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Classification of Textural CharacteristicsALINA SURMACKA SZCZESNIAK

General Foods Corporation, Technical Center, Tarrytown, New York(Manuscript received July 23, 1962)

SUMMARYDefinition of texture is reviewed, and a system for classification of textural

characteristics of foods is described. The system is based on fundamentalrheological principles, and at the same time is suitable for routine use. Tex-tural characteristics are defined and classified into mechanical and geomet-rical qualities as well as those related to the moisture and fat content of aproduct. The mechanical characteristics are subdivided into the primary pa-rameter s of hardne ss, cohesive ness, viscosity, elasticity, and adhesive ness, andinto the secondary parameters of brittleness, chewiness, and gumminess. Itis pointed out that popular terms used to describe texture often de note degreesof intensity of these characterisiics. The proposed classification lends itselfto use with both objective and subjective methods of texture characterization.

INTRODUCTIONTexture is the least well described of themany organoleptic food attributes of concern

to a food scientist. One should mentio n, ofthe several reasons that caused this situa-tion, the lack of an adequat e bri dge betweentheoretical rheology a nd practical applica-tions, and the fact that most of the work infood texture reported to date dealt with aspecific textural characteristic in a specificfood product. This led to confusion evenregardin g the definition of texture itself.Only in the last few years have some at-temllts been made to define tekture, andthese are not devoid of the influence of theprevailing trend toward identifying texturewith just a few characteristics most commonto a particular food in question. For ex-ample, in discussing the texture of frozenpeas the food scientist will usually meantoughness of the skin; in referring to thetexture of meat he will usually mean tencler-ness versus toughness and chewiness; in de-scribing the texture of canned cherries hewill mean freedom from cracks and thewholeness of the fruit.One of the most urgent problems in tex-ture technology is the development of arational system and nomenclatur e for de-scribing and translating textural qualities

a Presented at the 22nd Annual Meeting of theInstitute of Food Technologists, Miami Beach,Florida, June 10-14, 1962.

into precisely defined, measurable properties.Foods represent very complex rheologicalsystems, and applications of the principles oftheoretical rheology (Burgers and ScottBlair, 1949 ; Erich, 195s; Scott Blair, 1949,1953 ; Scott Blair and Reiner. 1957) lead,in many cases, to difficult theoretical andexperimental problems that are too time-consuming for applied industrial research.This situation is further complicated by thefact that, once obtained, rheological descrip-tion of foods must be translated into pr ac-tical terms. Laboratory and field practicehas resulted in the creation of a popularnomenclature that serves to describe texturalcharacteristics in terms of simple words.However, this nomenclature has several in-herent prob lems. In many cases, a numberof terms are used to describe the same char-acteristic. In other cases, the same term isused to describe several characteristics.Also, the same word may have differentmeanings to different people.This study was made to review the defini-tions of tex@re used by workers in the field,and to develop a system of texture nomen-clature that could serve as a bridge betweenfundamental rheological principles and pop-ular nomenclature and lend itself to appliedresearch on a vast variety of food products.

Definition of TextureWebsters dictionary defines texture as thecharacteristic disposition or connection of threads

t 38.5 1

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386 CLASSIFlCATIOS OF TEXTURAL CHARACTERISTICSin a woven fabr ic or the disposition or mannerof union of the particles of a body or substance.

Hall and Fryer (1953) considered texture andconsistency as components of mouthfeel. They didnot give clear definitions of these terms, but said :It . . . appears that the term tcxtwre has cometo mean how hard or how soft as well as howlarge or how small the individual kernels in themass of . . . food actually are. If the individualkernels are large and hard, the texture is saidto be grainy or gritty. If the individual kernelsare small and soft, the texture is said to besmooth. Consistency has come to refer to thecondition of the individual kernels cohering to-gether and adhering to foreign substances, thatis, the stickiness, g ummine ss or viscosity, of the. . . food . .

Smith (19 47) distinguished between textureand consistency in a similar manner, although hisdefinitions are even less specific. He consideredtexture to be the rigidity of solid units of food,and consistency, a combination of the size andtexture of the solid units, the viscosity of thefluid, and the proportions of solids to fluid. Theterm texture is thus applied to solids whileconsistency refers to a mixture of fluid andsolid foods.

Consistency is often termed body. Davis(1937) wrote : . . . it is preferable to definebody as that quality which is perceptible totouch; texture as that which is evident to theeye, excluding color. Under the term body areincluded several factors of which fir mness (viscos-ity and modulus), springiness (elasticity) andsmoothness (homogeneity) are most impor-tant. .

Martin (1955) defined the texture of candy asthe sum or resultant of several physical proper-ties including density, hardness, plasticity or elas-ticity, and consistency. Isherwood (1960) in-cluded in the texture of fruit and vegetables suchcharacteristics as toughness, stringiness, slicingquality, and crispness.

Ball et nl. ( 1957) stressed the need for a gooddefinition of texture. In an attempt to find a defi-nition that could be used dependab ly in gradingmeat for quality by the usual organoleptic pro-cedures, they constructed two tentative definitions :the sight and the feel definitions. The sightdefinition reads: Texture of meat is the micro-scopic appearan ce of muscle tissue from the stand-point of smoothness o r fineness of grain . . .The feel definition is worded: The texture ofcooked meat is the feel of smoothness or finenessof the muscle tissue in the mouth.

These definitions point out two important ele-ments of texture: the physical structure of thematerial (its geometry) and the way the material

handles and feels in the mouth (its me chanicaland surface properties).

A rigorous definition of texture will have toawait a better und erstanding of the basic principlesinvolved, especially those concerned with rheo-logical or mechanical properties of foods. For thepurpose of this work, texture was considered asthe composite of the structural elements of foodand the mantler in which it registers with physio-logical senses. Thus, this definition includes theconcepts of texture and consistency (orbody) as defined by other workers. The termstructural elements as used in this definitionrefers to the microscopic and molecular structuresas well as to the macroscopi c structure that canbe sensed visually. Sound effects, such as thoseoccurring during eating of popcorn or celery, arerelated to the physical constitution of the foodand should be considered as part of the over-alltextural effect.

Classification of Textural CharacteristicsA compilation of terms used in popular texture

terminology, an analysis of their meaning s, andthe definitions of rheological concepts served asa background for the development of an organizedsystem for classification of textural characteristics.In studying the definitions of terms popularly usedin description of texture, one finds that whereassome refer to what might be called pr imarycharacteristics, many refer to secondary charac-teristics, i.e., those that could be adequately de-scribed by two or more of the primary terms. Inaddition, many popular terms actually denotedegrees of the same characteristic and could beconsidered points on a scale.

When terms used to describe texture were com-piled and their dictionary and rheological defini-tions were studied, it became apparent thattextural characteristics could be grouped intothree main classes:

1) Mechanical characteristics2) Geometrical characteristics3) Other characteristics (referring mainly to

moisture and fat content of the food)Mechanical characteristics are manifested by

the reaction of the food to stress. They are meas-ured organoleptically by pressures exerted on theteeth, tongue, and roof of the mouth duringeating. Geometrical characteristics refer to thearrange ment of the constituents of the food, andare reflected mainly in the appeara nce of the foodproduct. They are mostly sensed visually. How-ever, these characteristics are often sufficientlypronounced to produce an oral sensation throughthe sense of touch and pressure. In the categoryof other characteristics are included mouthfeel

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SZCZESNIAli 3s7factors that cannot be easily resolved on the basis visions. There are, however, two general groupsof mechanical and geometrical properties. . of qualities: A) those related to size and shape of

Mechanical characteristics. The mechanicalcharacteristics are probably the most important indetermining the manner in which the food handlesand behaves in the mouth. They can be dividedinto the following five basic parameters:

A) Hardness, defined as the force necessar y toattain a given deformation.

B) Cohesiveness, defined as the strength of theinternal bonds making up the body of theproduct.

C) Viscosity, defined as the rate of flow perunit force.

D) Elasticity, defined as the rate at which adeformed material goes back to its unde-formed condition after the deforming forceis removed.

E) Adhesiveness, defined as the work necessaryto overcome the attractive forces betweenthe surface of the food and the surface ofother materials with which the food comesin contact (e.g., tongue, teeth, palate, etc.)

The first four characteristics are related toforces of attraction acting between particles offood and opposing disintegration, whereas ad-hesiveness is related to surface properties.

In addition to the above five primary parameters,it appears to be desirable to characterize foodtexture in terms of three additional secondary pa-rameters in order to make the characterization asmeaningful as possible to individuals accustomedto popular terminology, while at the same timekeeping it in agreement with basic rheologicalprinciples. These secondary parameters are :B-I) Brittleness.defined as the force with which

the material fractur es. It is related to theprimary parameters of hardness and co-hesiveness. In brittle materials, cohesivenessis low and hardness can vary from lowto high. Brittle material s, especia lly whenpossessing a substantial degree of hard-ness, often produce sound effects on masti-cation (e.g., celery, toasted bread)

B-Z) Chewiness, defined as the energy requiredto masticate a solid food product to astate ready for swallowing. It is relatedto the primary parameters of hardness,cohesiveness, and elasticity.

B-3) Gummilzess, defined as the energy requiredto disintegrate a semisolid food product toa state ready for swallowing. It is relatedto the primary parameters of hardness andcohesiveness. With semisolid food prod-ucts, hardness is low.

Geometrical characteristics. Geometrical char-acteristics do not lend themselves to clear-cut di-

the particles, and B) those related to shape andorientation. A number of characteristics in thefirst group could be placed on a hardness scale.

While some of the geometrical characteristics(e.g., smooth) refer to homogeneous food prod-ucts, many are applicable to systems containingmore than one phase. For example, cellular re-fers to a highly organized structure composed ofcell walls filled with a gas (e.g., egg-white foam)or with a liquid (e.g., watermelon). The mechani-cal characteristics of each one of the phasesshould be considered for a thorough texture analy-sis of su ch a food.Other characteristics. This group comprisesmouthfeel qualities related to the percepti on ofthe moisture and fat content of the food :

A) Moisture contentB) Fat content

1) Oiliness2) Greasiness

These qualities may also be considered to be con-cernedwith the lubricating properties of the prod-uct. The popular terms used to describe themoisture content of a food reflect not only thetotal amount of moisture perceived but its rateand manner of release. With fat content, thetotal amoun t of fat and its melting point, asrelated to mouth-coating properties, be come im-portant. The secondary parameters of oiliness andgreasiness have been set up to distinguish betweenthese two characteristics. The secondary param-eter of oiliness refer s to the intensity of theoily feeling in the mouth, which might berelated to the surface tension and changes inviscosity of the product affected by movements ofthe tongue. The secondary parameter of greasinessreflects the solidity and difficulty of removal of afatty film coating the mouth cavity, both of which,in turn, are related to the melting point of thecomponent fat.

Popular Nomenclature and the DevelopedClassification

Table 1 shows the relations between the popularnomenclature of food texture and the developedclassification of textural characteristics. Becauseof the large number of terms used in popularterminology, only the most characteristic ones areincluded in this table to illustrate the principle.It will thus be seen that the ter ms soft, firm,, andhard are actually degrees of the characteristic ofhardness, and the terms crumbly, crunchy, andbrittle are degrees of the characteristic of brittle-ness. Dry, moist, wet, and watery are terms de-noting different levels of moisture. No popular

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385 CLASSTFICATION OF TEXTURAL CHARACTERISTICSTable 1. Relations b etween textural paramete rs and popular no menclature.

MECHANICAL CHARACTERISTICSPrimary paramctcrs Scco+kdary parametersHardnessCohesiveness Brittleness

ChewinessGumminess

Viscosity-ElasticityAdhesiveness

Poplklar ter?lZsSoft+Firm-tHardCrumbly+Crunchy+BrittleTender+Chewy-+ToughShort+Mealy+Pasty+GummyThin+ViscousPlastic+ElasticSticky+Tacky+Gooey

GEOMETRICAL CHAR.4CTERISTICSClassParticle size and shapeParticle shape and orientationOTHER CHARACTERISTICSPrinlary parameters Secondary parametersMoisture contentFat content Oiliness

Greasiness

ExamplesGritty, Grainy, Coarse , etc.Fibrous, Cellular, Crystalline, etc.

Popzklar terllzsDry+Moist+Wet+WateryOilyGreasy

terms are available to describe different degreesof oil or fat content. This p roblem is solved inpractice by adding qualifying words such asslightly, moderate ly, very, etc., to the termsoily and greasy. Combinations of severalparameters result in popular terms like juicy,which is a.combinati on of the geometri cal charac-teristic of cellularity and a high moisture content.

Adaptation to Objective a nd SubjectiveEvaluation of Texture

The proposed classification of texturalcharacteristics lends itself to use with in-strumental (Friedman et al., 1963) and or-ganoleptic (Brandt et al., 1963) methods oftexture evaluation, and has served as a basisfor the development of quantitative ratingscales (Szczesniak ef al., 1963). Use of thesame nomenclature for both objective andsubjective evaluation of texture facilitatedestablishmen t of a correlation between thetwo methods (ibid). In addition, experiencehas indicated that the proposed classifica-tion can be used with ordinary laboratorypanels provided clefinition of the basic pa-rameters is properly understood.To determine the adequacy of the devel-oped classification and the relation betweenthe individual parameters, quantitative de-scription of the texture of 22 selected foodproducts using this system and the standardscales (Szczesniak et al., 1963) was sub-jected to scatter-d iagram analysis. The re-sults indicated that there was no duplication

between the paramet ers and that the ratingsfollowed the established definitions. A rela-tion was found between hardness and brittle-ness : products noted for brittleness alwaysfell on the upper portion of the hardnessscale. Similarly, an inverse relation wasfound between hardness and moisture con-tent: relatively moist materials fell on thelower portion of the hardness scale, whereasonly low-moisture materials possessed highdegrees of hardness. The parameters of vis-cosity and brittleness were found to be mu-tually exclusive. This is understandablesince only solids can have the characteiisticof brittleness, whereas the characteristic ofviscdsity is limited, in this classification, toliquid and semisolid products. Scatter di-agrams of viscosity and brittleness versushardness show that the brittleness scale maybe a continuation of the viscosity scale. Astrong positive relation was found betweengumminess and adhesiveness. This does notmean that the two parameters are mutuallyexclusive, but that thege two characteristicsoften occur simultaneously. An interestingcorrelation was obtained between gumminessand moisture content, and adhesiveness andmoisture content. In both cases, a high rat-ing on either one of the scales could be ob-tained only with a low rating on the otherscale. For example, a highly adhesive mate-rial was always rated as being low in mois-ture, whereas products with a low degree of

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SZCZESNIAE; 389adhesiveness could fall on any part of the Report on the Principles of Rheologicalmoisture scale. The same situation existed Nomenclature, Joint Committee on Rheologywhen scatter diagrams were prepared for of the Intern. Count. of Sci. Unions. Proc.the elasticity scale versus the adhesiveness Intern. Congr. Rheology, Holland, 1948.and gumminess scales. Davis, J. G. 1937. The rheology of cheese, butterand other milk products (the measurement of

Concluding Remarks body and texture). J. Dalrj! Resear ch 8.7CA more rigorous evaluation of the pro-

posed classification of textural character-istics will have to await the accumulation ofmore data and a more extensive use of thesystem. No doubt, slight modification ofthis classification may become necessary asproblems are encountered in its practicaluse. It is believed, however, that the basicconcepts described may be a step in thedirection of supplyin g the food scientist witha rational tool for a scientific descriptio n offood testure.

ACKNOWLEDGMENTSincere appreciation is expressed to Dr. Robert

V. MacAllister, presently with Clinton Foods,Clinton, Iowa, for inspiring discussions and con-structive criticism that made possible the finalformulation of the thoughts described in thispaper, and to the late Professor Bernard E.Proctor, Massachusetts Institute of Technology,

LtJ.Eirich, F. Q., Editor. 1958. Rheology, Theory and

Applications. Academic Press, New York.Friedman, H. H., J. E. Whitney, and A. S. Szczes-

niak. 1963. The texturometer, a new instru-ment for objective texture measure ment. J.Food Sri. 28, 390.

Hall, R. C., and H. C. Fryer. 1953. Consistencyevaluation of dehydrated potato granules anddirection for microscopic rupture count pro-cedure. Food Tcchnol. 7, 373.

Isherwood, F. A. 1960. Some factors involved inthe texture of plant tissues. In Texture inFoods. Sot. Chem. Ind. Monograph No. 7,135.

Martin, L. F. 1955. Application of research toproblems of candy manufacture. Advances inFood Research 6, I.

Scott Blair, G. W. 1949. A Survey of Generaland Applied Rheology. Sir Isaac Pitman andSons, London.

Scott Blair, G. W. 1953. Foodstuffs, Their Plas-ticity, Fluidity and Consistency. Intersciencewhose interest and encouragement contributed to Publishers, Amsterdam, New York.bringing this work to its present stage of devel- Scott Blair, G. W., and M. Reiner. 1957. Agri-

opment. cultural Rheology. Routledge and KeganREFERENCES Paul, London.

Ball, C. O., H. E. Clauss, and E. F. Stier. 1957.Smith, H. R. 1947. Objective measurement of

Factors affecting quality of prepackaged meat. quality in foods. Food Technol. 1, 345.IB. Loss of weight and study of texture. Szczesniak, A. S., M. A. Brandt, and H. Fried-Food Technol. 11. 281. man. 1963. Development of standard ratingscales for mechanical parameters of texture

Brandt, M. A., E. Skinner, and J. Coleman. 1963. and correlation between the objective andTexture profile method. J. Food Sci. 28, 404. the sensory methods of texture evaluation.

Burgers, J. M., and G. W. Scott Blair. 1949. J. Food Sri. 20, 397.