surface active agents: influence on the rheological properties of food emulsions
introduction
surface active agents, commonly known as surfactants, play a pivotal role in the formulation and stabilization of food emulsions. these compounds are amphiphilic, meaning they possess both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions. this dual nature allows surfactants to reduce interfacial tension between immiscible phases, such as oil and water, thereby facilitating the formation and stabilization of emulsions. the rheological properties of food emulsions, which include viscosity, elasticity, and flow behavior, are significantly influenced by the type and concentration of surfactants used. this article delves into the impact of surface active agents on the rheological properties of food emulsions, providing a comprehensive analysis supported by product parameters, tables, and visual aids.
1. fundamentals of surface active agents
1.1 definition and classification
surface active agents are molecules that adsorb at the interface between two immiscible phases, reducing the interfacial tension. they are classified based on their charge:
- non-ionic surfactants: do not dissociate in water and are generally less sensitive to ph and ionic strength.
- anionic surfactants: carry a negative charge and are effective in a wide range of ph conditions.
- cationic surfactants: carry a positive charge and are often used for their antimicrobial properties.
- amphoteric surfactants: possess both positive and negative charges, depending on the ph of the solution.
1.2 mechanism of action
surfactants stabilize emulsions by forming a protective layer around dispersed droplets, preventing coalescence. this is achieved through:
- reduction of interfacial tension: surfactants lower the energy required to create new surfaces, facilitating droplet formation.
- formation of a mechanical barrier: the surfactant layer around droplets prevents them from merging.
- electrostatic repulsion: charged surfactants create repulsive forces between droplets, further stabilizing the emulsion.
2. rheological properties of food emulsions
2.1 viscosity
viscosity is a measure of a fluid’s resistance to flow. in food emulsions, viscosity is influenced by:
- droplet size and distribution: smaller droplets increase viscosity due to greater interfacial area.
- surfactant concentration: higher surfactant levels can increase viscosity by enhancing droplet interactions.
- continuous phase properties: the viscosity of the continuous phase (e.g., water or oil) also affects the overall emulsion viscosity.
2.2 elasticity
elasticity refers to the ability of an emulsion to return to its original shape after deformation. surfactants can enhance elasticity by:
- forming a viscoelastic interface: the surfactant layer can exhibit both viscous and elastic properties, contributing to the overall elasticity of the emulsion.
- promoting droplet-droplet interactions: surfactants can facilitate the formation of a network structure within the emulsion, increasing elasticity.
2.3 flow behavior
flow behavior describes how an emulsion deforms under stress. common flow behaviors include:
- newtonian: viscosity remains constant regardless of shear rate.
- pseudoplastic (shear-thinning): viscosity decreases with increasing shear rate.
- dilatant (shear-thickening): viscosity increases with increasing shear rate.
- thixotropic: viscosity decreases over time under constant shear stress.
3. influence of surfactants on rheological properties
3.1 effect of surfactant type
different types of surfactants have varying effects on the rheological properties of emulsions:
| surfactant type | effect on viscosity | effect on elasticity | effect on flow behavior |
|---|---|---|---|
| non-ionic | moderate increase | moderate increase | pseudoplastic |
| anionic | significant increase | significant increase | pseudoplastic |
| cationic | moderate increase | moderate increase | newtonian |
| amphoteric | variable | variable | variable |
3.2 effect of surfactant concentration
the concentration of surfactants in an emulsion can significantly alter its rheological properties:
| surfactant concentration (%) | viscosity (mpa·s) | elasticity (pa) | flow behavior |
|---|---|---|---|
| 0.1 | 50 | 0.5 | newtonian |
| 0.5 | 100 | 1.0 | pseudoplastic |
| 1.0 | 200 | 2.0 | pseudoplastic |
| 2.0 | 300 | 3.0 | thixotropic |
3.3 effect of droplet size
droplet size, influenced by surfactant type and concentration, also affects rheological properties:
| droplet size (µm) | viscosity (mpa·s) | elasticity (pa) | flow behavior |
|---|---|---|---|
| 1 | 150 | 1.5 | pseudoplastic |
| 5 | 100 | 1.0 | pseudoplastic |
| 10 | 50 | 0.5 | newtonian |
4. practical applications
4.1 dairy products
in dairy emulsions like milk and cream, surfactants such as lecithin and mono- and diglycerides are used to stabilize fat droplets, enhancing texture and mouthfeel.
4.2 salad dressings
surfactants in salad dressings prevent oil and water separation, ensuring a consistent viscosity and flow behavior.
4.3 bakery products
in baked goods, surfactants improve dough handling properties and final product texture by stabilizing fat and water phases.
conclusion
surface active agents are indispensable in the formulation of food emulsions, significantly influencing their rheological properties. the type and concentration of surfactants, along with droplet size, play crucial roles in determining viscosity, elasticity, and flow behavior. understanding these relationships allows for the optimization of food product formulations, ensuring desired textures and stability. future research should focus on the development of novel surfactants with enhanced functionality and minimal impact on health.
references
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- dickinson, e. (2003). hydrocolloids at interfaces and the influence on the properties of dispersed systems. food hydrocolloids, 17(1), 25-39.
- friberg, s. e., larsson, k., & sjoblom, j. (2004). food emulsions. marcel dekker.
- garti, n., & leser, m. e. (2001). emulsification properties of hydrocolloids. food hydrocolloids, 15(4-6), 533-541.
- dalgleish, d. g. (2006). food emulsions—their structures and structure-forming properties. food hydrocolloids, 20(4), 415-422.