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

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are much more sensitive than the conventional ELISA

tests. Still, due to specialized training requirements,

expensive equipment, and access demands to DNA

sequence databases, the techniques are not used

routinely unless there is no other method adequate

to solve the problem.

0049 Following the DNA changes, treatments that

induce DNA fragmentation can be identified.

0050 To determine the cultivar, the restriction-fragment

length polymorphism (RFLP) concept is applied. Re-

striction enzymes are used to cut the strands of

DNA at specific points where a specific sequence of

nucleotides is recognized. The fragments separated by

microelectrophoresis are stained or characterized

using specific radioactive gene probes. The presence

of common wheat in durum wheat is based on a

specific DNA sequence on the D-genome, a set of

chromosomes found only in common wheat.

0051 DNA-fingerprinting analysis can be used to iden-

tify fish species, but extensive databases on the

sequence of DNA of many commercially important

species are a prerequisite.

0052 Detection of inserted DNA is based on a compari-

son with the genetically modified and nongenetically

modified material. Several controls are necessary to

ensure that the PCR reaction is successful. Products

such as coarsely ground cereals, flour, soya lecithin,

unrefined fats, chocolate, soy sauce, tofu, cakes

and other baked products, cornflakes and popcorn,

meat and sausages, tomato, and ketchup are among

the products that can be tested for illegal labelling or

admixture with genetically modified organisms.

Detection of Irradiated Foods

0053 Methods to identify whether or not a food product

has been treated with ionizing radiation without re-

quiring comparison with the respective non-treated

counterpart are based on different principles, e.g.,

thermoluminescence, electron spin resonance (ESR)

and gas chromatography of irradiation-induced com-

pounds in fats. In 1996, the European Committee for

Standardization (CEN) adopted five standards for

methods to detect irradiated foods. The standards

are applicable to herbs and spices, shrimps and

prawns, frog legs, poultry and other meats, liquid

egg, fruits, and vegetables.

0054 Thermoluminescence methods (EN 1788) are

based on the emission of light when energy trapped

in crystalline lattices during irradiation is released by

heat. In food products, such lattices are silicate min-

erals originated from exposure to wind or soil (herbs

and spices) or other sources such as the minerals from

the sea sand found in the intestine of shrimps. ESR

detects stable radiation-specific radicals produced

upon irradiation of food, which are trapped in solid

and dry components such as bones or cellulose. ESR

does not apply equally effectively to all foods. CEN

standard 1786 ‘for food containing bone’ was valid-

ated for beef, chicken, and trout bones. The favorable

detection limit of the method permits the identifica-

tion of radiation, even in complex cases (e.g., chicken

burgers). Radicals from cellulose are also used as a

means for identification in some plant products,

but only when the signal is positive (CEN 1787).

The sensitivity of the method is dose-dependent.

Further validation of the procedure is expected for

foods containing crystalline sugars and crustaceae.

Gas-chromatographic determination of radiolytic

hydrocarbons (EN 1784) or 2-alkylcyclobutanones

(EN1785) seems very useful in detecting irradiation

of products containing even low levels of lipids.

Detection of ‘Organic’ Foods

0055No chemical or other method seems potent enough to

distinguish foods produced organically from conven-

tional foods. Therefore, an extensive and formalized

system of documentation, inspection, and certifica-

tion, like that which has been applied to EU countries,

seems more realistic for the authenticity control.

See also: Analysis of Food; Carbohydrates:

Determination; Chromatography: High-performance

Liquid Chromatography; Gas Chromatography;

Combined Chromatography and Mass Spectrometry;

Consumer Protection Legislation in the UK; Fatty

Acids: Analysis; Food and Drug Administration; Food

Safety; Genetically Modified Foods; Irradiation of

Foods: Legal and Consumer Aspects; Legislation:

History; International Standards; Additives; Contaminants

and Adulterants; Codex

Further Reading

AOAC International (1995) Official Methods of Analysis,

16th edn. Gaithersburg, MD: AOAC International.

Ashurst PR and Dennis MJ (eds) (1996) Food Authentica-

tion. London: Blackie Academic and Professional.

Gachet E, Martin GG, Vigneau F and Meyer G (1999)

Detection of genetically modified organisms (GMOs)

by PCR: a brief review of methodologies available.

Trends in Food Science and Technology 9: 380–388.

Gordon MH (ed.) (1990) Principles and Applications of

Gas Chromatography. Chichester, UK: Ellis Horwood.

Guillou C, Remaud G and Martin GJ (1991) Application of

deuterium NMR and isotopic analysis to the character-

ization of foods and beverages. Trends in Food Science

and Technology 2: 85–89.

Hummel-Liljegren H (ed.) (1990) Lebensmittelrechts-

Handbuch. Munich: CH. Beck’s Verlagsbuchhandlung.

Lindeberg J (1996) Capillary electrophoresis in food

analysis. Food Chemistry 55: 73–94.

Macrae R (ed.) (1988) HPLC in Food Analysis, 2nd edn.

London: Academic Press.

54 ADULTERATION OF FOODS/Detection

Masotti P and Zonta F (1999) Food irradiation: an update

of legal and analytical aspects. Italian Journal of Food

Science 11: 305–316.

Nielsen SS (1998) Food Analysis, 2nd edn. Gaithersburg,

MD: Aspen.

Piggot JR (ed.) (1986) Statistical Procedures in Food

Research. London: Academic Press.

Simpkins W and Harrison M (1995) The state of art

in authenticity testing. Trends in Food Science and

Technology 6: 321–328.

Singhal RS, Kulkarni PR and Rege DV (1997) Handbook of

Indices of Food Quality and Authenticity. Lancaster, PA:

Technomic.

Van de Voort FR and Ismail AA (1991) Proximate analysis

of foods by mid-FTIR spectroscopy. Trends in Food

Science and Technology 2: 13–17.

Williams PC and Stevenson SG (1990) Near-infrared

reflectance analysis: food industry applications. Trends

in Food Science and Technology 1: 44–48.

AERATED FOODS

G M Campbell, Satake Centre for Grain Process

Engineering, Manchester, UK

Background

0001 Many foods, such as bread, ice cream, and whipped

cream, gain their distinctive texture and appearance

from the presence of bubbles. This article describes

the processes used to create aerated foods, considers

methods for characterizing aerated foods, and identi-

fies the factors contributing to the stability of aerated

foods during and after manufacture. The creation of

aerated foods uses a wide range of aeration processes,

including low-viscosity whipping, beating, and

shaking, high-viscosity dough and paste mixing,

gas injection, and slow and rapid gas generation or

expansion through fermentation, leavening agents,

extrusion, and puffing. A variety of whisks and

mixers are used, both in the kitchen and on the com-

mercial scale, to aerate foods. The air content and

bubble size distribution depend on mixer design and

operation. Air contents of aerated foods can be deter-

mined from density measurements, and cover the

range from a few percent for some confectionery

products to greater than 95% of the total volume of,

for example, popcorn or rice cakes. Aerated food

systems are inherently unstable, and the bubbles

must be stabilized against drainage of the liquid

films, coalescence of bubbles, and disproportiona-

tion. Surface-active materials such as emulsifiers,

lipids, proteins, fat crystals, and ice crystals stabilize

bubbles in foods.

Aerated Foods

0002 Most food processing, either domestic or commer-

cial, is concerned with creating desirable, distinctive,

or novel textures, along with an attractive appear-

ance. Many of the most appealing foods deliver their

characteristic texture and appearance by exploiting

the presence of bubbles. Examples include bread,

cakes, ice cream, breakfast cereals, meringues,

whipped cream, waffles, souffle

s, aerated chocolate

bars, beer, champagne, popcorn, and many others

(See Beers: History and Types; Biscuits, Cookies,

and Crackers: Nature of the Products; Bread: Bread-

making Processes; Butter: The Product and its Manu-

facture; Cakes: Nature of Cakes; Cereals: Breakfast

Cereals; Cheeses: Types of Cheese; Cream: Types of

Cream; Eggs: Use in the Food Industry; Ice Cream:

Properties and Analysis; Snack Foods: Range on the

Market; Soft Drinks: Chemical Composition).

Bubbles in foods offer no nutritional benefit; they

represent pure luxury and proclaim the skill of

the chef or industrial counterparts. Aeration trans-

forms food from merely flavorsome fuel into tex-

tural novelty. The textures achieved vary from the

soft but strong crumb of bread to the crispness

of breakfast cereals, the crunch of honeycomb, the

smoothness of whipped cream, the ‘melting bubbles’

of aerated chocolate bars, and the tingle of carbon-

ated beverages.

0003The results of food aeration are:

0004an increase in volume of the product;

0005a change in the texture and rheology of the product,

aiding mastication and giving a different mouth

feel and appearance;

0006enhanced ability to hydrate rapidly or to take up

sauces, due to increased surface area;

0007possibly a modification of digestibility;

0008the possibility of a reduced shelf-life, as the finely

divided air bubbles or increased porosity may

enhance oxidation reactions and affect fat and

flavor ingredients;

0009a decrease in the intensity of flavors.

AERATED FOODS 55

Aerated foods offer product differentiation and

marketing advantage in the highly competitive,

innovative, and dynamic food market. They also in-

spire delight and praise in the dining room. Their

creation requires detailed understanding, empirical

and fundamental, of the complex interactions be-

tween food chemistry and physics in the kitchen or

in the manufacturing environment.

Aeration Processes and Equipment

0010 Aerated foods are produced in one of three ways: (1)

liquid is forced around air to form bubbles; (2) gas is

sparged into liquid to form bubbles; (3) gas is gener-

ated within the food to create bubbles. The first of

these can be divided into low- and high-viscosity

systems, while the third method can be divided into

slow and rapid generation of gas, giving a total of

five categories of aeration method. Within these

five broad categories a very wide range of specific

processes and operations are used, including whip-

ping cream, beating eggs, dough and paste mixing,

widgets in beer, fermentation, gas injection, frying,

vacuum puffing, and extrusion. Table 1 describes

the range of aeration methods used within these five

categories, and lists examples of foods created using

these processes. Several aeration operations may

contribute together or consecutively during the

process; for example, in breadmaking, bubbles are

incorporated into the high-viscosity dough during

mixing (a type 1b process); these bubbles are inflated

slowly by carbon dioxide gas generated by yeast

fermentation (type 3a), and are further inflated

by steam generation and thermal expansion during

baking (also type 3a), while also undergoing coales-

cence and rupture along with setting of the matrix

structure.

0011The major food aeration methods are as described

below.

Type 1(a)

. 0012Whipping, beating, or shaking of low-medium

viscosity liquids to entrap air.

0013Pressure beating (dissolution of air or gas under

pressure), for example, in a syrup, fat mixture, or

chocolate, for confectionery manufacture.

Type 1(b)

. 0014Mixing of doughs or high-viscosity pastes, in which

air bubbles are entrapped as surfaces come to-

gether. The high viscosity of the dough or paste

prevents rising and disengagement of bubbles. In

raised bread, the bubbles incorporated by mixing

act as nucleation sites for the CO

produced during

fermentation. During creaming of butter and sugar,

sugar crystals aid aeration; fat crystals in cake

batters and ice cream act similarly, with the particle

tbl0001 Table 1 The five categories of processing method used to produce aerated food products

Category Processingmethods Foodproducts

1 (a) Low-intermediate-viscosity

whipping

Beating, whipping, shaking, pressure

beating

Whipped cream, milkshakes, batters (for cakes, pikelets,

crumpets, Yorkshire puddings), souffle

, beaten egg

whites, meringues, marshmallow, chocolate bars,

mousses, dairy desserts, frozen desserts, milkshakes,

sherbet, frappe

, fruit fool

1 (b) High-viscosity mixing or

layering

Dough mixing, paste mixing,

entrapment, pulling, layering

Bread doughs, biscuit doughs, meat doughs, soft butter,

creamed butter and sugar, ice cream, chocolate bars,

puff pastry, choux pastry, croissants, vol au vents,

fondant, cream fillings, icing, pulled taffy, peanut butter

2 Gas injection Sparging, gas injection during

pressure beating, widgets,

pressurized containers, steam

injection

Carbonated soft drinks, widgets in beer, cappuccino,

instant whipped cream, ice cream, bubble gum, boiled

sweets

3 (a) Slow in situ generation or

expansion of gas

Fermentation, chemical raising

agents (baking powder, sodium

bicarbonate), steam generation

and thermal expansion during slow

cooking processes, vacuum

expansion

Proving bread dough, baked goods, unleavened bread,

doughnuts, waffles, puff pastry, choux pastry, vol au

vents, pizza bases, soda bread, cakes, crumpets,

crackers, beer, wine, carbonated soft drinks, ginger

beer, Swiss cheese, honeycomb, chocolate bars, boiled

sweets

3 (b) Rapid in situ generation or

expansion of gas

Steam generation and thermal

expansion during frying, extrusion,

rapid dry heating, puffing, vacuum

expansion, microwaving, pressure

release

Popcorn, popped sorghum, puffed rice, puffed wheat,

cornflakes, extruded breakfast cereals, poppadoms,

crispbreads, snacks, potato crisps, micronized wheat,

coatings, chocolate bars, beer, wine, soft drinks, bubble

and squeak

56 AERATED FOODS

size of the crystals affecting the size and number of

bubbles entrained.

0015 Entrapment of air between sheeted layers, as in

pastries and croissants, or between pulled strands,

as in pulled taffy and candies.

Type 2

.0016 Gas injection, e.g., air or nitrogen injection in ice

cream and sugar confectionery, carbon dioxide in-

jection in soft drinks, nitrous oxide injection in

instant whipped cream, steam frothing of cappuc-

cino.

Type 3(a)

.0017 Fermentation, in which aeration is achieved

through carbon dioxide production by yeast in

bread, beer, and wine, or by Propionibacterium in

Swiss cheeses.

0018 Steam generation during slow to moderate

cooking, baking, or frying. Steam generation is

often accompanied by thermal expansion of the

gases already in the bubbles, and by evaporation

of other dissolved components (e.g., CO

, ethanol).

0019 The use of chemical raising agents such as baking

powders in cakes, or sodium bicarbonate in soda

bread, honeycomb, or dulce de leche.

0020 Vacuum expansion, followed by rapid cooling to

set the expanded product, e.g., chocolate bars.

Type 3(b)

.0021 Rapid dry heating or toasting of small or thin

products to induce blistering or slight puffing.

0022 Frying in very hot oil, such that internal steam is

formed rapidly, causing the product to puff.

0023 Expansion extrusion, in which superheated prod-

uct under pressure emerges suddenly from an

extruder, such that internal moisture immediately

vaporizes into steam bubbles, to produce crisp

snacks, cereals, and sugar confectionery.

0024 Puffing, in which products such as breakfast cereals

containing superheated moisture are subjected to a

sudden release of pressure.

In addition, special cases exist, such as popcorn, freeze-

dried coffee, and candy floss (cotton candy). Despite

the wide variety of processes, aeration equipment com-

prises primarily mixers of various designs, extruders

(See Extrusion Cooking: Principles and Practice), or

specialized puffing or expansion equipment. Most

other aeration processes are achieved by heating or

by chemical raising agents. (See Leavening Agents.)

0025Mixers for food aeration include high-speed whisks

and beaters with stainless-steel wire assemblies for

egg foams, whipped cream, and cake batters, and

high- and low-speed heavy-duty mixers for doughs

and pastes. Pressure beating delivers greatly acceler-

ated aeration and produces fine foams, with air con-

sumption of up to 1000 l h

1

. Blades can be mounted

horizontally within the mixer bowl, or, more usually,

vertically from the top or through the base of the

bowl. Industrial-scale high-speed whisks operate at

speeds of around 200 rpm with a specific power

input of around 50 W kg

1

. Low-viscosity mixers

must use high speeds to entrain air and break up

the bubbles, while in dough mixing bubbles are en-

trained unavoidably simply through the action of

surface renewal during mixing. In both high- and

low-viscosity mixing operations the air content and

bubble size distribution depend on the balance be-

tween entrainment and disentrainment of air, along

with bubble break-up and coalescence.

0026In modern breadmaking processes the bubble

structure created in the dough directly affects the

baked loaf structure. Some dough mixers use pres-

sure-vacuum mixing, in which the dough is mixed

initially under high pressure to provide additional

oxygen (which contributes to development of the

gluten network), followed by mixing under a partial

vacuum to reduce the air content in the dough. Some

batch dough mixers, e.g., the Biplex, operate initially

at slow speed to blend and hydrate the ingredients,

then at high speed to develop the dough.

0027Continuous dough mixers lack the versatility of

batch mixers to modify the bubble structure in the

dough and in the resulting bread. Continuous, tubu-

lar, pressurized scraped-surface aerator-freezers with

a residence time of about 30 s are used in icecream

manufacture to give air contents of up to 50% by

volume.

Characterization of Aerated Foods

0028Aerated foods can be characterized in terms of their

rate of aeration, air content, bubble size distribution,

the resulting texture, and the stability of the aerated

structure. Foamability (the ease with which a foam is

formed, in terms of rate and air content) and foam

stability are usually applied to transient food foams

such as beer and wine foams and beaten egg whites.

Foamability may be measured by sparging gas or by

rapid whipping to determine the time required to

form a prescribed volume of foam, while foam stabil-

ity is characterized by the half-life of the foam, the

rate of foam drainage, or the change of conductivity

of the foam. Foamability and foam stability are often

inversely related – a greater foam volume is achieved

AERATED FOODS 57

at the expense of a less stable foam. Stability is con-

ferred through a range of stabilization mechanisms,

discussed below.

0029 For more solid aerated foods, texture is of greater

importance, and rheometers and textural measure-

ments, including sensory evaluation, are applied.

Rheometers may be empirical, imitative, or funda-

mental. Crisp aerated foods can be characterized by

calculating the apparent fractal dimension of the

jagged stress–strain curve resulting from crushing.

Mechanical properties of solid aerated foods depend

on the mechanical properties of the matrix, the

amount and distribution of the air, and whether the

foam is of open or closed gas cells. Closed gas cells

occur in aerated chocolate bars, for example, in

which each bubble is discrete, while bread has

an open-cell or sponge structure, in which the gas

cells are interconnected to form a porous network.

Mechanical properties of foams, such as Young’s

modulus, collapse stresses, crushing strength, and

fracture toughness can be related to the density of

the foam by a power law model:



ð1Þ

where s is a general mechanical property, s

is the

mechanical property of the matrix material in the

foam, and r and r

are the density of the foam and

of the matrix material, respectively. The exponent n is

usually in the range 2–3. For constant matrix proper-

ties, mechanical properties vary with foam density

raised to the power of n, such that as air content

increases, the foam becomes less strong.

0030 Air contents of aerated foods range from 2% for

some confectionery products to greater than 95%

for popcorn, rice cakes, and beer foam, with every

air content in between. The air content of highly

aerated fluid fluids such as icecream and whipped

cream is characterized in terms of the overrun, OR,

calculated as:

OR ¼

Weight of unwhipped product 

weight of whipped product

 100%

ð2Þ

where weights are measured in a container of con-

stant volume. The overrun represents the additional

air added. In aerated foods with lower air contents,

the void fraction of air as a fraction of the total

volume is more usually calculated as:

a ¼ 1 

 100% ¼

OR þ 100



 100%

ð3Þ

where r is the density of the product, and r

is the

gas-free dough density. Table 2 gives typical values of

the density and air content of a range of aerated

foods, from which the other parameters describing

aeration can be calculated. Figure 1 shows typical

air contents and specific volumes of a selection of

aerated food products.

0031Density measurements indicate the gross air con-

tent of a food, but not how that air is distributed. The

size distribution of bubbles or gas cells determines the

texture, appearance, and mass transfer behavior of an

aerated food. The size distribution can be determined

by image analysis of an aerated surface or of thin

slices of the product. When thin slices of a food

material are taken, the holes appearing on the slice

are, on average, smaller than the bubbles from which

they came. The hole size distribution measured is

therefore different from the true bubble size distribu-

tion, which must be reconstructed using stereological

techniques.

Stabilization of Aerated Foods

0032The lifetimes of aerated food products range from a

few seconds for wine foams, to minutes for beer

foams and souffle

s, hours for whipped cream, days

tbl0002Table 2 Typical values of density and air content of aerated

foods (dependent in all cases on temperature, composition, and

processing factors)

Food Density (g cm

3

) Void fractionof air (%)

Popcorn <0.07 >95

Rice cakes 0.11–0.13 90–92

Puffed rice 0.13–0.17 88–90

Extruded products 0.10–0.33 75–90

Meringue 0.17–0.18 88–90

Beaten egg whites 0.15–0.20 80–85

Baked loaf 0.20–0.35 72–85

Sponge cake 0.25–0.35 70–80

Risen dough 0.25–0.40 68–80

Marshmallow 0.35–0.45 68–75

Cake 0.35–0.40 68–72

Whipped cream 0.40–0.60 40–60

Ice cream – hard 0.54–0.55 50

Cake batter 0.55–0.80 30–50

Aerated chocolate bar 0.70–0.80 40–45

Nougat 0.80–0.90 30–40

Fruit fool 0.75–0.8 25–30

Ice cream – soft 0.78–0.8 28

Milkshake 0.90–0.95 9–13

Micronized wheat 1.15–1.25 7–11

Bread dough (unrisen) 1.15–1.20 4–8

Wheat grains 1.25–1.35 2–3

Adapted from Campbell GM and Mougeot E (1999) Creation and

characterisation of aerated food products. Trends in Food Science and

Te c hn o l o g y 10: 283–296, with permission.

58 AERATED FOODS

for bread, weeks for cakes, and several months for

Swiss cheeses, cereals, chocolate bars, and ice cream.

Aerated systems are inherently unstable, and the

bubble structure must be stabilized against collapse.

In solid aerated foods such as crackers, snacks, and

chocolate bars, stability is achieved through the solid

matrix. These products are however delicate due to

their fine aerated structure, and their high surface

area makes them prone to oxidative deterioration or

picking up atmospheric moisture; these factors ultim-

ately limit their shelf-life. During the formation of

these products, however, and in other more liquid

food foams, the bubbles must be stabilized against

their thermodynamic tendency to coalesce. Stability

is conferred to aerated foods through the action of

indigenous or added emulsifiers, proteins (as in beer

foams, egg foams, proving bread doughs, cakes), fat

or ice crystals or other particles (whipped cream, ice

cream, puff pastry), or through a high-viscosity or

solid continuous matrix (chocolate bars, meringues,

breakfast cereals, Swiss cheese, mousse).

0033In low-viscosity foams, three distinct mechanisms

of destabilization occur: drainage, coalescence, and

disproportionation. Drainage occurs when liquid

flows from the thin lamellae between bubbles into

the Plateau borders that form at the meeting point

of three bubbles. Plateau borders form a continuous

pathway through the foam, allowing liquid to drain

until the bubble lamellae thin sufficiently to allow

coalescence. If the foam is stable against coalescence,

drainage will continue until a relatively dry poly-

hedral foam skeleton remains. Higher viscosity in

the liquid slows both drainage and coalescence.

0034Coalescence between bubbles is thermodynamic-

ally favorable because of the resulting reduction in

surface area. The presence of surface active materials

(e.g., emulsifiers and proteins) stabilizes against co-

alescence; pure liquids, free of surfactants, are unable

to produce stable foams. The presence of hydrophilic

particles may stabilize a foam, while hydrophobic

particles destabilize foams by thinning the film

between bubbles, as illustrated in Figure 2.

100

Air content (%)

Specific volume (cm

−1

)

Popcorn

Rice cakes

Puffed rice

Meringue

Beaten egg whites

Baked loaf

Sponge cake

Risen dough

Marshmallow

Cake

Whipped cream

Icecream − hard

Aerated chocolate bar

Cake batter

Nougat

Icecream − soft

Fruit fool

Milkshake

Micronized wheat

Bread dough (unrisen)

Wheat grains

fig0001 Figure 1 Typical air contents and specific volumes of a selection of aerated foods. Open columns, specific volume; filled columns,

air content. Adapted from Campbell GM and Mougeot E (1999) Creation and characterisation of aerated food products. Trends in Food

Science and Technology 10: 283–296, with permission.

AERATED FOODS 59

0035 Disproportionation in foams is the growth of large

bubbles at the expense of smaller ones, equivalent to

Ostwald ripening in emulsions. The gas pressure in

smaller bubbles is greater than that in larger bubbles,

due to the contribution of surface tension, g,as

described by the Laplace equation:

¼ P

ð4Þ

where r is the bubble radius, and P

and P

are the

pressures inside and outside the bubble, respectively.

The higher Laplace pressure in smaller bubbles causes

a higher gas solubility in the neighboring region. This

results in a mass transfer driving force between small

and large bubbles that causes diffusion of gas to the

latter. The process is self-accelerating, as the loss of

gas makes small bubbles even smaller, the Laplace

pressure higher, and the gas solubility greater. Ultim-

ately small bubbles implode and the foam coarsens.

However, the increasing concentration of surface-

active components at the surface of the shrinking

bubble slows down and can even arrest the process.

Disproportionation is a major factor in foam stability

of carbonated beverages such as beer, due to the high

solubility of carbon dioxide in water. The presence of

a small proportion of nitrogen in the gas phase stabil-

izes against disproportionation, as the loss of carbon

(a)

(b)

fig0002 Figure 2 Effect of particles on foam stability: (a) stabilization of

bubble lamellae by hydrophilic particles which draw liquid to the

particle and oppose film thinning; (b) destabilization by hydro-

phobic particles.

(a)

(b)

(c)

(d)

fig0003 Figure 3 Illustration of film stability between bubbles with (a) no surfactant present; (b) low-molecular-weight surfactants such as

lipids; (c) protein-stabilized bubbles; and (d) mixed protein–lipid systems.

60 AERATED FOODS

dioxide by disproportionation from a small bubble

lowers the partial pressure of carbon dioxide

remaining in the bubble, lowering its concentration

in the surrounding liquid, and thus ultimately remov-

ing the driving force for diffusive mass transfer.

0036 Most aerated foods are not low-viscosity foams,

and in most aerated foods, bubble coalescence is the

most significant destabilization mechanism. Stability

against bubble coalescence in both low- and high-

viscosity systems is conferred by the presence of sur-

face-active materials such as low-molecular-weight

lipids and high-molecular-weight proteins (See Emul-

sifiers: Organic Emulsifiers). These two types of sur-

factant stabilize against coalescence via different

mechanisms, as illustrated in Figure 3. Two adjacent

bubbles will coalesce when the lamella between them

thins and breaks as a result of drainage or mechanical

disturbance (Figure 3a). When the lamella thins,

it experiences greater stress which rapidly causes

further thinning and breakage of the film. If a low-

molecular-weight surfactant is present at the surface,

its concentration decreases at the point of film thin-

ning. This creates a surface tension gradient which

causes surfactant molecules from the adjacent area

to diffuse rapidly to the depleted region, sweeping

liquid into it and restoring the thickness of the

region (the Marangoni effect), and thus safeguarding

against coalescence (Figure 3b). The effectiveness of

low-molecular-weight surfactants thus depends on

their rates of surface lateral diffusion. Large-molecu-

lar-weight surface-active molecules such as proteins

have low lateral diffusivities; they stabilize against

coalescence via a different mechanism, in which

they interlink to form a rigid layer at the surface

(Figure 3c). If both proteins and low-molecular-

weight surfactants such as lipids are present, com-

peting for space at the bubble interface, these two

stabilization mechanisms can interfere; the presence

of protein prevents rapid surface diffusion of lipid

molecules, while the presence of lipids prevents the

formation of a rigid interlinked protein network

(Figure 3d). Such mixed systems are responsible

for the reduction in egg foam volume when a small

amount of egg yolk is allowed in with the whites,

the reduction in loaf volume when small amounts of

polar lipids are added to bread dough formulations,

and the disastrous effects of lipids on beer foam

stability. In other cases low-molecular-weight lipids

may act cooperatively with proteins to improve foam

stability.

See also: Beers: History and Types; Biscuits, Cookies,

and Crackers: Nature of the Products; Bread:

Breadmaking Processes; Butter: The Product and its

Manufacture; Cakes: Nature of Cakes; Cereals: Breakfast

Cereals; Cheeses: Types of Cheese; Cream: Types of

Cream; Eggs: Use in the Food Industry; Emulsifiers:

Organic Emulsifiers; Extrusion Cooking: Principles and

Practice; Chemical and Nutritional Changes; Ice Cream:

Properties and Analysis; Snack Foods: Range on the

Market; Soft Drinks: Chemical Composition

Further Reading

Aguiliera JM and Stanley DW (1999) Microstructural

Principles of Food Processing and Engineering, 2nd

edn. Maryland, USA: Aspen.

Barham P (2001) The Science of Cooking. Berlin, Germany:

Springer-Verlag.

Blanshard JMV and Mitchell JR (1988) Food Structure – Its

Creation and Evaluation. London, UK: Butterworths.

Campbell GM and Mougeot E (1999) Creation and char-

acterisation of aerated food products. Trends in Food

Science and Technology 10: 283–296.

Campbell GM, Webb C, Pandiella SS and Niranjan K

(1999) Bubbles in Food. St Paul, Minnesota, USA:

Eagan Press.

McGee H (1984) On Food and Cooking: The Science and

Lore of the Kitchen. London, UK: Harper Collins.

Rosenthal AJ (ed.) (1999) Food Texture. Maryland, USA:

Aspen.

Sadoc JF and Rivier N (1999) Foams and Emulsions. The

Netherlands: Kluwer Academic.

Stauffer CE (1999) Emulsifiers. St Paul, Minnesota, USA:

Eagan Press.

Wan PJ, Cavallo JL, Saleeb FZ and McCarthy MJ (1990)

Food Emulsions and Foams: Theory and Practice.

AIChE Symposium Series no. 277, vol. 86. New York,

USA: American Institute of Chemical Engineers.

AERATED FOODS 61

AEROMONAS

S J Walker, Campden & Chorleywood Food Research

Association, Gloucestershire, UK

Background

0001 The genus Aeromonas belongs to the bacterial family

Vibrionaceae and it has been suggested that this

genus should be considered to contain some bacterial

pathogens for humans, which may cause food

poisoning. Other closely related genera within the

family are Vibrio and Plesiomonas species. Both P.

shigelloides and some Vibrio species are known to

cause human gastroenteritis. With Aeromonas

species, however, this pathogenic ability is not yet

proven. These bacteria have traditionally been con-

sidered to be waterborne, although they may be isol-

ated from a wide range of environments. (See Vibrios:

Vibrio cholerae; Vibrio parahaemolyticus; Vibrio

vulnificus.)

Aeromonas

Species

0002 Historically, the taxonomy of this genus has been

confused, but the genus may be divided into two

main groups. The first is the nonmotile species A.

salmonicida. This microorganism is a pathogen for

many species of fish and particularly the salmonids,

but is not considered to be of significance in food

safety or spoilage. Therefore, this species will not be

considered further. The second group is the motile

species. A phenotypic classification in 1981 divided

this group into the species A. hydrophila, A. caviae,

and A. sobria, which could be distinguished by simple

biochemical tests. DNA studies have resulted in the

genus being recognized as a separate family, Aero-

monadaceae, as part of the Proteobacteria. At least

16 DNA-DNA hybridization groups (equivalent to a

genomospecies) are recognized in the genus. The tax-

onomy of Aeromonas continues in this state of flux.

0003 Much of the published literature on Aeromonas

has not distinguished the motile species or has used

out-of-date nomenclature. Consequently, the name A.

hydrophila will be used as a general name for the

motile species in this article. The name is derived

from Greek, i.e., Aeromonas (a gas-producing unit),

hydrophila (water-loving).

A. hydrophila

as a Pathogen?

0004 The potential for A. hydrophila to cause disease

has been recognized since 1960. However, the

significance of A. hydrophila as a direct cause

of human gastroenteritis is a matter of debate. Al-

though recognized as an opportunistic pathogen,

it is only since the 1980s that there has been in-

creased attention to its clinical significance in healthy

individuals.

0005Some studies have found that the incidence of

A. hydrophila in diarrheal stools is significantly

greater than in those from control groups (e.g.,

10.6% and 0.6% of samples positive respectively),

and it may often be obtained in high numbers.

These observations are most pronounced in infants

and young children. The presence of a bacterium in a

diarrheal stool, however, does not confirm its role as

the causative agent of the syndrome. Other pathogens

(including bacteria, viruses, or parasites) may initiate

disease and alter the gut environment, and only then

may A. hydrophila become prominent in the bacterial

fecal flora. Much clinical data are of limited use as, in

many cases, A. hydrophila was not looked for speci-

fically, whilst, in other incidents, the presence of other

pathogens could not be ruled out. The confusion in

taxonomy and methodology for A. hydrophila may

also have impaired the abilities of clinical microbiolo-

gists to isolate this bacterium from stool samples. A

major survey on infectious intestinal disease in the UK

found that A. hydrophila was as frequently isolated

from control cases as from those presenting at general

practitioners.

0006Many strains of A. hydrophila possess a wide range

of potential virulence factors (i.e., strategies which

may aid pathogenicity). These include the production

of adhesins, cytotoxic enterotoxins (which lead to

fluid secretion in the gut), protease enzymes, and

hemolysins (aerolysins), although not all may be

present in some isolates. Similar virulence factors

are present in many recognized foodborne bacterial

pathogens. The virulence factors of A. hydrophila

produce typical responses in both in vitro and in vivo

model systems for pathogenicity. Human feeding

studies with A. hydrophila, however, have been

inconclusive, with only two of 57 volunteers getting

diarrhea after consuming high levels of enterotoxin-

producing strains. The validity of this study has been

challenged as the strains used may not have possessed

all the other virulence factors necessary for gut

colonization and so disease.

0007Consequently, in spite of the abundance of

circumstantial evidence for pathogenicity, the case

to include A. hydrophila as a foodborne pathogen

resulting in gastroenteritis has not been conclusively

proven.

AEROMONAS

Symptoms

0008 Human infection with A. hydrophila may produce a

wide range of localized and systemic illnesses. The

symptoms are dependent on the bacterium, host,

inoculum size, and infection site. Many of the motile

Aeromonas species have been implicated in human

illness and it is likely that many of them have been

mistakenly reported as A. hydrophila in the past.

0009 The most common type of infection involves the

gastrointestinal tract and this is the illness of most

concern to the food industry. Gastroenteritis may

occur at all ages and in both sexes, although it is

most commonly reported in young children. Symp-

toms are generally more pronounced in the very

young, elderly, and immunocompromised. The most

common symptom is an acute profusely watery diar-

rhea which may last for several days. Other associ-

ated symptoms include fever and vomiting. Generally,

the disease is self-limiting and symptoms subside

within 1 week. In a few cases, the disease may persist

for more than 10 days and is classified as chronic.

This results in weight loss and may require hospital-

ization. Chronic disease may last for several months.

A second manifestation of gastroenteritis is a ‘dysen-

tery-like’ illness characterized by diarrhea containing

blood and/or mucus and cramping abdominal pain.

Hospitalization and antimicrobial therapy are often

required. More rarely, a ‘cholera-like’ illness may

occur when the diarrheal stool is turbid and contains

white grains (‘rice-water stools’). This results in rapid

and extensive dehydration, usually requiring hospi-

talization and aggressive antimicrobial therapy.

0010 Other types of A. hydrophila illness include wound

infection, septicemia, and localized infections.

Wound infections are most common after traumatic

injury and in many cases the injury has been associ-

ated with water or soil. Infection is most common in

the hands, arms, and legs. In most cases with healthy

adults, the infection remains localized but, in the

immunocompromised, it may become more serious

and result in amputation, septicemia, or even death.

Septicemia and the infection of a variety of bodily

sites occur most commonly, but not exclusively, with

immunocompromised individuals. The symptoms of

A. hydrophila septicemia are similar to those pro-

duced by other Gram-negative bacterial pathogens

and the mortality level in the immunocompromised

is approximately 50%. Both septicemia and localized

infections generally require antibiotic therapy and

often hospitalization.

Incidents of Foodborne Disease

0011 Although there has not yet been a fully confirmed

foodborne outbreak of A. hydrophila gastroenteritis,

a number of suspected incidents have been reported.

Three outbreaks affecting between three and 472

individuals have implicated oysters as the vector of

transmission. Levels of the bacterium isolated ranged

from 9 per 100 g to 2 10

per gram of food, al-

though in the former case, the value obtained was

after frozen storage for 1 year.

0012Cooked prawns have been implicated in three small

incidents (1–20 cases) in the UK. Levels obtained in

one outbreak were approximately 5 10

cells of

A. hydrophila per gram of food. Several incidents

implicating both treated and untreated water have

also been reported.

0013A number of the incidents involving A. hydrophila

cannot be fully confirmed as the feces of indi-

viduals were not specifically tested for this bacter-

ium. Confirmed outbreaks are more likely to be

reported with the increased awareness of this

bacterium. (See Food Poisoning: Tracing Origins

and Testing.)

Ecology

0014A. hydrophila may be isolated from a wide range of

environmental sources, but has traditionally been

considered to be an aquatic bacterium. It has been

isolated from fresh waters, estuaries, coastal waters

and, occasionally, open ocean waters. The incidence,

numbers present, and survival of the bacterium are

affected by the pH, temperature, salt content and

dissolved oxygen in the water. Several studies have

noted that the presence/level of A. hydrophila in

sources of drinking water does not correlate well

with the usual indicators of water quality (e.g., fecal

indicator organisms). Furthermore, A. hydrophila

may be isolated (at low levels) from chlorinated and

unchlorinated potable water supplies. A statistical

relationship between A. hydrophila in water supplies

and health risks has been suggested, but there is no

firm evidence of serious health effects of humans after

exposure to such drinking water. The presence of

A. hydrophila in potable water supplies may be of

concern to food-processing operations, as it may

lead to product and/or equipment contamination.

Adequate chlorination of the water will insure

destruction of the bacterium.

0015The presence of A. hydrophila in water undoubt-

edly contributes to its common occurrence in fish and

seafood. The bacterium is a common component of

the intestinal microflora of healthy fish. It is likely

that poor processing techniques and poor hygienic

practices may result in the contamination of fish

flesh and seafood products. (See Fish: Spoilage of

Seafood; Shellfish: Contamination and Spoilage

of Molluscs and Crustaceans.)

AEROMONAS