Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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0054 Cardiac arrhythmias The question of whether caf-

feine consumption increases the frequency or severity

of ventricular arrhythmias is still partially unresolved

as there may be particularly sensitive patients: the

consensus is now that a ‘moderate’ coffee consump-

tion, corresponding to 200–500 mg of caffeine per

day, is unlikely to increase the frequency of arrhyth-

mias both in healthy subjects and in patients suffering

from tachycardia or fibrillation.

Other Health Questions Associated with Coffee

Consumption

0055 Coffee and the gastrointestinal function According

to a complete review of all the available data

‘ . . . there is no evidence for an association of coffee

ingestion with peptic ulcer disease or functional dys-

pepsia.’ The increase in heartburn due to a reduction

of the lower esophagal sphincter pressure and conse-

quent gastric acid reflux, associated with both regular

and decaffeinated coffee consumption by sensitive

individuals, has not yet been explained, and no clear

link has been shown with the presence of specific

substances in the coffee brew. An increase of the distal

colon motility by about 30% of subjects within min-

utes after coffee consumption has been described in

one study. In another study, consumption of two cups

of coffee after an overnight fast produced no change

in mouth-to-cecum transit time in healthy subjects.

Coffee consumption produces contractions of the

gallbladder, as is also the case with other foodstuffs.

The size of the population suffering gastrointestinal

discomfort after drinking coffee, the symptoms them-

selves, and the mechanism remain unclear, with the

possible exception of dyspeptic complaints of patients

suffering from duodenal ulcer.

See also: Caffeine; Calcium: Properties and

Determination; Cancer: Diet in Cancer Prevention;

Coffee: Physiological Effects; Dietary Fiber:

Physiological Effects; Flavor (Flavour) Compounds:

Structures and Characteristics; Hypertension:

Physiology; Magnesium; Mycotoxins: Occurrence and

Determination; Niacin: Physiology; Phytic Acid:

Nutritional Impact; Polycyclic Aromatic Hydrocarbons;

Potassium: Properties and Determination; Zinc:

Physiology

Further Reading

Boekema PJ (1999) Coffee and gastrointestinal function:

Facts and fiction – A review. Scandinavian Journal of

Gastroentherology Suppl. 230.

Bucheli P and Taniwaki MH (2002) Review – Research on

the origin, and on the impact of post-harvest handling

and manufacturing on the presence of ochratoxin A in

coffee. Food Additives Contaminants 19: 655–665.

Clifford MN (1999) Review – Chlorogenic acids and other

cinnamates – nature, occurrence and dietary burden.

Journal of the Science of Food Agriculture 79: 362–372.

Committee on Diet and Health, National Research Council

(1989) Diet and Health, pp. 465–471.Washington, DC:

National Academy Press.

Martin PR, De Paulis T and Lovinger DM (2001) Non-

caffeine di-cinnamoylquinide constituents of roasted

coffee inhibit the human adenosine transporter. Proceed-

ings of the 19th ASIC Conference, CD-rom.

Nehlig A (1999) Are we dependent upon coffee and caf-

feine? A review on human and animal data. Neurosci-

ence and Biobehavioral Reviews 23: 563–576.

Schilter B, Holzha

user D and Cavin C (2001) Health bene-

fits of coffee. Proceedings of the 19th ASIC Conference,

CD-rom.

Viani R (2000) Coffee: Ullmann’s Encyclopedia of Indus-

trial Chemistry, CD-rom.

Walker R (1997) Quality and safety of coffee. Proceedings

of the 17th ASIC Conference, pp. 51–60.

Cognac See Brandy and Cognac: Armagnac, Brandy, and Cognac and their Manufacture; Chemical

Composition and Analysis of Cognac

Cole Crops See Vegetables of Temperate Climates: Commercial and Dietary Importance; Cabbage and

Related Vegetables; Leaf Vegetables; Oriental Brassicas; Carrot, Parsnip, and Beetroot; Swede, Turnip, and

Radish; Miscellaneous Root Crops; Stem and Other Vegetables

1516 COFFEE/Physiological Effects

COLLOIDS AND EMULSIONS

M M Robins and P J Wilde, Institute of Food

Research, Norwich, UK

Introduction

0001 In the strictest sense, colloids are dispersions of two

or more immiscible materials, where the characteris-

tic length of the inclusions (particles) is between 1 nm

and 1 mm. However, in foods, the term ‘colloids’ gen-

erally includes dispersions containing larger particles,

and foams, where the bubble size may approach milli-

meters. Food dispersions thus encompass suspensions

such as sugar fondant (solid sugar particles in a satur-

ated sugar solution), emulsions such as milk, cream,

sauces, and spreads, and foams such as are found in

bread, beer, and icecream. Icecream is an archetypal

food dispersion, containing dispersed particles of

solid (sugar and ice), liquid (fat), and gas (air). The

key property that these foods have in common is the

presence of particles, and the consequent high inter-

facial area between the particles and the continuous

phase. The properties of food colloids are dominated

by the particles and their interactions, which in turn

are controlled by the properties of the interfacial

region.

0002 Although other food-relevant systems may contain

microscopic particles, such as suspensions of yeast

used in brewing, this article is confined to inanimate

colloids whose composition and properties are formed

naturally, as in milk, or deliberately engineered to

manufacture a particular food, such as a dressing.

0003 Figure 1 shows examples of the structure of the

common forms of food colloid. Suspensions are

usually formed by crystallization processes, which

are outside the scope of this article (See Crystalliza-

tion: Basic Principles). Similarly, foams have charac-

teristic behavior and properties that are covered more

fully in a companion article. Here we focus on emul-

sions, which are the most common type of food col-

loid.

0004 Emulsions are liquid-based dispersions containing

droplets of oil or water dispersed in water or oil

respectively. In order to maintain the droplet struc-

ture, the oil–water interface is coated with a layer of

emulsifier. In oil-in-water (O–W) emulsions the sta-

bility is highly dependent on the emulsifier. However,

in water-in-oil (W–O) systems (such as butter) the

continuous phase is partially solid, which enhances

stability and gives the food a firm texture. Crystal-

lization of the dispersed phase may also be used to

generate texture, but in an indirect way, as partial

crystallization of the oil droplets in cream and ice-

cream enhances their whipping properties.

0005Below we outline the composition, formation, and

characterization of food emulsions and refer briefly

to their safety and nutritional aspects.

Formulation

0006Food emulsions can be simple O–WorW–O systems,

or multiple emulsions where the dispersed phase is

already an emulsion (W–O–W, O–W–O). They often

contain a complex mixture of ingredients including

proteins, polysaccharides, emulsifiers, water, and oil.

Each of these ingredients contributes to the overall

properties of the emulsion, as detailed in the

following section.

Aqueous Phase

0007The role of the aqueous phase is chiefly as a solvent to

the water-soluble components (salts, emulsifiers, pro-

teins, polysaccharides, etc.), as either the continuous

phase in O–W emulsions or as the dispersed phase

in W–O emulsions. Water molecules possess strong

orientation-dependent attractive hydrogen bonds,

which result in a high interfacial tension. This makes

breaking the dispersed phase into fine droplets diffi-

cult. Thus, the interfacial tension forces need to be

reduced by the presence of emulsifiers. The pH, ionic

strength, and emulsifier concentration influence the

formation and physical characteristics of the emul-

sion, by altering the size of droplets and the inter-

actions between them. The viscosity of the aqueous

phase can be altered by the addition of hydrocolloids

(gelling or thickening agents, usually polysaccharides

such as starch or xanthan gum), which can also affect

emulsion formation, rheology, and stability. This is

utilized extensively in low-fat O–W emulsion-based

food products such as dressings, yogurts, and

desserts.

Oil Phase

0008The oil phase in food emulsions, whether from animal

or plant sources, is mainly in the form of triglycerides.

The oil also contains levels of di- and monoglycerides,

polar lipids, and free fatty acids. The latter three

lipids tend to be surface-active, more water-soluble,

and sometimes used as food emulsifiers. The three

most common fatty-acid chains are the saturated

palmitic (C16) and stearic (C18) and the unsaturated

oleic (C18:1) and linoleic (C18:2). Plant oils tend to

COLLOIDS AND EMULSIONS 1517

contain more unsaturated fats than those of animal

origin (with the exceptions of coconut and palm oils),

and therefore tend to be liquid at room temperature.

Fat is an essential nutrient, and the oil phase imparts a

characteristic mouth feel to products such as cream,

butter, and cheese. The oil phase is also used as a

solvent for the more lipophilic emulsifiers, oil-soluble

nutrients (e.g., oil-soluble vitamins and essential fatty

acids) and some flavor and aroma compounds.

0009 An important property of the oil phase is the

solidification or crystallization of the fat. Solid fat in

an emulsion has different rheological and textural

properties to liquid oil, particularly in W–O emul-

sions. In addition, fat crystals can form structures

that change the texture of fat-continuous products

such as chocolate, butter, and cheese. Crystallization

of the fat in the dispersed phase of ice cream and

whipped cream allows the oil droplets to form solid

networks that enhance structure and texture. The

viscosity of the dispersed oil phase is also important

for homogenization, as droplets made up of high-

viscosity oil are less deformable and therefore more

difficult to homogenize into smaller droplets.

Emulsifiers

0010The role of emulsifiers is to form an adsorbed layer

around the emulsion droplets. This adsorbed

layer lowers the interfacial tension, which aids

emulsification, and stabilizes the droplets against

flocculation and coalescence. Emulsifiers need to be

surface-active, so that they will naturally assemble at

an interface. They therefore need to be amphiphilic

(i.e., possess both hydrophilic and hydrophobic

parts). Emulsifiers can be either low-molecular-weight

Foam

(e.g. beer)

Key

Gas bubble

Aqueous phase

containing

stabilizers, salts,

flavors,

preservatives, etc.

Emulsifier layer

around droplets.

Oil phase

containing oil-

soluble flavors and

nutrients.

Fat crystals

Sugar crystals

Oil-in-

water

emulsion

(e.g. salad

cream)

Water-in-

oil

emulsion

(e.g.

margarine)

Suspension

(e.g.

chocolate)

fig0001 Figure 1 Types of food colloid and their structures.

1518 COLLOIDS AND EMULSIONS

emulsifiers or macromolecular polymers, as shown in

Figure 2. The low-molecular-weight emulsifiers have

a small hydrophilic head group and one or more

hydrocarbon chains; they include mono- and digly-

cerides, lecithins, polysorbates, and sugar esters. The

most common polymeric emulsifiers in food systems

are proteins, mainly from milk, but other sources

include soya, meat, and fish, depending on the appli-

cation. Other polymeric emulsifiers include modified

cellulose and block copolymers, although block co-

polymers are not approved for food use.

Stabilizers

0011 Although emulsifiers help stabilize emulsions, the term

‘stabilizer’ in the context of emulsions is specifically

aimed at polymers which are added to the aqueous

continuous phase to improve the stability of O–W

emulsions. These polymers are often in the form of

gums such as xanthan, guar, carageenan, etc. Addition

of the polymers in sufficient concentrations to form a

network results in the effective encapsulation of the

emulsion droplets by the polymer. This immobilization

prevents flocculation of the droplets and reduces

creaming.Lowerconcentrationsofpolymercaninduce

flocculation by the depletion mechanism. Depending

on the conditions, this flocculation can create a solid

network of emulsion particles with enhanced viscoe-

lastic properties (e.g., ice cream and whipped cream).

Under different conditions, flocculation by polymers

can induce rapid creaming or sedimentation.

Other Ingredients

0012 From a purely physical standpoint, we tend to ignore

components in an emulsion that do not contribute

to its physical characteristics. However, in food

emulsions the physical properties are not the only

issue: flavor, sensory, and safety characteristics are

also important. To this end, food emulsions contain

salts, sugars, flavors, colors, and preservatives. How-

ever, these components highlight issues in colloids

and emulsions that would not normally be of concern

in other food products. Salt is perhaps the most im-

portant component as this can affect the solubility of

functional ingredients such as proteins and polysac-

charides. Specific interactions between, for example,

calcium and caseins cause precipitation of the pro-

teins. Salt also affects the electrostatic colloidal sta-

bility by screening the electrostatic repulsion forces

between particles. Sugars are usually inert as far the

physical properties of emulsions are concerned, but

they can lower the dielectric constant of the continu-

ous phase, which affects colloidal interactions, and

certain sugars, such as trehalose, are known to pro-

tect proteins against denaturation during heating and

drying. Flavors, colors, and preservatives are usually

present at relatively low levels, and generally do not

influence the physical characteristics. However, some

components are volatile and partitioning can often

occur, from one phase to the other. It is important to

understand these processes, for example for efficient

use of preservatives, so that they remain in the phase

that allows them to achieve the desired function.

Formation

0013Formation is clearly a critical stage in the life of an

emulsion. Many of the physical parameters of the

emulsion are established here. If the emulsion is to

remain stable for significant periods, then the emul-

sion droplets need to be small enough to slow down

Hydrophobic groups exposed to oil phase

Unfolding

Adsorption

Aqueous phase

Oil phase

(a) (b)

Diffusion

Hydrophobic

chains at core

of micelle

Hydrophobic

groups at core

of molecule

fig0002 Figure 2 Emulsifier structure and adsorption of (a) proteins/polymers and (b) low-molecular-weight emulsifiers/surfactants.

Hydrophobic groups are shaded in gray. Proteins form a folded structure in solution, keeping the hydrophobic amino acids away

from aqueous solution. Upon adsorption, they unfold to expose these groups to the oil phase. Similarly, surfactants and emulsifiers

form micelles, holding the hydrophobic chains in the core of the structure, which then dissociate and adsorb to the interface.

COLLOIDS AND EMULSIONS 1519

the creaming and separation processes (typically

<1mm diameter), which will be discussed later. The

main factors which determine the ‘quality’ of the

newly formed emulsion which will be discussed here

are: (1) the adsorption of emulsifiers to the droplet

surface; (2) the homogenization process; and (3)

stability during formation.

Adsorption

0014 Emulsion formation requires the dispersion of one

phase into small droplets. This results in a massive

increase in interfacial area between the dispersed and

continuous phase. Classically, interfacial tension is

defined as the work required to increase the surface

area by a unit amount. The interfacial tension be-

tween the two phases is therefore extremely import-

ant. Interfacial tension forces act on droplets to

minimize surface area, hence, liquid droplets form

spheres in the absence of other forces. Lowering the

interfacial tension means that less energy is required

to increase the interfacial area by a unit value. This is

achieved through the use of emulsifiers, which adsorb

at the interface between the two phases and lower the

interfacial tension by disrupting the hydrogen bonds

between the water molecules at the interface. Emulsi-

fiers can be simple lipid-like molecules or complex

macromolecules such as proteins; however, they are

all amphiphilic so that they naturally align themselves

at the interface, as shown in Figure 2. The interfacial

tension is determined by the structure of the emulsi-

fier and the interfacial concentration, therefore the

most effective emulsifiers are those whose structure

allows the formation of a dense, close-packed inter-

facial layer. The dynamics are also very important

because the homogenization process is rapid, so it is

the dynamic interfacial tension over time scales < 1 s

that is most important, not the equilibrium value.

Rapid adsorption of the emulsifier to lower the inter-

facial tension will thus result in more effective

emulsification of the dispersed phase during the

homogenization process.

Homogenization

0015Homogenization is the process by which the dis-

persed phase is broken up into small droplets (See

Agitation). Normally, rapid mixing of the ingredients

is used to create a coarse premix. This is sufficient to

break up the dispersed phase into large droplets, and

allow adsorption of the emulsifiers prior to final

homogenization. Figure 3 shows the three main

methods of homogenization. Mechanical methods

such as rotor–stator systems induce high shear fields

to break up droplets. High-pressure homogenizers are

now very common, and simply force the premix

through a narrow orifice or valve at high pressures

(typically 10–100 MPa or 1500–15 000 lb in

2

). For-

cing the emulsion through a valve at high pressure

creates turbulence and very high shear forces, thus

breaking up the droplets. However, shear forces are

not the most efficient way of breaking up droplets, as

the shear field tends to lose energy rotating the drop-

let. Forcing the droplets through a simple orifice in-

duces elongational flow forces that break up droplets

more efficiently. The droplet size produced is, within

certain boundaries, generally inversely proportional

to the energy input during homogenization. More

accurately, it is related to the energy dissipated into

droplet break-up. This is affected by the interfacial

tension, and viscosities of the dispersed and continu-

ous phases. As continuous-phase viscosity increases,

Stator

Outlet

Rotor

Motor drive

(a) (b) (c)

Valve

Outlet

High pressure

Continuous phase

Controlled pore

membrane

Dispersed

phase

Inlet

fig0003 Figure 3 Basic principles of homogenization methods. (a) Rotor–stator device or colloid mill. The rotor, incorporating grooves or

blades, spins at high speed, generating high shear fields to break up droplets. (b) High-pressure homogenizers force the droplets

through narrow valves or orifices to break up droplets through high shear or elongational forces. (c) Porous membranes are used to

extrude the dispersed phase into the flowing continuous phase.

1520 COLLOIDS AND EMULSIONS

droplet size will decrease, yet as dispersed-phase vis-

cosity increases, the droplets become more difficult to

break and droplet size consequently increases. The

most energy-efficient process is the microporous ap-

proach, where the dispersed phase, or premix, is

forced through a porous substrate into the continuous

phase. The droplet size produced is dependent on the

interfacial tension and the pore sizes. These micro-

porous methods are still being developed and have

not yet been widely utilized in the food industry, due

to the low throughput.

Stability during Formation

0016 Coalescence or fusing of droplets is a major instabil-

ity process in emulsions, which will be discussed later.

During formation, droplets are exposed to aggressive,

turbulent conditions designed to break up droplets.

These conditions also serve to increase the number

and intensity of collisions between droplets. This in-

creases the probability of recoalescence, particularly

during the critical period when the adsorption process

is not complete. Therefore, emulsifiers that adsorb

slowly are particularly prone to this problem, and if

the homogenization method is not optimized, then

energy is wasted through recoalescence after break-

up. This problem can be reduced through the control

of the hydrodynamic conditions immediately after

homogenization. This can be achieved by gently dis-

persing the droplets into a larger volume by either

hydrodynamic methods or inert filler particles. This

increases the interdroplet distance and reduces the

probability of coalescence. However, in a commercial

process it may not be convenient to form dilute emul-

sions and other strategies may be needed to produce

acceptable emulsions.

0017 In summary, by controlling the emulsifier type,

concentration, and homogenization conditions, it is

possible to control the droplet size distribution. It

is the size and physical properties of the droplets

which are the principal factors controlling the behav-

ior of the whole emulsion, such as the rheology,

stability, texture, and other quality characteristics of

food emulsions, as described in the next section.

Characterization and Properties of

Colloids and Emulsions

Particle/Droplet Size

0018 The size of the dispersed particles or droplets is a key

factor in the properties and stability of the dispersion.

For example, for a product to appear turbid, the

constituent particles must be able to scatter visible

light, which means that their diameter needs to be at

least 0.4 mm. However, very large particles tend to be

less stable to the processes of flocculation (See Floc-

culation) and creaming, or impart a gritty mouth feel

to the product. The particles in colloidal foods are

generally very polydisperse, with a factor of 10 or

more between the sizes of the largest and smallest

particles present. It is thus very important to generate

the optimum particle size distribution for each prod-

uct, and to insure that it does not change during

storage.

0019Typically, O–W food emulsions contain oil drop-

lets of diameter in the range 0.5–2 mm, and W–O

products such as spreads contain water droplets up

to 20 mm. Solid dispersed particles, such as sugar

crystals in chocolate, have diameters in the range

0.1–50 mm, although grittiness is detectable when

the particles exceed about 20 mm.

0020Measurement of the particle size in colloidal foods

or emulsions depends on the type of product and the

size range of interest. Most foods can be examined

microscopically, using optical methods for larger

particles (> 3 mm) and electron microscopy in the sub-

micron range. Microscopy is ideal for qualitative as-

sessments of particle size and shape (See Microscopy:

Light Microscopy and Histochemical Methods; Scan-

ning Electron Microscopy; Transmission Electron

Microscopy; Image Analysis), but quantitative meas-

urement of a size distribution is less straightforward.

Other direct methods such as sieving are convenient

only for solid particles at the larger end of colloidal

sizes. Indirect particle-sizing methods include optical

and electrical techniques. The most common tech-

nique for droplet size determination is laser diffrac-

tion, based on the principle that the angle by which

light is scattered from a particle is (inversely) related

to its size. Measuring the intensity of light at a range

of scattering angles enables the size distribution of the

scattering particles (droplets) to be estimated. An-

other popular method for characterizing particle size

distribution is the electric sensing-zone technique. In

this method the particles are passed singly through a

small gap between electrodes, and the volume of the

particle is estimated from the change in capacitance

during transit.

0021The light-scattering and electrical capacitance tech-

niques both require very low particle concentrations

(typically < 0.01% v/v), and recently there have been

developments in methods that can characterize

droplet size in concentrated emulsions. The principal

techniques are acoustic, since ultrasound propagates

readily through concentrated dispersions and detailed

analysis of the frequency-dependent attenuation spec-

trum of ultrasound enables estimation of the particle

sizes present. A similar technique is electroacoustics,

where (charged) particles in a concentrated dispersion

are made to oscillate in an electric field, which

COLLOIDS AND EMULSIONS 1521

generates sound waves whose characteristics are re-

lated to the particle charge and size.

Stability

0022 Emulsions are inherently unstable and, given suffi-

cient time, will eventually separate into bulk layers

of oil and water. However, temporary stability can be

achieved, and foods are engineered to remain com-

paratively stable for the required shelf-life. The mech-

anisms by which emulsions change in structure with

time are shown in Figure 4. The figure also shows

schematic drawings of the structural changes in the

emulsions due to each type of instability. It is clear

that creaming and flocculation involve the physical

rearrangement of droplets into a layer or clumps

(aggregates) respectively, whereas coalescence and

disproportionation exhibit changes in the primary

droplet size. Since emulsions are not thermodynamic-

ally stable systems, there are slow changes taking place

from the start, and eventually the droplets grow to

such an extent that the emulsion breaks into bulk

layers of the component phases.

0023The interaction forces between droplets are crucial

for the stability of the emulsion. If the droplets are

highly charged, and there is a low ion concentration

in the continuous phase, the droplets experience

repulsive forces when they approach each other,

and these forces keep them separated as individual

Instability

type

Creaming

Flocculation

Coalescence

Disproportion-

ation

Schematic mechanism

Factors promoting

instability

Large density difference

between droplets and

continuous phase; large

droplets; low continuous-

phase viscosity

Droplets move

under gravity

to the top

(oil-in-water)

or base

(water-in-oil)

of the container

Droplets clump

together into

aggregates, but

retain original

droplet size

Droplets clump

together and

then merge to

form larger

droplets

Swelling of

larger droplets

by molecular

transfer of oil

from smaller

droplets, which

eventually

disappear

Attractive interactions

between droplets; low

charge and/or high ionic

strength; presence of

adsorbing polymer to bridge

between droplets; presence of

nonadsorbing polymers to

induce depletion

flocculation

Flocculation or creaming to

bring droplets together; an

interfacial layer that

ruptures easily

Polydispersity in droplet

size; significant solubility of

disperse phase (oil) in

continuous phase; excess

emulsifier; high interfacial

tension

fig0004 Figure 4 Emulsion instability processes.

1522 COLLOIDS AND EMULSIONS

particles. If an emulsion is stable to flocculation on a

certain time scale, coalescence is also prevented

because the droplets are repelled before the interfaces

get near enough to merge. Certain additives used

in food, e.g., polysaccharide ‘stabilizers’, may cause

flocculation of the droplets in order to impart a par-

ticular texture, and then the interfacial composition is

important in preventing coalescence of the adjoining

droplets. The interfacial properties that are important

in preventing coalescence of droplets in close contact

are steric hindrance, interfacial viscoelasticity (of

polymers and proteins), and interfacial mobility (of

emulsifiers and surfactants).

Rheology

0024 The rheological properties of emulsions are import-

ant functional properties, both during processing and

in the final product where the texture is an important

component of mouth feel. The response of the emul-

sion to mechanical stress may mimic simple liquids,

as in dilute emulsions such as milk. At the other

extreme, they may be designed to impart a solid-like

texture, as in mayonnaise.

0025 The rheological properties are controlled primarily

by the concentration and size of the droplets, and by

the composition of the continuous phase. (See Rhe-

ology of Liquids). Emulsions of low droplet con-

centration, where the droplets are unflocculated

and the continuous phase is a simple liquid, will be

of low, constant viscosity at a given temperature

(Newtonian behavior). As the droplet concentration

increases, and/or flocculation occurs, the emulsion

becomes progressively more non-Newtonian. Its

response to flow is now dependent on the magnitude

of the applied stress, and the time scales of applica-

tion. These characteristics are deliberately designed

into many products, so the food rests on a plate

without spreading, but is capable of flowing when

dispensed. The apparent lack of response to a low

stress (such as gravity) is sometimes attributed to the

presence of a ‘yield’ stress, implying that up to a

certain stress level the emulsion is solid-like and

flows only when the applied stress exceeds a critical

value.

Flavor

0026 Controlling the flavor of foods is described elsewhere,

but in emulsions the droplet composition and struc-

ture are additional variables. In many traditional

foods, flavor components are present in both the oil

and aqueous phases, and the full flavor is only

released by droplet breakdown in the mouth. The

droplet size and interfacial composition (emulsifiers)

are also important factors.

Safety

0027Microbiological food safety relies on either steriliza-

tion or the existence of chemical conditions hostile

to microbial growth. In an emulsion the foodborne

bacteria can only grow in the aqueous phase, but the

oil phase can affect the chemical environment for

growth. This is because many antimicrobial agents,

e.g., organic acids, are able to partition into the oil

phase, thus reducing their preservative action in the

aqueous phase. Food manufacturers are increasingly

taking emulsion structure into account when formu-

lating preservative systems.

Nutrition

0028The dietary impact of emulsions is the high fat

content. Reducing fat intake is the principal target

in the fight to reduce chronic heart disease in the

developed world. Therefore one approach is to

reduce the fat content in food emulsions, but retain

the texture and flavor of the original product. A

simple way is to increase the viscosity of the pro-

duct with thickeners; however, the rheological behav-

ior and hence perceived texture is very different. A

better way is to produce dispersed droplets that

behave like emulsion droplets, but with a lower fat

content, by using gels, coacervates, or multiple emul-

sions (oil droplets containing water droplets). In

margarines and spreads, large volumes of water can

be dispersed into the fat continuous phase through

the use of appropriate emulsifiers and processing.

Another approach is to use fat replacers, which

are not broken down and digested in the gut; how-

ever, use is limited due to other physiological side-

effects.

0029Another important nutritional factor is the absorp-

tion of oil-soluble nutrients. Oil-soluble vitamins and

antioxidants are crucial for health. They are thought

to protect against heart disease and cancer. Their

successful absorption is dependent on the processing

of the raw materials, emulsion structure, and inter-

facial composition. Colloid science is now an import-

ant part of trying to understand the mechanisms

involved.

See also: Aerated Foods; Crystallization: Basic

Principles; Emulsifiers: Organic Emulsifiers; Phosphates

as Meat Emulsion Stabilizers; Uses in Processed Foods;

Fats: Production of Animal Fats; Uses in the Food

Industry; Flocculation; Homogenization; Margarine:

Types and Properties; Milk: Liquid Milk for the Consumer;

Processing of Liquid Milk; Physical and Chemical

Properties; Rheology of Liquids; Stabilizers: Types and

Function

COLLOIDS AND EMULSIONS 1523