Derived from the Latin textura, whichmeans a weave, texture originally was takento refer to the structure, feel, and appearanceof fabrics. It was not until the 166Os that itstarted to be used to describe "the constitu-tion, structure or substance of anything withregard to its constituents, formative ele-ments" (Oxford English Dictionary, 1989).Various attempts to define food texture haveculminated in some international agreementwith the development of the internationalstandard ISO 5492 (International Organiza-tion for Standardization [1992]), which dealswith the vocabulary used for sensory evalua-tion. This defines texture as "All the me-chanical, geometrical and surface attributesof a product perceptible by means of me-chanical, tactile and, where appropriate, vi-sual and auditory receptors." Clearly, foodtexture is about perception, making it aboveall other things a human experience. It isabout our perception of a foodstuff that origi-nates in that product's structure and how theproduct behaves when handled and eaten.Furthermore, it incorporates all the attributes(mechanical, geometric, and surface) of thefood, suggesting that the experience of tex-ture is one of many stimuli working in combi-nation.

Bearing such a definition in mind, is it rea-sonable to presume that such a complex array

of interactions of stimuli could be measuredinstrumentally? This is a moot point, forwhile many samples can be subjected to as-sorted instrumental testing techniques, onwhat basis shall we acknowledge that the re-sults produced have any relation to humanperception? Prior to the 1940s, it was gener-ally considered that sensory measurements offood texture were purely subjective and assuch generally unreliable. Variation in indi-viduals as well as variability of any one per-son from day to day seemed to make the sen-sory analysis of food an art and not worthy ofserious scientific study. It was inconceivablethat an individual's response could be any-thing other than personal, hedonic, and preju-diced by that person's beliefs and biases.Since our scientific ethos was founded on re-producibility, most serious researchers werepersuaded to rely on instrumental testingtechniques carried out under standardizedconditions. Such techniques were consideredto be reliable, with relatively small inherentvariation or error. An attitude developed inwhich instrumental tests were referred to as"objective" while sensory work was deni-grated as being "subjective."

In these early years of texture measure-ment, the study of texture focused on the ef-forts of the rheologist, who measured flowand deformation of food materials. In his dis-

Relation between Instrumental andSensory Measures of Food Texture

Andrew J. Rosenthal

CHAPTER 1

cussion paper "Is Rheology Enough for FoodTexture Measurement?" Bourne (1975) sug-gested that rheological measurements, whichoften focus on a single large deformation, re-sulting in the sample's breaking into pieces,were inadequate in defining food texture.When an individual eats a foodstuff, thesample is chewed beyond this initial break-down, and the stimuli that result form part ofthe overall texture sensation. While the initialbite is an important aspect of texture, so tooare the subsequent bites, the viscosity, sticki-ness, and consistency of the food as it mixeswith saliva. Equally important are aspects ofthe food's appearance, the mechanical prop-erties, and sounds that occur when it ishandled, cut, and eaten. Clearly, rheology isnot enough to explain all the rich and com-plex aspects of texture that are experiencedby humans.

Attitudes about the objectivity of sensoryresearch began to change shortly after theSecond World War. The U.S. Army hadmade a considerable investment in develop-ing nutritious rations for its troops, only tofind that many of them did not appreciatewhat they were being offered to eat. A fo-cused research program was developed bythe U.S. Quartermasters Corps to look at is-sues of food acceptability and choice. Devel-opment of controlled sensory testing proce-dures, the separation (in people's minds)between "sensory" and "affective" tests, andadvances in multivariate statistical tech-niques that were made possible by the appli-cation of relatively powerful computers allhelped to bring the perception of sensory test-ing into scientific respectability.

Without a doubt, texture plays a key role inour appreciation of food. Our perception offood texture often constitutes a criterion bywhich we judge its quality and is frequentlyan important factor in whether we select anitem or reject it. We squeeze and prod fruits

and cheese to gauge their ripeness and tiltbottles to estimate the viscosity of their con-tents. Texture can be expressed in the soundsthat foods make when handled, to the extentthat we listen to foods to estimate their qual-ity—for example, listening to the sound of amelon when it is tapped. Familiarity with aproduct brings knowledge about how its tex-ture and behavior change during processingand storage. With continued contact with aspecific food, certain individuals have devel-oped great expertise, to the extent that theybecome expert judges of that product's qual-ity. Such individuals (e.g., master crafts-people) often determine the texture of foodswith empirical test methods such as prodding,tapping, or squeezing. These so-called"foreman's finger tests" have become estab-lished measures of quality for some productsand may even set market prices. Surely, if anindividual can assess texture by prodding,then a machine that prods and measures theforce involved will give a reproducible mea-sure of that aspect of texture and thereforequality. By applying suitable calibration, weare able to create machines that might pro-vide information that formerly only our ex-pert could offer.

Having accepted the idea that a machinemight take the place of a human to assessfood texture, we must remember that beforeall else, food texture is essentially a humanexperience that arises from our interactionwith food. It is worthwhile to be cautiouswhen considering physical test procedures,for data can be collected by subjecting anymaterial to any procedure, but results fromthe test do not necessarily mean anything interms of texture. Such a comment is not in-tended to disqualify instrumental tests thatmay not relate directly to human perception,for such procedures may be valid for all kindsof other reasons. For example, viscosity of aliquid is frequently measured to gain an idea

of resistance to flow and therefore the pump-ing requirements to push the fluid down apipe. However, as we will see later, undersome conditions instrumental measurementsof viscosity bear little relation to the experi-ence as perceived in the mouth,

THE PROCESS OF FOOD TEXTUREPERCEPTION

Human perception of food has been de-scribed as a cyclic process that starts with ananticipation originating primarily from visualcues but also flavored by our prior experi-ences. Various aspects of appearance, such ascolor, size, and shape, as well as aspects ofstructure (e.g., openness), preempt our physi-cal interaction with the food (Kramer, 1973).Though not always associated with textureperception, visual cues provide a gauge ofviscosity (Shama, Parkinson & Sherman,1973) and the "wobbly" behavior of semi-solid, jelly like foods. Our participation leadsto manual manipulation either directly orwith tools (e.g., cutting with a knife). Visu-ally perceived changes in the food with han-dling add to our impressions of the food'stexture. Even before the food is in the mouth,we have gathered a substantial amount ofknowledge about the food's texture from vi-sual, tactile, and even auditory stimuli.

Initial perception in the mouth (i.e., with-out biting) is at a relatively low shear rate.Two categories of sensation have been iden-tified: those due to touch, which occur re-gardless of any shearing, and those that re-quire a small amount of deformation. Withno shear at all, we gather impressions aboutthe food's homogeneity, such as the pres-ence, size, and shape of particles or air cells.At slightly higher shear rates caused bymovement of the tongue, the food is de-formed and flows. Under these conditions,characteristics such as elasticity, stickiness

to the palate, and viscous behavior are per-ceived (Sherman, 1969).

During the first few chews, much of thestructure is broken. Brittle materials fracture,fibrous materials are torn, and the food iskneaded and mixed with saliva to form a co-herent bolus (Heath, 1991). During thesechewing cycles, a high degree of shearing isachieved, and a wide variety of textural char-acteristics are perceived. Attributes that re-late to physical makeup (e.g., hardness-soft-ness) and deformation and breakdown (e.g.,brittleness, plasticity, crispness, andsponginess) are detected (Sherman, 1969).Movement of the jaw during subsequentchewing cycles of the so-called "mushphase" are more regular than in the earlystages of mastication. During this period, sa-liva is secreted into the bolus as it is furtherkneaded prior to swallowing. Textural at-tributes perceived are those that relate to theparticular nature (e.g., smoothness, coarse-ness, powderiness, lumpiness, and pastiness),consistency (e.g., creaminess, wateriness),and adhesion to the palate (e.g., stickiness).

Hutchings and Lillford (1988) developed amodel to explain the breakdown path towhich a food is subjected before it is readyfor swallowing. They reinforced the idea thatthe breakdown path is a dynamic process oc-curring over a period of time and postulatedthat the key attributes that affect the processare the degree of lubrication and the structurethat the food possesses. During mastication,the structure is broken down by mechanicalaction. Lubrication is due in part to the re-lease of saliva during chewing but also arisesfrom the composition of the food in terms ofmoisture and fat, both of which act as lubri-cants. Changes in structure and lubricationoccur until a threshold is reached, at whichpoint the material is swallowed.

After swallowing, we perceive a residualmasticatory impression that arises from the

remains of disintegrated food and any mouth-coating materials. Such attributes includemeltdown properties on the palate, greasi-ness, gumminess, and stringy sensations(Sherman, 1969).

Since perception is a cyclic process, the in-formation gathered during the handling, bit-ing, chewing, and swallowing feeds back intoour anticipation of the next portion.

COMPARISON OF HARDWARE FORINSTRUMENTAL AND SENSORYMEASUREMENT

Many stimuli contribute to our perceptionof texture, including visual and auditory cuesas well as those related to touch and move-ment. Visual and auditory cues are gatheredthrough specialized sense organs—the eyesand ears. In contrast, the sensors of materialcharacteristics are spread throughout thebody, sometimes being categorized as thosesensitive to touch (somasthesis) and move-ment/position (kinesthesis). Various skin or-ganelles have been identified, such as Pacin-ian corpuscles, Meissner's corpuscles,Ruffini endings, and free nerve endings.While some of these organelles have been as-sociated with the perception of certain at-tributes (e.g., Pacinian corpuscles with per-ception of pressure), it is often thought thatthe law of specific nerve endings does not ap-ply to skin senses and that all types of nerveendings contribute to our general perceptionof texture. In addition to the tactile sense or-ganelles in the hard and soft palate, tongue,gums, and periodontal membrane surround-ing the teeth, there are vitally important nerveendings in the oral muscles and joints. Sig-nals from these nerves provide informationon jaw position, muscle tension, and length.

In comparison to the sensing apparatus ofthe human body, instrumental testing devicesrely on transducers to convert material and

physical measurements into visual or electri-cal outputs that can be either observed directlyor fed into data-recording/processing equip-ment. Instrumentation depends on the type oftest being performed, but for mechanical test-ing, it often involves strain gauges and loadcells to measure forces and position or move-ment detectors. Crucial to the type of test beingapplied is the geometry of the test cell and howthe sample is held. Tests carried out on solidand viscoelastic materials are often done un-der compression, shear, torsion, or tension(see Chapter 4 of this volume).

Successful transducers usually have a lin-ear response that, through calibration withstandards, can represent defined physicalcharacteristics in terms of absolute units. Incontrast, human perception is governed bypsychophysical phenomena, which tend to benonlinear. For example, Weber's law statesthat the magnitude of a "just noticeable dif-ference" is proportional to the intensity of thestimulus already present. Thus, the bodyadapts to the forces that are exerted, beingmost sensitive when small forces are applied.Moreover, the response to a stimulus is mostnoticeable when a change in that stimulus oc-curs; if the stimulus is held constant, the re-sponse lessens with time. The overall conse-quence is that we are receptive to relativedifferences and changes but rather poor atidentifying any absolute measures.

Factors like temperature are frequently in-fluential on rheological behavior and conse-quently most apparatus for measurement ofsuch parameters will be accuratelythermostated. Typical instrumental method-ology would involve introducing the sampleto the device and leaving it to reach a steadytemperature before the physical test is ap-plied. But this approach is quite differentfrom what occurs in the mouth during eating.While the temperature within the center ofthe body is fairly constant at 370C, the mouth

is normally a few degrees below that. Foodthat is introduced is rarely at the same tem-perature, and there follows a brief change inthe temperature of the food, which may leadto a change in its physical behavior. A well-known example of how this temperaturechange affects the texture of food is providedby chocolate. As with most triglyceride mate-rial, cocoa butter can be crystallized in a vari-ety of polymorphic forms. The desirable Vcrystal has a relatively sharp melting pointaround 33.50C (Schlichter-Aronhime &Garti, 1988). When chocolate at around 2O0Cis introduced to the mouth, the temperaturestarts to rise and the fat crystals start to meltin the mouth, giving rise to the sensation ofdissolving and melting away. Similar sensa-tions are important in ice cream and marga-rines (see Chapter 7 this volume).

In addition to thermal melting, the presenceof saliva within the mouth leads to a dissolu-tion of water-soluble materials. About 1.5litres of saliva are secreted each day. Saliva isa dilute non-Newtonian liquid that containsdigestive enzymes (a amylase) as well as avariety of unusual proteins and polypeptides.Histatins, for example, are a group of polypep-tides found only in saliva that have excep-tional antibacterial properties (Schenkels,Veerman, & Amerongen, 1995). Saliva acts asa lubricant as well as a solvent, allowing foodmaterials to be effectively dissolved and bro-ken down during mastication. Most rheologi-cal test equipment does not have any facility tointroduce a solvating lubricant onto thesample during testing.

The speed of movement of the jaw and thetongue within the mouth is a critical factor inour perception of food texture. Tornberg et al.(1985) showed that when chewing meat, thejaw can move at between 200 and 400 cm/min in contrast to typical instrumental testingmachines, which operate around at 20 cm/min. Bearing in mind that only a few liquid

foods behave as Newtonian, the others willexhibit shear dependent viscosity, and in sucha situation the apparent viscosity will dependon the rate at which the liquid is beingsheared. Shama and Sherman (1973) foundthat shear rates in the mouth are applied in arange from 0.1 to 1000 s~!. Since most foodsare non-Newtonian, their apparent viscositywill depend on the shear rate applied. It istherefore necessary to match shear rates ap-plied in an instrumental test with those thatmight be experienced in the mouth, or themeasured viscosity may not equate to whatwould be perceived orally. Shama andSherman showed that the shear rate applied inthe mouth actually depends on the viscosityof the food, such that low-viscosity foods re-ceive relatively high shear rates while high-viscosity foods tend to be sheared moreslowly (Figure 1-1). Consequently, some it-eration is needed to find appropriate shearrates that measure the apparent viscosity as itwould be perceived in the mouth, and the ap-propriate shear rate may vary from one foodto another.

Takahashi and Nakazawa (1991) studiedthe swallowing process by introducing pres-sure transducers into the palate of volunteers.Using a laboratory viscometer at shear ratesthat occur in the mouth, they determined theviscosity of a variety of carboxymethyl cellu-lose solutions. Different volumes of these so-lutions (<25 cm3) were then offered to thevolunteers, and the retaining time that thesamples were in the mouth, palatal pressureduring swallowing, and work involved inswallowing were monitored. The swallowingpressure changed only from about 0.1 to 0.2mPa over a viscosity range of 10 to 10,000mPa»s. The time that the sample was retainedin the mouth and the work involved in swal-lowing remained almost constant up to a criti-cal viscosity of 1 Pa0S, above which both thetime retained and the work increased mark-

edly. The effect of sample volume variedwith solution viscosity, low-viscosity liquidsbeing swallowed in one deglutition (<15 cm3)while high-viscosity liquids were swallowedin several small volumes.

The action of mastication and secretion ofsaliva combine in the breakdown path untilthe bolus is suitable for swallowing(Hutchings & Lillfbrd, 1988). Clearly, this isa time-dependent process and one in whichthe nature of the food, and hence its texture, ischanging. Few instrumental tests considerthis kind of broad time frame; in fact, mostfocus on the first or the first and second bites.While a considerable amount of the structuralbreakdown may occur during these early

parts of mastication, other sensory attributesexperienced closer to the time of swallowingfrequently do not get evaluated.

At this stage, it is worth returning to theISO definition of food texture. A key aspectof food texture is that it can arise from a com-bination of physical properties; sensationssuch as wetness have been attributed to thesimultaneous stimuli of cold and pressure,while stickiness is due to the sequence of agentle pressure followed immediately bypulling on the skin (Szczesniak, 1990). De-spite these multifarious sensations, few tex-ture-measuring instruments focus on morethan one physical property at a time; this hasresulted in investigators' examining different

SHEARRATEt(S-1)

Figure 1-1 Bounds of Shear Stress and Shear Rate Associated with Oral Evaluation of Viscosity.Source: Reprinted with permission from F. Shama and P. Sherman, Identification Stimuli Controlling theSensory Evaluation of Viscosity: II Oral Methods, in Journal of Texture Studies, Vol. 4, pp. 111-118, ©1973.

SH

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physical aspects in isolation, each yielding afacet of the experience but none an equiva-lence to the entire sensation. For example,Dacremont, Colas, and Sauvageot (1991)measured the sounds traveling to the ear dur-ing eating. By placing a conventional micro-phone adjacent to the ear as well as a minia-ture contact microphone inside the ear, theywere able to distinguish between the soundsthat are conducted through the bone and thosethat travel as waves through the air. Such anapproach allows some distinction to be madebetween sounds that originate from differenttypes of food. Crispy foods (described ascroustillant by the French authors) producehigh-pitched sounds with frequencies in ex-cess of 5 kHz. Such sounds travel predomi-nantly through the air. In contrast, low-fre-quency crackly and crunchy sounds(described as craquant and croquant by theauthors) travel predominantly through thebone. Distinction between crackly andcrunchy sounds is postulated to be due tocrunchy foods' creating vibrations that stimu-late touch sensors (Dacremont, 1995).Vickers (1988) reviewed the research intoacoustical properties of crisp foods in relationto their sensory properties. She pointed outthat acoustic data alone provide only a partialexplanation of sensory crispness. By combin-ing acoustic data with force-deformation fail-ure tests in multiple regression, one can ob-tain a better correlation with sensorycrispness for some foods. It could of coursebe argued that examining each physical phe-nomenon individually allows a separation ofthe relative contribution that each has to theoverall experience.

APPROACHES TO INSTRUMENTALMEASUREMENT OF TEXTURE

Scott-Blair (1958) categorized the instru-mental techniques used to measure food tex-ture into three groups:

1. empirical tests, which measure some-thing physical under well-defined con-ditions

2. imitative tests, which attempt to simu-late the conditions to which the mate-rial is subjected in the mouth

3. fundamental tests, which measurewell-defined physical properties suchas viscosity or elastic modulus

Advances in medical instrumentation havecreated a fourth category of techniques thatexamine the neurophysiology of the eatingexperience.

The following discussion will predomi-nantly (though not exclusively) focus ontechniques used to obtain "textural" informa-tion about gelatin gels. While the topic of hy-drocolloid gel testing is reviewed more com-prehensively in Chapter 10 of this book, thecomparison of techniques considered herehelps to clarify the nature of the various typesof tests that exist.

Empirical Tests

By definition, empirical tests are devel-oped by experimentation and observation,and as such they may lack a rigorous scien-tific basis. However, this does not disqualifytheir use, and in some sectors of the food in-dustry, empirical tests act as standards thatare used to grade food quality. For example,the Adam's consistometer is used as a stan-dard test to ascertain the quality of apple pu-ree or creamed corn. It consists of a flat hori-zontal plate with a series of concentric,equally spaced rings etched on the surface.The sample is placed in a short tube posi-tioned over the central ring, and the tube islifted, allowing the food to spill out and flowradially across the plate. Clearly, the distancecovered by the puree in a given time dependson a variety of phenomena such as hydro-static force, particle size and shape, friction,

and surface tension. Consequently, there isno simple scientific explanation for the flowbehavior, yet it does relate well to quality ofthe food product.

Empirical tests have been criticized as usu-ally being specific to particular narrowranges of products. Each empirical test mea-sures characteristics of the food in an arbi-trary way, and since they are uniquely differ-ent from each other, they tend not to comparewell and cannot be used predictively (Bagley& Christiansen, 1987).

The Bloom Gelometer (Bloom, 1925) wasonce an industry standard for testing gelatin.The importance of the test is typified by thefact that gelatins are both characterized andsold on the basis of their gel strength, whichis given in Bloom units (with very weak gela-tin gels being around 100 Bloom, while firmgels are in the region of 250 Bloom). Whiledeveloped to test gelatin gels, the instrumenthas been applied to other gel-like materials.

One of the original patent drawings of theBloom Gelometer is shown in Figure 1-2.Itemized parts on the sketch are referred to inthe following discussion. The essence of thetest is that a cylindrical plunger of fixedweight and 12.7 mm in diameter (Item 10) isplaced on the top of the gel (Item 9). Theplunger is connected via a rod (Item 11) to avessel (Item 13) into which lead shot can begradually added from a dispensing mecha-nism (Items 33-37). When the force actingdown on the gel is enough to force theplunger to penetrate to a depth of 4 mm, theflow of lead shot is halted by an intricate elec-trical magnetic mechanism (Items 41-55).The Bloom strength of the test sample is ob-tained from the weight of the plunger, thelead shot-collecting vessel, and the lead shotthat was required to force the plunger 4 mminto the surface of the gel.

As with all effective empirical tests,sample preparation and test procedures are

methodically specified. In this case, the gela-tin is made up to a concentration of 6.67% (w/w) and must be matured at a temperature of1O0C for between 16 and 18 hours prior totesting. Failure to hold a sample long enoughcan lead to spurious results; for example, agelatin gel held at 1O0C and tested after 5Vzand 10 hours can give values that differ by20% (Szczesniak, 1975).

While the Bloom Gelometer was an indus-trial standard, the basis of measuring theforce required to push a 12.7-mm-diameterplunger 4 mm deep into a gel is very much anarbitrary measure. This begs the question,Does the Bloom Gelometer measure texture?Strictly speaking, no. The Bloom strength ofthe gel cannot describe the combinations ofexperience that occur when a gel is eaten. Yetthere is undoubtedly a relationship betweenthe Bloom strength of a gel and the texturethat it exhibits. Moreover, the Bloom strengthis useful in classifying gels for different fooduses, ranging from soft gels suitable for chil-dren to hard pastilles. In addition to food use,the Bloom Gelometer is important in the stan-dardizing gelatins for other industries, suchas adhesives and photographic emulsions.

Imitative Tests

Imitative tests attempt to mimic mastica-tion with some kind of machine that chewsthe food. The machine is instrumentated toprovide measurements of stress and/or strainduring the test sequence. In the past, a varietyof such tests were created. Some actually in-corporated human dentures that rocked overeach other, imitating the movement of thejaw. While there is some sense in creating atest cell with a geometry similar to the humanmouth, gaining data from the machine de-pends on other factors such as the type andposition of the sensors and the relative mo-tion of the jaws. However, some minor modi-

June 9, 1925.O. T. BLOOM

1,540,979

MACHINE FOR TESTING JELLY STRENGTH OF GLUES. GELATINES, AND THE LIKB

Filed April U, 1923 3 Sheets-She*t l

Figure 1-2 The Bloom Gelometer (Based on Original Patent Sketch).

fications, such as replacing the dentures withplungers of known cross-section area so thatdefined stresses can be applied, allow them toyield valuable data for comparative applica-tions such as quality assurance.

An imitative test that has caught the imagi-nation of many food technologists because itpurports to provide standardized values offood texture is Texture Profile Analysis(TPA), created at General Foods in the mid-1960s. In a series of papers, Szczesniak andher coworkers defined a variety of texturalterms, which are explained in Exhibit 1-1(Szczesniak, 1963). By using nine trained as-sessors who were familiar with these terms,various products were first ranked and thenrated. For each of the sensory attributes, nineproducts were selected that were deemed toexist at equal distances from each other andthat spanned the sensory continuum. For ex-ample, their scale for hardness ranged fromcream cheese (with a sensory rating of 1),through hard-boiled egg white, frankfurter,yellow cheese, olives, peanuts, raw carrot,peanut brittle (the candy part), and rockcandy (rated as 9). Each standard had defineddetails of preparation or brand names, as wellas details of testing temperature (SzczesniakBrandt, & Friedman, 1963). Standards weretherefore established that enabled anyone toconsider each of the textural attributes at dis-tinct defined levels.

In addition to these sensory standards, acompressive force-deformation instrumentwas described (Friedman, Whitney, &Szczesniak, 1963). Based on an instrumentthat deformed the food via a pivotal motion(resembling the human jaw), the GeneralFoods Texturometer used a flat-endedplunger to contact the food sample. Theplunger rocked back and forth at a rate of 108cm per minute, and the food sample, with astandard height of 0.5 inch (0.0127 m), wasdeformed by 75%. A two-bite cycle was em-

ployed, and the stress that developed in thefood sample was measured as the sample wascompressed. After this "first bite," the loadwas removed from the sample and allowed torelax somewhat. As the plunger pulled awayfrom the surface of the sample, any tensiondue to stickiness was observed. The secondbite then compressed the sample again beforeallowing it to relax for a second time. The re-sistance during deformation of the food wasmonitored throughout this two-bite cycle. Anidealized stress-strain TPA curve is shown inFigure 1-3. In an attempt to relate the senso-rial texture definitions already defined (Ex-hibit 1-1), a mathematical function was at-tributed to each term on the basis of the datafrom the texturometer's stress-strain curve.This was achieved through "careful experi-mentation and consideration of the importantdependent variables" (Friedman et al., 1963,p. 393). Illustrations of how these instrumen-tal measures were determined have been in-corporated into Exhibit 1-1. For example, co-hesiveness was defined as the ratio of thework required to compress the sample on thesecond bite to the work required to compressthe sample on the first bite. As such, it wasdetermined by comparing the ratio of the areaunder the two peaks that correspond to thetwo bites. The proof that the instrument per-formed in accordance with the sensory panel

Second biteFirst bite

Forc

e

Figure 1-3 Aspects of a Texture Profile AnalysisCurve

Time

Parameter

Hardness

Elasticity

Adhesiveness

Cohesiveness

Brittleness

Chewiness

Gumminess

Sensorial Definition

Force required to compress a food betweenthe molars.

The extent to which a compressed foodreturns to its original size when the loadis removed.

The work required to pull the food awayfrom a surface.

The strength of the internal bonds makingup the food.

The force at which the material fractures.Brittle foods are never adhesive.

The energy required to chew a solid fooduntil it is ready for swallowing.

The energy required to disintegrate asemisolid food so that it is ready forswallowing.

Exhibit 1-1 Parameters Measured by Texture Profile Analysis

= Hardness x Cohesiveness xElasticity

= Hardness x Cohesiveness

Brittleness

COHESIVENESS = B/A

Cycle = Second contact - First contactElasticity = Cycle for inelastic material

- Cycle for food

Hardness

Instrumental Definition

was achieved by correlating the sensory rat-ing against the Texturometer output. Best-fitlines or curves were then fitted to the data(Szczesniak et al., 1963).

It is evident that the development of TPAhas proved a valuable aid to assessing foodtexture. However, care should be exercised inaccepting the results for purposes other thancomparative evaluation. The technique isclearly imitative of what goes on in themouth. But it should be noted that quite apartfrom the differences, identified earlier, be-tween instrumental and human testing (e.g.,temperature control, saliva), the relationshipsbetween some of the sensory characteristicsthat TPA purports to measure are not linear.For example, between the sensory hardnessratings of 1 and 2 there are about 10Texturometer units, while between the sen-sory hardness ratings of 8 and 9 there areabout 70 (Szczesniak et al., 1963). Anotherfeature of TPA as measured with theTexturometer is that the instrument has arocking action that results from the pivotalconstruction (analogous to the temporoman-dibular joint). The area of contact between ahorizontal-plane sample and the flat-facedplunger will therefore vary, being small atfirst and then rapidly increasing until it is allin contact with the food.

Despite these shortcomings, the greateravailability of other force-deformation com-pressive testing machines on the market hasencouraged transposing TPA to other instru-ments for comparative tests. For example,Bourne (1966) substituted an Instron Univer-sal Testing Machine and performed TPA tocompare pears at different stages of ripeness.

As long as TPA is limited to comparativetesting, some of its shortcomings may notmatter, but if workers overlook such pointsand treat the instrumental analogues of thesesensory terms as absolute values, the basisbecomes risky. Szczesniak and Hall (1975)

recognized this potential for abuse and statedthat "proper use of the Texturometer is stillmuch of an art since the operator must supplythe thinking of which the instrument is notcapable" (p. 118).

Szczesniak (1975) used TPA to examinegelatin and carageenan gels. So that somecommonality existed between the experimen-tal gels, they were formulated to the same gelstrength as measured with a BloomGelometer. While the Bloom strengths werethe same, differences in TPA hardness andadhesiveness were apparent. An interestingaspect of their study is that TPA measure-ments were carried out over a range of tem-peratures. The result of varying the test tem-perature from 1O0C to 2O0C was a dramaticdrop in the gelatin gel hardness, presumablydue to melting. In contrast, the carrageenangels, which did not melt, showed only a smalldecrease. Cohesiveness ratings of thecarageenan and the gelatin gels appeared al-most indistinguishable from each other andhardly changed along the temperature gradi-ent.

Henry, Katz, Pilgrim, and May (1971) car-ried out TPA on a range of semisolid foods(such as gelatin gels). In addition to the char-acteristics identified in Exhibit 1-1, they de-fined five further variables, which related tothe material's behavior during the upstrokewhile the plunger is being pulled away fromthe sample. In addition to the instrumentaltesting, they also performed sensory evalua-tion on the foods. Performing correlationanalysis on each of the attributes allowed theidentification of four factors that accountedfor 95% of the sensory variance. Each ofthese factors correlated well with specific at-tributes. For example, 33% of the sensoryvariance was explained by the factor referredto as "stringy and sticky." Multiple regres-sions were then performed on the four sen-sory factors and each of the instrumental

measurements. It was found that the sensoryfactors could be explained by just eight of theinstrumental measurements, suggesting thatin the case of semisolid foods, much of thedata obtained through TPA may well be re-dundant.

Fundamental Tests

Fundamental tests measure innate physicalproperties of materials such as their Young'smodulus or Poisson's ratio. Such tests are sci-entifically rigorous, and data are expressed inwell-defined scientific units. Moreover, rela-tionships between fundamental properties ofmaterials allow prediction of values for oneproperty based on known values for others(see Chapter 4 of this book).

Coefficient of viscosity is a fundamentalproperty of Newtonian liquids, and its mea-surement therefore constitutes a fundamentaltest of food texture. However, with non-Newtonian liquids, comments made earlier inthis chapter need to be considered if one is tocontemplate drawing parallels with sensa-tions of viscosity in the mouth. In practice, ifone is to quote apparent viscosities ofnon-Newtonian liquids, one needs to definethe shear rates that were applied in perform-ing the test. Moreover, exact details of testprocedures become crucial if one deals withtime-dependent flow behavior. Since thixot-ropy is a quite common rheological behaviorin food systems, some measure of shear his-tory needs to be established prior to instru-mental testing for the results to mean any-thing useful (let alone to provide afundamental measure of the food's texture).

Saunders and Ward (1953) devised an in-genious and simple method of measuring theelastic modulus of gelatin gels. The Saundersand Ward apparatus (Figure 1-4) consists ofa U-tube, one limb of which is approximately2 cm in diameter and the other limb of which

is a capillary. The gel is poured into the widetube (radius R) and allowed to set in the appa-ratus. The length of the gel (L) needs to beconsiderably greater than its radius. The cap-illary tube (radius a) contains a drop of alow-surface tension, colored liquid, such thatany movement of the gel results in the dis-placement of the drop of liquid.

Once the gel is formed in the U-tube, theentire apparatus can be placed into a tempera-ture-controlled bath, To the open end of thewide tube a known small pressure (P) is ap-plied. Since the gel adheres to the walls of thetube, the stress that results from the appliedpressure causes the gel to shear as if it weremade up of concentric cylinders: those near-est the center of the tube are displaced themost, whereas those at the tube wall do notmove at all. This parabolic deformation ismeasured by the difference in the height ofthe dye (h) in the capillary tube. Relating the

Figure 1-4 Saunders and Ward's Apparatus

Capillary withinternalradius 'a'

L

applied pressure to the geometry and dis-placement of the gel allows calculation of theelastic modulus (G).

G= ^8La2h

Do such fundamental measurements relateto food texture? It has been argued that someaspects of food texture such as food break-down can be treated essentially as an issue ofmechanical failure. Since there are well-established theories that relate mechanicalfailure of a material when stressed to its fun-damental physical properties, it follows thatthe breakdown behavior of food should bemeasurable from such characteristics (see,e.g., Chapter 5 of this book). However, whilecontributing to food texture, mechanical fail-ure represents only a small part of the experi-ence, predominantly during the early stagesof mastication.

A knowledge of fundamental physicalproperties of food can be very valuable forreasons other than establishing food tex-ture—for example, developing packagingmaterial to prevent bruising of fruits.

Physiological Tests

In recent years, considerable interest hasbeen paid to measuring physiological activityduring eating. Various techniques have beenused to investigate characteristics such as jawmovement, muscle activity (electromyogra-phy), and sounds made during mastication(gnathosonics) (Boyar & Kilcast, 1986a).Advances in these areas have been linked todevelopments in medical technology: for ex-ample, early work with electromyographycaused considerable discomfort with the useof needle electrodes, but fortunately, adhe-sive surface electrodes have now taken overfor most oral measurements.

Various muscles can be monitored, but thetwo most commonly used are the masseterand the temporal, both of which are relativelyclose to the surface of the face and can be feltthrough the side of the cheek.

Techniques like electromyography havebeen used to investigate human physiologicalresponse to eating gelatin gels. Boyar andKilcast (1986b) had assessors linked to anelectromyograph eat both gelatin andcarageenan table jellies. By integrating theelectromyograph peak height during chew-ing, one gains an appreciation of the totalmuscle activity. In the case of gelatin, theforce involved during chewing rose duringthe first couple of seconds and then began todecline as the gel was dispersed in the saliva,melting and dissolving away. In contrast, thecarageenan gels took longer to break up in themouth.

In another study, electromyography wasused to study relatively concentrated gelatinand starch gels formed into various confec-tionery products such as wine gums. The inte-grated muscle activity during chewing of astarch:gelatin wine gum mixture (50:50) be-gan to break down rapidly in the mouth. Incontrast, the pure gelatin wine gum requiredan initial rise in integrated muscle activity forabout 20 seconds before the gum began tomelt and dissolve away. Other gelatin confec-tionery products were also examined (gelatinchew and whipped gelatin chew), and thesewere found to behave in the same way(Kilcast & Eves, 1991).

RELATIONSHIP BETWEENINSTRUMENTAL AND SENSORYMEASURES OF TEXTURE

The advantages and disadvantages of bothsensory and instrumental techniques havebeen discussed, and while these techniquesare clearly different, an understanding of the

relation between them is necessary to enablecorrelating one with the other, as is desirablein formulating procedures such as qualitycontrol. Many researchers have investigatedsuch relationships, and the approaches takenhave been comprehensively reviewed byKapsalis, Kramer, and Szczesniak (1973);other review articles consider some of the re-lationships that have been found to exist (e.g.,Szczesniak, 1987).

Before considering how sensory and in-strumental measures of food texture interact,it is worthwhile to reflect on what is beingmeasured and compared. Bearing in mindthat texture can arise from multifariousstimuli and that most instrumental measure-ments tend to concentrate on one property ofthe food, we should not assume that there willnecessarily be any relationship between aninstrumental measurement and the sensoryexperience.

Some distinction needs to be made betweentrained and consumer panels of assessors. Theformer are often composed of individuals whohave been selected on the basis of their sensorydiscrimination and descriptive power. More-over, they are often trained to develop andfine-tune their power of perception. As such,they may be far more discriminating betweensubtle nuances of texture than members of thegeneral public. Both trained and consumerpanels have a role to play, yet that role is verydifferent. One should not assume that the mea-surements of texture that come from a trainedpanel are similar to those perceived by a con-sumer panel. The consumer may not detect thesame detail as the trained assessor, and as suchit is probably appropriate that the consumer'sopinion be the final arbiter in assessing accept-ability. Nevertheless, in the past, some presti-gious research institutes have claimed thattheir trained panel can identify quality charac-teristics such as acceptability in the same wayas the general public.

Concepts of quality such as ripeness fre-quently involve changes in texture. Identify-ing correlations between sensory and instru-mental tests can prove useful for qualityassurance applications.

Texture arises from the mechanical, geo-metrical, and surface attributes of foods, andsince these are themselves dependent onstructure and chemical composition, it wouldseem reasonable to attempt to monitorchanges in chemical composition as a mea-sure of food texture and possibly quality.Szczesniak (1973) reviewed some of theseindirect measurements of food texture. Thepower of such chemical techniques can be il-lustrated by considering changes to bananaand plantain during ripening. During ripen-ing, the flesh softens due to the starchymaterial's being broken down into sweetsimple sugars. Alcohol-insoluble solids, ameasure of complex carbohydrate, (such asstarch, pectin, or cellulose) decrease, makingthem an excellent index of maturity. Otherindirect indices include moisture content ofvegetables and free fatty acids in tomatoes.

When considering correlations betweensensory and instrumental measurements, it isappropriate to consider the linearity of thepsychophysical response. While Weber's lawrelates to the magnitude of the just noticeabledifference, Fechner's law shows that manystimuli bear a log relation to perception.However, the perception of some stimulidoes not follow Fechner's law, and othermodels have been put forward to explain theirbehavior, such as that of Stevens, which es-sentially follows a power law, and that ofBeidler, which exhibits an S-shaped curverising to a maximum (Meilgaard, Civille, &Carr, 1991). Fitting the right model to a par-ticular stimulus requires an element of pre-liminary experimentation since each requiresthat data be collected in a different way (e.g.,magnitude estimation for Stevens' model).

Elejalde and Kokini (1992) examined thepsychophysics of in-mouth viscosity percep-tion. Using magnitude estimation, theyshowed that perception of viscosity followeda power law relationship and correlated wellwith shear stress (R2 = 0.96) and that shearstress in the mouth is in fact the sensorymechanism for oral evaluation of viscosity,since the slope is very close to unity.

CONCLUSION

Food texture is essentially a human experi-ence arising from our interaction with food—its structure and behavior when handled. Tounderstand our response to food structure andits breakdown involves us in a matrix of dis-

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