Chapter 5 Flashcards
Are commercial dressing a W/O or O/W emulsion? And how much oil is generally inside?
A W/O emulsion with circa 30-40% oil
Is mayonnaise a W/O or O/W emulsion? And how much oil is generally inside?
O/W emulsion with circa 70-80% oil
Are the following products W/O or O/W emulsions, milk, margarine and cream?
Milk - O/W
Margarine - W/O
Cream - O/W
Why are emulsion not stable?
Because to create an interface energy is required. The more interface is created, due to maybe smaller droplets created, the more energy is required. Therefore, coalescence takes place to reduce this energy as system will strive to reduce Gibbs free energy.
What is Laplace pressure?
Is the pressure at the interface. Smaller droplets have higher pressure.
What are the four physical instability issues encountered in emulsion?
- Gravitational separation which leads to creaming or sedimentation depending on densities. Increase viscosity of continuous phase can slow down the process.
- Flocculation (or bridging/depletion flocculation in presence of biopolymers) were droplets come together but maintain their individual integrity. Phenomena is reversible.
- Coalescence where droplets come together to form larger droplets. Phenomena irreversible. Partial coalescence may be desired to entrap air bubble and provide with certain sensory properties.
- Ostwald Ripening, small droplets diffuse matter to larger droplets. This does not occur in O/W emulsion as oil solubility in water is extremely low.
What is an emulsifier and with is its role?
Is an amphiphilic molecule which consists of both hydrophobic and hydrophilic parts. Its role is to decrease the interfacial tension.
How do low molecular weight emulsifiers or surfactants prevent droplets from coming close together?
Steric hindrance and electrostatic repulsion
What is meant for the critical micelle concentration CMC?
When surfactants are added in amounts greater than CMC then an excess is present which will aggregate in micelles.
What is a requirement for a surfactant in an O/W emulsion and which hydrophilic-lipophilic balance (HLB) value should it have?
A requirement is that the surfactant must be soluble in the continuous phase. The phase in which the surfactant is dissolved usually becomes the continuous phase and this is called the Bancroft rule. A high HLB is required as it represent the ratio of hydrophilic and hydrophobic group. The higher the value the higher the affinity to the aqueous phase.
How do particles stabilize emulsions?
So called, Pickering emulsions, stabilize by steric hindrance and form egg shell like structure around droplet. Bancroft rule also applies, meaning that the phase that wets the particles best will be the continuous phase.
What influence competition for adsorption?
Relative surface activities, adsorption rate, availability.
How can you calculate the number of droplets?
V tot / V droplet = N of droplet
Which value of average droplet size from which equation is better for which case?
So in reader they give general equation with n and m. If n=1 and m=0 then the number average is used. If n=3 and m=0 then the volume average is use and if n=3 and m=2 then the Sauter diameter is used. The latter is useful if we are interested on the surface (amount of surfactant, emulsion stability, ect.). The volume average on the other hand is useful for volume related application (amount of oil in the product ect.)
Why is the span of distribution important?
Because it indicates the presence of very large droplets which causes instability of emulsions.
Mention examples of low molecular surfactants with ionic and non-ionic properties.
Ionic = Sodium dodecyl sulfate (SDS), Lecithin, Citric acid ester.
Non ionic= Polysorbate 80 (Tween 80), monoglyceride, diglyceride.
What happens to the interfacial pressure when the emulsifiers adsorbs?
It increase. The interfacial tension on the other hand decreases. The interfacial load increases. Usually interfacial load is higher for protein than for surfactants.
Order the following emulsifier on order of how fast they can adsorb to the surface. low molecular surfactants, particles and proteins
Low molecular surfactants, protein and particles. This is in order of how fast they adsorb. However they are also in order of how stable they are. Particle adhere more slowly but are the most stable.
Give some examples of particles
For O/W emulsion -> Soy protein particles, modified starch granules, lactoferrin particles.
For W/O emulsion-> fat crystals, hydrophobized starch
What is the role of hydrocolloids?
Increase viscosity or gel continuous phase in order to avoid creaming and coalescence. Some examples are starch, carrageenan. However depletion interactions may occur.
What may happen to vinaigrette when added to dry lettuce?
It can phase invert from W/O to O/W.
Explain shortly the three types of rotor-stator machines.
- Stirred Tank -> useful for droplet of size >10um not smaller.
- Colloid Mill-> smaller droplets can be achieved. If mixture is highly viscous then flow will be laminar (Re<370) otherwise turbulent.
- Toothed mill -> used in chemical and pharmaceutical industry. Also give rise to small droplets.
Explain shortly the two types of high pressure homogenizers.
Valve and nozzle system. Used to produce droplet of size <1um. Extensional flow, turbulence and cavitation causes breaking of droplets into smaller.
What is cavitation and what problems can cause?
Cavitation is the formation of vapour bubbles within a liquid at low-pressure regions that occur in places where the liquid has been accelerated to high velocities. This vapor then creates a lot of turbulence and causes droplets to break. It should be limited as it may cause radicals and compromise product quality.
Explain shortly ultrasound and where it should not be used.
Using of sound to produce cavitation and turbulence to disrupt particles into smaller. Should not be used for unsaturated fatty acids or other chemical liable molecules due to cavitation. Only suitable for low volume products, not for large scale production.
What does We<1 mean and We>1
We represent the ratio between the externally applied stress and the internal stress (Laplace). If we>1 smaller droplets are made, if we<1 then droplets are stable.
What is the difference between simple shear flow and extensional shear flow?
Simple shear flow is the flow over a plate or tube while extensional shear flow is the flow through a small opening and is also more effecting in breaking droplets.
What is the difference between turbulent and laminar flow?
Turbulent flow has Re>370. It uses eddies to disrupt droplets. The eddies must be the same or smaller size compared to the droplet. This technique is less energy efficient compared to laminar but it is used in large scale production including toothed mill, colloid mill, stirred tank and homogenizer.
What are the complication that can occur during emulsification?
Emulsification is a balance between droplet breaking and immediate coalescence. Therefore often, multiple passages must be applied. This causes longer processing times and energy expenditures.
How can a company make infant formula mimic better the structure of human milk? Hint: Fat in human milk is presented in larger droplets.
Until now companies use homogenizers. However in order to produce larger droplets which will be stabilized by the addition of a milk membrane, a rotor-stator mixer is a better option.
What type of emulsification can be used only for alcoholic beverages?
Emulsification by supersaturation
Shortly describe the two type of membrane emulsification?
- Cross-flow membrane emulsification-> Suitable for small volume, high value production with a low volume fraction of dispersed phase. If a O/W or a W/O emulsion is desired the membrane should be hydrophilic or hydrophobic respectively. XME membrane is usually used which allows for both emulsions to be made.
- Dead-end (premix) membrane emulsification -> pre-made emulsion is pushed through a membrane with pores resulting in droplets being reduced in size. The membrane should have relative narrow pore size distribution.
Does increasing crossflow reduce particle size in SPG membrane?
Increasing crossflow has relative no affect on droplet size as SPG membrane work with spontaneous detachment of the droplet due to a difference in Laplace pressure occuring. The spontaneous detachment can occur when the radius of the droplet is twice the pore size.
Using an XME membrane how can one decrease droplet size?
Increasing crossflow and decreasing pore size and addition of surfactant to reduce surface tension.
What are some advantage and disadvantages of cross flow and dead end membrane emulsification?
Cross flow:
Advantage -> for small operations, low energy consumption
Disadvantage-> small throughput, and small volume fraction
Dead end:
Advantage-> large throughput, large volume fractions, phase inversion possible
Disadvantage-> internal fouling if e.g. protein stabilizes emulsion. Protein will adhere to interface of membrane and reduce pore size. Whole membrane must be replace.
How can phase inversion be made in a premix membrane emulsification?
If a O/W is passed through a hydrophilic membrane and O/W emulsion is obtained. If passed through a hydrophobic membrane then a W/O emulsion is obtained. Surfactant also must be able to support this system.
Why is microfluid emulsification still not used that much?
Very low throughput. However make droplets all of the same size and is helpful to conduct analysis on surfactant adsorptions in dynamic conditions which is not possible with other processes.
Explain shortly the two technique for phase inversion?
Catastrophic phase inversion include the addition of the dispersed phase slowly while agitating. The viscosity will increase and large hysteresis is present. At a certain point the emulsion flips. The other option is to add the dispersed phase more quickly which leads to faster increase in viscosity and smaller hysteresis.
The other methods includes using transitional phase inversion where by using temperature or pH change one can change surfactant solubility from the continuous to the dispersed phase resulting in phase inversion. Will show no hysteresis.
What is the pupose of W/O/W emulsion?
To prepare light products, with less fat.
