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PDF | Emulsions take place partially or completely in the structures of many natural and processed foods or some foods are already emulsified in certain stages.
Table of contents
- Emulsions: making oil and water mix
- Stability of food emulsions
- Freeze-Thaw Stability of Food Emulsions | Food and Applied Bioscience Journal
- Centre for Food education & Research
If the particles strongly repel each other, the colloidal system will be stable. A summary of the breakdown processes, details of each process, and methods of its prevention are given as follows. Creaming and sedimentation take place when gravitational or centrifugal forces exceed the thermal motion of the droplets. If droplet density is lower than that of the medium, heavier droplets move faster to the top. On the contrary, they will move to the bottom when their density is larger than that of the medium. The closeness of the droplets favors breakdown of the interface. Eventually, the droplets can build up a close-packed arrangement at the top or bottom, giving rise to creaming or sedimentation, respectively.
For a deeper discussion, see Refs. The recovery of a creamed emulsion may be made by simply shaking or prevented by the following ways: 1 reducing the droplet size because the gravitational force is proportional to the cube of the droplet size; 2 increasing the viscosity of the continuous phase because it causes a slowdown of droplet movement; and 3 adding thickening agents high-molecular-weight polymers such as carrageenans, alginates, etc.
Coalescence is a growth process during which the emulsified droplets join together to form larger ones. Contrary to creaming, coalescence implies the irreversible fusion of droplets into larger ones, which implies disruption of the interdroplet liquid film.
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So the driving force for coalescence is the film fluctuations. The high mobility of molecules at the interface gives rise to fluctuations in the interface film, which can cause the film to break [ 30 ], and thus the two droplets spontaneously merge, causing coalescence.
Emulsions: making oil and water mix
For a deeper study of coalescence, see Ref. Repulsive interactions can be modified by changing the charge at the surface, or by using a surfactant that provides a different thickness. Apart from that, coalescence is frequently caused by an incomplete covering of the droplets with surfactant molecules, so replace or increase surfactant can commonly avoid coalescence. Flocculation is the process by which droplets without changing droplet size are aggregated into larger units. It occurs when there is not sufficient repulsion to keep the droplets apart to distances where the van der Waals attraction is weak.
The van der Waals attraction is inversely proportional to the droplet-droplet distance of separation [ 29 ]. Flocculation is determined by the magnitude of attractive versus repulsive forces.
Stability of food emulsions
On the contrary, coalescence is determined by the stability of the interdroplet film. The van der Waals attractive forces depend on temperature, ionic strength, and charge of the interfacial layer [ 31 ], which strongly affects emulsion stability. Emulsions can be stabilized by electrostatic repulsion using ionic surfactants or by steric stabilization adding large polymers to the surrounding aqueous phase.
Phase inversion is the process by which the dispersed phase and the medium are exchanged. Phase inversion usually takes place through a transition state including multiple emulsions. It can be minimized by choosing a suitable surfactant. Finally, another process affecting emulsion stability is the Ostwald ripening , caused by the finite solubility of the liquid phases. Thus, liquids considered as being immiscible usually have no negligible solubilities. Specifically, in emulsions, curvature effects in the smaller droplets give rise to larger solubility than the larger ones.
This difference in solubility between small and large droplets is the driving force for Ostwald ripening. The increase in solubility takes place when the droplet curvature increases, that is, when the droplet size decreases [ 32 ]. So the smaller droplets disappear and are deposited on the larger ones, resulting in larger droplets that grow at expenses of smaller ones. There is a range of possibilities for modification in the properties of the emulsion which influence the stability and functional behavior of the colloidal system.
Different protocols to ensure food storage stability can be found in Ref.
Freeze-Thaw Stability of Food Emulsions | Food and Applied Bioscience Journal
All of them are based on the analysis of the following emulsion properties:. Droplet size distribution and droplet concentration are one of the most characteristic features of an emulsion not only due to the fact that most of the instability process are driven by droplet-droplet interactions, but also to the bulk properties such as taste, color and texture , which depends mainly on these two parameters [ 22 ].
In particular, the speed of creaming depends on the effective particle size. Composition of the stabilizing layer at the interface has an essential role to ensure emulsification and stability. The main component of the interfacial layer is the emulsifier or surfactant. The most used scale to classify emulsifiers is the hydrophilic-lipophilic balance HLB , that is, a parameter relating molecular structure to interfacial packing and film curvature. The HLB value ranges from 0 to This parameter was introduced by Griffin [ 33 ] in order to obtain an empirical Eq.
Also relevant can be the presence of solid particles and cosurfactant at the interface layer. Specifically, food emulsions frequently carry particulate material which is located at oil-water interface favoring emulsion stabilization [ 34 ].
Centre for Food education & Research
Pickering-type food emulsions are emulsions consisting of droplets coated by a layer of adsorbed solid particles at the interface. The addition of a cosurfactant usually allows to enhance the effectiveness of surfactant. Furthermore, cosurfactants can be used to fine-tune the formulation, for example, by expanding the temperature or salinity range of microemulsion stability. However, many factors can affect the both rates. Both rates depend on their intrinsic rate constants k and on the concentrations of AOs at the reaction site. On the one hand, the radical scavenging activity of an AO depends on its chemical structure.
Therefore, all these factors and their effects need to be taken into account to enhance the AO efficiency in depth. Substituents play a key role on the hydrogen atom donating capacity of AOs and understanding on their conformational, electronic, and geometrical characteristics is of vital significance to comprehend the relationship among AO structure and AO activity [ 36 , 37 ]. Structure of phenolics that allows conjugation and electronic delocalization, as well as resonance effects also can improve the radial scavenging activity of AOs. Though the chemical properties and reactivities of relevant AOs toward free radicals are becoming well comprehended, it remains less clear how these properties translate into multiphasic systems.
Left: optical microscope image of the droplets of an olive oil-in-water emulsion. Right: partition of AOs between the oil, interfacial where lipid oxidation primarily occurs , and aqueous regions of the emulsion [38, 44]. The physical impossibility of separating the interfacial region from the aqueous and oil regions of emulsions makes that any attempt to determine antioxidant distributions needs to be done in the intact emulsions, that is, without sample pretreatment.
Application of the pseudophase kinetic model to emulsions provides a natural explanation, based on molecular properties, of the effects of a variety of parameters nature and type of the oil, HLB, temperature, acidity, etc. The reaction of choice was the reduction of a hydrophobic arenediazonium ion, whose reactive group is located in the interfacial region of the emulsions, and that can be monitored by a sampling method. Results obtained for a series of AOs caffeic, gallic, protocatechuic acids, and hydroxytyrosol series show that their distribution can be correlated with their antioxidant efficiency [ 35 , 41 — 43 ].
This finding may have important consequences for the food industry because it opens the possibility of choosing the most efficient AOs for a particular food system on a scientific basis, resulting in an increase of the shelf-life of the product. Results should contribute to enhance current understanding of how antioxidant structure and physical location within the food system affect their efficiency and should provide basic information on the factors controlling antioxidant distributions and efficiencies, allowing a more rational selection of AOs and emulsifiers in food stabilization to maintain the organoleptic properties of foods.
The solvent properties of the reaction site affect both the intrinsic rate constant for the reaction between AO and free radicals, k i nh , and the partitioning of AOs between the different regions of the food emulsions. Among others: Emulsifier nature: the emulsifier electrical nature and HLB of the emulsifier can affect the concentration of AOs at the reaction site. Distribution results showed that the HLB of the emulsifier can modify the partition of moderate to high hydrophobicity AOs and the main parameter controlling the partition of AOs is the emulsifier concentration [ 45 ].
Acidity of the aqueous region: the acidity of the medium can change substantially the partitioning of phenolic AOs. At the typical acidities of foods, phenolic AOs may be neutral or partially ionized and the ionic forms of the AOs are usually oil insoluble but much more aqueous soluble than the neutral forms, changing the partition of AOs and, as a consequence, the antioxidant efficiency [ 46 ]. Oil nature: oxidation rates of monounsaturated fatty acids are much slower than those of polyunsaturated fatty acids.
In this way, foods enriched with omega-3 can be seriously compromised by the oxidation of lipids due to their high degree of lipid unsaturation [ 35 ]. This thesis reports on studies of the stabilizing effect of insoluble protein particles on two types of emulsions, the classical oil-in-water emulsions and the next generation water-in-water emulsions. Water-in-water emulsions have the potential application as oil-free emulsions. Author keywords: Anti-solvent precipitation, emulsions, particles, Pickering stabilization, insoluble protein, plants, water-water interfaces, dynamic light scattering, microscopy, food.
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