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Food Science > Food Emulsions and Foams > Flashcards

Food Emulsions and Foams Flashcards

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1
Q
  • Ex. Frozen desserts, margarine, milk, butter
  • either w/o or o/w
  • need to be stable through adding emulsifiers/stabilizing agents
A

Food emulsions

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2
Q
  • beaten egg white
  • contain gas (air or CO2) dispersed into liquid
A

Food Foams

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3
Q

Definition of emulsion

A
  • 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
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4
Q

Function of emulsifier

A

coats the emulsion droplets and prevents them from coalescing or combining with each other

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5
Q
  • 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
A

Colloidal Dispersion

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6
Q
  • A colloid that pours
  • a two-phase system with a solid dispersed phase in a liquid continuous phase
A

sol

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7
Q
  • a two-phase system containing an elastic solid with a liquid dispersed phase in a solid continuous phase
A

gel

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8
Q
  • a substance that enables two normally immiscible liquids to be mixed together without separating on standing
A

emulsifier

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9
Q

Classifications of emulsifiers

A
  • cationic
  • anionic
  • non-ionic
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10
Q

Two liquid/gas droplets merge/merging to form one larger droplet

A

coalescence

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11
Q

O/W Emulsion Definition

A
  • more common
  • continuous phase = water
  • Ex. salad dressings, mayonnaise, cake batter, frozen desserts
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12
Q

W/O Emulsion Definition

A
  • continuous phase: oil
  • Ex. butter, margarine, some icings
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13
Q

How does surface tension work?

A
  • 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
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14
Q
  • 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
A

surface tension

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15
Q
  • when a surface is between two liquids
  • ex. water and oil
A

interfacial tension

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16
Q

The higher the tensions the ___________ it is to mix two phases together

A

harder

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17
Q
  • 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
A

Surface-Active Molecules / Surfactants

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18
Q

Structure of Surfactants

A
  • 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
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19
Q

Method of action of surface-active molecules

A
  • molecule ADSORBS at the surface
  • molecule REDUCES the attractive forces of the water molecules for THEMSELVES
  • makes it easier to EXPAND/spread the surface
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20
Q

Definition of adsorb

A

to bind to a surface

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21
Q

Non-Food Surfactants

A
  • Detergents
  • Water flows over surface, forming thin sheet
  • Water DOESN’T gather in droplets
  • Not used as food ingredients
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22
Q

Food Surfactants

A
  • Proteins
  • Lipids
  • Some spices
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23
Q

Characteristics of proteins used as food surfactants

A

depends on AA composition: whether they are hydrophilic or hydrophobic

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24
Q

Lipids as food surfactants

A
  • Ex. lecithin
  • polar head and apolar tail
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25
Q

Spices as food surfactants

A
  • dry mustard
  • dry paprika
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26
Q
  • 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
A

Non-Surface-Active Molecules

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27
Q

Formation of emulsion

A
  • 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
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28
Q

Types of emulsifiers used in O/W Emulsions

A
  • emulsifiers that are more easily dispersed in water
  • these tend to reduce the interfacial tension of the water more than that of oil
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29
Q

Types of emulsifiers used in W/O Emulsions

A
  • emulsifiers that disperse more readily in oil phase
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30
Q

Principles of formation of a stable O/W Emulsion

A
  1. disperse emulsifier in aqueous phase
  2. oil is added, and interfacial tension of each liquid is reduced by the emulsifier
  3. beating and homogenization break the oil phase into droplets surrounded by water
  4. small droplets are formed, protected by an interfacial layer of emulsifier
  5. interfacial area of the oil becomes very large, aqueous phase spreads to surround each oil droplet
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31
Q

Functions of emulsifier

A
  1. reduce interfacial tension
  2. formation of a stable film that protects the emulsion droplets and prevents separation of the emulsion
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32
Q

Three fates of droplets after collision in an emulsion:

A
  1. film stretches/breaks, droplets coalesce into a larger droplet
  2. the emulsifier layers surrounding the droplets interact and an aggregate is formed
  3. the droplets move apart again
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33
Q

an unstable system with a greater concentration of a material in solution than would exist at equilibrium

A

supersaturation

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34
Q

dissolution of small crystals or sol particles and the redeposition of the dissolved species on the surfaces of larger crystals or sol particles

A

Ostwald ripening

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35
Q
  • 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
A

Coalescence

36
Q

Process whereby dispersed molecules or particles form aggregates

A

Aggregation

37
Q
  • 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
A

Creaming

38
Q
  • 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
A

Completely coated emulsion-

39
Q
  • an emulsion that is more likely to coalesce
  • formation of large droplets
  • ultimately in the separation of the emulsion
A

incompletely coated emulsion

40
Q

Characteristics of emulsifiers that facilitate the emulsion formation

A
  • adsorb at the interface between two liquids
  • reduce the interfacial tension of each liquid, enabling one liquid to spread more easily around the other
41
Q

Characteristics of emulsifiers that promote emulsion stability

A
  • form a stable, coherent viscoelastic interfacial film
  • prevent or delay coalescence of the emulsion droplets
42
Q

All emulsifiers are ___________

A

surfactants

43
Q

Not all surfactants make good ______________

A

emulsifiers
- not all surfactants are able to form stable film at the interface and prevent coalescence

44
Q
  • 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
A

Large macromolecules

45
Q
  • 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
A

small molecules

46
Q

Difference between an emulsifier and a surfactant

A
  • 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
47
Q

Why are proteins the best natural emulsifiers

A
  • 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
48
Q

Examples of natural emulsifiers

A
  • egg yolk proteins
  • milk caseins
  • meat proteins
  • soy proteins
  • lecithin
49
Q

Egg yolk proteins as natural emulsifiers

A
  • lipoproteins are associated with each other
  • associate with phospholipids (lecithin) in micelles
50
Q

Milk caseins as natural emulsifiers

A
  • present at fat globule surface
  • give the stability of homogenized milk
51
Q

Lecithin as natural emulsifiers

A
  • 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
52
Q

Synthetic Emulsifiers and Surfactants

A
  • 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
53
Q

Method of action of small-molecule surfactants

A
  • 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
54
Q

Surfactants with low HLB

A
  • 3-6
  • more hydrophobic/lipophilic
  • used to form w/o
  • Ex. glycerol monostearate, sorbitan monostearate
55
Q

Surfactants with high HLB

A
  • 8-18
  • more hydrophilic
  • o/w emulsions
  • ex. polyoxyethylene sorbitan monostearate (TWEENS 60), sodium stearoyl-2-lactylate
56
Q
  • unstable emulsion that separates fairly soon after formation
  • Ex. french dressing: combine ingredients and shake vigorously —-> oil droplets are not protected and soon coalesce
A

Temporary Emulsions

57
Q
  • 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
A

Permanent Emulsions

58
Q

Milk as an emulsion

A
  • 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
59
Q

Factors affecting emulsion stability

A
  • sufficient emulsifier
  • droplet size
  • changing pH
  • viscosity –> addition of hydrophilic gums
  • storage and handling
  • temperature
  • violent shaking
60
Q

How sufficient emulsifier affects emulsion stability

A

sufficient amount needed to completely coat the surface of all the droplets in order to ensure stability

61
Q

How droplet size affects emulsion stability

A
  • important because larger droplets are more likely to coalesce
62
Q

How changing the pH affects emulsion stability

A
  • adding acid/changing the ionic strength by adding salts
  • may reduce the stability of interfacial film (especially if it is made of protein)
63
Q

How viscosity affects emulsion stability

A
  • 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
64
Q

How storage and handling affects emulsion stability

A
  • emulsions will break into two phases with wrong handling conditions
65
Q

How temperature affects emulsion stability

A
  • 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!)
66
Q

How violent shaking affects emulsion stability

A
  • likely to disrupt emulsions
67
Q
  • 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
A

foam

68
Q

Density difference between two phases in foams

A
  • much larger density difference than in emulsions
  • tendency for:
    1) liquid continuous phase to drain due to gravity
    2) gas bubbles to escape
69
Q

Difference of droplets between foams and emulsions

A
  • Foam: bigger
  • Emulsion: smaller
70
Q

Difference of continuous phase between foams and emulsions

A
  • foam: thinner
  • emulsion: thicker
71
Q

Mechanism of action of whipping in foam formation

A
  • 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
72
Q

Mechanism of action of foaming agent in foam formation

A
  • 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
73
Q

How is foam stability measured?

A
  • 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
74
Q

Foam stability factors

A
  • 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
75
Q

Prevention of loss of foam stability

A
  • 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
76
Q
  • the ability of the foaming agent to adsorb at the interface
  • the ability of reducing interfacial tension
  • amount of energy during whipping
A

volume of foaming agents

77
Q
  • the ability of the foaming agent to produce a stable interfacial film and a viscous continuous phase
A

stability

78
Q

Qualities of a good foaming agent

A
  • easily adsorb at the interface
  • reduce interfacial tension
  • form a stable interfacial film that resists rupture
79
Q

Are all surfactants able to form stable foams?

A

no
- but all surfactants are able to reduce surface tension and produce foams

80
Q

Egg white as a foaming agent

A
  • 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
81
Q

Gelatin as a foaming agent

A
  • 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
82
Q

Effect of sugar on foam stability

A
  • increases liquid viscosity
  • protects proteins from excessive denaturation and aggregation at the interface
83
Q

Effect of acid on foam stability

A
  • reduces pH —> reduces charge on protein —> stronger and more stable interfacial film
  • delay foaming process —> added at the end
84
Q

Effect of gums thickening agents on foam stability

A
  • increase viscosity
85
Q

Method of action of foam suppressants

A
  • 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
86
Q

Method of action of anti-foaming agents

A
  • 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

Food Science (5 decks)

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