Texture is a major quality attribute of plant foods that is dictated by the underlying structure of plant tissues. However, texture is often poorly defined and measured using inappropriate methods, leading to confusion. A comprehensive understanding of plant structure, texture, and objective measurement techniques is needed to fully characterize texture and overcome quality problems in plant foods. Advances in understanding plant microstructure, using multiple measurement methods, and data analysis can provide insights into texture and structural relationships.

1. Review
Textureis a major quality a~ibute that determinesthe accept-
ance of plant foods. HoweceG the term is stillofien pcx~ly
defined and applied. As texture is dictated by the underlying
composition and organization of plant tissues,it is crucial for
food scientiststo he awareof the structureof plant foods.The
texture of plant foods can be attributed mainly to the struc-
tural integrityof thecell wall and middle larnella,aswell asto
the turgor pressuregeneratedwithin cells by osmosis.Recent
models of the cell wall envision a celiulose-hemicellulose
structuraldomain embeddedin a seconddomainconsistingof
pectic substances,while a third domain contains covalently
crosslinked protein units. Textural problems in plant foods,
arising from diffusion, ripening and processingfactors, for
example, are directly related to the architecture of the plant
cell. Thus, an increasedunderstandingof the structural basis
of texture and also of the fundamentals of texture measure-
ment should assistin overcoming quality problems in plant
foods.
As the eating habits of consumers have become more
sophisticated, the importance of texture as a quality
attribute has become increasingly apparent. Especially
with the advent of bioteclmology, the modification of
the texture of plant foods to either improve or optimize
overall product quality now extends beyond the realm of
the food scientist or the plant breeder. The payment of
greater attention to texture as a quality attribute will
provide an opportunity for the production of superior
produce throngh ncv~,l and varied approaches. However,
filture developments depend upon a comprehensive
understanding of texture, its dictates and principles of
measurement by all those new to this field. A lack of
understanding of fundzu-ncntal p~.ciplcs mc,vihTtbly
leads to problems of semantics that could deter success-
ful advances and add to the ambiguity that already
exists around the 'Gestalt" of texture. This would seem
to be an appropriate forum to present recent develop-
merits in the understanding of plant food structure, its
relation to texture, and aspects of objective texture
measurementupon which it has a direct bearing.
What is texture?
Texture comprises all physical characteristics sensed
by the feeling of touch that are related to deformation
under an applied force and are measured objectively in
terms of force, distance and tuneL Unfortunately, tex-
ture is generally misunderstood, and there is a need for
the standard and consistent use of terminology (see
Box 1). The fiterature is replete with examples of tex-
ture having been measured using inappropriate methods.
I~ohedL JadamaandDavidW.9aahy(correspondingauthor)areatthe
DepartmentofFoodScience,UniversityofGuelph,Guelph,Ontario,Canada
N1G2Wl (fax:÷1-519-824-6631;e-maihdstanleyQuoguelph.ca).
Perspectivesin the
texturalevaluationof
plant foods
RobertL.JackmanandDavidW. Stanley
This further adds to the confusion over what texture
truly represents. Texture arises from the arrangement of
various chemical speciesby physical forces into
micro- and macros~ac.m_res,and is the external manifes-
tation of these structuresL A rmlimentary knowledge of
what is actually being meaanrod and of which forces
predominate is germane to an appreciation and under-
standing of texture.
Objeclbe texture measuremeal
What should be measured?
While it might be approwiate in some cases, such as
quality control, to search for a single parameter that
reflects the overall texture .of a commodity, this
approach frequently fails. Textm'e is a complex attribute
that is influenced by nmnerous factors, not the least of
which are the complexity and dynamics of the plant
material itself. The relafiouship between objective tex-
ture measurements and sensoe3 properties (in addition
to the relationship between the objective meam:ementa
and the structural elements that they are intended to
measure) is often, as a result, poorly characterized or
understood. Objective texture-measuring methods mad
neces~ri!y to ~ empL~c~.
Only ~ a result of the appEcalion of a number of
objective methods, which are based on different win-
ciples (e.g. compression vcrs~]s shear) and measure
properties of differing scale O~-g-large- versus small-
deformation measurements),can a commodity's texture
be fully characterized. However, a complete textural
evaluation is rather ambitious and may be logistically
impossible. More impotently, not all these data may
be required because some mcasurenmnts are probably
redundant (mulficorrelated parameters) or more, or less,
sensitive than others. On the other hand, with only par-
tial characterization there is a real risk of false con-
ciusiousbeing drawn or of results being misinterpreted~.
For example, the current controversy over the role of
polygalacturonase (PG; EC3.1.1.15) in the softening of
tomato fruit7 can be partially attributed to the incom-
plete texture characterization of fruit and constituent tis-
sues as a result of the use of inapprol~iate and/or insen-
sitive methods of objective ~exture measurement. A
strong correlation has been demonsuated between large-
deformation texture measurements that involve the
flat-plate compression o1" puncture of whole fruit and
Trendsin FoodScience&TechnologyJune1995Wol.6]

2. changes in extractable PG concentration that are associ-
ated with ripening and are visible at the microstmctural
leveW. However, such an apparent cause-and-effect
relationship must be viewed with caution given the great
difference in .~le between what large-deformation
methods measure and what we think (or wish) they are
measuring.
Are we measuring what we think we are measuring?
All those involved in texture evaluation are vulner-
able to the age-old trap of 'If I can measure it, then it
must be real'. Visual examination of plant ma~rials as
they are being subjected to an applied force or defor-
marion provides critical information necessary for the
interpretation of data associated with fracture or fail-
ure°~. The use of microscopy (light or electron) inter-
faced with photography or video-imaging equipment
similarly can provide information at the mlcrostructural
level. Microscopic techniques should be more widely
utilized in texture evaluation3.
Non-destructivetexture measurement: a misnomer?
All plant nmterials exhibit viscoelastic behavionr; that
is. their textural response is dictated by both elastic and
viscoOscharacteristics, and is time-dependentzg'l°.Thus,
textural parameters measured objectively under one set
of test conditions will generally differ from those
measured using another set of conditions. Also, any ap-
plication of force or stress will induce a certain mount
of permanent deformation, the extent of which depends
on the duration of the applied force or stress. The classi-
fication as 'non-destructive' of objective methods that
involve any amount of compression, tension or shear is
misleading. Because of the viscoelastic nature of plant
materials, no amount of deformation or force applied to
them over a measurable time period can be regarded as
totally non-destructive.
The standard destmcfive~type tests (e.g. puncture,
flat-plate compression, flexure) often ~eld bJ_~Myvari-
able data, which cannot be regarded as accurately
reflecting the properties of the whole fruit. This varia-
bility is attributable to a range of both physiological:°
and operationalI,Is,n factors, not the least of which are
inslrumental accuracy and precision. As a consequence,
there has been renewed interest in the application of
resonance frequency measmen~nts for the 'non-
destructive' evaluation of the texture of fruit and veg-
etables'3-1s,primarily for adaptation to on-line sorting or
grading processes. Mechanical or sonic vibration pro-
duces characteristic resonance responses, that is spheri-
cadand torsional vibration-mode shapes with unique fre-
quencies; the two lowest frequencies correlate with fruit
firmness and overall elastic behavionr. Stiffness factors
f2.m andf2.m2~have become widely used as indices
of textural quality, where f is the lowest resonant
frequency associated with either spherical or torsional
vibration modes, and m is the mass of the commodity.
Theoretical considerations have, to date, limited the
application of these methods to fruit and vegetables that
are approximately spherical. However, recent work with
188 Trendsin FoodScience&TechnologyJune1995IVol.61

3. pseudo- or non-spherical shapes could result in their
wider use. When combined with the finite-element
method, various models or shapes can be evaluated and
boundary conditions simulated to determine opOanum
sensor type and locatiouIs-Is. The application of high-
speed video imaging, interfaced with high-resolution
computer graphics and image-analysis softwme, could
augment "conventional" modal analysis. The application
of vibrationsof a predeterminedfrequency band to fruit,
such as narrow band random excitation around an arbl-
U'arily chosen or average modal frequency, could
increase the sensitivity of resonance indices for on-line
sortingof fruit into differentclassesof textural quality~4.
Advances in data analyses
The degree to which two objective texture measure-
merits agree has recendy been addressed by Pelegt9,
who suggested optimally translating the readings of one
method to the scale of the other by minimizing the sum
of squares of the differences between the absolute
values of a derived fast Fourier transform series. The
readings of one method are then expressed in terms of
another, even if they are based on different physical
principles.This approach has merit in consolidatingdata
derived for a given commodity by different methods.
On the other hand, multivariate statistical analysis
(MVA) techniques could provide a more powerful tool
to ferret out those methods and parameters that are
potentially most 'useful' for different applications. The
use of MVA for the comprehensive evaluation of a
range of objective methods could simplify the textund
c ~ o n of food materials and lead to the devel-
opment of standardized metho0s. Such methods will
inevitably be needed by countries involved in the 'free
trade' of high-value produce. To date, however, few
systematic texture studies have been carried out using
thesetechniques.
The apparently irresular and/or complex force-defor-
marion and freqncncy-response profiles of a variety of
food materials (e.g. those that are brittle or of irregular
size or shape) are particularly amenable to fractal analy-
sis, whereby they can be characterized in terms of their
fractal or u-integer dimeusion2°. Objects, images or
behaviour with (~logical) features that are scale
invariant are readily described using fractal geometry.
The use of fractals in image analysis is mentioned in the
preceding article by Kaltb eta/.; the application of frac-
tals to various aspects of food science, including texture
evaluation, has only recentlybeen addressedza.
The stmchnul bash d ptant food texture
Texture is derived ~-om a structural hierarchy~. The
physicomechanical ~ e s exhibited at each success-
ive level are dependent on the properties of elements in
the preceding level, their relative concentrations, the
physical forc~ involved in their interaction, and the
manner in which these elements are spatially arranged.
Plant structures at every level of scale are most often
anisotrc~c, heterogeneous and non-continuous, and
exhibit considerable variability in construct. Textural
cheracterization of such malmigs is Ilam pmblema~ at
best. Still, a fundamental uadm-~mlling of plato swat-
ture at v~'ious k-vets of scale c:m ~ amiad~cmioa of
appropriate methedsof textu~ meumemem.
Ceils are the fundamental ~uc~u~ unit
Plant tissues used for l~d, ~ ~
nuts, stems, seeds or tubers, comaia u cell types
(mainly parenchynm, bat also colknchyma, sckma-
chyma and vasodar b*-~L~__).Pmm:ICmmcelrmme poly-
hedral or spherical in shape told meam~e 50-5001Jura
across (Box 2). The cells are associated with one
anotherowing to mutual imum~e raisingfrom lheircon*
m~eat within a limiting epidermisor ,IdL Depmding
on the typo, spatial mraagemeat aad ndmive drapes of
Trendsin FoodScience& TechnologyJune1995Wol.61 189

4. constituent cells,plant tissues contain significant amounts
(1-25%) of intercellular air space, which can have a
considerableimpact on texture~. Intracellularly. import-
aut food components (starch, proteins, lipids) are com-
partmentalized within inclusion bodies. Also present
within the cell are: a large central vacuole containing a
watery solution of salts and sugars; various organelles
that regulate cell metabolism; and a matrix of organized
polymeric proteins, collectively referred to as the
cytoskeleton, which may function to provide a structural
framework for cytoplasmic processes. The cytoskeleton
is continuous with tic cell mambnme or plasmalemma
and, perhaps, the cell wail. In addition to the subcellular
membranes surrounding the vacuole, plasdds and
organelles, the plasmulemma surrounding the cell con-
trols the translocation of water and solutes. The contri-
butions of the plasmalemma and cytoskeleton to overall
texture are considered to be weak, although the deterio-
ration of membianes due, for example, to chill injury in
suscep~ble produce can lead to textural problems2S;
also, the polysaecharidesof the cell wall are synthesized
in association with cell and organullar membranes.
Exterior to the plasmalcmma are the primary cell wall
and middle lamella, both of which contain polysacchar-
ides and smaller amounts of glycoproteins and phenolic
compounds. Much of the texture of plant foods can be
attributed to the structural integrity of the primary cell
wall and the middle lamella, and to the turgot generated
within cells by osmosis. The cell wall of the average
parenchyma cell is thin (0.1-10pan) but strong, thereby
limiting expansion due to the presence of the intra-
cellular fluid and, as a result, generating turgor press-
ares of ~0.3-1 MPa and associated wall stresses of
~100-250MPa. This internal pressure must be borne
mainly by the wall, if burstingof the cell is to be avoided.
Middle lamellae between adjacent cells act much like
adhesives, and bear some of the compressive or tensile
stress, but readily transmit shear. Middle lamellae are
heat I~Hc a,~din their absenceplant cells separate easily.
Secondary cell walls, if present, are deposited, after the
cessation of cell growth, outside the plasmalcmma but
internal to the primary cell wall; their presence in plant
tissues is associated with the development of 'woodi-
ness', such as in asparagus.
The chemical foundation of structure and associated
texture
Cell walls contain numerous polymeric compounds
that are capable of bearing an applied stress Gable l).
With the exception of cellulose, these compounds, when
extracted, are water soluble. Yet, in the cell wall they
are organized into a water-insoluble matrix capable of
bearing considerable stresses while simultaneously per-
mitring growth. Cellulose, [3(1-->4)-v-polyglucan, forms
the skeletal scaffolding of the wall through the forma-
tion of microfibrilsof-5-!5 nm Lncli~neter and several
thousand units long. Hemicellulose consists of rigid,
highly branched rod*shapedpolymers of neutral sugars,
such as xylan, xyloglucan and [3(1-->3) or 1~(1--->4)
mixed ghicans, which are ~200rim in length and link
with cellulose, pectin and lignin by hydrogen bonding.
Pectin, which is found in highest concentrations in the
middle lamella, contains "smooth' zones of partially
esterified, PG-lablle a-galacturonic acid residues (homo-
galacturonan; -100nm in length) in addition to PG-
resistant 'hairy' zones (rhamnogalacturonans) of varying
Polymer Solebtty b wateP Clwgeat Fit 7 lp'mes (%)¢ (kols (%y
Cellulose Insoluble O 30 30
Hemicelluloses
Xyloglucan Soluble 0 4 25
Xylan Soluble 30 5
Mixed glucans Soluble 0 30 07
Pectir~
Homosalacturonan Soluble - 15
RhamnogalacturonanI Soluble - 5 15
RhamnogalacturonanII Soluble - 5
Arab/nogalactanproteins Soluble - Variable Variable
Extensin Soluble + 0.5 -5
mDatau~en fiom Refs26 and27
bSolubil~d~' e(Uacliono(6hepolymer~)mthewaU
¢E~o~sedasI~e~pproxim~epercentageonadp/basis.Growingpriman/wallscontain-65%water;maturecellwallshavealowerwatercontent
190 Trendsin FoodScience& TechnologyJune1995 IVol. 6]

5. degrees of polymerization and neutral sugar content,
which may also contain phenolic acid or other side
chains that facilitate crossllnking.
Glycopmteius are also present, at -5-10% of the dry
weight of the walls of dicotyledon fells, with cafoo-
hydrate constituting as much as 67% of the glycoprotein
mass. Several classes of these cell-wall proteins are rec-
ognized, the extensins being the most well known2s~.
These constitute a group of glycoproteins rich in
hydroxyproline; they possess the repeating pentapeptide
sequence Ser-(Hyp)4and an extended, rod-like polypro-
line II helical structure of ~8Ohm in length. Exteusins
are uniformly distributed across the cell wall, but do not
occur in the middle lamella. They form crosslinks with
other cell-wall polymers, and perhaps also pectin, and
are therefore thought to contribute to the physico-
mechanical integrity of the wall. For mature cells, the
list of cell-wall polymers must he extended to include
lignin, the second most abundant material on earth after
cellulose. Lignins are phenyl propanoid polymers of
varying molecular weight, which can account for as
much as 20-30% of the dry weight of plant tissue.
Lignln formation begins in the primary cell walls or
middle lamellae, but the greatest concenu~tion occurs in
secondary cell walls where polymerization and the for-
marion of a composite material, in which the linear wall
polysaccharides are encased in a lignln 'cage', occurs at
the ~xi,enseof the water content.
The cell wall is not a static structure
Older concepts of the cell wall as a static, inert, load-
bearing structure have given way to recognition that it is
a dynamic organelie vital to cell growth, metabolism,
attachment, shape and resistance to both disease and
suess. How are the diverse elements of the cell wall
organized to form a structure capable of performing all
of these functions? A recent model proposes that three
structurally independem but interacting domains consti-
tute a single layer of the growing cell wall, and that sev-
end layers condense to form the wall in toto3°. In tiffs
model (Fig. 1), hemicellulosusconstitute the main inter-
locking component. Their highly branched but linear
conformation is conducive to orientation between cellu-
lose mlcrofibrils, to which they bind. The resulting
cellulose-hemifellulose domain constitutes 50-65% of
the dry weight of the wall; it is embedded in a second
domain consisting of pectic substances, which accounts
for an additional 30% of the wall mass. Pectin crosslink-
ing can also occur as a result of oxidative coupling of
phenolic constituents such as ferulate. However, more
often the cmsslinking of the helical homogalachironan
chains of de.esterified pectin occurs by Ca~÷-bridging
to form junction zones. De-esterification is mediated
by the enzyme pectinesterase (EC 3.1.1.11); however,
not all sites of de-esterification become crosslinked.
P-_hamnogalactaronan I represents a portion of the pec-
tin polymer rich in arabinogalactan side chains, which
can interrupt the Caz÷junctions. A third structural do-
main contains extensin units covalently crossllnked
and oriented radially within the wall matrix. Extensin
crossn-k~-g ~sthought m be inv~ed in ~ ~ cen
wall in a fixed shape once cell ~s~h is complex.
During growth, the cell must ,mqmd, defemiag the
wall, yet retaining the strea~ to wiemml ~t<,~ imm-
ure. Cellulose microfibrib ase depmited in a directed
orientation as the hemifellaleee network is mzymafi-
cally cleaved, leading to stngchingof the wall as far as
the ceUulose-hemicellulceeinteractions will allow. The
alignment of the cellulose micmfibrils tramvem~ ia a
shallow helix permits wall en~moa to occtw longi-
tudinally: the non-fellulosic pelymccha~k matrix in
which the microfibrils are emi~lded dictates the de-
gree to which they are pulled a~ doring extemion. A~
the wall stretches, chemical bonde or associations are
broken and stress relaxation takes place, resulting in a
reduction in turgor pressure. Expansioe of the cell fol-
lows, as it absorbs water in respome to the rednctiee in
turgot pressure. This mechani~ of cell expansion dur-
ing growth has been confirmed by various rheologlcal
studies3~. The mechanism of buw extemias become
crosslinked in the wall matrix at the cessation of fell
growth is still not clear.
This current model of plant fell wall architecture is
consistent with cell growth, ~md arises from investi-
gations into wall expansion mecbRni~.m,:.Such a model
can facilitate the study and understanding of plant food
texture, but it must he regarded as being somewhat
incomplete, if only because it ~ missing the sueauntl
component lignin, which wo~ be expected to play a
major role in the texture of th~e plant tismes in which
constituent cells develop a seccmdery wall. 18 addition,
this model of the cell wall (Fig. 1) disregards the tex-
tural impact of some non-stngttnl metabolic ~
and any possible interaction with the cytoskeleton.
Nevertheless, the model serves as a ftmus ~ wb2ch
we may proceed towards a bettor understandingof plant
tissue texture-structurerelationships.
Plant tissue texture-structure
Diffusion control
The transfer of moisture m plant tissues d~ring unit
opemtions such as drying, rehydratinn, solld-liquid
extraction and absorption is a complex process that is
dependent on diffusion and often limited by cell-wall
permeability: the texture of the final product can be
markedly affected by such operations3. Although fell
walls can be modified to increase diffusion by heating,
grinding and/or enzymatic ¢.eatment, this it; ~ always
practical or effective. The apidication of ~'tovel NMR
imaging techniques has been useful in obtaining more
accurate physical da~ and in lxobing the role of
the suucture of plant mal~al in moisture transport3z.
However, more information about the role of cell walls
in the diffusion process is required. For example, how is
wall porosity controlled? The pore size that limits unre-
stricted di_ffi~ion through the |nn~'y cell wall appears
to he determined by the pectin mau'ix3°,and is ~4nm in
diameter. In~m:asingthe pore size to >10nm by enzymatic
digestion has been shown m increase the molecular
weight cutoff for the unrestricted diffusion of proteins,
Trendsin FoodScience& TechnologyJune1995Wol.6] 191

6. Xyloglucan PC;A-junctlon RC I with Extensin
zone arabinogalactan
side chains
Fbl
A modelof theexpandingprimarycell wall of floweringplantsexceptgrasses.A singlelayerisrepresented;
severalsuchlayerscondenseto formthewall. Threethick cellulosicmicrofibrilsarealignedin parallelbut
in a helicalformationaroundelongatingcells.Theyarecrosslinkedwith bumicellulosicxyloglucanpolymers
thathavebeenpartiallycleavedto permitmicmfibril separation.Thisdomain isembeddedin a secondone
consistingof a matrixof pecticpolygalactumnicacid(PGA),which formsjunction zonesin the presenceof
Ca2.andthamnogalacturonanI (RGI)with attendantarabinogalactansidechains.A third domaincontains
extensinmolecules,which areinsertedradiallyto stabilizethe separatedmicmfibrilsand limit further
stretchinguponthecessationof growth.Reproducedwith permissionfromCarpitaandGibeaut3°.
The food applications of this
concept relate to texture preser-
vation during dehydration pro-
cessegThe coUapseofcell walls dur-
ing dryingcan lead to a poor-quality
product upon rehydration. The ad-
dition of various biopolymers such
as dextran, amylose and amylo-
pectin during processing has been
shown to improve the quality of
the rehydrated product33. These
biopolymers are thought to add
strength m the cell wall, thereby
enabling it to maintain its integrity
during the dehydration process.
This mechanismof texture preser-
vation could be analogous to that
res~usible for the drought-resist-
ant, or 'resuscitation', behaviour
of several plants indigenous to
South Africa. A drought-induced
stress in these plants leads to a
surge in the content of extensins
or polyflavonoid taunins3L The
helicoid three-dimeusional struc-
tures of these compounds function
like springs to prevent the cell
walls from cracking during cell
dehydration, and to allow for sub-
sequent resuscitation of the plant
tissues.
Calcium as a texture enhancer
Calcium has a major effect on
the structure of the pectin m~itrix.
The interaction of calcium and
pectin has recently been investi-
gated by differential scanning
calorimetry~. A phase transition,
thought to reflect the melting
point of the pectin gel, was ob-
served at -53°C in cell walls of
grovdng soybean tissue. The ad-
dition of Ca2~increased this tran-
sition temperature but reduced the
enthalpy involved to that found
for mature tissue. These data were
interpreted to mean that much
of the pectin in the cell walls
of growing plant tissue is de-
esterified but not crosslinked, and
that available Caz+ binds to the
pectin backbone to give a mole
rigid wall. In mature tissue, either
more Ca2÷ is present or the wall
contains a higher degree of esteri-
fled pectin, making it less respon-
from 17kDa to >100 kDa. Clearly, the nature of the pec- sive to added cations. Food scientists use calcium salts
tin and its configuration in the cell wall have a marked as additives, to prevent tissue sloughing and to render
influence on the porosity of the wall matrix, pectates tess heat labile-~s.
192 Trendsin FoodScience& TechnologyJune1995 [VoL 6]

7. Woolliness and mealiness
development
'Woolliness' is a physiological
disorder of many stone fruit,
wherein they fail to ripen normally
following storage at chilling tem-
peratures. The disorder may be akin
to the development of 'meatiness'
in other fruit that are susceptible
to chill injury such as tomatoes
and kiwi fruit. Woolly or mealy
fruit lack juicin~s ~",d fiavour,
and exhibit a dry, soR texture da-
spite containing the same overall
moisture content as non-chilled
fruit of the same physiological age.
Recent research3vsuggeststhat this
disorder results from an altered
pattern of pectin breakdown dur-
ing storage and subsequent ripen-
ing. Pectinesterases are present in
fruit tissues throughout growth and
maturation through to senescence;
the enzymes probably continue to
be active at chilling temperatures.
A proportion of the de-esterilied
pectin that occurs, especially in
unripe fruit, is not crosslinked; the
free carboxyl groups can serve as
sites for association with ions and/
or water that are translocated from
the cytosol owing to chill-induced
membrane leakage. The swelling
of cell walls and middle lamellae
that occurs as a consequence of
chilling is due mainly to the ab-
sorption of water, which is either
physically entrapped within the
wall matrix or 'bound'. The trans-
fer of fruit to non-chilling tempera-
tures triggers normal ripening
processes, including the de novo
synthesis of PC; in some fruit.
This enzyme, combined with the
increased moisture content of the
wall, leads to accelerated soften-
ing as a result of cell 'de-bonding'
rather than rupture. This is ex-
pressed as a mealy texture since
cells do not release the internal
Cell relaxation
and rupture
<
Cell
de-lx~ing
Ripenin8 time
Rg.2
Texture-structurerelationshipsin ripeningtomatofruit. (above),Thegraph(redrawnfromlackmanand
Stanhyza)depictsdecreasingpunctureforce(althoughothertex~Llreparameterscouldhavebeenused)
with increasingripeningtime,consistentwith tissuesoftening.Notethatatransitionoccursin the modeof
tissuefailurewhen it becomeseasierfor cellsto separateorde.t~l Ihanto rupture,cotrespo~ing to the
degradationof the middlelamellato a critical level.(below),Transmissionelectronmicrographs(courtesy
of A.G.Uarangoni):(a)and(b)con'espondto pointsaandb, respectively,onthegraphabove.Notethe
separationanddegradationof themiddle lamellae(ML)in (b),thedpenedsa~. Scalebars= ! lain.
fluids normally associated with the perception of juici- occurs in ripening tomato tissue undergoes a transition
ness and succulence, whence it becomes easier for cells to separate or de-
The progressive softening of fruit is a normal conse- bond than to rupture. This point coincides with the
quence of ripening, and has been attributed to a decrease degradation of the middle lamella to a critical level.
in cell turgot pressure and to a reduction in the molecu-
lar weight size distribution of cell-wall hemicelluloses Tomato fruit ripening
and middle-lamellar pectin. Microscopic examination of The relative contributions of various mechanisms to
the middle iamella in ripening tomato fruit shows strnc- the overall sofie,qing process in tomatoes has been the
rural separation and degradation that correlate with soft- subject of considerable cona'oversy. Recently, these
ening (Fig. 2), the process by which structural failure different mechanisms were consolidated into a single
Trendsin FoodScience& TechnologyJune1995iVol. 6J 193

8. rheological model through the measurementof the creep
behaviour of pericasp tissue4. Instantaneous elasticity of
tissue was attributed to the combination of turgor and
primary cell-wall strength, as dictated by cellulose;
viscoelasticproperties to hemicelluloseand pectin com-
pouents; and steady-state viscous behaviour to exos-
mosisand to increased wall fluidity arising from thebreak-
clownof cell-wall and/or middle-lamellarpolymers. The
loss of turgor, pectin breakdown and overall increase in
wall fluidity each accounted for 25-30% of the overall
apparent softening of tissue, while the downshift in
molecular weight/size distribution of hemicelluioses
accounted for an additional 10-15%. During normal
ripening, loss of tissue elasticity oeeuned as viscous
properties became predominant. Pectin is thought to be
altered significantly; these changes are exacerbated by
chill injury~. The putative relationshipbetween the bio-
chemical or microstmctural features of the tomato tissue
and its rheolosical behaviour rationalized the contri-
bution of multiple mechanisms to the overall softening
process, and identifiedthose mechanisms with the great-
est potential impact on textu~ changes.
Consumerperception of texture is often subconscious,
requiring that it be unacceptable or otherwise differ-
ent from that expected before it is even noticed.
Paradoxically, the pleasure experienced when eating
fresh or processed plant foods is largely attributed to
their texture. The importance of texture as a quality
attribute b accented by the enormous real and economic
losses incurred during harvesting and subsequent hand-
ling, distribution and storage that ultimately arise from
textural deterioration and concomitant physicomechani-
cal damage. In light of these losses, an obvious goal is
the successfulmanagementof textural properties at each
postharvest step. A greater appreciation of texture as a
complex quality attribute, and of the structural basis
from which it is manifest, will focus research on achiev-
ing this goal.
The authors are grateful to Dr N.C. Carpita and
Blackwell ScienceLtd for permissionto reproduceFig. I,
to Food Science and Nutrition Press to rewoduce part
of Fig. 2, and to Dr A.G. Marangoni for the previously
unpublished micrographs in Fig. 2. Original research
conducted in this laboratory was supported, in part, by
the Natural Sciences and Engineering Research Council
of Canada and the Ontario Ministry of Agriculture and
Food.
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