Texture in food volume 2

Texture in food

Texture in food

Volume 2: Solid foods

Related titles from Woodhead’s food science, technology and nutrition list:

Texture in food Volume 1: Semi-solid foods (ISBN 1 85573 673 X)

Understanding and controlling the texture of semi-solid foods such as yoghurt and ice cream is a complex process With a distinguished international team of contributors, this important collection summarises some of the most significant research in this area The first part of the book looks at the behaviour of gels and emulsions, how they can be measured and their textural properties improved The second part of the collection discusses the control of texture in particular foods such as yoghurt, ice cream, spreads and sauces.

Understanding and measuring the shelf-life of food (ISBN 1 85573 732 9)

The shelf-life of a product is critical in determining both its quality and profitability This important collection reviews the key factors in determining shelf-life and how they can be measured.

Taints and off-flavours in foods (ISBN 1 85573 449 4)

Taints and off-flavours are a major problem for the food industry The first part of this important collection reviews the major causes of taints and off-flavours, from oxidative rancidity and microbiologically-derived off-flavours, to packaging materials

as a source of taints The second part of the book discusses the range of techniques for detecting taints and off-flavours, from sensory analysis to instrumental techniques, including the development of new rapid, on-line sensors.

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Texture in food

Volume 2: Solid foods

Edited by David Kilcast

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List of abbreviations

Part I Consumers, texture and food quality

1 Measuring consumer perceptions of texture: an

overview

D Kilcast, Leatherhead Food International, UK

1.1 Introduction: texture and food quality

1.2 Perception and sensory assessment of food

texture

1.3 Tests and test procedures

1.4 Instrumental measurement of texture

1.5 In vivo texture measurement

J-F Meullenet, University of Arkansas, USA

2.1 Introduction: problems with consumer descriptions oftexture

2.2 Investigating consumer descriptions of texture 2.3 Tests and test procedures

2.4 Understanding consumer preferences

2.5 Challenges to understanding consumer

preferences

2.6 Future trends

2.7 Conclusions

2.8 References

3 Texture and mastication

A C Smith, Institute of Food Research, UK

P Mallikarjunan, Virginia Polytechnic Institute and

State University, USA

4.1 Introduction

4.2 Characterization and determination of crispness4.3 Methods of data correlation, evaluation and analysis4.4 Case-study: breaded chicken nuggets

4.5 Future trends

4.6 References

Part II Instrumental techniques for analysing texture

5 Force/deformation techniques for measuring texture

R Lu and J A Abbott, USDA Agricultural Research Service, USA

6 Sound input techniques for measuring texture

L M Duizer, Massey University, New Zealand

7 Near infrared (NIR) diffuse reflectance in texture measurement

S Millar, Campden and Chorleywood Food Research Association, UK

7.1 Introduction

7.2 Application of NIR to cereals and their products7.3 Application of NIR to fruit and vegetables

7.4 Application of NIR to meat

7.5 Application of NIR to other foods

7.6 Conclusions and future trends

7.7 Sources of further information

7.8 References

8 Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) in texture measurement

A K Thybo, A H Karlsson, H C Bertram and

H J Andersen, Danish Institute of Agricultural Sciences,

P M Szczypinski, Technical University of Lodz, Poland and S Donstrup, Aarhus University Hospital, Denmark

8.1 Introduction

8.2 Methods and analysis

8.3 Application of NMR: texture determination of solidfoods

8.4 Application of MRI: texture determination of solidfoods

8.5 Future trends

8.6 References

9 Modelling food texture

L M M Tijskens and H Luyten, Wageningen University and Research Centre, The Netherlands

9.1 Introduction

9.2 Factors affecting texture

9.3 Effects of enzymes on texture

9.4 Applying models to predict texture

10 Plant structure and fruit and vegetable texture

K W Waldron, Institute of Food Research, UK

10.1 Introduction

10.2 Measurement of texture

10.3 Plant structure

10.4 Cellular basis of crispness, juiciness and mealiness infruit tissue

10.5 Cellular stability during processing

10.6 Improving cell adhesion

10.7 Future trends

10.8 Acknowledgements

10.9 References

11 Plant compounds and fruit texture: the case of pear

T Kojima, S Fujita and M Tanaka, Saga University, Japan and P Sirisomboon, King Mongkut’s Institute

of Technology Ladkrabang, Thailand

11.1 Introduction: variations in pear texture

11.2 Measuring and modelling fruit firmness

11.3 Chemical compounds affecting firmness: the example

12.4 PPOs, PODs and texture development

12.5 Controlling PPO and POD activity

12.6 PPOs and PODs: implications for food texture 12.7 Future trends

12.8 Sources of further information

12.9 References

13 Improving fruit and vegetable texture by genetic transformation

G Tucker, University of Nottingham, UK

13.1 Introduction

13.2 Tools of genetic modification

13.3 Approaches to the manipulation of texture: thetomato

13.4 Other approaches to the manipulation of texture13.5 Future trends

14.2 Vegetable texture determined by starch

14.3 Vegetable texture determined by cell wall

15.2 Vacuum infusion technology

15.3 Applications to improve texture

M Suutarinen and K Autio, VTT Biotechnology, Finland

16.1 Introduction: the effects of freezing and thawing

17 Improving the texture of processed fruit: the case

of olives

I Mafra, University of Porto and M A Coimbra, University

of Aveiro, Portugal

17.1 Introduction: the texture of table olives

17.2 Factors affecting the texture quality of raw olives17.3 Influence of processing on table olives

19 Analysing and improving the texture of cooked rice

S K Kim, Dankook University and C O Rhee,

Chonnam National University, Korea

19.1 Introduction

19.2 Criteria for evaluating rice quality

19.3 Hydration of rice

19.4 Factors affecting cooking quality

19.5 Testing texture quality

19.6 Problems and challenges

19.7 Sources of further information and advice

19.8 References

20 Improving the texture of pasta

B A Marchylo and J E Dexter, Canadian Grain Commission and L J Malcolmson, Canadian International Grains Institute

20.1 Introduction

20.2 Measuring the texture of cooked pasta

20.3 Influence of raw materials

20.4 Influence of processing

20.5 Trends in consumer preference

20.6 References

21 Improving the texture of fried food

C-J Shieh and C-Y Chang, Da-Yeh University and C-S Chen, Chao-Yang University of Technology, Taiwan

21.1 Introduction

21.2 Measuring texture

21.3 Factors influencing texture

21.4 The use of response surface methodology (RSM)21.5 A case study: fried gluten balls

21.6 Conclusions

21.7 References

Contributor contact details

(* = main point of contact)

Institute of Food ResearchNorwich Research Park, ColneyNorwich

NR4 7UAUK

Tel: +44 (0) 1603 255286Fax: +44 (0) 1603 507723E-mail: andrew.smith@bbsrc.ac.uk

Chapter 4

Dr P MallikarjunanBiological Systems EngineeringDepartment

312 Seitz HallVirginia Polytechnic Institute andState University

Blacksburg VA 24060USA

Tel: +1 (540) 231 7937Fax: +1 (540) 231 3199

Tel: +44 (0) 1386 842157Fax: +44 (0) 1386 842150E-mail: s.millar@campden.co.uk

Chapter 8

Dr A K Thybo*

Department of Food ScienceDanish Institute of AgriculturalSciences

DK-5792 AarslevDenmark

Tel: +045 63 90 43 05Fax: +045 63 90 43 95E-mail: anette.thybo@agrsci.dk

Dr A H Karlsson, Dr H C.Bertram and Dr H J AndersenDepartment of Food ScienceDanish Institute of AgriculturalSciences

DK-8830 TjeleDenmark

Tel: +045 89 99 19 00Fax: +045 89 99 15 64E-mail: HanneC.Bertram@agrsci.dk

Dr S DonstrupAarhus University HospitalDepartment of BiomedicalEngineering

DK-8200 Aarhus NDenmark

Institute of Food Research

Norwich Research Park

Tel: +81 952 288 750Fax: +81 952 288 768E-mail: kojimat@cc.saga-u.ac.jp

Dr P Sirisomboon*

Department of AgriculturalEngineering

Faculty of EngineeringKing Mongkut’s Institute ofTechnology LadkrabangBangkok 10520

Thailand

Tel: +66 2737 300 ext 5120Fax: +66 2326 4178E-mail: kspanman@kmitl.ac.th

Chapter 12

Dr H J Wichers and Dr C BoeriuAgrotechnology & Food InnovationsBornsesteeg 59

6708 PD WageningenThe Netherlands

Tel: +31 317 475228Fax: +31 317 475347E-mail: harry.wichers@wur.nlcarmen.boeriu@wur.nl

Chapter 13

Professor G TuckerUniversity of NottinghamSchool of Biosciences

Sutton Bonington Campus

Chapter 17

Professor M A Coimbra*

Department of ChemistryUniversity of Aveiro

PT 3810-193 AveiroPortugal

Tel: +1 351 234 370 706Fax: +1 351 234 370 084E-mail: mac@dq.ua.pt

Dr I MafraREQUIMTELaboratory of BromatologyFaculty of PharmacyUniversity of Porto

R Anibal Cunha, 164

PT 4050-047 PortoPortugal

Tel: +351 22 2078902Fax: +351 22 2003977E-mail: isabel.mafra@ff.up.pt

Chapter 18

Dr S P CauvainCampden & Chorleywood FoodResearch AssociationChipping CampdenGloucestershireGL55 6LDUK

Tel: +44 (0) 1386 842 000Fax: +44 (0) 1386 842 150E-mail: s.cauvain@campden.co.uk

Grain Research Laboratory

Canadian Grain Commission

1000-303 Main StrectWinnipeg MB R3C 3G7Canada

Tel: +1 (204) 983 8584Fax: +1 (204) 983 2642E-mail: lmalcolmson@cigi.ca

Chapter 21

Dr C-J Shieh and Dr C-Y ChangDepartment of Food EngineeringDa-Yeh University

Cheng-HwaTaiwan 515

Taiwan

Fax: +886 4 2374 2341E-mail: csc@mail.cyut.edu.tw

AACC American Association of Cereal Chemists

MassL, MassR masseter left, right

PIHMI paired increasing-height multiple-impacting

SEM scanning electron microscope

TempL, TempR temporalis left, temporalis right

Part I

Consumers, texture and food quality

1.1 Introduction: texture and food quality

In prosperous societies, we have available an enormous and ever-increasingrange of foods, and manufacturers find themselves in an intensely competitivesituation In less well-developed societies, hunger will be the constant drivingforce, and our diet will be determined by availability of any food that satisfiesour basic nutritional needs It is increasingly clear that if we are to understandwhat drives consumers’ choice of food, no single factor can be considered inisolation from others For some years, psychology researchers have beendeveloping models to understand consumer behaviour (e.g Shepherd andSparks, 1994) Although there are many possible circumstances under whichnon-sensory factors such as price and nutritional image can have dominanteffects, the sensory characteristics of foods are central to their continuedpurchase

The importance of a holistic approach is also becoming more clear when thecomponents of sensory perception are considered During the sequences ofactions that constitute food consumption, we perceive a whole range of differentcharacteristics relating to the appearance, flavour and texture of the food.Numerous tools are available for investigating the sensory properties of foods,and the information required must be carefully defined if appropriate tools are

to be selected Systematic development of new products will inevitably depend

on the use of different tools at different stages of the development cycle

1.1.1 The human senses

It is generally accepted that human beings have five senses in operation,namely sight, smell, taste, touch and hearing, although warmth, cold, movement

and pain may also be considered as senses of importance in a food context(Fig 1.1) Foods are complex mixtures of chemical compounds, arrangedinto structural units The perception of the sensory characteristics of foodsresults from the stimulation of all our senses to some extent by thephysicochemical properties of the foods The sensory characteristics of foodare generally grouped into three categories, namely appearance, flavour andtexture These categories are, however, not independent of one another Forexample, colour, which is obviously an important appearance characteristic,can be shown to have an influence on flavour perception; consumers willassign higher scores for flavour intensity to darker foods than to lighterfoods The interaction between appearance and flavour is referred to as

‘visual flavour’ Similarly, textural characteristics such as viscosity caninfluence the perception of flavour, and some flavour characteristics, e.g.acidity, can affect textural characteristics One means of defining flavour,texture and appearance is by taking into account the fact that each can beattributed to the stimulation of one or possibly two of the senses On thisbasis the International Standards Organisation (ISO, 1992) has proposedworking definitions for flavour, texture and appearance, as given below

• Appearance: sensory characteristics of foods perceived largely by way of

the visual sense Input from other senses, especially smell, may contribute

• Flavour: the combination of taste and odour Pain, heat, cold, tactile and

visual sensations may also contribute

• Texture: sensory characteristics perceived largely by way of the senses of

movement and touch Input from other senses, especially vision and taste,may sometimes contribute

The above definitions give little information on how the senses are used inthe perception of quality attributes Appearance is sometimes, mistakenly,equated only with colour, and yet many other visual aspects of form, shape,translucency, etc., may influence our use of the visual senses Taste (gustation)

SOUND

TEXTURE APPEARANCE

FLAVOUR

Taste Odour Irritant Vision Gustation Olfaction Trigeminal Touch Hearing

Fig 1.1 Schematic diagram of the human senses and their operation in the

perception of food quality.

is strictly defined as the response by the tongue to soluble, involatile materials.These have classically been defined as four primary basic taste sensations:salt, sweet, sour and bitter, although umami, the sensation associated withmonosodium glutamate, is now widely recognised as a basic taste This list

is frequently extended further to include sensations such as metallic andastringency The taste receptors are organised groups of cells, known as tastebuds, located within specialised structures called papillae These are locatedmainly on the tip, sides and rear upper surface of the tongue Taste stimuliare characterised by the relatively narrow range between the weakest and the

strongest stimulants (ca 104), and are strongly influenced by factors such as

temperature and pH (Meilgaard et al., 1999).

The odour response is much more complex, and odours are detected asvolatiles entering the nasal passage, either directly via the nose or indirectlythrough the retronasal path via the mouth The odorants are sensed by theolfactory epithelium, which is located in the roof of the nasal cavity Some150–200 odour qualities have been recognised, and there is a very wide

range (ca 1012) between the weakest and the strongest stimulants (Meilgaard

et al., 1999) The odour receptors are easily saturated, and specific anosmia

(blindness to specific odours) is common It is thought that the wide range

of possible odour responses contributes to variety in flavour perception.Both taste and odour stimuli can be detected only if they are released effectivelyfrom the food matrix during the course of mastication

The chemical sense corresponds to a pain response through stimulation ofthe trigeminal nerve This is produced by chemical irritants such as gingerand capsaicin (from chilli), both of which give a heat response, and chemicalssuch as menthol and sorbitol, which give a cooling response With the exception

of capsaicin, these stimulants are characterised by high thresholds Thecombined effect of the taste, odour and chemical responses gives rise to thesensation generally perceived as flavour, although these terms are often usedloosely

Texture is perceived by the sense of touch, and comprises two components:somesthesis, a tactile, surface response from skin, and kinesthesis (orproprioception), which is a deep response from muscles and tendons Formany foods, visual stimuli will generate an expectation of textural properties.The touch stimuli themselves can arise from tactile manipulation of the foodwith the hands and fingers, either directly or through the intermediary ofutensils such as a knife or spoon Oral contact with food can occur throughthe lips, tongue, palate and teeth, all of which provide textural information

1.1.2 Texture and food enjoyment

Most studies which have investigated the importance of different sensorymodalities on consumer acceptability conclude that flavour is the mostimportant modality, followed by texture and then appearance (e.g Moskowitzand Krieger, 1995) Such conclusions do not reflect the enormous efforts

that the food industry devotes to designing appealing textural characteristics,and to maintaining those characteristics to long-term production Researchwith consumers in the USA carried out by Szczesniak and Kahn (1971)showed that awareness of texture lies at a subconscious level, and that texturalproperties are taken for granted If the expectations of texture are violated,however, awareness of textural defects is accentuated, and texture becomes afocal point for criticism and rejection of the food Expectations are beingincreasingly recognised as important factors in food choice by consumers(e.g Vickers, 1991; Cardello, 1994).

1.1.3 The interactive role of texture

In addition to its direct contribution to consumer acceptance, texture has avitally important secondary effect, through modulation of flavour release Ifflavour components are to be perceived, they must be released from the foodmatrix in order to reach the appropriate receptors This release of flavour isintimately related to the way in which the food structure breaks down in themouth, and consequently to both the initial texture of the food and the change

in texture throughout mastication (Section 1.2) In addition, the structuralfactors that deliver a specif ic texture can also influence appearancecharacteristics, for example the glossy surface of chocolate confectionery

1.1.4 Texture and product design

Texture and food structure are inextricably linked; the micro- and structural composition of foods will determine the sensory perception, andany change in structure carries the risk of changing perceived texture andviolating consumer expectations Industry therefore needs to take great care

macro-to ensure that products with key textural characteristics, such as snack foodsand confectionery products, are manufactured to consistently high quality.This can present an enormous challenge for foods relying on primarycomponents such as meat and vegetables that are naturally subject to highvariability, and for any processed food manufactured on high-volume productionlines Product modifications, for example to produce low-fat variants, canintroduce structural changes that can generate substantial textural modifications.Industry therefore needs methods to measure textural characteristics However,designing suitable measurement systems requires an understanding of themechanisms by which texture is perceived

1.2 Perception and sensory assessment of food texture

1.2.1 Oral breakdown processes

The importance of the interaction between the texture of foods and theirperceived flavour can be clearly seen if the sequence of events during food

consumption is considered A strong expectation of the flavour and texturecharacteristics can be generated before the food is introduced into the mouth.

As food enters the mouth, and is either bitten or manipulated between tongueand palate, catastrophic changes occur to the structure of the food that stronglyinfluence the way in which tastants and odorants are released from it Ofparticular importance are temperature increase (cold foods) or decrease (hotfoods) and dilution by saliva Salivary introduction also serves to lubricatethe food bolus The factors that influence such release are under active study

(e.g Overbosch et al., 1991), and include:

• rate and mode of production of new surface

• rate of production of saliva

• dissolution and dispersion of the food

• release of involatile tastants and volatile odorants

• transport of volatiles to the nasal cavity

1.2.2 Oral food management

Even the complex picture of food breakdown described previously is anover-simplification of actual oral processes (Heath and Prinz, 1999), whichshow substantial differences for different foods and between individuals Thefirst bite by the incisors is an important stage which generates an earlytextural response that can influence subsequent chewing actions The food isthen transported between the cheek teeth, the jaw closes and main-sequencemastication starts Hard foods are comminuted into particles, which are thenformed into a soft bolus with saliva Before this bolus is swallowed, thetongue is used to clear any remaining particles Hutchings and Lillford (1988)have described two thresholds that need to be satisfied before swallowing isinitiated: a food particle size threshold and a lubrication threshold Finally,debris can be left in the mouth after swallowing, and further clearance andswallows may be necessary

1.2.3 Mechanisms of texture perception

Either of the two mechanisms described in Section 1.1.1 (proprioception andsomesthesis) can operate during the mastication process, depending on thenature and texture of the food The texture of solid foods is perceived primarilythrough proprioception, as the food is chopped by the incisors and ground bythe molars As the physical state of the food changes dramatically duringmastication, both mechanisms can be operative In particular, during themastication of solid foods somesthesis becomes important as the bolus isformed and manipulated However, even at the early stages of mastication(and prior to mastication, through use of fingers and lips), somesthesis cangive important textural sensations The texture of semi-solid and liquid foods

is perceived primarily through somesthesis, from the action of the tongue

and the soft palate, and is usually expressed by the term mouthfeel.

1.2.4 Sensory assessment of texture

A sensory stimulus to a human subject produces a set of physiological sensationsthat are interpreted by the brain as perceptions The sensations will varyconsiderably between subjects, reflecting the natural physiological differences

in the population, and the interpretation by the brain will be modified bypsychological differences Perceptions are recorded as actions by the subject,

or as verbal responses This may be the selection of a certain sample from agroup (as in a difference test), words to describe the nature of a perception,and numbers to measure the size of a perception To ensure that the subjects’responses relate as closely to their perception as possible, it is necessary touse carefully controlled environmental conditions and test procedures tocarry out experiments In particular, it is important to minimise the manysources of psychological bias that can produce unwanted influences onresponses, and it is frequently necessary to minimise the spread of physiologicalresponses characteristic of biological systems through careful panel selectionand training procedures

1.3 Tests and test procedures

1.3.1 Procedures

A basic classification of the main sensory test procedures is shown inFig 1.2 The primary classification is into analytical tests and hedonic tests.Analytical tests use trained panels as a form of analytical instrument togenerate information on the sensory properties of the food, whereas hedonictests measure the response of untrained consumers to the sensory properties

in terms of liking or acceptability Different psychological procedures form

Sensory test methods

Acceptability scaling Paired preference

Qualitative Profile Paired comparison

Duo-trio

Triangle

Fig 1.2 Classification of the main sensory testing procedures.

the basis for this test distinction, and the information produced is distinct butcomplementary.

Discrimination tests are commonly used to establish, for example, if aformulation modification has changed the sensory quality Although thetests can be a sensitive measure of change, and more recently have beengeneralised from their traditional use as difference tests to permit testing for

similarity (Meilgaard et al., 1999), they generate relatively little information.

The most informative analytical procedures identify the sensory propertiesthat are characteristic of that food, and quantify the individual characteristics;these are termed sensory profile methods Numerous sensory profile methodshave been developed for foods, but the most important in practical use arethe Texture Profile Method, Quantitative Descriptive Analysis and the SpectrumMethod (reviewed in Kilcast, 1999)

1.3.2 Difference tests

Paired comparison test

In the most common form of the test, two coded samples are presented eithersequentially or simultaneously in a balanced presentation order (i.e AB andBA) There are two variations on the test In the directional difference variant,the panellists are asked to choose the sample with the greater or lesseramount of a specified characteristic The panellists are usually instructed tomake a choice (forced-choice procedure), even if they have to guess, or theymay be allowed to record a ‘no-difference’ response

Duo-trio test

In the most common variant of the duo-trio test, the panellists are presentedwith a sample that is identified as a reference, followed by two coded samples,one of which is the same as the reference and the other different Thesecoded samples are presented in a balanced presentation order, i.e

The panellists are asked to identify which sample is the same as the reference.The duo-trio test is particularly useful when testing foods that are difficult toprepare in identical portions Testing such heterogeneous foods using thetriangle test, which relies on identical portions, can give rise to practicaldifficulties, but in the duo-trio test there are no major difficulties in asking

the question: Which sample is most similar to the reference?

Triangle test

Three coded samples are presented to the panellists, two of which are identical,using all possible sample permutations, i.e

3-AFC (Alternative Forced Choice) test

This less common procedure uses one-half of the same sample permutationsfrom the triangle test in a triad format, but either the difference of interestbetween the samples is revealed to the panellists in advance, or the panellistsidentify the nature of any difference in advance In the test itself, the panellistsare then asked to identify the sample (or samples) with the specified

characteristic For example, a typical instruction might be: One of these

samples is more bitter than the others; please identify this sample O’Mahony

(1995) has identified the reasons why this test can be more sensitive than thetriangle test, but the test suffers from the need to identify the nature of thedifference positively in advance

R-index test

This short-cut signal-detection method (O’Mahony, 1979; 1986) is less wellused but has found applications in industry The test samples are comparedagainst a previously presented standard, and rated in one of four categories.For difference testing, these categories are standard, perhaps standard, perhapsnot standard and not standard The test can also be carried out as a recognitiontest, in which case the categories are standard recognised, perhaps standardrecognised, perhaps standard not recognised and standard not recognised.The results are expressed in terms of R-indices, which represent probabilityvalues of correct discrimination or correct identification The method isclaimed to give some quantification of magnitude of difference, but its usehas not been widely reported in the literature One important limitation isthat a relatively high number of judgements is needed in this form of test,leading to the risk of severe panellist fatigue

Difference from control test

The test is of particular value when a control is available; the panellists arepresented with an identified control and a range of test samples They areasked to rate the samples on suitable scales anchored by the points ‘notdifferent from control’ to ‘very different from control’ The test results areusually analysed as scaled data

1.3.3 Quantitative descriptive tests

The Texture Profile Method

The Texture Profile Method was developed by the General Foods Companyspecifically to define and measure the textural parameters of foods Panellistsare selected on the basis of ability to discriminate known textural differences

in the specific product application for which the panel is to be trained (solidfoods, beverages, semi-solid foods, skin care products, fabrics and papergoods) Panellists selected for training are exposed to a wide range of productsfrom the category under investigation, to provide a wide frame of references.The characteristics of the product, the order of appearance and the degree towhich each is present are determined Attributes are usually evaluated in thefollowing order:

1 surface characteristics (can be visual);

2 initial compression (perceived on first bite);

3 masticatory phase (perceived during chewing);

4 residual phase (changes made during mastication and often perceivedafter swallowing)

In addition to the mechanical (e.g firmness, adhesiveness, viscosity,springiness, cohesiveness) and geometrical (e.g flakiness, grittiness, beady,crystalline) characteristics evaluated during the initial compression andmasticatory stages, auditory characteristics such as crunchiness, crackliness

or crispness might be evaluated The panel verdicts are derived by groupconsensus or by statistical analysis of the data Results are displayed intabular or graphic form

Quantitative Descriptive Analysis (QDA)

QDA is a total system covering sample selection, panellist screening, vocabulary

development, testing and data analysis (Meilgaard et al., 1999) Variants of

the original QDA procedures are probably used more than any other profilingprocedure The QDA technique uses small numbers of highly trained panellists.Typically, 6 to 12 people are screened for sensory acuity and trained toperform the descriptive task to evaluate the product Three major steps arerequired: development of a standardised vocabulary, quantification of selectedsensory characteristics and analysis of the results using parametric statistics.Development of the vocabulary is a group process for creating a completelist of descriptors for the products under study Panellists freely describe theflavour, appearance, odour, mouthfeel, texture and aftertaste characteristics

of different samples No hedonic (good or balanced ), general (full or typical )

or intensity-based (strong or weak) terms are permitted Terminology should

be consistent from product to product and tied to reference materials Thereferences decrease panellist variability, reduce the amount of time necessary

to train sensory panellists, and allow calibration of the panel in the use ofintensity scales References should be simple, reproducible and clear to the

panellists, and illustrate only a single sensory descriptor They can besingle chemical substances or finished products, and are made availableduring both the training and the testing phase, at various concentrations

When the panellists have agreed a vocabulary, further training is performed.The number of training sessions is dependent on the subject’s performance,product and attribute difficulties and the time allowed for QDA testing.Panel training increases panellist sensitivity and memory and helps panellists

to make valid, reliable judgements independent of personal preferences Oncethe training sessions have established satisfactory panel performance, andafter removal of ambiguities and misunderstandings, the test samples can beevaluated This is usually carried out in replicated (commonly three) sessions,using experimental designs that minimise biases

In each session, the mean is calculated for group and individual judgements

of each attribute The results are then subjected to univariate statistics (e.g.analysis of variance) or multivariate statistics (e.g principal componentanalysis) Test results may also be visualised via bar charts or line graphs

The Spectrum Method

This more recent method provides a tool with which to design a descriptiveprocedure for a given product category The method resembles QDA inmany respects; for example, the panel must be trained to fully define allproduct sensory attributes, to rate the intensity of each and to include otherrelevant characterising aspects such as change over time, difference in theorder of appearance of attributes, and integrated total aroma and/or flavourimpact

Panellists develop their lists of descriptors by first evaluating a broadarray of products that define the product category The process includesusing references to determine the best choice of term and to best definethat term so that it is understood in the same way by all panellists Wordssuch as vanilla, chocolate or orange must describe an authentic vanilla,chocolate and orange character, for which clear references are supplied Allterms from all panellists are then compiled into a list that is comprehensivebut not overlapping

The Spectrum Method is based on an extensive use of reference points.The choice of scaling technique may depend on the available facilities for

computer manipulation of data and on the need for sophisticated data analysis.Whatever the scale chosen, it must have at least two, preferably three or five,reference points distributed across the range.

from foods (Overbosch et al., 1991; Shamil et al., 1992) during mastication.

Such studies are particularly important in the reformulation of foods thatresults in structural modifications, and in changes that can occur on storage.These structural modifications are often accompanied by textural changes,and these often result in complex perceptual phenomena that are directconsequences of the changes in texture with time producing different flavourrelease phenomena The use of time-intensity for flavour measurement iswell established, and there have also been studies to measure textural changes

using the method (Burger, 1992; Duizer et al., 1993).

A major limitation of time-intensity methods is that only a single attributecan be tracked with time, and, if a number of important attributes are thought

to be time-dependent, separate sessions are needed for each attribute.Difficulties encountered in time-intensity profiling prompted the development

of a hybrid technique, progressive profiling(Jack et al., 1994) In this technique,

assessors carried out a profile on a set of texture descriptors at each chewstroke over the mastication period Such a method has a number of potentialadvantages: several attributes can be assessed in one session; scaling isreduced to a unidimensional process; and the most important aspects of theshape of a time-intensity curve are retained

1.4 Instrumental measurement of texture

Sensory methods are, for the foreseeable future, the primary means of measuringthe range of textural characteristics of food that are important to consumeracceptance The highly labour-intensive nature of sensory analysis has inevitablyled to the development of instrumental methods designed to measure foodproperties that relate to relevant sensory characteristics These methods havebeen classified in various ways, according to the type of measurement andthe type of food

1.4.1 Empirical, imitative and fundamental measurement

Instrumental methods have been classified into three main categories: empirical,imitative and fundamental

Empirical methods

Empirical tests often measure ill-defined variables that are indicated by practicalexperience to be related to some aspect of textural quality Devices have beendeveloped within different sectors of the industry that are appropriate tospecific product types Even for the same product type, different foodmanufacturers have developed their own in-house devices Fuller details ofthe devices described in this section are given in Bourne (2002)

• Puncture or penetration devices measure either the force needed to push

a probe into the food to a specified depth or the penetration distanceachieved by application of a specified force Examples include Magness-Taylor testers (for fruit), the Bloom Gelometer and the FIRA Jelly Tester(for gels), the cone penetrometer (for fats) and the Christel Texture Meter(for peas)

• Shearing devices measure the force needed for one or more blades toshear through the food The maximum force is often assumed to measuretoughness, firmness or fibrousness Instruments include the Warner-BratzlerShear (for meat), the Kramer Shear Cell (general-purpose) and the FMCPea Tenderometer

• Compression devices measure the force needed to achieve a givencompression or the compression achieved at a given force Examplesinclude the Baker Compressimeter (for bread) and the ball indenter (forfats) In extrusion tests, the food is forced through one or more orificesand the maximum force, average force or work done over a specifiedperiod is measured The measured values are assumed to relate to firmness,toughness, consistency or spreadability Examples include the FIRA-NIRDExtruder (for fats) and various cells used in conjunction with general-purpose instruments

• Cutting devices use wires or blades (sometimes rotating) to cut throughthe food and measure the maximum force developed or the time needed tocut through a standard size of sample Measurements are assumed torelate to fibrousness, firmness or hardness The FMBRA Biscuit TextureMeter is a rotating blade device used to measure biscuit hardness

• Flow and mixing devices are used to give a measure of viscosity orconsistency of liquid and semi-liquid foods They often measure the extent

to which samples flow or spread under specific geometric conditions, e.g.the Bostwick Consistometer and the Lyons Gel Flow Meter

Although such empirical devices are often simple, inexpensive and portable,precision and reproducibility are generally poor, and the measured parametersare poor measures of perceived texture Extensive use is still made of them

in industry, however, mainly for quality control purposes

Imitative methods

Imitative methods of measurement mimic the conditions to which the material

is subjected in practice during eating The Volodkevich bite tenderometerattempted to mimic the action of teeth on food It recorded the force of biting

on a piece of food as a function of the deformation incurred Two wedgeswith rounded points were substituted for teeth, the lower being fixed to aframe The upper wedge moved with a linear motion through the arc of acircle by a lever, squeezing a sample between the wedges

A device using human dentures served as the prototype for the General

Foods (GF) Texturometer (Friedman et al., 1963), in which the dentures are

replaced by a plunger The location of the sensing element was moved fromthe articular arm to the sample area to eliminate gravity forces, and theoscilloscope was replaced by a chart recorder, enabling easy and permanentrecording of any chosen number of consecutive chews In this device, thedriving mechanism no longer imparts a combined lateral and forward motion

to the lower jaw, although it still drives the plunger through the arc of acircle

Although the GF Texturometer remains in use to a small extent in NorthAmerica and in Japan, the general-purpose testing machines designed foruse with foods, exemplified by those made by Stable Micro Systems, Stevens,Lloyd and Instron, are commonly used in the food industry in most countries.The instruments differ in their mechanical construction and in their dataacquisition and data analysis capabilities, but they have a number of importantfeatures in common All have a crosshead containing a load cell, which isdriven vertically at a range of constant speeds, and which can cycle over afixed distance or load range Probes can be attached to the crosshead forpenetrating, shearing or crushing food, which can be held in a variety ofcells The load is recorded relative to time or to penetration/deformationdistance, and displayed on a suitable recorder Computer control of theinstrument and sophisticated and rapid computer analysis of the data areincreasingly common

A major advantage of such instruments is that flexibility of design allowsthem to be used for a wide range of foods This is particularly useful forcompanies that are handling or manufacturing a varied product range Loadcells can be changed to give a high level of accuracy for relatively soft foodsthrough to very hard foods Probes and sample holders can easily be changed

to accommodate measurements on different product types An additionaladvantage is that such instruments can often be adapted for fundamentaltexture measurement

Fundamental methods

Fundamental methods involve measuring well-defined physical properties

of food, which, if measured properly, are independent of the method ofmeasurement The most common fundamental parameters are Young’s modulus,shear modulus, bulk modulus and Poisson’s ratio (for solids) and viscosity

(for liquids) Fundamental parameters for solids can be measured on purpose testing machines, but such measurements require a carefully designedexperimental set-up and are consequently slow In addition, foods are generallyheterogeneous and do not exhibit simple elastic behaviour Fundamentalparameters can be measured on liquids using suitable instrumentation, forexample the Weissenberg Rheogoniometer and the Carri-Med Rheometer.Again, however, liquid foods rarely exhibit simple viscosity behaviour.Such fundamental measurements are valuable in investigating the physicalproperties of food, but are too complex for routine use and, in common withother instrumental measurements, rarely correlate well with perceived texture.Some reasons for this can be identified on examining some of the physiologicalfactors associated with chewing.

general-1.4.2 Application to solid foods

Development of measurement methods

For most solid foods, key sensory attributes can be defined that are known to

be highly important in defining consumer acceptability, for example crispness

in salad vegetables and snack foods, tenderness in meat, and snap in chocolate.The evolution of measurement methods has followed the need both to controlthese attributes in routine production and to understand how they can bedesigned into new products

The computerised modern instruments that utilise force-deformationprinciples are used almost universally in research functions and, with particularsuccess, in quality control (QC) functions A good example of this is the use

by the French company Isigny Sainte-Mère of Stable Micro Systems XT2 Texture Analysers for on-line measurement of the firmness of Camembertcheeses to sort for different maturing conditions (Toursel, 1996) The reasonfor the successful applications in QC is easy to see In such applications, it

TA-is more important to be able to detect changes in measurable parameters than

to measure precisely specific textural parameters A change in any measuredparameter outside set control limits can act as a signal that some aspect ofthe food production cycle has drifted Of course, this introduces the risk thatactions could be taken on the basis of changes in parameters that have littleimportance to consumer acceptability, but the measurements would normallytrigger sensory tests that would minimise any production holds A moreserious risk is that the measurement system would not identify textural changesthat were unmeasurable by that system, and, unless routine sensory testswere also carried out, defective material could be supplied to consumers Forexample, a standard texture instrument capable of measuring the characteristicsnap of chocolate might not detect the dryness that characterises stale chocolate.The deficiencies of texture measurement instruments become particularlyapparent when they are used for R&D purposes One practical problem isthat no single instrument is likely to be able to measure the food properties

that are detected by both the tactile and the deep senses One route to addressthis problem is to investigate more closely those physical processes that giverise to the texture sensations In a study of stickiness in confectionery products,Kilcast and Roberts (1998) explained the need to understand both the meaning

of the word ‘stickiness’ by consumers, and in what context the phenomenon

is perceived, as well as the physical processes that contribute to the variousphenomena commonly described as stickiness They showed that the perception

of stickiness can occur through combinations of both adhesive and cohesivefailure (Fig 1.3) Most industrial problems are associated with cohesivefailure, which leaves unwanted material behind on surfaces Both productrheology and the surface energy of the surfaces can contribute to the observedsticking phenomena, and under critical conditions it is possible to minimisesticking by either changing the surfaces involved or changing productcomposition or operating conditions The research led to a test procedureadapted from that developed by Chen and Hoseney (1995) for measuring thestickiness of dough that can be used to study the stickiness of caramels over

a wide range of conditions Figure 1.4 shows the cell, which consists of a

Adhesive

failure

Cohesive failure

Fig 1.3 Adhesive and cohesive failure mechanisms during the force–deformation

testing of food The sequence to the left (start, a1, a2) illustrates adhesive failure; the

sequence to the right (start, c1, c2) illustrates cohesive failure.

water-jacketed cylinder holding the caramel sample at the required temperature.The screw piston is used to extrude the caramel through a perforated stainlesssteel cap to give a fresh surface The instrumental measurements are carriedout by lowering a cylindrical probe onto the caramel surface, and thenwithdrawing it and measuring the force-deformation characteristics.For solid foods giving the proprioceptive (deep) response, the principles

of fracture mechanics have been applied widely, and are outlined in thefollowing section

Fracture mechanics in food texture measurements

The science of fracture mechanics was originally developed to explain thefracture of brittle materials such as glass The breakage of a material isinfluenced by the relationship between the applied force and the bond holdingthe material together Bonds will break more readily under the high localstress concentration around the tip of a sharp blade, for example The basicpremise underlying fracture mechanics is that all solids contain inhomogeneities,which exist in the form of flaws, or cracks The magnitude and distribution

of these defects govern the strength of the material Fracture occurs whenthese defects grow and traverse the solid, creating new fracture surfaces.Early studies showed the potential of wedge fracture testing to brittle and

semi-brittle foods such as apples and cheeses (Vincent et al., 1991).

One difficulty in applying the principles of fracture mechanics to foodlies in the great complexity of food structure, and in the complex viscoelasticbehaviour of most foods, although recent research has demonstrated the

feasibility of these techniques (Dolores Alvarez et al., 2000) However,

deformation tests applied to carefully prepared food samples have been shown

to give improved correlations between instrumentally measured propertiesand sensory measures The principles of fracture mechanics have been extended

to the measurement of fracture toughness of complex foods, such as the

pastry casing of spring rolls (Sim et al., 1993) Lillford (2001) has reviewed the work relating to fracture mechanisms in food, and Lucas et al., (2002)

have proposed physiological models relating to the fragmentation andswallowing of food particles

1.4.3 Analysis and validation of instrumental measurements

Statistical methods

A physical measurement of textural characteristics can be of practical valueonly if it is shown to relate to some relevant sensory texture measure Therelationship should take the form of a statistic that represents the fit betweenthe instrumental measurement and the sensory attribute, or an equation thatrelates the instrumental measure (or set of measures) to the required sensoryattribute Two basic procedures are used

• Pearson product moment correlation coefficients (r), where a perfect positive correlation gives r = +1, a perfect negative correlation gives r = –1, and

no correlation gives r = 0 The square of the correlation coefficient (r2)gives a measure of the data variance accounted for by the linear correlation,and is a measure of the value of the correlation

• Multiple linear regression (MLR), in which the variable of interest (e.g.sensory attribute) is expressed as a linear combination of other variables(e.g instrumental parameters) The variable combination is usually foundusing stepwise selection procedures The method is of greatest value whenthe number of data points exceeds the number of attributes of interest.The complexity of both instrumental and sensory data, however, increasinglydemands the use of multivariate statistical procedures Many techniques areavailable, but, when examining for structure and relationships in data sets,the most common technique is principal component analysis (PCA).PCA is a data reduction technique that replaces a large number of originalvariables by a smaller number of linear combinations, whilst still explaining

a substantial proportion of the original variation in the data Essentially, PCA

projects an n-dimensional space onto a 2-dimensional plot Other multivariate

analyses, such as partial least squares (PLS) analysis are increasingly beingused for combined data sets For example, in a study of the sensory andinstrumental characteristics of Reggiano grating cheeses, PLS was used toshow that sensory texture correlated best with strain at breaking point (Hough

et al., 1996).

Novel methods for analysing force–deformation data

Analytical software included in the modern force-deformation test systems

is capable of parameterising the curve shapes generated by many foods Themost extreme deviation from the idealised force–deformation curves iscommonly found in testing brittle foods, which are characterised by very

jagged curves In a series of papers (e.g Barrett et al., 1992), Peleg and

co-workers have described different mathematical approaches to analysing thesehighly irregular curve shapes, which can give rise to difficulties inparameterisation One approach has been to carry out a Fast Fourier Transform

on the force–deformation curve, giving a power spectrum of underlyingfrequencies This procedure gives a qualitative representation of the jaggedness

of the original curve, but cannot give a quantitative representation Suchquantification can be carried out through fractal analysis The fractal concept

is based on the geometry of self-similar objects expressed in terms of integer dimensions The fractal dimension is determined from the slope, inlogarithmic co-ordinates between the length of the force–deformation contourand the corresponding measurement scale The latter measurement was found

non-to be convenient in giving a measure of overall jaggedness in terms of asingle number, but the power spectrum gave more information on the location

of the fracture, and its shape could be related more directly to structure and

texture The use of the power spectrum was subsequently described in relation

to the measurement of crunchiness in friable baked products (Rohde et al.,

1993)

A further innovative attempt to analyse jagged force–deformation curveshas been to use a technique used in speech analysis and medical diagnosis,symmetrised dot-pattern (SDP) displays (Peleg and Normand, 1992) Thisconcept is based on the premise that, whereas humans find it difficult toanalyse the visual appearance of highly irregular shapes, they are highlysensitive to changes in symmetric patterns In the method, the recorded dataare transformed into several symmetrically arranged sets of points, eachreflected by a mirror plane, in an analogous way to the production of symmetricvisual images by a kaleidoscope The technique was used in the hope that thedisplays could be used to identify crunchiness in the same way that they can

be used to identify vowels, but, in practice, the SDP displays were so sensitive

to minor details that every signature appeared unique (Peleg, 1998)

1.5 In vivo texture measurement

The limitations in trying to mimic the events occurring during masticationusing relatively simple instruments have long been appreciated An alternativeapproach that has gained credence in recent years has been to attempt torecord signals generated by or within the human subject that may relate tothe texture of the food being masticated

1.5.1 Electromyography (EMG) and associated techniques

EMG involves the use of a polygraph to measure electrical signals generated

in muscles that are active during mastication For certain muscles that lieclose to the surface of the skin, for example the masseter muscle, which isactive during the chewing of solid foods, this activity can be related to aspecific muscle Other oral activity, for example tongue movement, is controlled

by groups of muscles that are deeper-lying Monitoring of signals from thislatter type of musculature ideally requires implanted electrodes, whereassignals from the masseter muscle can be readily recorded using surfaceelectrodes

Early attempts to use EMG in the study of food texture were limited bydifficulty in interpreting the complex data patterns produced In the absence

of suitable computerised acquisition and analysis equipment, visual inspection

of the raw data was carried out For example, motor pauses (or silent periods)were more frequent with hard foods than with soft foods (Boyar and Kilcast,1986) The development of more sophisticated EMG equipment and computersystems, however, has permitted much deeper analysis of EMG data andtheir relationship to food texture An added potential advantage of this technique

is studying changes in food texture in the mouth throughout the whole chewing

cycle Research projects carried out at the Leatherhead Food ResearchAssociation (e.g Eves, 1990; Kilcast and Eves, 1991) were carried out toinvestigate the potential use for EMG as a means of characterising foodtexture, and the way in which texture changes during mastication.

Following this work, a number of papers have appeared in the literature inwhich EMG has been used to investigate food texture Further studies on theuse of EMG in the confectionery sector were reported by Smalls (1992), andindicated good correlations between EMG and Instron measurements

Applications to cheese texture were described by Jack et al (1993) In this

work, EMG was used in conjunction with sensory and instrumentalmeasurements of the texture of a range of Cheddar cheeses, but inconsistencybetween subjects resulted in difficulties in correlating EMG and Instronmeasurements In a review of texture measurements for use in product

development, Jack et al (1995) described the use of EMG in conjunction with other methods Duizer et al (1996) used a combination of EMG, time-

intensity measurement and instrumental measurement to investigate beeftenderness The results indicated that the effects of early mastication should

be compared with the effects of late mastication

More fundamental aspects of the use of EMG in understanding the oralbreakdown process have been reported by Brown and co-workers (e.g Brown

et al., 1994; Brown, 1995; Brown et al., 1998) These studies focused primarily

on understanding the chewing behaviour of consumers rather than on texturemeasurement EMG was used either as the sole technique, or in combinationwith synchronous measurement of jaw movement (kinesthesiology) by a set

of transducers mounted on a head-frame to track the movement of a smallmagnet attached to the lower front incisors Several oral techniques, includingEMG, have also been used by Mioche and co-workers to study the mastication

process (e.g Mathevon et al., 1995; Peyron et al., 1996; Mioche and Martin,

1998) A more recent study within the EU HealthSense project has seen theuse of EMG to investigate differences in chewing patterns between young

and elderly populations (Kohyama et al., 2002).

An unusual related technique for studying mastication behaviour has beenreported by Jack and Gibbon (1995) The technique, electropalatography(EPG), is used to measure tongue movement during eating and swallowing,and comprises an artificial plate, moulded to the individual’s hard palate,embedded in which are 62 electrodes covering the entire palate surface.Tongue contact with the electrodes generates a signal that can be used tomonitor the movement of the tongue Experiments were carried out withliquid, semi-solid and gelled foods The authors concluded that the techniquecould be used for liquid and semi-solid foods, but that bulky or sticky foodsprevented the tongue making contact with the palate

1.5.2 Sound emission

Early studies on food-crushing sounds (Drake, 1963) showed that soundsfrom crisp foods differed from those of non-crisp foods, primarily in terms