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

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Further Reading

ASTM (2000) ASTM Standards on Color and Appearance

Measurement, 6th edn. West Conshohocken, PA: Ameri-

can Society for Testing and Materials.

BSI/ISO (1999) BS 5929-10:1999, ISO 11037: 1999

Methods for Sensory Analysis of Food. General

Guidance and Test Method for Assessment of the Colour

of Foods. London: British Standards Institution.

CIE (1986) Publication Commission Internationale de

l’Eclairage no. 15.2 Colorimetery, 2nd edn. Vienna:

CIE Central Bureau.

Francis FJ and Clydesdale FM (1975) Food Colorimetry:

Theory and Application. Westport, CT: AVI.

Hunt RWG (1998) Measuring Colour, 3rd edn. Surbiton,

UK: Fountain Press.

Hunter RS and Harold RW (1987) The Measurement of

Appearance, 2nd edn. New York: John Wiley.

Hutchings JB (1999) Food Colour and Appearance, 2nd

edn. London: Lippincott Williams & Wilkins.

Mackinney G and Little AC (1962) Color of Foods. West-

port, CT: AVI.

McDonald R (1997) Colour Physics for Industry, 2nd edn.

Bradford, UK: Society of Dyers and Colourists.

MacDougall DB (2002) Colour in Food – Improving Qual-

ity. Cambridge: Woodhead Publishing Co. Ltd.

Texture

M C Bourne, New York State Agricultural Experiment

Station, Cornell University, Geneva, USA

A S Szczesniak, New York, NY, USA

A Quality Factor

0001 Texture is a sensory property of foods which, together

with appearance, aroma, and basic taste, has a pro-

found effect on consumer acceptance of foods. Each

of these properties is composed of a number of notes.

Texture lies between taste and aroma in this respect,

several dozen different texture notes being detectable

in foods, which is more than the four taste notes of

sweet, sour, salt, bitter, and less than the hundreds of

odor notes that can be recognized.

0002 Textural perception occurs directly through the

tactile (touch) and kinesthetic (movement) senses,

and indirectly through the senses of vision and hear-

ing. In contrast to color and flavor, there are no

specific sensory receptors for texture. Texture is an

important quality attribute in almost all foods and

most important in foods that are bland in flavor, or

have the characteristics of crispness or crunchiness. It

contributes to the satisfaction of chewing and the

pleasure of eating. Good textures are indicators of

excellent food preparation. Highly valued is textural

contrast, i.e., the presence of two or more contrasting,

but compatible, characteristics that can occur within

a meal, on the plate, or within a food product.

0003Texture may be defined as ‘that group of physical

characteristics that arise from the structural elements

of the food, are sensed primarily by the feeling of

touch, are related to the deformation, disintegration,

and flow of the food under a force, and are measured

objectively by functions of mass, time, and length.’

This definition teaches that texture has its roots in

structure (molecular, microscopic, macroscopic) and

the manner in which this structure reacts to applied

forces. It also emphasizes that texture is a multi-

dimensional property comprising a number of

sensory characteristics.

0004A large number of terms are popularly used to

describe textural sensations. Table 1 organizes many

of these terms into a manageable system that

facilitates understanding their interrelationships. It

classifies textural properties into mechanical charac-

teristics (reaction of the food to stress), geometrical

characteristics (the feeling of the size, shape, and

arrangements of particles in the food, sometimes

called ‘particulate properties’), and other characteris-

tics (relating to the sensations of moisture, fat, and oil

in the mouth). Table 2 provides physical and sensory

definitions of the mechanical characteristics.

Sensory Evaluation

0005Since, by definition, texture is a sensory property, the

most logical approach to its description and quantifi-

cation is by sensory evaluation. In the early days,

panels with various degrees of training were used to

score specific textural characteristics or ‘texture’ in

general. The scoring methods used were either numer-

ical intensity scales (frequently 0–7, with 0 denoting

absence and 7 a very high intensity of a specific

characteristic) or hedonic scales (ranging from ‘dis-

like extremely’ to ‘like extremely’). The latter should

not be used when the objective is to describe texture

in terms of the characteristics present or to quantify

their intensity.

0006The sensory perception of texture is a dynamic

process which involves the rate and magnitude of

the applied forces, and also the effects of temperature,

saliva, and time. The time element includes the

repeated application of destructive forces in the

masticatory process, and the duration of the food’s

contact with saliva and mouth temperature.

0007The multiparameter nature of texture and the pro-

cess dynamics of its sensory perception during masti-

cation form the basis for the sensory texture profile,

SENSORY EVALUATION/Texture 5167

the schematic for which is shown in Figure 1. The

method is used to define the texture characteristics

present, the intensity of each, the order in which they

appear, and all the changes that occur from the first

bite through completion of mastication. Texture pro-

file analysis is presently the preferred sensory method

for texture characterization because it is the only

method that provides a complete analysis of all the

textural properties of a food. Using reference samples

and standard scales for specific parameters, highly

trained panels provide a descriptive and quantitative

‘fingerprint’ of the product’s texture. The training

and maintenance of the panel may be tedious and

expensive, but the quality of the generated data

usually compensates for it. The basic principles can

be adapted to different products in different situ-

ations including untrained consumer panels. Two

examples of sensory texture profiling are shown in

Table 3.

0008Most other sensory methods can be regarded as

partial texture profiles or modifications of the basic

procedure. The two main methods are: Quantitative

Descriptive Analysis (QDA) and Spectrum. QDA

differs from texture profiling in that it uses unstruc-

tured line scales for quantifying the intensity of

texture notes present, and the data are generated by

averaging the ratings given by the individual panelists

rather than through panel discussion and a consensus.

Another difference is that the panelists generate their

own sets of terms. In contrast, the Spectrum method

uses an expanded set of terms within the standard

lexicon of texture profiling. It employs 15-point

intensity scales anchored by reference samples.

The extensive panel training and the active role

tbl0001 Table 1 Classification of textural characteristics and their relationship to popular nomenclature

Characteristics Primary parameters Secondaryparameter Popular terms

Mechanical Hardness Soft, firm, hard

Cohesiveness Brittleness Crumbly, crunchy, brittle

Chewiness Tender, chewy, tough

Gumminess Short, mealy, pasty, gummy

Viscosity Thin, thick

Springiness Plastic, elastic

Adhesiveness Sticky, tacky, gooey

Geometrical Class Examples

Particle size and shape Gritty, grainy, coarse, etc.

Particle shape and orientation Fibrous, cellular, crystalline, etc.

Other Moisture content Dry, moist, wet, juicy

Fat content Oiliness Oily

Greasiness Greasy

Adapted from Szczesniak AS (1963) Classification of textural characteristics. Journal of Food Science 28: 385–389, with permission.

tbl0002 Table 2 Definitions of the mechanical parameters of texture

Physical Sensory

Hardness Force necessary to attain a given deformation Force required to compress a substance between molar teeth

(solids) or the tongue and palate (semisolids)

Cohesiveness Strength of internal bonds Amount of sample deformation before rupture when biting with

molars

Fracturability Forces necessary to fracture the material Force with which the material crumbles, cracks, or shatters

Chewiness Energy required to disintegrate a solid food to

a state ready for swallowing

Number of chews required to masticate a sample at one chew

per second and constant rate of force application to reduce

it to a consistency suitable for swallowing

Gumminess Energy required to disintegrate a semisolid

food to a state ready for swallowing

Denseness that persists throughout mastication of a

semisolid food

Viscosity Rate of flow per unit force Force required to draw a liquid from a spoon over the tongue

Springiness Rate at which a deformed material goes back

to its undeformed condition following removal

of the deforming force

Degree and speed with which the material returns

to its original height following partial compression

with molar teeth

Adhesiveness Work necessary to overcome the attractive forces

between the surface of the food and other

surfaces with which the food comes in contact

Force of the tongue required to remove the material that

adheres to the mouth (generally the palate, but also lips,

teeth. etc.) during the normal eating process

5168 SENSORY EVALUATION/Texture

of the panel leader are similar to those of texture

profiling.

Instrumental Evaluation

0009 Although sensory methods are the final arbiter of

textural quality, instrumental methods are the most

frequently used for texture measurement because they

are cheaper and less time-consuming. To be success-

ful, any instrumental measurement must correlate

highly with the sensory evaluation. It needs to be

recognized that instrumental methods measure one

or more textural properties of a food before it is

placed in the mouth, but cannot follow the changes

that occur during mastication. Most instrumental

methods are ‘one-point’ measurements, i.e., they

measure only one dimension of the texture, albeit

usually a dominant textural property. Because there

is a wide range of types of foods and textures, people

use a wide variety of methods to assess texture sen-

sorially. Before they put food into the mouth they use

their fingers and hand to squeeze, stir, bend, or punc-

ture the food. After the food is put into the mouth the

chewing rate, the amount of lateral movement of the

mandible (jaw), and the movements of the tongue

are automatically adjusted to handle that particular

food in the most convenient way. A large number

of texture-testing instruments have been described

in the literature and almost 100 are available com-

mercially. Table 4 classifies objective methods of

Visual

First bite

Mechanical

Fracturability

Firmness Viscosity

Geometrical and surface properties

Geometrical

Any, depending on product

Masticatory

(late)

(early)

Chewiness

Gumminess Adhesiveness

MoistureBreakdown

Type Rate Adsorption Release

Fat

Release Mouth-coating

Mouth-coating

Type Amount

Swallowing

Ease of Throat-coating

Throat-coating

Residual

Mouth-coating

Type Amount Type Amount

General feel on:

Mouth Throat

fig0001 Figure 1 Sensory texture profiling technique. Reproduced from Sensory Evaluation: Texture, Encyclopaedia of Food Science, Food

Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press, with permission.

SENSORY EVALUATION/Texture 5169

food texture measurement on the basis of the vari-

able(s) measured and the principle of the test. Force-

measuring instruments are the most common, but

other principles, such as distance, time, and energy,

are also used. There are a few examples where a

chemical analysis correlates well with a textural prop-

tbl0003 Table 3 Basic sensory texture profile ballot for meatballs and soda crackers

Texturenotes Meatballs Soda crackers

Initial

Mechanical

Hardness (9-point scale) 3.4 4.0

Fracturability (7-point scale) 0.7 2.5

Viscosity (8-point scale) Not applicable Not applicable

Geometrical Lumps, with a grainy surface Flaky and puffy

Other Moist, uncut surface is slippery and cut surface is

not slippery

Dry

Masticatory

Mechanical

Gumminess (6-point scale) 1.2 0

Chewiness 17.7 chews 16 chews

Adhesiveness (5-point scale) 1.2 0.7

Geometrical Coarse, grainy; some fibrous particles are present Flaky

Other Moist Dry

Residual

Rate of breakdown Large lumps break down fast;

grains break down at a medium rate

High

Type of breakdown Lumps turn into a nonhomogeneous, grainy paste,

and grain size decreases. Some stringly fibrous

grains are present; they become more noticeable

towards the end and require more effort to chew

It breaks down into little rough sheets,

then changes into a smooth dough

Moisture absorption Initially moist. Saliva mixes easily with slurry and

the bolus becomes progressively more

moist. Residual grains feel dry

It absorbs a lot of saliva slowly and

change into a moist dough

Mouth-coating Slight residual oiliness. A few particles stick

between the teeth and around the mouth

Little pieces stick to the mouth and gums

From Bourne MC (1982) Food Texture and Viscosity: Concept and Measurement. New York: Academic Press, with permission.

tbl0004 Table 4 Objective methods for measuring food texture

Principle Measured variable Dimensionalunits Examples

Force Force (F) mlt

2

Puncture Fmlt

2

Fruit pressure testers

Extrusion Fmlt

2

Shear press, Tenderometer

Shear Fmlt

2

Warner– Bratzler shear

Crushing Fmlt

2

Tensile Fmlt

2

Extensograph

Torque Fmlt

2

Rotary viscometers, Farinograph, Struct-o-Graph

Snapping Fmlt

2

Deformation Fmlt

2

Distance

Length l Penetrometer, Bostwick Consistometer

Area l

USDA Consistometer

Volume l

Bread volume, Succulometer

Time Time (T) t Ostwald Viscometer, Biscuit Texture Meter

Energy Work (F  D) ml

2

Area under force–distance curves

Ratio F or D or T measured twice Dimensionless Specific Gravity

Multiple F and D and Tmlt

2

, l, t Instron, Lloyd, Zwick, TA.XT2

Chemical analysis Concentration Dimensionless (%) Alcohol-insoluble solids

Miscellaneous Anything Anything Optical density, crushing sounds

Adapted from Bourne MC (1996) A classification of objective methods for measuring texture and consistency of foods. Journal of Food Science 31: 1011–

1015, with permission.

5170 SENSORY EVALUATION/Texture

erty. Finally, the sounds that are generated are an

important dimension of the textural quality of crisp

and crunchy foods. Characteristic sounds are gener-

ated when these foods are compressed. The sounds

are of low frequency and come in a series of sharp

bursts, that occur in rapid succession. Crunchy foods

exhibit notes of a lower frequency than crisp foods.

0010 The key element that differentiates force-measuring

instruments is the geometry of the test cell which

holds the sample and applies a force to it (cutting,

puncture, compression, extrusion, etc.). Recognition

of the above fact has led to the widespread use of

strength-of-materials-testing machines that provide a

sophisticated driving mechanism, force sensor, and

recording of the force–time relationship. They are

more expensive than the simple instruments that

have been in use for many decades, and they came

into use in the late 1960s. They are called universal

testing machines (UTMs) because, with appropriate

test cells attached, all test procedures that require

rectilinear movement can be performed on the same

machine. With the advent of computer retrieval and

analysis of the force–time data their use is rapidly

becoming routine for quality purposes also. One

UTM, the TA.XT2 texture analyzer, has been ex-

pressly developed for measuring textural properties

of foods. Its operation is so simple, yet precise, that it

is now widely used for both research and quality

control purposes.

0011 The following criteria are recommended as a guide

to selecting a texture-testing instrument or the type of

test to program into an UTM.

0012 Purpose – research or quality assurance.

0013 Nature of product – rheological type, hetero-

geneity.

0014 Required accuracy – high variability inherent in

unprocessed foods often necessitates many replica-

tions.

0015 Cost – including operation and maintenance.

0016 Time – routine use requires rapid testing.

0017 Location – ability to withstand adverse environ-

mental conditions when placed in factory sur-

roundings.

0018 Nature of sensory evaluation method used by

people (squeeze in hand, cut with incisors, crush

between molars, roll with the tongue against the

hard palate, etc.)

0019 This should narrow the field to the most promising

two or three test principles. The final candidates

should be tested over the full range of textures nor-

mally encountered with the food and correlated with

sensory evaluation. A statistical analysis of the results

should identify which principle and instrument are

the best for each particular application. The final

step is to establish the test conditions that give the

strongest resolution between different samples and

then standardize them. These include sample size,

test cell dimensions, force range, speed of travel of

moving parts, chart speed, temperature, and perhaps

other factors.

Texture Profile Analysis

0020The methodology of texture profile analysis (TPA)

brings instrumental texture evaluation a step closer

to sensory testing. It involves compressing a bite-size

piece of food two or more times in a reciprocating

motion that simulates the action of the jaw, and

quantifying from the resulting force–time curve a

number of textural parameters that correlate well

with sensory evaluation. The method, originally de-

veloped for the General Foods Texturometer, has

been adapted to universal testing machines. Figure 2

shows a generalized TPA force–time curve. Hardness

is defined as the peak force on the first compression

cycle (first bite); fracturability is defined as the first

significant break in the curve on the first bite. The

areas under the curve during the first bite and

the second bite are a measure of the work done in

the compression. The ratio of these two areas (A2/A1)

is defined as cohesiveness. The negative force peak in

the first decompression is defined as adhesive force

and the negative areas as adhesive work. The distance

that the product extends in the decompression is de-

fined as stringiness, and the distance that the product

recovers its height between the first and second com-

pressions is defined as springiness. Two other param-

eters are derived by calculation: gumminess is defined

as the product of hardness  cohesiveness; chewiness

is defined as the product of hardness  cohesiveness

 springiness. Gumminess is the energy required to

disintegrate a semisolid food while chewiness is the

energy required to masticate a solid food. It is incor-

rect to report gumminess and chewiness for the same

product.

0021In the early days, instrumental texture profile

analysis was an academic curiosity because of the

long time required to extract the information from

the force–time curves. With computer retrieval and

analysis of the data, the time requirement has been

reduced to the point where it can become a simple

routine test. The TA.XT2 texture analyzer is particu-

larly well suited to do this, and as a result measuring

TPA in the TA.XT2 is now a widely used procedure.

Rheological Measurements

0022Historically, rheology has been defined as the study

of the deformation and flow of matter or the response

SENSORY EVALUATION/Texture 5171

of materials to stress. The science of rheology, proven

so useful in the field of high polymers, has many

applications to food, including raw materials (e.g.,

grains, meats, fruits), intermediate products in the

manufacturing process (e.g., bread dough, cheese

curd, sausage emulsions), and finished products

(all foods). However, rheology does not cover all

aspects of texture. The size reduction that occurs

during mastication is not rheology, neither are the

sensations of oiliness, moistness, and particle size

and shape.

0023 Most people believe that there is a sharp distinction

between solids (which do not flow) and liquids

(which flow). In fact, the distinction between solids

and liquids is far from clear because many liquids

possess some of the properties of solids, and many

solids possess some of the qualities of liquids. The

science of rheology specializes in the study of

these complex materials (of which there are many

examples in foods) that are partly solid and partly

liquid.

0024 The flow of liquids may be divided into several

broad classes:

0025Newtonian flow, in which the shear rate* is dir-

ectly proportional to the shear stressy. Examples

are edible oils, sugar syrups, milk, and honey. The

measurement of the properties of these foods is

straightforward, since viscosityz is independent

of shear rate.

0026Plastic or Bingham flow, in which a minimum

shear stress, known as the ‘yield stress,’ must be

exceeded before flow begins. Examples are tomato

ketchup, whipped egg white, mayonnaise, margar-

ine, and butter.

First bite Second bite

Downstroke DownstrokeUpstroke

Hardness 1

Hardness 2

Fracturability

Stringiness

Area 3

Area 1

Force

Adhesive force

Area 2

Springiness

Upstroke

Time

fig0002 Figure 2 A generalized texture profile analysis curve. From Bourne MC (1978) Texture profile analysis. Food Technology 32: 63, with

permission.

*Shear rate (denoted by the symbol g

and expressed in s

1

) is the

velocity gradient established in a fluid that results from the appli-

cation of a shear stress.

yShear stress (denoted by s and expressed in Pa) is the force per unit

area applied tangential to the plane on which the force acts.

zViscosity (denoted by Z and expressed in Pa s) is the internal

friction of a fluid or its tendency to resist flow (Z ¼ s/

gg); it should

only be used for Newtonian fluids. Apparent viscosity (denoted by

) is the viscosity of a non-Newtonian fluid expressed as the

viscosity of a Newtonian fluid at a specified shear rate (e.g., Z

is the apparent viscosity of a non-Newtonian fluid at a shear

rate of 50 s

1

5172 SENSORY EVALUATION/Texture

3.0027 Pseudoplastic flow, in which an increasing shear

force gives a more than proportionate increase in

shear rate, i.e., apparent viscosity decreases with

increasing shear rate. Salad dressings exemplify

this type of flow.

0028 Dilatant flow, in which equal increments in shear

stress give less than equal increments in shear rate,

i.e., apparent viscosity increases with increasing

shear rate. This type of flow is rare in foods but

is found in high-solid suspensions of raw starch

and some chocolate syrups. (Refer to individual

foods.)

0029 The differences between these types of flow are

shown in Figure 3. The shear-stress versus shear-rate

plot (Figure 3a) is the more common manner of pre-

sentation; another that is sometimes used is shown in

Figure 3b, in which the axes are interchanged.

Time Dependency

0030 For some fluids the shear stress is a function of both

the shear rate and the time it is subjected to shear. For

thixotropic products the apparent viscosity decreases

with time of shearing. This condition is frequently

found in food systems such as gum solutions and

starch pastes. Rheopectic products increase in

apparent viscosity with time of shearing. This type

of behavior is rare in foods.

Viscoelasticity

0031 As discussed above, most foods combine some of

the properties of ideal liquids, which exhibit only

viscosity (flow) and ideal solids which exhibit only

elasticity (deformation). These are called viscoelastic

foods. In characterizing these systems rheologically it

is necessary to measure both the viscous component

(loss modulus, G

) and the elastic component (stor-

age modulus. G

). A food with high G

and low G

behaves more like a solid than a liquid, while a food

with low G

and high G

behaves more like a liquid

than a solid. An elastic solid such as rock candy will

have a G

value of zero, while a Newtonian liquid

such as sucrose syrup will have a G

value of zero.

0032Traditionally, viscoelasticity was measured by per-

forming creep tests in which a weight was placed on

the test material and the change in specimen height

was monitored over a period of time. Presently, visco-

elasticity is usually measured by an oscillation test in

which a sample of known dimensions (if solid) or filled

into the space between a cone and plate or parallel

plates or a cup with standard dimensions (if liquid) is

subjected to repeated small sinusoidal deformations

that do not fracture the sample. Analysis of the

resulting shear stress versus time curves yields the

numerical values of G

and G

. Modern instrumenta-

tion provides a computerized system for quantifying

these moduli. The rate and degree of deformation may

be varied to provide information on the internal struc-

ture and mechanical behavior of the material. (See

Food Acceptability: Affective Methods.)

See also: Food Acceptability: Affective Methods;

Sensory Evaluation: Sensory Characteristics of Human

Foods; Food Acceptability and Sensory Evaluation;

Sensory Difference Testing; Sensory Rating and Scoring

Methods

Plastic or Bingham

Pseudoplastic

Newtonian

Dilatant

Shear stress (σ)

Shear stress(σ)Shear rate (γ)

Shear rate (γ)

fig0003 Figure 3 (a) Shear-stress versus shear-rate plots for various types of flow. (b) Shear-rate versus shear-stress plots for the same

types of flow. From Bourne MC (1982), Food Texture and Viscosity: Concept and Measurement. New York: Academic Press, with

permission.

SENSORY EVALUATION/Texture 5173

Further Reading

Aguilera JM and Stanley DW (1999) Microstructural Prin-

ciples of Food Processing and Engineering, 2nd edn.

Gaithersburg, Maryland: Aspen Publishers.

Bourne MC (2002) Food Texture and Viscosity: Concept

and Measurement, 2nd edition. New York: Academic

Press.

Christensen CM (1984) Food texture perception. Advances

in Food Research 29: 159–199.

De Man JM, Voisey PW, Rasper VF and Stanley DW (eds)

(1976) Rheology and Texture in Food Quality. West-

port, Connecticut: AVI Publishing.

Lawless HT and Heymann H (1999) Sensory Evaluation of

Food – Principles and Practices. Gaithersburg, Mary-

land: Aspen Publishers.

Meilgaard M, Civille GV and Carr BT (1999) Sensory

Evaluation Techniques, 3rd edn. Boca Raton, Florida:

CRC Press.

Mohsenin NN (1970) Application of engineering tech-

niques to evaluation of texture of solid food materials.

Journal of Texture Studies 1: 133–154.

Mun

oz AM and Civille GV (1987) Factors affecting percep-

tion and acceptance of food texture by American con-

sumers. Food Reviews International 3: 285–322.

Rao MA and Steffe JF (1992) Viscoelastic Properties of

Foods. New York: Elsevier Applied Science.

Rosenthal AJ (1999) Food Texture – Measurement and

Perception. Gaithersburg, Maryland: Aspen Publishers.

Sherman P (ed.) (1979) Food Texture and Rheology. New

York: Academic Press.

Szczesniak AS (1987) Correlating sensory with instrumen-

tal texture measurements – an overview of recent devel-

opments. Journal of Texture Studies 18: 1–15.

Szczesniak AS (1991) Textural perceptions and food

quality. Journal of Food Quality 14: 75–85.

Szczesniak AS and Kahn E (1971) Consumer awareness of

and attitudes to food texture I. Adults. Journal of Tex-

ture Studies 2: 280–295.

Aroma

D S Mottram and J S Elmore, The University of

Reading, Whiteknights, Reading, UK

Aroma

0001 The sense of smell (olfaction) is one of the two chem-

ical senses in humans, the other being the sense of

taste. The term ‘odor’ has been defined by the British

Standards Institute as the quality of the sensation

perceived, via the olfactory organ situated in the

nasal cavity, from certain volatile compounds; aroma

is an odor with a pleasant connotation. The olfactory

organ is extremely sensitive, and hence the concen-

trations of volatile compounds in foods contributing

to aroma (odorants) may be very low. These complex

mixtures of volatile compounds may be analyzed

by extraction, followed by separation and character-

ization.

Olfaction

0002The olfactory receptors are sited in the olfactory epi-

thelia, two small patches of mucous membrane in the

recesses of the nasal passages (Figure 1). The response

of these receptors to airborne molecules determines

the odor of a substance. Molecules entering the nose

find their way to the olfactory epithelia where they

come into contact with cilia (hairs) on the receptor

cells. These produce a change in electrical potential

and a signal travels along nerve fibers to the olfactory

bulb, part of the brain. There are approximately 100

million olfactory receptor cells in the human nose.

0003Olfaction plays a primary role in the perception of

flavor in foods and beverages. When food is eaten

taste, smell and mouthfeel act in concert to produce

the integrated sensation of flavor. The aroma of the

food is not just perceived in inhaled air. When food is

in the mouth, odorants can reach the olfactory recep-

tors by the retronasal route, entering the nasal cavity

by the same route as expired air.

Olfactory nerve

Olfactory bulb

Olfactory epithelium

Turbinate bones

Odor:

retronasal route

Odor:

nasal route

Nasal cavity

Tongue

fig0001Figure 1 Representation of a section through the nasal and

oral cavities showing the olfactory organ. From Thomson DMH

(1986) The meaning of flavour. In: Birch GG and Lindley MG (eds.)

Developments in Food Flavours, pp. 1–21. London: Elsevier Applied

Science, with permission.

5174 SENSORY EVALUATION/Aroma

Odor Sensitivity

0004 Odor perception is considerably more sensitive than

the perception of taste, by factors of one million to

one billion. As we are able to differentiate a large

number of odorants, aroma plays the major role in

delineating the characteristic flavor of a particular

food. The inability to differentiate flavors is usually

associated with anosmia (lack of a sense of smell)

rather than ageusia (lack of a sense of taste).

0005 It has been suggested that the triggering of one

human olfactory neurone by a powerful odorant may

require only eight molecules, and as few as 40

molecules can produce an identifiable sensation. The

odor threshold of a compound is defined as the min-

imum concentration at which the odor of a compound

can be detected. The range of thresholds exhibited by

odorants extends over at least 10 orders of magnitude

(Table 1). One compound with one of the lowest

reported odor thresholds is bis-(2-methyl-3-furyl) di-

sulfide, which has a meaty aroma and can be detected

at concentrations of 2 parts in 10

parts of water.

This concentration is equivalent to about 20 mg in the

water of a 50-m swimming pool of 1000 m

capacity!

At the other end of the range, at least 1 mg of ethanol

needs to be present in 10 ml of water before the smell

of ethanol can be detected.

Chemical Nature of Aroma Compounds in

Foods

0006 The range of chemical classes that contribute to food

aromas is diverse in both chemical composition and

physical properties, and includes aliphatic, alicyclic,

aromatic, and heterocyclic compounds. All aroma

compounds have some volatility, but this ranges

from permanent gases to compounds with very little

vapor pressure and molecular weights up to 300.

Approximately 10 000 different volatile compounds

have been identified in foods and beverages; complex

cooked foods such as coffee and meat contain in

excess of 1000 aroma compounds. Many of these

compounds are common to a number of different

foods, and the contribution of any compound to the

characteristic aroma of a particular food will depend

on a number of factors, including:

0007odor character;

0008concentration in product;

0009odor threshold;

0010vapor pressure;

0011adsorption on the food matrix;

0012interaction with other components;

0013synergism with other volatiles.

0014Some volatiles have little aroma and do not con-

tribute to food flavor, whereas others define the

aromas of specific foods (e.g., benzaldehyde –

almonds; citral – lemons). However, for most foods,

the aroma depends on contributions from a complex

mixture of volatiles comprising different chemical

classes. (See Essential Oils: Properties and Uses;

Flavor (Flavour) Compounds: Structures and

Characteristics.)

Characterization of Aroma Volatiles

0015In attempting to identify those compounds that char-

acterize different food aromas, flavor scientists have

analyzed the volatile composition of many different

foods. This usually involves the extraction of volatiles

from the food matrix, their concentration, and the

separation and identification of the individual com-

ponents (Table 2). As far as possible, the extract

should contain only the volatile components in the

same relative proportions as the original food, with-

out the introduction of artefacts. The wide range of

odor thresholds can result in quantitatively minor

tbl0001 Table 1 Odor thresholds of some aroma compounds in

aqueous solution

Compound Threshold (ppb, mgl

1

)

Ethanol 100 000

2,5-Dimethylpyrazine 1 800

Butyric acid 250

Limonene 10

Hexanal 5

Ethyl 2-methylbutyrate 1

Methanethiol 0.2

b-Ionone 0.007

2-Isobutyl-3-methoxypyrazine 0.002

Bis-(2-methyl-3-furyl)disulfide 0.00002

2,3,6-Trichloroanisole 0.0000003

tbl0002Table 2 Stages in the analysis of aroma volatiles

Step Analytical techniques

Isolation and concentration Headspace analysis

Adsorption

Distillation

Extraction

Separation Gas chromatography

Preparative liquid chromatography

Identification Chromatographic retention time

Mass spectrometry

Infrared spectroscopy

Nuclear magnetic resonance

spectroscopy

Chemical synthesis

Reference compounds

Sensory characterization Gas chromatography olfactometry

Sensory panel

SENSORY EVALUATION/Aroma 5175

components making a major contribution to the

aroma, whereas some large components will have no

sensory significance. Successful isolation and identifi-

cation of minor components with aroma significance

is one of the major challenges of aroma analysis.

Isolation and Concentration of Aroma Volatiles

0016 The components of food that are responsible for

aroma are present in extremely small quantities com-

pared with the major constituents, of which water is

usually the most abundant. A number of isolation

techniques exist, all based on utilizing the volatile

nature of the aroma compounds to separate them

from the food matrix.

0017 Headspace analysis The concentration of volatiles

in the headspace vapor above a food or beverage will

be very low. However, it will contain a representative

mixture of those compounds that contribute to the

aroma of the product, and therefore, analysis of that

headspace will provide the best method of obtaining a

sample representative of the food aroma. However,

direct introduction of a given volume of the vapors

above a food sample on to a gas chromatographic

column is rarely used, because the volatiles are not

present in sufficiently high concentrations and be-

cause water from the sample will often cause interfer-

ence. Purging the headspace volatiles with a stream of

inert gas (nitrogen or helium) and condensing the

volatiles in a series of traps cooled by ice, solid carbon

dioxide, or liquid nitrogen will result in a concen-

trated aroma condensate. Extraction of the conden-

sate with a small amount of a suitable solvent provides

an aroma extract suitable for chromatographic analy-

sis. Instead of using cold traps, headspace volatiles

can be collected on suitable adsorbents.

0018 Adsorption The ability of certain solid surfaces to

reversibly adsorb volatile organic molecules is widely

used in the analysis of aroma compounds. Activated

charcoal was the first to be used, but now, certain

porous polymers, in particular Tenax TA, are fre-

quently employed. Like charcoal, all of these mater-

ials readily adsorb volatiles while having little affinity

for water, making them particularly useful in the

analysis of samples with a high water content.

0019 In a typical collection, a stream of a purified inert

gas (helium or nitrogen) sweeps the volatiles from the

sample flask into a small tube containing 10–200 mg

of the adsorbent. Adsorbed volatiles can be heat

desorbed directly on to a gas chromatographic

column by placing the adsorbent trap in a specially

modified injection port, thus avoiding any loss of

components or unnecessary dilution. Cooling the

front of the column (‘cryofocusing’) with solid carbon

dioxide or liquid nitrogen during this desorption will

avoid any loss in chromatographic resolution. Alter-

natively, volatiles may be removed from the trap by

passing a small amount of a low-boiling-point solvent

through the adsorbent. Concentration of the collected

solution by careful evaporation of the solvent pro-

vides an aroma extract for gas chromatography (GC).

0020Solid-phase microextraction This is an adaptation

of traditional adsorption methods in which a small

fused silica fiber, coated in an adsorbent material, is

mounted in a syringe-like device (Figure 2). The

needle is pushed through a septum, and the fiber is

exposed to the headspace above the food or beverage

sample, which is sealed in a suitable container. Vola-

tile compounds are adsorbed on to the fibre, and at

the end of the extraction, the fiber can be removed

from the sample vessel and directly desorbed into the

split/splitless injector of a gas chromatograph. Again,

cryofocusing of the aroma volatiles on the front of the

gas chromatographic column is performed to insure

no loss of resolution. Solid-phase microextraction

(SPME) has evolved rapidly since its introduction in

the early 1990s, and fibres coated with Carboxen

are suitable for the analysis of a wide range of

aroma compounds. SPME can be automated, and its

ease of use, relatively low cost, and affinity for a large

range of compounds would suggest that it will

become a widely used technique for the isolation of

aroma volatiles.

Pierce sample

septum with

SPME needle

Expose fiber and

commence

extraction

Retract fiber at the end

of extraction

fig0002Figure 2 Principle of solid-phase microextraction.

5176 SENSORY EVALUATION/Aroma