CJW-+Chapter+2

Food Colloids: Emulsions and foams
Introduction Nature of an Emulsions Types of Emulsions Stability of Food Emulsions Emulsions Destabilisation Emulsion Formation

**__Introduction __** Emulsion-based food products exhibit different physicochemical and organoleptic characteristics, e.g. appearance, aroma, texture, taste and shelf life These happened as a result of different ingredients and processing conditions used. The manufacture if the emulsion-based food product with specific quality attributes depends on the selection of the most appropriate raw materials( water, oil, emulsifiers, thickening agent, minerals, acids, bases, etc) and processing conditions (mixing, homogenization, pasteurization, sterilization, etc).

**__Surface and Interfacial Tension __**

**//Surface tension //** : A force to reduce the surface area acts on the surface of the water.

**//Interfacial tension //** : A kind of surface tension acts on the interface (when two immiscible substances in contact) so that they separate from each other. When an emulsion is formed, surface contact area increases, the total interfacial tension also increase.

**//Surfactant //** <span style="background-color: #ffffff; font-family: 'Times New Roman',serif;"> weakens interfacial tension and changes the properties of an interface.

**//<span style="font-family: 'Times New Roman',serif;">Food emulsifier //** **<span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">: **
 * 1) <span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">Surfactant for food which distinguishes from other surfactants for other purposes.
 * 2) <span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">Consists of hydrophilic and lipophilic (hydrophobic) parts within it
 * 3) <span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">Reduce the interfacial tension (make the mixing of oil and water easy

**__<span style="font-family: 'Times New Roman',serif;">Natural of an Emulsion __** <span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">An emulsion consists of two immiscible liquids (oil & water), with one of the liquids dispersed as small spherical droplets in the other. <span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">Examples of common food emulsions: <span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">Cream, butter, margarine, mayonnaise, salad dressing, sausage, ice cream, cake, chocolate, spread, milk and egg yolk.

<span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">An emulsion consists of <span style="background-color: #ffffff; color: #800080; font-family: 'Times New Roman',serif;">**three main components:**

__**<span style="font-family: 'Times New Roman',serif;">A **____**<span style="font-family: 'Times New Roman',serif;">queous Phase **__ <span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">Role as a solvent to the water- soluble components, as either the continuous phase in O/W emulsions or as the dispersed phase in W/O emulsions. The interfacial tension forces of water need to be reduced by the presence of emulsifiers. The formation of the emulsion is influenced by the pH, ionic strength and emulsifier concentration by altering the size of droplets and the interactions. Addition of hydrocolloids can affect the viscosity as well as emulsion formation, rheology and stability.

**__<span style="font-family: 'Times New Roman',serif;">Oil Phase __** <span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">The oil phase is mainly in the form of triglycerides. It contains levels of di- and monoglycerides, polar lipids and free fatty acids. These three lipids tend to be surface-active, water-soluble and used as food emulsifier. Plant oils contain more unsaturated fats than those of animal origin, therefore liquid at room temperature. Fat is essential nutrient, it also imparts mouth feel to product and some flavor and aroma compounds. <span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">Important property: solidification or crystallization of the fat which has different rheological and textural properties to liquid oil (W/O emulsion). It can also change the texture of fat-continuous products. Crystallization of the fat allows the droplets to form solid networks that enhance structure and texture. The viscosity of the dispersed oil phase is also important for homogenization.

**__<span style="font-family: 'Times New Roman',serif;">Surface Active Agent (Surfactant) __** <span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">Emulsifier has the role to form an adsorbed layer around the emulsion droplets which lowers the interfacial tension. It aids for emulsification and stabilizes the droplets against flocculation and coalescence. Emulsifiers need to be surface-active, therefore is amphiphilic. Emulsifiers can be either low molecular weight emulsifiers or macromolecular polymers. The most common polymeric emulsifiers in food are proteins.
 * <span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">Emulsifier **

<span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">Stabilizer is specifically aimed at polymers that are not surface-active. It is added to the aqueous continuous phase to impart long-term stability of O/W emulsion. Examples are xanthan, guar, carragenan, locust bean gum, Arabic, etc. Addition of the polymers in enough concentrations results in the effective encapsulation of the emulsion droplets by the polymer. This immobilization prevents flocculation of the droplets and reduces creaming.
 * <span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">Stabilizers **


 * //<span style="font-family: 'Times New Roman',serif;">Multiple Emulsions // **<span style="background-color: #ffffff; font-family: 'Times New Roman',serif;">: In some emulsions the dispersed phase itself may contain globules of other phases, and can be either O/W/O type or W/O/W type.

**__<span style="font-family: 'Times New Roman',serif;">Types of Emulsions __**

<span style="font-family: 'Times New Roman',serif;">Classification is based on the volume percent of the internal phase.

<span style="font-family: 'Times New Roman',serif;">Internal phase ratio (IPR) (phase-volume ratio F) is defined as

<span style="font-family: 'Times New Roman',serif;">Φ = Vi / (Vi + Ve) where Vi= volume of internal phase

<span style="font-family: 'Times New Roman',serif;"> Ve= volume of external phase

<span style="color: #0000ff; font-family: 'Times New Roman',serif;">Φ < 0.3 Low IPR o/w: milk, ice cream and cheese

<span style="color: #0000ff; font-family: 'Times New Roman',serif;"> w/o: butter and margarine

<span style="color: #0000ff; font-family: 'Times New Roman',serif;">0.3 < Φ < 0.7 Medium IPR e.g. Heavy cream

<span style="color: #0000ff; font-family: 'Times New Roman',serif;">Φ> 0.7 High IPR e.g. Mayonnaise and salad dressing

|| ||
 * || <span style="font-family: 'Times New Roman',serif;">Oil in water emulsion || <span style="font-family: 'Times New Roman',serif;">Water in oil emulsion ||
 * <span style="font-family: 'Times New Roman',serif;">Diagram || <span style="font-family: 'Cooper Black',serif;">Emulsion O/W
 * <span style="font-family: 'Times New Roman',serif;">Symbol || <span style="font-family: 'Times New Roman',serif;">o/w || <span style="font-family: 'Times New Roman',serif;">w/o ||
 * <span style="font-family: 'Times New Roman',serif;">Characteristic || <span style="font-family: 'Times New Roman',serif;">Conducts electricity, can be diluted with water || <span style="font-family: 'Times New Roman',serif;">Feel greasy, can be diluted with oils or solvent ||
 * <span style="font-family: 'Times New Roman',serif;">Example || <span style="font-family: 'Times New Roman',serif;">Milk, ice cream || <span style="font-family: 'Times New Roman',serif;">Margarine, butter ||

**<span style="font-family: 'Times New Roman',serif;">Stability of Food Emulsions **

<span style="font-family: 'Times New Roman',serif;">The stability requirements of different food emulsions vary; both short-term and long-term stability are needed. Short-term stability is usually provided by using small surfactants that reduce surface energy and facilitate emulsion formation. Long-term stability is usually provided by employing macromolecules, such as proteins and polysaccharides.

__<span style="font-family: 'Times New Roman',serif;">1. Increase in total interfacial area __

<span style="font-family: 'Times New Roman',serif;">Reduction in interfacial tension via addition of emulsifiers allows emulsion formation with considerably less energy input than it would required without emulsifier. The disperse phase is broken up into small droplets through homogenization process which results in a massive increase in interfacial area. As a result, molecules tend to migrate from the interface into the bulk phase and reduce the actual contact area between dissimilar molecules.

__<span style="font-family: 'Times New Roman',serif;">2. Electrostatic Stabilization: the DLVO Theory __

<span style="font-family: 'Times New Roman',serif;">The DLVO (Derjaguin-Landau-Verwey-Overbeek) theory of colloid stability proposes a balance of the repulsive electric double layer forces (positive by convention) and the attractive van der Waals’ forces (negative by convention) that exist between all matters. These additive forces maybe expressed as a potential energy versus separation curve.

<span style="font-family: 'Times New Roman',serif;">A positive resultant corresponds to an energy barrier and repulsion



<span style="font-family: 'Times New Roman',serif;">A negative resultant corresponds to attraction and hence aggregation



__<span style="font-family: 'Times New Roman',serif;">3. Fine Particles __



<span style="font-family: 'Times New Roman',serif;">Basic salts, plant cell fragments (spices) can stabilize an emulsion by adsorbing at the interface to form a physical barrier around the emulsion droplets. A balance must be achieved between the interfacial tension of the solid and oil and that of the solid and water. Fine particles are usually used in o/w emulsion such as in salad dressing and mayonnaise.

__<span style="font-family: 'Times New Roman',serif;">4. Steric stabilization __



Macromolecules such as proteins and gums (e.g. xanthan and Arabic gum) form film around an emulsion droplet to establish a physical barrier to coalescence. Such molecules can extend several nanometers into solution and can prevent the droplets from approaching closely by a mixture of mechanical and thermodynamic effects. When a protein is used as an emulsifier agent, it unfolds and re-orients its amino acid chain.

__<span style="font-family: 'Times New Roman',serif;">5. Liquid crystal stabilization __

<span style="font-family: 'Times New Roman',serif;">Micelles formation occurs when the excess molecules are positioned face to face gather and there is no change in the surface tension.

<span style="font-family: 'Times New Roman',serif;">The concentration to start micelle formation is called critical micelle concentration (cmc) and the properties of the solution change greatly with a change of this concentration.

<span style="font-family: 'Times New Roman',serif;">When the conc. exceeds cmc, spherical micelles (a) appear at first and disperse into water. Further increase in the conc. cause rod-shape micelles (b). Finally lamellar micelles with higher structures called liquid crystal are produced.



<span style="font-family: 'Times New Roman',serif;">Under certain conditions, emulsifier can form a protecting multilayer around the emulsion droplets. The sudden increase in emulsion stability with enhanced emulsifier conc. a liquid crystalline phase could be separated from the emulsion. It has been shown that the presence of a liquid crystalline state reduces the rate of coalescence, even if the droplet flocculation occurs.

<span style="font-family: 'Times New Roman',serif;"> __6. Stabilization against Gravitational Separation__


 * <span style="font-family: 'Times New Roman',serif;">Minimize density difference **<span style="font-family: 'Times New Roman',serif;">—it is possible to prevent gravitational separation by “matching” the densities of the oil and aqueous phases. This is commonly used for stabilizing beverage emulsion. Density matching can be achieved by mixing natural oils with brominated vegetable oils, so that the overall density of the oil droplets is similar to that of aqueous phase.


 * <span style="font-family: 'Times New Roman',serif;">Reduce droplet size **<span style="font-family: 'Times New Roman',serif;">—can enhance the stability against gravitational separation. For example, homogenization of raw milk retards the creaming in food emulsion by droplet size reduction.


 * <span style="font-family: 'Times New Roman',serif;">Modify rheology of continuous phase **<span style="font-family: 'Times New Roman',serif;">—Increasing the viscosity of the liquid surrounding a droplet decreases the velocity at which the droplet moves (Stoke’s law). E.g. by adding a thickening agent such as Arabic gum, xanthan gum, etc

**<span style="font-family: 'Times New Roman',serif;">Emulsion Destabilisation **
<span style="font-family: 'Times New Roman',serif;">During storage, due to the density differences between most edible oils and water, oil phase might be concentrate at the top of the food emulsion- Creaming. The process is reversible, with only gentle stirring is necessary to restore the original emulsion.
 * <span style="font-family: 'Times New Roman',serif;">1. Creaming **

<span style="font-family: 'Times New Roman',serif;">Creaming does not destabilize the emulsion, but high conc. of droplets in the creamed layer promotes interactions that lead to flocculation and coalescence.

<span style="font-family: 'Times New Roman',serif;">The rate of creaming can be lowered by reducing the droplet size, lowering the density difference between oil and the aqueous phase, and increasing the viscosity of the medium. The creaming rate also dependent on the volume fraction of the dispersed phase and usually slow in conc. emulsions. It is usually involve in o/w emulsion with 0.1< Φ < 0.5 and droplet size from 2 to 5µm.

<span style="font-family: 'Times New Roman',serif;">Flocculation is defined as process by which two or more droplets aggregate without losing their individual identity. Larger droplets will flocculate faster and it is promote by creaming. It can be also affected by the pH and ionic strength of the aqueous environment. Interactions among protein, polysaccharide and water soluble surfactants can also affect the stability of the emulsion.
 * <span style="font-family: 'Times New Roman',serif;">2. Flocculation **

<span style="font-family: 'Times New Roman',serif;">Bridging flocculation occurs in the presence of macromolecular in an emulsion. When there is insufficient emulsifier in the emulsion, large surfactant molecules will adsorb to two different droplets. Bridging flocculation causes an emulsion to cream more rapidly.

<span style="font-family: 'Times New Roman',serif;">Depletion flocculation: Polysaccharides that introduce into emulsion to control the viscosity and yield stress, which are not surface active, maybe too large to fit in to the space between emulsion droplets, and then exert an osmotic effect that pulls water from between the emulsion droplets.

<span style="font-family: 'Times New Roman',serif;">Coalescence is the process when two or more droplets collide to each other and results in the formation of one larger droplet and is dominating when Φ is high. Coalescence involves breaking the interfacial film and is irreversible. It can be affect by solubility of the emulsifier, pH, salts, emulsifier concentration, phase-volume ratio, Extensive droplet coalescence can lead to the formation of a separate layer of oil on top of a product, which is known as “oiling off”
 * <span style="font-family: 'Times New Roman',serif;">3. Coalescence **

<span style="font-family: 'Times New Roman',serif;">Ostwald ripening occurs with polydispersed droplets. Collisions between two droplets may lead to one bigger droplet and one smaller one. Eventually, the small droplets become very small and become solubilized in the continuous medium. High solubility of the oil in the aqueous phase is required; therefore Ostwald ripening is uncommon in food where triglycerides are not normally soluble in water but it do occurs rapidly in frozen foods and in w/o emulsion where water is partially soluble in polar triglyceride oils.
 * <span style="font-family: 'Times New Roman',serif;">4. Ostwald Ripening **

<span style="font-family: 'Times New Roman',serif;">The viscosity of an emulsion will increase gradually as more and more of a given phase is added until a critical volume is reached. If it exceeding the critical volume, emulsion will invert, i.e., the discontinuous phase will become the continuous phase.
 * <span style="font-family: 'Times New Roman',serif;">5. Phase Inversion **

<span style="font-family: 'Times New Roman',serif;">It is an important factor determining the repulsive energy between droplets. The lower the conc. of inorganic electrolytes (salts), the higher will be the repulsive energy and more stable the emulsion toward flocculation (aggregation). This property leads to compromises between flavor and stability. <span style="font-family: 'Times New Roman',serif;">
 * <span style="font-family: 'Times New Roman',serif;">6. Ionic Strength **

**__<span style="font-family: 'Times New Roman',serif;">Emulsion Formation __**

<span style="font-family: 'Times New Roman',serif;">The process o converting two immiscible liquids into an emulsion is known as Homogenization. In food processing operations, emulsion is prepared in two steps. First oil and water are converted to a coarse emulsion which contains fairly large droplets. Next, the size of the droplets is further reduced using another homogenizer.

<span style="font-family: 'Times New Roman',serif;">To form a stable emulsion for a reasonable period of time, we must prevent the droplets from merging together after they have been formed (by having enough emulsifier). Two main functions of emulsifiers during homogenization process are:


 * 1) <span style="font-family: 'Times New Roman',serif;">1. Decrease the interfacial tension of oil and water, at the same time reduces the amount of energy required to deform and disrupt the droplets.
 * 2) <span style="font-family: 'Times New Roman',serif;">2. Form protective coating which prevent coalescing.

<span style="font-family: 'Times New Roman',serif;">The size of the droplets produced during homogenization depends on:


 * 1) <span style="font-family: 'Times New Roman',serif;">1. The ratio of emulsifier to dispersed phase
 * 2) <span style="font-family: 'Times New Roman',serif;">2. The time required for the emulsifier to move from the bulk phase to the droplet surface
 * 3) <span style="font-family: 'Times New Roman',serif;">3. The probability that an emulsifier molecule will be adsorbed to the surface of a droplet during an encounter between it and the droplet.
 * 4) <span style="font-family: 'Times New Roman',serif;">4. The amount that the emulsifier reduces the interfacial tension
 * 5) <span style="font-family: 'Times New Roman',serif;">5. The effectiveness of the emulsifier membrane in protecting the droplets against coalescence