J Sci Food Agric 1997, 75, 463È471

Effect of Water Activity on the MechanicalProperties of Selected Legumes and NutsAntje Borges and Micha Peleg*Department of Food Science, University of Massachusetts, Amherst, MA 01003, USA

(Received 3 September 1996 ; revised version received 5 March 1997 ; accepted 28 April 1997)

Abstract : Two legumes (kidney beans and chickpeas) and two nut kernels(almonds and hazelnuts) were compressed intact with a Universal testingmachine at various levels in the range of 0É11È0É85 at an ambient temperatureawof 22¡C. Mechanical characterisation of the four was done through four param-eters ; the slope of the initial part of forceÈdisplacement curve, considered ameasure of sti†ness, the failure force, considered a measure of strength, the defor-mation at failure, considered a measure of brittleness or deformability and thearea under the forceÈdisplacement curve corresponding to 20% compression,considered a measure of toughness. Among these parameters only the lattercould be determined and used e†ectively in all four seeds and along the wholewater activity range. The jaggedness of the forceÈdisplacement curve was alsodetermined in terms of its apparent fractal dimension calculated with two di†er-ent algorithms for veriÐcation. There was a noticeable di†erence between themechanical behaviour of the two groups. The nut kernels, apparently because oftheir high oil content, had a much smoother forceÈdisplacement curve, and thee†ect of absorbed moisture on their texture was generally more moderate thanon that of the legumes. But in all the four types of seeds tested, each mechanicalproperty had its own speciÐc moisture dependency, an observation consistentwith previous reports on cereal-based products.

J Sci Food Agric 75, 463È471 (1997)No. of Figures : 9. No. of Tables : 4. No. of References : 13

Key words : compression, plasticisation, moisture sorption, seeds, almonds,beans, chick-peas, hazelnuts

INTRODUCTION

Beans and nuts are major food commodities ; the formeras a major component of the diet and the second as oras an important ingredient of “pleasure foodsÏ such asbaked products, sweets and ice creams. Their mechani-cal properties have been studied although not as exten-sively as those of many other foods (Woodroof 1979 ;Rosengarten 1984 ; Austin and Klein 1989). Both beansand nuts are more conveniently tested in their originalform. Obtaining from them a specimen with a moresuitable geometry such a cylinder or cube is a very diffi-cult task, technically, and any attempt to shape themmay result in fracture or a†ect their mechanical proper-ties. Proper analysis of mechanical tests performed onspecimens with the curved contours encountered in

* To whom correspondence should be addressed.Contract/grant sponsor : USDA-NRICGP.Contract/grant number : 9203438.

seeds must be based on the stresses and strain spacialdistribution which can be a cumbersome task even inthe age of computers. The classical solutions of thecontact stress problems may also be inappropriate whenlarge plastic deformations are involved. Provided thatthe dimensions and geometry of seeds of a certain typedo not vary by much, tests performed on them whileintact can be sufficient to establish the e†ect of mois-ture, for example, on their texture. Moreover, at leastwhen dry, certain seeds are brittle to such an extent thattheir forceÈdisplacement curves can be irregular andirreproducible, a situation encountered in the mechani-cal analysis of brittle snacks and particles of cerealorigin (Barrett et al 1992 ; Wollny and Peleg 1994 ;Harris and Peleg 1996). In these it was shown thatuseful information can still be derived from suchrelationships provided that the general shape of thecurve and its jaggedness are treated separately. The

4631997 SCI. J Sci Food Agric 0022-5142/97/$17.50. Printed in Great Britain(

464 A Borges, M Peleg

objectives of this study were to test the applicability ofsuch an approach in the testing of intact seeds and thepossibility of monitoring the e†ects of moisture on theirtexture through mechanical parameters derived fromsuch an analysis.

EXPERIMENTAL

Materials

Kidney beans and chickpeas (legumes) and almondsand hazelnuts (nuts) were purchased at a local super-market. Their dimensions, determined by a caliper, andweight are given in Table 1. They were placed in evac-uated desiccators with saturated salt solutions of LiCl,

NaBr,CH3COOK, MgCl2 , K2CO3 , Mg(NO3)2 ,NaCl an KCl to produce water activities ofNaNO2 ,

0É11, 0É23, 0É33, 0É43, 0É52, 0É55, 0É65, 0É75 and 0É85,respectively (Greenspan 1977), at 22¡C. They were testedafter 5È8 days, when their weight, monitored period-ically, reached a constant value.

Mechanical testing

Each seed was placed on its longest side and was com-pressed between two metal plates using an InstronUTM model 1000. The displacement rate was10 mm min~1 and the sampling rate 10 Hz in all theexperiments. The almonds and hazelnuts were com-pressed to about 65% of their original height (35%deformation) and the force was recorded through a50 kg load transducer. The beans and chickpeas werecompressed to 50% of their original height and theforce was recorded through a 500 kg load transducer.The voltageÈtime data output of the machine was con-verted to digitised forceÈdisplacement data using aStrawberry Tree Card and a program written by MarkD Normand. All the tests were performed with at leastÐve replicates.

Data processing

The digitised data were processed using the Systat 5.2.1package for statistical calculations and graphics. The

fractal analysis was performed on the force columns,saved as text Ðles, with the MacFractals programwritten by Russ (1994).

Mechanical and jaggedness parametersSince neither stress nor strain could be meaningfullycalculated the mechanical properties of the seeds werecharacterised with the following empirical measureswhich will be further discussed with the results. The waythey were deÐned is illustrated in Fig 1.

Pseudo-deformability modulusThe slope of the linear region of the forceÈdisplacement,where it existed (see below), was determined by linearregression (r2[ 0É99) and served as an apparentmeasure of sti†ness. (Unlike a proper modulus that hasstress units, this pseudo-modulus had a Newton per mmunits and is therefore only a relative measure ofsti†ness.)

Breaking forceThe force indicating the Ðrst major failure event wasrecorded and treated as an empirical measure ofstrength. (Conventional strength is expressed by thefailure stress.)

Break deformationThe absolute deformation corresponding to the break-ing force (see above) was monitored and served as an

Fig 1. Determination of various mechanical parameters fromthe forceÈdeformation curve of a legume seed/nut kernel com-

pressed intact.

TABLE 1Physical properties of the tested seeds

Kidney beans Chickpeas Almonds Hazelnuts

Mass (g) Mean 0É58 0É67 1É30 1É11SD 0É09 0É05 0É10 0É09

Height (mm) Mean 6É6 9É4 8É1 12É3SD 0É6 0É2 0É6 1É3

Length (mm) Mean 15É6 11É8 24É4 14É4SD 0É6 0É6 1É2 0É7

Width (mm) Mean 8É0 9É3 13É1 13É6SD 0É6 0É3 0É6 0É4

E†ect of water activity on the mechanical properties of legumes and nuts 465

Fig 2. Typical compressive forceÈdeformation relationships of kidney beans tested intact at various water activity levels.

empirical measure of deformability. (The conventionalmeasure of deformability is a breaking strain.)

Absorbed energyThe area under the Ðrst part of the forceÈdisplacementcurve was determined using a special option of theprogram with which the Instron was controlled and thedata retrieved. The 20% displacement was selectedbecause none of the samples has failed at or prior tothis compression level. Thus, the relative “toughnessÏ ofthe materials under the di†erent conditions could becompared without the need to account for their di†er-ent failure properties. The units of this arbitrary andrelative measure of toughness were Joules. (Again, aconventional measure of toughness is determined fromthe area under the stressÈstrain curves ending in failureand expressed in energy per unit volume units.)

T he apparent fractal dimensionThe degree of jaggedness of the forceÈdisplacementcurve was expressed in terms of the apparent fractaldimension determined by the Richardson plot and(DfR)

by the box counting method, also known as the Kolmo-gorov dimension They were determined simulta-(DfK).neously for mutual veriÐcation. It should be mentionedthat mechanical signatures of the kind analysed are nottruly fractal in the mathematical sense. However, as hasalready been shown previously (Borges and Peleg 1996 ;Damrau et al 1997) these algorithms still provide con-sistent measures of jaggedness. Their applicability canbe veriÐed by testing the linearity of the plots fromwhich they are calculated. In all the cases reported inthis work they were linear for all practical purposes.The apparent fractal dimension is a measure of jagged-ness on a scale from 1É0, a smooth Euclidean curve, to2É0, the asymptotic upper theoretical limit, where thecurve is so jagged that it tends to occupy the area onwhich it is drawn.

RESULTS AND DISCUSSION

Typical forceÈdisplacement relationships of the beans,chickpeas, almonds and hazelnuts tested in this work

466 A Borges, M Peleg

Fig 3. Typical compressive forceÈdeformation relationships of chick peas tested intact at various water activity levels.

are shown in Figs 2È5. At the low levels the beansÏawand chickpeasÏ relationships were considerably irregularand jagged, a characteristic clearly lost as moisture wasadsorbed (see below). In contrast the almondsÏ andhazelnutsÏ relationships, although irregular to someextent did not undergo a morphological change of suchobvious magnitude. This qualitative di†erence betweenthe two legumes and two nuts can be attributed to theinÑuence of their very di†erent oil content ; 1È6% and52È62%, respectively (McCarthy 1984). Although not aplasticiser in the sense that water is, the presence of aconsiderable amount of oil does a†ect the texture. Sinceoil hardly interacts with any absorbed water, it reducesthe latterÏs relative e†ect which is primarily the plastici-sation of the main hydrophilic components of thestructureÈthe carbohydrates and proteins.

Plots of the quantitative mechanical of the four typesof seeds vs the corresponding level are shown in Figsaw6È9. The vertical bars mark the experimental range ofeach parameter. With few exceptions the scatter wasconsiderable, which could be expected in light of the

size and shape and possibly structural variability withineach group of seeds (Table 1). In certain cases thescatter was too large for any trend to be conclusivelyestablished qualitatively, let alone quantitatively (seebelow).

Because of the considerable di†erences in size andgeometry the absolute magnitude of the mechanicalparameters cannot be used for comparisons between thegroups. A possible exception is the apparent fractaldimension, of either type when the curves have a similarnumber of points. This is because the dimension has auniversal scale and because the data from which it iscalculated are normalised Ðrst. Despite the above men-tioned limitations imposed by the unequal size andmorphology, certain mechanical features could still beidentiÐed and even expressed quantitatively.

Mechanical parameters

As could be expected, wherever the sti†ness parameters,as deÐned, could be determined its magnitude fell when

E†ect of water activity on the mechanical properties of legumes and nuts 467

Fig 4. Typical compressive forceÈdeformation relationships of almonds (kernels) tested intact at various water activity levels.

moisture was absorbed. The drop was moderate in thealmonds and hazelnuts for the reasons previously dis-cussed, that is, because of their high oil content. Thebreaking force was an insensitive measure in the twonuts and only in the chickpeas did it show a clear rela-tion to the water activity. In all the cases the breakdeformation increased as moisture was absorbed. Thebreak deformation is an obvious measure of plasticisa-tion. In principle its magnitude can increase indeÐnitelywhen the material is plasticised to such an extent that itdoes not fail at all. This stage was reached at levelsawabove 0É75, and is the reason for the absence of datapoints in this range. The toughness measure appears tobe the only parameter that could be determined in allfour materials and all over the experimental range.awIts magnitude was steady at the low region but itawdropped as the level increased. The toughnessawmeasure vs relationships of the two beans clearly hadawa sigmoid shape of the kind previously reported incertain cereal products (Wollny and Peleg 1994 ; Harrisand Peleg 1996). It could be described by a modiÐed

Fermian model whose Ðt is shown in Figs 6 and 7 andTable 2. That is :

Y \ (Y0[ Yr)/(1 ] exp((aw [ awc)/b))] Yr (1)

where is the parameter magnitude at zero humidity,Y0the inÑection point of b is a constant rep-awc Y (aw),

resenting the slope of around and theY (aw) awc Y rresidual value of at the very high levels. TheY (aw) awnutsÏ corresponding relationships did not have asigmoid shape and they could be e†ectively Ðtted by thesimple empirical model

Y (aw) \ Y0[ k awn (2)

where k and n are constants. The actual Ðt of the modelcan be seen in Figs 8 and 9 and Table 3.

A similar type of an empirical power equation couldbe used to describe the e†ect of moisture on the breakdeformation, ie

Y (aw)\ Y 0@ ] k@awn@ (3)

468 A Borges, M Peleg

Fig 5. Typical compressive forceÈdeformation relationships of hazelnuts (kernels) tested intact at various water activity levels.

TABLE 2Regression parameters of the modiÐed Fermi equation (eqn (1)) applied to selected mechanical

properties of kidney beans and chickpeas

awc

b Y0 Yr

c r2

Kidney beans“ToughnessÏa 0É75 0É003 150 mJb 40 mJb 0 0É990Df (Richardson) 0É44 0É054 1É25 1É05 0 0É999Df (Kolmogorov) 0É45 0É055 1É28 1É10 0 0É999

ChickpeasPseudo-modulus 0É52 0É11 660 N mm~1 0 690 N mm~1 0É997Breaking force 0É58 0É15 475 Nb 100 Nb 0 0É991“ToughnessÏa 0É70 0É052 510 mJ 72 mJ 0 0É987Df (Richardson) 0É44 0É083 1É13 1É06 0 0É999Df (Kolmogorov) 0É51 0É074 1É17 1É09 0 0É991

a DeÐned as the area under the forceÈdisplacement curve corresponding to 20% displacement.b Determined directly from the experimental data.

E†ect of water activity on the mechanical properties of legumes and nuts 469

TABLE 3Regression parameters of eqns (1) and (2) applied to two mechanical properties of almonds and

hazelnuts

Pseudo-modulus (eqn (1)) “T oughnessÏ (eqn (2))

Almonds Hazelnuts Almonds Hazelnuts

awc 0É67 0É58 k 197 mJ 114 mJb 0É22 0É17 n 3É6 2É0Y0 240 N mm~1 b 70 N mm~1 b Y0 195 mJ 144 mJYr 1 N mm~1 20 N mm~1 bc 0 0r2 0É993 0É992 r2 0É997 0É988

a DeÐned as the area under the forceÈdisplacement curve corresponding to 20% displacement.b Determined directly from the experimental data.

Fig 6. E†ect of water activity on selected mechanical proper-ties of kidney beans.

TABLE 4Regression parameters of eqn (3) applied to the break defor-

mation of four seeds

Kidney beans Chickpeas Almonds Hazelnuts

k@ (mm) 23 3É4 0É71 1É1n@ 8É9 5É6 10 1É9Y0 (mm) 0É15 0É59 0É71 1É1r2 0É988 0É988 0É995 0É998

Fig 7. E†ect of water activity on selected mechanical proper-ties of chick peas.

The Ðt of the model is demonstrated in Figs 6È9 andTable 4. It captures the phenomenon that when fullyplasticised the seeds become ductile, ie they deformwithout exhibiting failure. At such levels though theawseeds will also be biologically or biochemically unstableand therefore, as far as storage is concerned may be oflittle practical consequences.

Jaggedness parameters

Only the jaggedness parameters of the two legumesshowed a clear dependency on the level (Figs 6 andaw

470 A Borges, M Peleg

Fig 8. E†ect of water activity on selected mechanical proper-ties of almonds (kernels).

Fig 9. E†ect of water activity on selected mechanical proper-ties of hazelnuts (kernels).

7). There was a general agreement between the twomeasures, RichardsonÏs and KolmogorovÏs. The awdependency of both could be described by the modiÐedFermian model, eqn (1), with and replacingDf , Df0 DFRY , and respectively. The regression parametersY0 Yr ,are given in Table 2. They clearly indicate that the jag-gedness and toughness losses occurred at signiÐcantlydi†erent levels and at di†erent rates with respect toawthe That di†erent mechanical properties need notaw .change in unison as a result of moisture sorption hasalso been found in a variety of baked and extrudedcereal products (Wollny and Peleg 1994 ; Harris andPeleg 1996.) The observation of the same in naturalproducts of very di†erent composition and structuresuggests that it might be a general phenomenon, andthat plasticisation by water has di†erent manifestationsin di†erent materials. As in the cereal products, brittle-ness loss, which is manifested by smoother curves(jaggedness loss) usually precedes the changes in tough-ness. The latter can even slightly increase by the pres-ence of moderate amount of water because the resultingpartial plasticisation prevents or hinders failure propa-gation. Thus, it enables the material to absorb higherlevels of mechanical energy (Harris and Peleg 1996).This is consistent with the observed increase in thespecimenÏs deformability (Figs 6 and 7). A phenomenonalso reported in cereal products by Georget et al (1995)and Nichols et al (1995).

CONCLUSIONS

Compressing intact seeds, legumes or nut kernels, fortheir textural evaluation is a convenient option.However, not every mechanical parameter derived fromsuch an essay is equally useful for their mechanicalcharacterisation, and some are even difficult or impossi-ble to determine in certain seeds. (Examples are thesti†ness of kidney beans and the breaking force ofalmonds and hazelnuts.) The presence of oil appears tobe a main cause of the absence of brittleness in nuts. Italso moderates any plasticising e†ect produced by mois-ture sorption. In contrast legumes behave, at least qual-itatively, in a manner very similar to that of cerealbased products. The degree of jaggedness of their forceÈdisplacement curves can be consistently expressed interms of an apparent fractal dimension and the latterserves as a sensitive indicator or brittleness loss as aresult of moisture sorption. The latter is also evident inthe increased prefailure deformation, an observationreported in certain cereal based products too. Theempirical relative measure of toughness, deÐned anddetermined as the area under the forceÈdisplacementcurve up to what corresponds to 20% compression, isthe only mechanical parameter used that was equallyapplicable to all four materials and over the whole

E†ect of water activity on the mechanical properties of legumes and nuts 471

range of the experimental water activities. This param-eter, or similar ones based on di†erent displacementlevels, have the advantage that it does not require aclear identiÐcation of the failure parameters. The lattercan be a difficult task, for di†erent reasons, when theexperimental forceÈdisplacement curve is eitherextremely irregular or very smooth. Exploring thepotential usefulness of “toughnessÏ parameters in mecha-nical characterisation can be a worthwhile undertaking.

The changes in the various mechanical and jagged-ness parameters as a result of moisture sorption indi-cates that each material has a di†erent plasticisationpattern apparently determined by its composition andstructure. Thus, monitoring changes in a single mecha-nical property cannot serve as the basis for predictingchanges in other properties, a conclusion previouslyreached when cereal-based products were tested bysimilar methods.

ACKNOWLEDGEMENT

Contribution of the Massachusetts Agricultural Experi-ment Station at Amherst. The support of the work bythe USDA-NRICGP (under grant 9203438) is gratefullyacknowledged.

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