubility of surfactant in water and oil; <7 means greater solubility in oil; >7 means greater solubility in water Critical micelle concentration (CMC): the surfactant concentration over which the surfactants form micelles and the interfacial tension remains constant 1

2 4 Basic concepts of emulsions Two types of surfactants: macromolecules and small molecules Surface active macromolecules: synthetic co-polymers (with both hydrophilic and hydrophobic segments), proteins, and polysaccharides 5 Types of emulsion Simple emulsion: regular emulsion oil-in-water (O/W); inverse emulsion water-in-oil (W/O) Multiple emulsion: O/W/O, W/O/W Homoge nization Simple emulsion Homogenization Multiple emulsion 6 Physical stability of emulsion McClements. Critical Reviews in Food Science and Nutrition 2007, 47: Two factors to prevent droplets from approaching each others: Electrostatic repulsion (surface charge) Steric hindrance (thick interfacial layer) 2

3 7 Chemical stability of emulsions: oxidative stability Pro-oxidative factors Interfacial layer Oil droplet Water phase Pro-oxidative factors: compounds such as oxygen molecules, free radicals, and transition metal ions that can trigger or promote lipid autooxidation at the surface of oil phase Interfacial layer protection is a key approach to improve oxidative stability 8 Effect of polysaccharides on emulsion stability Polysaccharides as thickeners Bridging flocculation Depletion flocculation Layer-by-layer interfacial fabrication 9 Polysaccharides as thickeners Due to the difference in density between the emulsion droplets and continuous liquid, buoyancy forces (F buoyancy = V d ( - 0 )g) lead to a gravity-driven vertical movement Buoyancy force is counteracted by a viscoelastic force developed by the surrounding medium (F viscous = 3du) At steady state, F buoyancy = F viscous. When viscosity () increases, velocity (u) decreases 3

4 10 Polysaccharides as thickeners Relationship between viscosity and polysaccharide concentration Log sp C* C < C* C C* C > C* C*: coil overlap concentration sp : Specific viscosity Log C [] []: Intrinsic viscosity 11 Bridging flocculation Polysaccharide with long chains at a low amount in emulsion Each chain may adsorb to different individual droplets, dragging them together, leading to bridging flocculation 12 Depletion flocculation An entropy-driven phase separation between an droplet-rich region and polysaccharide-rich region Depletion flocculation is directly related to the size of polysaccharide coils. The center of mass of large polysaccharide molecules are excluded from the thin region adjacent to the droplets surface, the depletion layer A local concentration gradient is generated Solvent among individual droplets diffuses away to the polysaccharide region, leading to aggregation of droplets 4

5 13 Depletion flocculation Polysaccharide concentration for depletion flocculation is much higher than that for bridging flocculation 14 Layer-by-layer interfacial fabrication Goal To form a thick layer at the oil-water interface to protect oil phase from pro-oxidative factors, such as oxygen molecules, free radicals, metal ions, thus to reduce lipid autooxidation To form a interfacial layer with controlled digestibility to modulate the release pattern of lipophilic compounds Approach Use a charged surfactant (lecithin, β- lactoglobulin) to form emulsion Then apply polysaccharide with opposite charge (chitosan, pectin, alginate) to form the second layer Can form an additional layer via electrostatic interaction 15 Use of polysaccharides as emulsifier Gum arabic Other polysaccharide with emulsification property Polysaccharide-protein conjugates Modified starch: starch octenyl succinate 5

6 16 Gum Arabic Emulsification properties Widely used as an emulsifier to stabilize flavor oil emulsion for the soft drink industry Treatment with protease results in a loss of emulsification capability Hydrophilic carbohydrate blocks Hydrophobic polypeptide chain 17 Other polysaccharides with emulsification property Some non-gum arabic polysaccharides show emulsification properties Hypothesis (1): the important role of the proteinaceous moiety for the emulsifying properties of polysaccharide (e.g. research by the group of Glyn Philips on the emulsification property of sugar beet pectin (Funami et al., Food Hydrocolloids 2007, 21: )) Hypothesis (2): the built-in hydrophobic moieties on the polysaccharide s backbone internal structure are not obligatory for active interfacial adsorption (Garti and leser, Polymers for Advanced Technologies 2001, 12: ) 18 Polysaccharide-protein conjugates Principle: to form covalent binding between polysaccharide (reducing ends) and protein (lysine amino group) through Maillard reaction Water Whey protein: surface active, but with low steric hindrance Polysaccharide: Large molecule, but not surface active Polysaccharideprotein conjugates: surface active, large molecule with steric hindrance Oil 6

7 19 Modified starch: starch octenyl succinate + Octenyl succinic anhydride Glucosyl unit Starch-OS 20 Modified starch: starch octenyl succinate Starch-OS in solution Starch-OS at interface 7