Introduction hydrophilic portion of the phospholipid toward the surface

 Introduction

            Emulsion is a mixture of two liquids
that is not naturally mix together which one of the liquids being dispersed as
small spherical droplets. There are two types of emulsion which is oil-in-water
and water-in-oil. Oil-in-water emulsion is the dispersed oil droplets in an
aqueous phase of a system. For example, milk dressing and mayonnaise. In
contrast, water-in-oil emulsion is the dispersed of water droplet in an oil
phase of a system. For example, margarine and butter. The substance that makeup
the droplets referred as dispersed phase while that makeup surrounding liquid
is called continuous phase. The process of converting two separate immiscible
liquids into emulsion is known as homogenization. Temporary emulsification is
created. Since oil and water is immiscible, the oil droplets will realign
themselves as predicted by Stokes’ Law and recreate surface tension. This is
because the difference surface tension makes them immiscible with one another.  

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Discussion

Interaction between water and oil
creates an interfacial tension. The water molecules able to form strong
hydrogen bond with neighbor in bulk water phase whereas oil molecule only able
to form weak van der Waals bond with each other in bulk oil phase. At oil-water
interface, oil molecule can only form weak van der Waals bonds with water
molecules. Emulsification takes place by reduction of interfacial tension
between two phases by emulsifying agent that creates a film over one phase that
forms globules, which repel each other. This causes emulsion to remain
suspended in the dispersion medium.

             Emulsion is thermodynamically unstable system
that need stabilizer to form emulsion that are kinetically stable by a
stabilizer. Emulsifiers are classified as stabilizer to stabilize emulsion. Common
emulsifier that usually use is lecithin which can be found in egg yolk. Lecithin
act as a surfactant that will decreasing or complete diminish the surface
tension to form emulsification. Lecithin in the egg yolk is commonly the only
emulsifier that added into the emulsion. However, the vegetable oil may contain
emulsifiers that inhibit crystallization. Lecithin will facilitate
agglomeration during spray drying. During spray drying process, the hydrophobic
portion of emulsifier will dissolve in the cocoa butter, orienting the
hydrophilic portion of the phospholipid toward the surface of particle. (Van
Nieuwenenhuyzen,1981). This increased affinity of the cocoa powder for water
aids dispersion and wetting.

Emulsifier have two components which
is hydrophilic head (water-loving) and hydrophobic tail (water-fearing). After
adding lecithin into the emulsion, hydrophilic head will associate with water
while hydrophobic tail with oil form micelles. When amount of micelles exceeds
a certain level known as critical micelle concentration or CMC.  Consequently, the surface tension of a
solution decrease with increasing surfactant concentration below the CMC, but
remain constant above it. Besides, this thermodynamically unfavorable condition
can be overcome by supplying free energy to the system to increase surface area
between oil and water molecules. The amount of free energy supplied must be
proportional to the increase in contact in area between oil and water
molecules. (Hiemenz and Rajagopalan 1997).

DG=gDA

(DG is free energy required; g
is a interfacial tension)

            There
are two categories of homogenization which is primary and secondary
homogenization. Primary homogenization is the conversion of two bulk liquid
into an emulsion whereas secondary homogenization is the reduction in size of
the droplets in an existing emulsion. The droplets produce by homogenization range in size between 1 to 10mm
in diameter. Emulsions normally have a droplet size range that can
be represented by a particle size distribution function. High speed mixing in
homogenization is required to prevent clumping of material.

Concentration of droplets in emulsion
plays an important role in stability of food. The droplets concentration
described as dispersed-phase volume fraction, which is equal to volume of
droplets divided by total volume of emulsion. Emulsion viscosity can be greater
than the viscosity of oil or the water because emulsions show non-Newtonian behavior.
This behavior is a result of droplet crowding or structural viscosity.
Viscosity of emulsion can be measured by using rotational rheometer with the
plate/cone sensor. Many food emulsions are not pure liquids or pure solids but
have rheological properties that are partly viscous and partly elastic (Walstra
2003, Rao 2013, van Vliet 2013). For example, emulsion of mayonnaise describes
as viscoelastic material that exhibit both viscous and elastic behavior
simultaneously. In a viscoelastic material, part of the energy is stored as a
mechanical energy within the material and does not instaneously form or deform
when force is removed.

Emulsion stability often relate the
ability of emulsion to resist changes in its properties with time. The most
common physical mechanism that are responsible for the instability of food
emulsion are creaming and sedimentation.  This process result when external forces
gravitational or centrifugal that is exceed the thermal or Brownian motion of
droplets.

A concentration gradient builds up in
the system such the larger droplet moves more rapidly to the top if the density
is less than the medium or to the bottom if the density is greater than the
medium. Flocculation occurs when two or more droplets aggregate without losing
their individual identity. Flocculation is promoted by creaming and occurs when
van der Waals attractive force between droplets exceed the electrostatic
repulsive forces. In addition, flocculation can lead to coalescence that
involves breaking of interfacial film and is irreversible. There is various factor
such as pH, solubility of emulsifier, salt, emulsifier concentration, phase
volume ratio, temperature and properties of film affect coalescence stability
of emulsion. The extensive coalescence can eventually lead to the formation of
separate layer of oil on the top of product which is known as ‘oiling-off”.
Phase inversion found commonly where volume of the fluid of two components is
the same or the volume of dispersed phase is larger than continuous phase. The
process of emulsion inversion occurs when the dispersed droplets unite but
retain the formerly continuous phase droplets within the mass. This happened in
dairy cream which has high concentration of butterfat and the resulting
inversion emulsion look like a paste or thick cream and typical example is
mayonnaise.

Conclusion

Emulsion is the mixture of two immiscible liquids and
stabilize by the emulsifiers. The
stability of oil-in-water emulsions was strongly influenced by the droplet size
and concentration. Some concentrated emulsion cause instabilities including, particle
size variation and creaming. Hence, emulsion mostly relate to food and
pharmaceutical industry.

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