Food Emulsions and Foams Flashcards
- Ex. Frozen desserts, margarine, milk, butter
- either w/o or o/w
- need to be stable through adding emulsifiers/stabilizing agents
Food emulsions
- beaten egg white
- contain gas (air or CO2) dispersed into liquid
Food Foams
Definition of emulsion
- colloidal system containing droplets of one liquid dispersed in another
- two liquids are immiscible
- dispersed phase = droplets
- continuous phase = the other liquid
- must contain an emulsifier
- similar to colloidal dispersons/sols, except dispersed phase is liquid
Function of emulsifier
coats the emulsion droplets and prevents them from coalescing or combining with each other
- A system in which particles of the colloidal size of any nature (solid, liquid, gas) are dispersed in a continuous phase of a different composition/state
- molecules are often too big to form true solutions
- particle size range of 1-100nm
- ex. cellulose, cooked starch, pectic substances, gums, some food proteins
Colloidal Dispersion
- A colloid that pours
- a two-phase system with a solid dispersed phase in a liquid continuous phase
sol
- a two-phase system containing an elastic solid with a liquid dispersed phase in a solid continuous phase
gel
- a substance that enables two normally immiscible liquids to be mixed together without separating on standing
emulsifier
Classifications of emulsifiers
- cationic
- anionic
- non-ionic
Two liquid/gas droplets merge/merging to form one larger droplet
coalescence
O/W Emulsion Definition
- more common
- continuous phase = water
- Ex. salad dressings, mayonnaise, cake batter, frozen desserts
W/O Emulsion Definition
- continuous phase: oil
- Ex. butter, margarine, some icings
How does surface tension work?
- Surface: net downward pull on molecule toward bulk of the liquid
- Center: molecule has forces acting on it from all directions —-> net force = 0
- water molecules have strong attractive forces among them: hard to penetrate, large force needed to pull apart molecules and expand the surface
- the force required to increase the surface area of a liquid or to spread it over a surface
- work required to increase a surface area divided by that area
- when a gas (air) surrounds the liquid surface
surface tension
- when a surface is between two liquids
- ex. water and oil
interfacial tension
The higher the tensions the ___________ it is to mix two phases together
harder
- reduce the attractive forces between liquid molecules to reduce the surface or interfacial tension
- Active at the surface of the liquid rather than the bulk of it
- molecules are amphiphilic
Surface-Active Molecules / Surfactants
Structure of Surfactants
- Hydrophilic charged/polar end: attached to water and has no affinity for oil
- Hydrophobic apolar end: favorable for oil, no affinity for water
- Overall amphiphilic
Method of action of surface-active molecules
- molecule ADSORBS at the surface
- molecule REDUCES the attractive forces of the water molecules for THEMSELVES
- makes it easier to EXPAND/spread the surface
Definition of adsorb
to bind to a surface
Non-Food Surfactants
- Detergents
- Water flows over surface, forming thin sheet
- Water DOESN’T gather in droplets
- Not used as food ingredients
Food Surfactants
- Proteins
- Lipids
- Some spices
Characteristics of proteins used as food surfactants
depends on AA composition: whether they are hydrophilic or hydrophobic
Lipids as food surfactants
- Ex. lecithin
- polar head and apolar tail
Spices as food surfactants
- dry mustard
- dry paprika
- molecules that are EITHER hydrophilic or hydrophobic
- molecules remain in the bulk of the liquid
- sugars: hydrophilic
- salt: dissociate into ions, hydrophilic
- DO NOT DECREASE INTERFACIAL TENSION
- increase interfacial tension, depending on ability to bind water molecules —-> increase molecular attraction
Non-Surface-Active Molecules
Formation of emulsion
- when oil, water, and emulsifier are mixed together
1. break up oil or water phase into small droplets that remain dispersed throughout the other liquid ***NEEDS ENERGY VIA MIXER OR HOMOGENIZER
2. emulsifier is absorbed at the surface of droplets, decreasing the interfacial tension and allowing the formation of more and smaller droplets
3. liquid with the higher interfacial tension will tend to form droplets, other liquid will flow around the droplets to form the continuous
Types of emulsifiers used in O/W Emulsions
- emulsifiers that are more easily dispersed in water
- these tend to reduce the interfacial tension of the water more than that of oil
Types of emulsifiers used in W/O Emulsions
- emulsifiers that disperse more readily in oil phase
Principles of formation of a stable O/W Emulsion
- disperse emulsifier in aqueous phase
- oil is added, and interfacial tension of each liquid is reduced by the emulsifier
- beating and homogenization break the oil phase into droplets surrounded by water
- small droplets are formed, protected by an interfacial layer of emulsifier
- interfacial area of the oil becomes very large, aqueous phase spreads to surround each oil droplet
Functions of emulsifier
- reduce interfacial tension
- formation of a stable film that protects the emulsion droplets and prevents separation of the emulsion
Three fates of droplets after collision in an emulsion:
- film stretches/breaks, droplets coalesce into a larger droplet
- the emulsifier layers surrounding the droplets interact and an aggregate is formed
- the droplets move apart again
an unstable system with a greater concentration of a material in solution than would exist at equilibrium
supersaturation
dissolution of small crystals or sol particles and the redeposition of the dissolved species on the surfaces of larger crystals or sol particles
Ostwald ripening
- The disappearence of the boundary between two particles in contact, or between one of these and a bulk phase
- Followed by changes of shape leading to reduction of total SA
Coalescence
Process whereby dispersed molecules or particles form aggregates
Aggregation
- the macroscopic separation of a dilute emulsion into a highly concentrated emulsion, in which interglobular contact is important
- continuous phase under the action of gravity or centrifugal field
Creaming
- an emulsion that coats all the emulsion droplets with a stable, cohesive, viscoelastic film
- able to stretch instead of break
- less likely to break when collided
Completely coated emulsion-
- an emulsion that is more likely to coalesce
- formation of large droplets
- ultimately in the separation of the emulsion
incompletely coated emulsion
Characteristics of emulsifiers that facilitate the emulsion formation
- adsorb at the interface between two liquids
- reduce the interfacial tension of each liquid, enabling one liquid to spread more easily around the other
Characteristics of emulsifiers that promote emulsion stability
- form a stable, coherent viscoelastic interfacial film
- prevent or delay coalescence of the emulsion droplets
All emulsifiers are ___________
surfactants
Not all surfactants make good ______________
emulsifiers
- not all surfactants are able to form stable film at the interface and prevent coalescence
- emulsifier with greater ability to extend over the droplet surface
- greater ability to interact with other groups within the same molecule/on different molecules
- able to form viscoelastic and stronger surface films
- Ex. protein
Large macromolecules
- non-emulsifier with lower interfacial/surface tension
- promote spreading/wettability
- do not usually able to form stable interfacial films by themselves
- sometimes mislabeled as emulsifier
small molecules
Difference between an emulsifier and a surfactant
- Both: facilitate formation of emulsion by 1) adsorbing at interface 2) reducing interfacial tension
- Only Emulsifier: promotes emulsion stability by 1) forming stable interfacial film 2) preventing coalescence
Why are proteins the best natural emulsifiers
- uncoil/denature and adsorb at the surface
- hydrophobic orient in oil
- hydrophilic orient in water
- series of loops, trains, tails may be envisioned at the surface to orient in this way
Examples of natural emulsifiers
- egg yolk proteins
- milk caseins
- meat proteins
- soy proteins
- lecithin
Egg yolk proteins as natural emulsifiers
- lipoproteins are associated with each other
- associate with phospholipids (lecithin) in micelles
Milk caseins as natural emulsifiers
- present at fat globule surface
- give the stability of homogenized milk
Lecithin as natural emulsifiers
- surfactant
- useful for promoting wettability
- aids in mixing of products (ex. hot drink mixes)
- DOES NOT usually form strong interfacial films by itself
- would not be emulsifier of choice unless other emulsifiers and stabilizers were also added
Synthetic Emulsifiers and Surfactants
- relatively small molecules compared to proteins
- used mainly to aid in dispersion of fat rather than to stabilize emulsions
- Ex. mono and diglycerides are added to shortening and cake mixes to aid in the dispersion of the shortening
Method of action of small-molecule surfactants
- molecules adsorb strongly to oil-water interface
- few steric constraints to prevent them from packing closely
- generate low interfacial tensions and are very effective at lowering Gibbs interfacial energy
- Do not generally give highly cohesive/viscous surface layers ——> easily disrupted
Surfactants with low HLB
- 3-6
- more hydrophobic/lipophilic
- used to form w/o
- Ex. glycerol monostearate, sorbitan monostearate
Surfactants with high HLB
- 8-18
- more hydrophilic
- o/w emulsions
- ex. polyoxyethylene sorbitan monostearate (TWEENS 60), sodium stearoyl-2-lactylate
- unstable emulsion that separates fairly soon after formation
- Ex. french dressing: combine ingredients and shake vigorously —-> oil droplets are not protected and soon coalesce
Temporary Emulsions
- stable emulsion that does not separate under normal handling conditions
- Ex. mayonnaise: oil (dispersed) in vinegar (continuous), egg yolk
- contains more dispersed phase (oil droplets) than the continuous phase
Permanent Emulsions
Milk as an emulsion
- contains about 3.5% fat in emulsified
- fresh milk: fat droplets are stabilized by a complex protein membrane = milk fat globule membrane
- “stable” emulsion when fresh
- milk will cream fairly quickly if left to stand
- fat droplets will rise through milk because of density difference
- cream layer is not a true separation of oil and water —> still an emulsion and interfacial film is still intact
- homogenization breaks the fat into smaller droplets ——> prevents creaming effect
Factors affecting emulsion stability
- sufficient emulsifier
- droplet size
- changing pH
- viscosity –> addition of hydrophilic gums
- storage and handling
- temperature
- violent shaking
How sufficient emulsifier affects emulsion stability
sufficient amount needed to completely coat the surface of all the droplets in order to ensure stability
How droplet size affects emulsion stability
- important because larger droplets are more likely to coalesce
How changing the pH affects emulsion stability
- adding acid/changing the ionic strength by adding salts
- may reduce the stability of interfacial film (especially if it is made of protein)
How viscosity affects emulsion stability
- thicker emulsion: slower movement of the molecules within the system + longer it will take for two phases to separate
- addition of hydrophilic gums needed to increase viscosity of system
- ***hydrophilic gums are not emulsifiers
How storage and handling affects emulsion stability
- emulsions will break into two phases with wrong handling conditions
How temperature affects emulsion stability
- warm: oil droplets become more fluid —-> coalescence is more likely
- cool to refrigeration temperatures:
1) enhance stability —-> solidification of oil droplets
2) decrease stability —-> may cause protein denaturation/disruption of interfacial film due to formation of ice crystals ——-> therefore we add gum to frozen emulsions (ice cream!)
How violent shaking affects emulsion stability
- likely to disrupt emulsions
- contains gas bubbles dispersed in a liquid continuous phase
- simple dispersion (egg white = dilute protein dispersion)
- complex dispersion: emulsified fat droplets, ice crystals, solid matter
- contribute to the volume and texture of foods
foam
Density difference between two phases in foams
- much larger density difference than in emulsions
- tendency for:
1) liquid continuous phase to drain due to gravity
2) gas bubbles to escape
Difference of droplets between foams and emulsions
- Foam: bigger
- Emulsion: smaller
Difference of continuous phase between foams and emulsions
- foam: thinner
- emulsion: thicker
Mechanism of action of whipping in foam formation
- incorporates gas into the liquid
- breaks up large bubbles into smaller ones
- spread liquid phase around the gas bubbles
- higher the energy, the smaller the bubbles and the greater the foam volume
Mechanism of action of foaming agent in foam formation
- foaming agent is in liquid phase
- adsorbs at the surface of the liquid —-> reduces surface tension and forms films around the gas bubbles
- sufficient to completely coat and stabilize the bubbles
How is foam stability measured?
- loss of foam volume over a period of time
- stable: volume does not change very much
- unstable: loss of air can lead to a considerable reduction in volume
Foam stability factors
- tendency of the liquid film to drain due to gravity
- tendency for the film to rupture and allow coalescence or escape of gas bubbles
- diffusion of gas from small bubbles to larger ones
- Evaporation of the continuous phase
Prevention of loss of foam stability
- increase viscosity to reduce drainage
- stabilize the bubbles with a stable interfacial layer
- liquid phase must have a low vapor pressure —-> doesn’t evaporate easily
- the ability of the foaming agent to adsorb at the interface
- the ability of reducing interfacial tension
- amount of energy during whipping
volume of foaming agents
- the ability of the foaming agent to produce a stable interfacial film and a viscous continuous phase
stability
Qualities of a good foaming agent
- easily adsorb at the interface
- reduce interfacial tension
- form a stable interfacial film that resists rupture
Are all surfactants able to form stable foams?
no
- but all surfactants are able to reduce surface tension and produce foams
Egg white as a foaming agent
- when whipped, proteins denature at the interface and interact with each other to form a stable, viscoelastic interfacial film
- some of proteins are glycoproteins containing carbohydrate
- carbohydrates are hydrophilic that bind water and increase the viscosity of the liquid
- carbs reduce drainage
- carbs increase stability
Gelatin as a foaming agent
- warm gelatin sol can be whipped to three times its original volume
- when cooled, gelatin solidifies to form a gel —-> traps the air bubbles and stabilizes the foam
Effect of sugar on foam stability
- increases liquid viscosity
- protects proteins from excessive denaturation and aggregation at the interface
Effect of acid on foam stability
- reduces pH —> reduces charge on protein —> stronger and more stable interfacial film
- delay foaming process —> added at the end
Effect of gums thickening agents on foam stability
- increase viscosity
Method of action of foam suppressants
- suppress foam volume because they adsorb at the interface —-> SUPPRESSING THE ADSORPTION OF THE DESIRED FOAMING AGENT + preventing it from forming a stable foam
- ex. egg yolk: fats, phospholipids, lipoproteins (hydrophobic) ——> adsorb at the surface in competition with the egg white proteins —–> prevent the formation of a stable foam
Method of action of anti-foaming agents
- break up foams/prevent them from forming
- added to fats and oils used in frying to prevent foaming during the frying process
- Ex. polydimethylsiloxane, a type of silicon
