Information and Technology for Sustainable Fruit and Vegetable Production
FRUTIC 05, 12 – 16 September 2005, Montpellier France
Micromechanical Behaviour of Apple Tissue in Tensile and
Compression Tests
M.C. Alamar, M.Zarzo, R.Suay and E. Moltó
Instituto Valenciano de Investigaciones Agrarias (IVIA), Ctra.Moncada-Náquera, Km.4.5
46113 Moncada (Valencia), Spain
E.Vanstreels, B.Verlinden, and B.Nicolaï
Flanders Centre/ Laboratory of Postharvest Technology, K.U. Leuven, Willem De
Croylaan 42, B-3001 Heverlee, Belgium
J. Loodts, E. Tijskens and H. Ramon
Laboratory for Agricultural Machinery and Processing, K.U.Leuven, Kasteelpark
Arenberg 20, B-3001 Heverlee, Belgium
Keywords: Malus domestica, elasticity, turgor pressure, cell structural parameters
Abstract.
The micromechanical behaviour of apple tissue was studied using a miniature
tensile stage positioned underneath a microscope that allowed for simultaneous
acquisition of force-displacement curves while the deformation of the individual cells
was followed and recorded. Tensile and compression tests were performed on small
samples of apple parenchyma from two different cultivars (Jonagored and
Braeburn) and two firmness categories (soft and firm). Turgor pressure in all
samples from every batch was equalised by soaking them overnight in an isotonic
mannitol solution. A number of micromechanical parameters such as Young’s
modulus (E), stress at failure (εmax) and strain at maximum stress (σmax) were
calculated. We found a clear difference in the shape of the strain-stress curves.
While S-shaped strain-stress curves were observed for apple specimens under
compression loadings, almost linear curves were obtained for samples under tensile
tests. Moreover, Braeburn apples were found to have a higher modulus of elasticity
in tensile tests than Jonagored apples, indicating a stiffer material. However, in
compressive tests these differences were not found. Fresh Jonagored apples proved
to be more compressible in compression tests and, to a lesser extent, more extensible
in tensile tests than fresh Braeburn apples. Unexpectedly, we also found higher
values for maximal stress for Jonagored apples in compressive tests. Deformations of
individual cells were found higher in tensile than in compression tests, while at tissue
scale it is the other way round.
INTRODUCTION
During harvesting, transport, storage and packing, fruits and vegetables are
subjected to mechanical damage which directly affects their quality and, as a
consequence, implies a loss in their commercial value. Traditionally, most of the
techniques for the evaluation of fruit mechanical behaviour have been focused on
obtaining information from a whole single fruit, and the average value of several
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measurements taken from the fruit was considered as representative of its behaviour.
However, fruits and vegetables are not a continuous material but a complex conglomerate
of tissues, which in their turn consist of cells. This cellular conglomerate is composed of
three primary components: parenchyma cells, an adhesive middle lamella between
adjacent cells and intercellular spaces. Cell walls provide mechanical strength to the
whole cell structure. Since macroscopic mechanical behaviour of fruits depends on
several microscopic properties (cell size, internal turgor pressure, cell wall mechanical
properties and thickness, etc), a micromechanical approach is useful to understand the
relative importance of these cellular and histological attributes on the overall mechanical
behaviour of fruits and vegetables.
Although there have been attempts to model cellular structures using finite
elements, these models have not yet been able to put together a large number of cells to
successfully predict tissue properties (Burrows, 1994). Wang et al. (2004) were working
at a isolated cellular level, as well, and determined the Young’s modulus of walls of
single suspension cultured tomato cells, by modelling force-deformation data.
Until now a significant amount of studies have been done on potato tuber
parenchyma tissue. Hiller and Jeronimidis (1996) assessed the fracture behaviour for two
different varieties and predicted critical crack lengths (for different turgor states) by
means of the compressive Young’s modulus and work of fracture. Konstankiewicz et al.
(2000) concluded that structural parameters like cell area and cell perimeter exert a
significant influence on mechanical parameters like strength and E-modulus. Hepworth
and Bruce (2000) measured the deformation of individual cell within potato living tissue
under uniaxial compression load. The deformations were successfully related to the
global tissue deformations, up to compressive deformations of 20% of specimen height.
Onion epidermal tissue has been a subject of research, as well. Mechanical properties and
molecular dynamics in single cell walls were studied by Fourier-transform infrared
spectroscopy under mechanically stressed conditions (Wilson et al., 2000). Recently, the
variation in mechanical properties and structural parametres of onion epidermal peels,
originating from different layers, has been investigated (Vanstreels et al., 2004).
A number of mechanical models have been proposed to explain the rheology of
fruit tissue (Pitt, 1982; Lin and Pitt, 1986; Qiong et al., 1989). McLauglin and Pitt (1983)
proposed a statistical model for the number of cycles to failure which indicated that the
likelihood of imminent failure remained approximately constant with the number of
cycles applied, and thus failure appeared to be primarily a random event rather than the
result of damage accumulation. Khan and Vincent (1993) found that cylindrical samples
which were tangentially compressed had a lower Young’s modulus than those
compressed radially. Most investigations have been based on uniaxial compression tests
of apple cylinders, though some research has been done on other samples. De Belie et al.
(2000) developed an approach in which thin strips of pear tissue were pulled apart under a
microscope so that deformation could be examined at cellular level. They used that
technique to study the influences of cell turgor and fruit ripening on tissue and cellular
characteristics. Rojas et al. (2002) proposed an empirical model to study the relative
contribution of structural parameters to the rheologhy of kiwifruit by performing large
deformation assays.
Despite the amount of mechanical experiments carried out on fruits and
vegetables, experiments where different tests were carried out simultaneously are hardly
described. In this paper, the micromechanical behaviour of small samples of apple
parenchyma tissue is studied using a miniature tensile stage positioned underneath a
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microscope. This allows for simultaneous observations of the cell structural parameters,
deformations and way of rupture while performing a mechanical test. Our research is the
first in its kind to directly compare results obtained from tensile and compression tests
and to study the influence of factors like turgor or stiffness of the apples in both tests.
Since tensile tests are more related to chewing experience, while compression is related to
bruise susceptibility, our work will provide additional information on the textural
properties of apples.
The objectives of our study were twofold: first, to establish a methology which
allows us to perform compressive and tensile tests on small samples of apple parenchyma
in a repeatable way, and second, to investigate the influence of cell structural parameters
on micromechanical properties. For this purpose micromechanical experiments were
carried out on two different apple cultivars with known differences in cell structural
parameters.
MATERIALS AND METHODS
Samples
Experiments were carried out on Jonagored and Braeburn apples of different
firmness. Apples stored at 21ºC for 12 days were named “shelf-life” or “soft” apples
(average firmness of 62 and 64 N, respectively) while “fresh” or “firm” apples” (average
firmness of 75 and 85 N) were stored under controlled atmosphere conditions until the
experiment was performed. These two cultivars were chosen since significant structural
differences between them have been described (Schotsmans, 2003). Jonagored apples
have less number of cells/mm2, more intercellular space (%) and larger cells than
Braeburn apples.
Apples were selected on the basis of uniformity, absence of damage or blemishes
and size (average diameter between 80-85 mm for Jonagored, and 60-65 mm for
Braeburn). Samples were taken in the form of radially cut rectangular beam specimens,
using both a slicer and parallel razor blades to ensure constant dimensions and smooth
surfaces. The dimensions of the apple specimens, 11mm length × 5 mm width × 2mm
height for tensile tests and 3mm × 11mm × 5mm samples for compression tests, as well as
the testing speed, 0.5mm/min. and 0.2mm/min., respectively, were selected to keep a
constant strain rate during the mechanical tests.
Determination of Isotonic Point
The average isotonic point of every set of apples (2 cultivars × 2 firmness) was
determined by measuring relative volume and weight changes of cylindrical samples of
apple tissue (12mm diameter by 5 mm high) after overnight soaking in one of several
concentrations of mannitol (from 0 to 0.8M). In order to minimize cellular degradation of
tissue strength during the experiment, and following the method described by Lin and
Pitt, 1986, the mannitol solutions were buffered with K2HPO4 (0.02 M) and KH2PO4
(0.02 M). Nine tissue cylinders were cut out from the cortex of the green side of each
apple and placed into each osmotic solution. Parenchyma samples represented ≈ 1% of
solution volume, ensuring a constant water activity value during soaking.
Before and after the soaking, both weight and dimensions (diameter and height) of
the samples were determined with an Analytical balance (Sartorious, CP124S, Germany)
and a digital calliper (Absolute digimatic, Mitutoyo, UK, Ltd.), respectively. The appletissue cylinders were carefully blotted with tissue paper to remove excess of water before
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the measurements. Eventually, the isotonic point was calculated by interpolation to zero
weight or volume change from the curve indicating the weight gain or loss of tissue as a
function of the mannitol concentration.
Mechanical Tests and Image Analysis
The mechanical testing was carried out on a miniature tensile stage (Deben
Microtest, Suffolk, U.K.) which, due to its reduced dimensions (12 cm length x 8 cm
width), could be mounted underneath a stereomicroscope (SMZ1000, Nikon, Japan)
equipped with a CCD camera (type TK-1360 colour ½ CCD, JVC). The whole setup
allowed the acquisition of force-displacement curves while the deformation of the
individual cells was followed and recorded.
The experiments consisted of performing tensile and compression tests on radial
cut rectangular specimens of apple parenchyma (see above). In total 20 samples coming
from 3 apples from each cultivar and each firmness group were analysed. The day before
the test was performed, the apples were taken out from the cool room and cut, avoiding
the outmost 5 mm of the fruit where the flesh is very heterogeneous. Cell turgor pressure
was equalized by soaking the samples overnight in an isotonic mannitol solution which
was previously determined per each apple set.
For both mechanical tests every sample was gently stained with methylene blue
(7.5mg/100ml mannitol isotonic solution) for 3min. While for compression tests no
specific adaptations were needed, for the tensile tests the apple samples had to be glued
with cyanocrilate adhesive to the two moving grips of the miniature tensile bench (Fig. 1).
After the glue was hardened and during the mechanical test, humid air was blown over
the samples to keep them from drying out. Loading and unloading cycles upon return to
zero stress were carried out beyond a certain travel. Then, samples were further elongated
or compressed until rupture.
Measurements of cell structural parameters (perimeter, area, aspect, length, width,
etc.) were obtained from the recorded images. A pre-processing procedure on the original
images was required. Thus, a specific program was developed in Matlab (MATLAB
version. 6.5, The MathWorks, Inc., Natick, Ma, USA) to digitalize the original images that
allowed for an easy and unambiguous determination of cell structural parameters with
image analysing software.
Data Analysis
Force and deformation data were converted to stress and strain data. From these
data a number of micromechanical parameters such as Young’s modulus in different parts
of the stress-strain curve (see later), stress at failure (εmax) and strain at maximum stress
(σmax) were calculated. Statistical analysis was carried out using a multifactor analysis of
variance and significant differences among treatments were determined.
RESULTS AND DISCUSSION
Isotonic Point
The isotonic point for fresh and shelf apples was –1.48 and –1.72 Mpa for
Jonagored and –1.23 and –1.48 MPa6 for Braeburn apples. Since turgor pressure of cells
in tissue affects their mechanical properties (Lin and Pitt, 1986; De Belie, et al., 2000;
Konstankiewicz and Zdunek, 2001), all the samples from every batch were equalised by
soaking them overnight in an isotonic mannitol solution. This allowed us to manipulate
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cell turgor pressure of apple samples without damaging the tissue. Fresh Jonagored
apples have a higher isotonic point than fresh Braeburn apples and for both cultivars there
is an increase in soluble solids after 12 days of storage at shelf-life conditions. The values
of isotonic conditions for fresh Jonagored apples agree with the value reported by Quiong
et al. (1989).
Sample Acquisition
Apple flesh is mechanically very anisotropic (Khan and Vincent, 1993; Abbott
and Lu, 1996), so we restricted our experiments to radially cut rectangular beam
specimens. Additionally, strain rate has been proved to have an influence on dynamic
failure properties (Bajema et al., 2000). In our study, a constant strain rate of 0.05 mm-1
for both tensile and compression tests was selected. Additionally, a staining step was
necessary to visualize and quantify cell deformations that occur during mechanical
testing. Preliminary experiments indicated that a staining procedure for such a small
period of time (≤ 4 min.) did not affect neither the mechanical nor structural properties of
apple samples. The staining time was selected for being the shortest at which good images
could be recorded.
No significant differences (α=0.01) among replicates, for any of the batches, could
be found, which confirms that sample acquisition and manipulation were carried out in a
repeatable way.
Micromechanical Properties and Image Analysis of Apple Parenchyma Tissue
Typical stress-strain curves of apple tissue subjected to tensile and compression
tests are shown in Fig. 2 and mean values of mechanical parameters are shown in Table 1.
1. Tensile Tests. An almost linear relationship was found between stress and strain values
for all samples analysed. For fresh samples an abrupt decrease in stress when the tissue
fails was observed while for shelf life samples the peak reached at maximum stress
became broader. Besides, for some soft samples, a certain tissue extension before the
specimen was completely broken was observed (Fig. 3). The different behaviour between
firmness categories would indicate an abrupt break of the tissue in firm apples and a
sequential rupture of cells groups in soft apples (De Belie et al., 2000).
When micromechanical parameters were calculated from these curves we found
statistically significant differences (p≤0.01) between cultivars and firmness classes for ET
(Young’s modulus for tensile test). Firm apples have higher ET than soft apples. Between
cultivars fresh Braeburn are stiffer than fresh Jonagored, but no differences between
varieties were found when the apples became soft. A considerable decrease in the
modulus of elasticity in both cultivars was also observed as they became soft. As
expected, higher values of σmax were found for fresh apples than soft apples. No
differences between cultivars could be found.
For fresh Jonagored apples slightly higher strain at maximum stress was found
than for fresh Braeburn apples, however after shelf life these differences had disappeared.
No significant differences in strain at maximum stress were found between soft and firm
apples.
Results obtained from penetrometer measurements indicated that the Braeburn
apples were harder than the Jonagored apples, which might be correlated with the higher
stiffness found for Braeburn apples in our tensile tests on microscale.
If we compare strain-stress curves obtained from apple tissue subjected to tensile
loading with those obtained from other vegetative tissues like Aristolochia brasiliensis
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(Kölher, 2000) or onion epidermis (Wilson et al., 2000; Vanstreels et al., 2004), we see
that the clear and typical biphasic behaviour is not found for the former. Such a behaviour
was explained in terms of cell walls composition where fibrils of cellulose are embedded
in a flexible pectin. Two processes were involved: firstly, fibrillar reorientation and
secondly, slippage of microfibrils past each other due to yield of the pectin matrix. In
apple parenchyma, the intercellular spaces represent an important part of the total volume.
This, added to the fact that we are working on specimens constituted by many cell layers
(where different cells undergo different stresses) could explain the differences in curves
shape between both experiments.
The cyclic part of all tests reveals that after unloading to zero stress certain plastic
deformation remains.
2. Compression Tests. Compression strain-stress curves for apple samples are S-shaped,
where stiffness varies with strain. Young’s moduli were denoted by EC1 for small strains
and by EC2 for higher strains.
Statistically significant differences (p≤0.01) were found between cultivars and
firmness categories for εmax and σmax. Firm Jonagored specimens reach higher values of
strain than firm Braeburn samples, however, there are no significant differences (α=0.01)
between cultivars for soft apples. Surprisingly, we found higher values for σmax for fresh
Jonagored than we found for fresh Braeburn. After shelf life the differences were still
found, although less pronounced.
For EC1 modulus a significant effect of cultivars was found and Braeburn apples
had higher values of EC2 than Jonagored apples. For EC2 modulus a significant effect of
firmness was found where firm apples have higher values of EC2 than soft apples.
However, no differences between cultivars were found.
From the video images recorded under the stereomicroscope, the cell deformations
of one apple of every batch were measured. It was found that those apples subjected to
tensile tests had higher deformations (judging from length deformations) than those under
compression loading. This is the other way round at tissue level, where apples subjected
to compression tests reached higher values of strain at maximum stress. Further work is in
progress to check this results with the rest of the apples.
CONCLUSIONS
In this work a methodology for performing tensile and compression tests on small
apple parenchyma tissue samples, which allowed for the simultaneous observation of cell
deformations, was developed. We have found a clear difference in the shape of stressstrain curves for tensile and compression tests. Thus, while S-shaped curves were
observed for apple specimens under compression loadings, almost linear curves were
obtained for samples under tensile tests. Besides, as expected, firm and soft apples were
found to behave significantly different where, under the same conditions, fresh apples
have higher modulus of elasticity and maximum stress. Braeburn apples were found to
have a higher modulus of elasticity in tensile tests than Jonagored apples, indicating a
stiffer material. In compressive tests these differences were not found. Fresh Jonagored
apples proved to be more compressible in compression tests and, to a lesser extent more
extensible in tensile tests than fresh Braeburn apples (higher strain at maximum stress).
Surprisingly, we also found higher values for maximal stress for Jonagored apples in
compressive tests. These results will be further investigated and interpreted in terms of
the cell structural parameters of the samples.
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Through cell measurement analysis it was found that at the cellular scale, unlike
what was found at the tissue scale, samples under tensile loading reached higher
deformations than samples subjected to compression tests.
A thorough investigation of cell structural parameters (amount of intercellular
space, way of rupture, etc.) in the different samples will be made and the influence of
these parameters on the observed micromechanical behaviour will be further investigated.
The information will be extracted from both the microscopic samples and the recorded
images. Eventually, the insights gained from this research will prove valuable when
developing mathematical models based upon the histological properties of tissues to
predict strain and failure of fruit tissue as a consequence of static loading and impact.
AKNOWLEDGMENTS
The authors would like to acknowledge the Consellería d’Agricultura, Pesca i
Alimentació (Generalitat Valenciana), the Fund for Scientific Research Flanders (F.W.O.
Vlaanderen), project G.0200.02 and the European Union (Training Site Marie Curie
Grant to M.C. Alamar Gavidia, project G056). We also thank the European Science
Foundation for a Short Term Scientific Mission grant to M.C. Alamar Gavidia (COST
Action 924, reference code COST-STSM-924-00788).
LITERATURE CITED
Abbott, J.A. and Lu R. 1996. Anisotropic mechanical properties of apples. Transactions
of the ASAE. 39(4): 1451-1459.
Bajema, R.W.; Baritelle, A.L.; Hyde, G.M.; Pitts, M.J. 2000. Factors influencing dynamic
mechanical properties of red “Delicious” apple tissue. Transactions of the ASAE.
43(6), 1725-1731.
Burrows, K.M. 1994. Finite elements analyiss of mechanical properties of plant cells.
Plant Biomechanics Summaries. Paris: Elsevier, 33.
De Belie, N.; Hallett, I.C.; Harker, F.R.; De Baerdemaeker, J. 2000. Influence of ripening
and turgor on the tensile properties of pears: a microscopic study of cellular and tissue
changes. J. Amer. Soc. Hort. Sci. 125(3), 350-356.
Harker, F.R. and Hallet, I.C. 1992. Phyisological changes associated with development of
mealiness of apple fruit during storage. HortScience, 27: 1291-1294.
Hepworth, D.G.; Bruce, D.M. 2000. Measuring the deformation of cells within a piece of
compressed potato tuber tissue. Annals of Botany Company. 86, 287-292.
Hiller, S. and Jeronimidis, S. 1996. Fracture in potato tuber parenchyma. Journal of
Materials Science. 31, 2779-2796.
Khan, A.A.; Vincent, J.F.V. 1993. Compressive stiffness and fracture properties of apple
and potato parenchyma. Journal of Texture Studies. 24, 423-435.
Kölher. 2000. Biphasic mechanical behaviour of plant tissues. Materials Science and
Engineering. 11, 51-56.
Konstankiewicz, K.; Pawlack, K.; Zduneck, A. 2001. Influence of structural parameters
of potato tuber cells on their mechanical properties. Institute of Agrophysics. 15, 243246.
Konstankiewicz, K.; Zdunek, A. 2001. Influence of turgor and cell size on the cracking of
potato tissue. Institute of Agrophysics. 15, 27-30.
Lin Ta-Te and Pitt, R.E. 1986. Rheology of apple and potato tissue as affected by cell
turgor pressure. Journal of Texture Studies 17, 291-313.
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McLauglin, N.B.; Pitt; R.E. 1983. Failure characteristics of apple tissue under cyclic
loading. Transactions of the ASAE, 27(1), 311-320.
Pitt, R.E. 1982. Models for rheology and statistical strength of uniformly stressed
vegetative tissue. American Society of Agricultural Engineers, 25(6), 1776-1784.
Qiong, G.; Pitt, R.E.; Bartsch, J.A. 1989. Elastic-plastic constitutive relations of the cell
walls of apple and potato parenchyma. Journal of Rheology. 33(2), 233-256.
Rojas, A.M.; Delbon, M.; Marangoni, A.G.; Gersshenson, L.N. 2002. Contribution of
cellular structure to the large and small deformation rheological behaviour of
kiwifruit. Journal of Food Science, 67(6), 2143-2148.
Schotsmans, W. 2003. Diffusion properties of pome fruit in relation to storage potential.
PhD dissertation, Katholieke Universiteit Leuven, Belgium.
Vanstreels, E., Verlinden, B., Alamar, M.C., Enninghorst, A., Loodts, J. K. A., Ramon,
H., and Nicolaï, B. M. Micromechanical behaviour of onion epidermal tissue. Acta
horticulturae: Proceedings of the 5th International Postharvest Symposium. Verona,
Italy, 6-11 June 2004. In press
Wang, C.X.; Wang, L.; Thomas, C.R. 2004. Modelling the mechanical properties of
single suspension –cultured tomato cells. Annals of Botany. 93, 443-453.
Wilson, R.H.; Smith, A.C.; Kacuráková, M.; Saunders, P.K. 2000. The mechanical
properties and molecular dynamics of plant cell wall polysaccharides studied by
Fourier-Transform infrared spectroscopy. Plant Physiology. 124, 297-405.
Tables
Table 1. Mean values of micromechanical parameters for apple tissue. ± standard
deviation are given next to each value.
Mechanical
parameters
σmax (Mpa)
ε (%)
E1
E2
ET
Compression
Braeburn
Soft
Firm
0.36±0.09 0.15±0.03
Jonagored
Soft
Firm
0.55±0.08
0.27±0.05
23.00±3.11 24.09±3.00 32.29±1.62
24.86±4.51
0.59±0.23
2.23±0.43
----
0.34±0.18
1.68±0.56
----
0.16±0.09
1.30±0.25
----
0.52±1.16
2.45±0.32
----
Tensile
σmax (Mpa)
0.33±0.08
0.09±0.03
0.36±0.05
0.13±0.08
ε (%)
E1
E2
ET
7.34±1.5
-------
8.39±2.16
-------
11.18±1.18
-------
9.36±2.53
-------
5.68±0.46
1.93±0.78
4.33±0.55
2.22±1.04
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Figures
Fig. 1. Miniature tensile stage (left) with the grips used for compression tests. Close up of
how the apple specimens for tensile tests were glued to the grips of the bench (right).
0.6
σmax
Stress (MPa)
0.5
σmax
0.4
0.3
a
b
EC2
ET
0.2
0.1
0
-0.1
0 EC1
5
εmax
10
εmax
15
20
25
30
35
Strain (%)
Fig. 2. Representative stress-strain curves of fresh apple tissue subjected to compression
(a) and tension (b) loading. EC1 and EC2: modulus of elasticity for small and higher strains,
respectively, for the compression curve; ET: Young’s modulus for the tension curve; σmax :
maximum stress; εmax: strain corresponding with the maximum stress. Images of the tissue
at the beginning and at the end of each test performance are also shown.
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0.45
A
0.4
0.35
Stress (MPa)
0.3
0.25
0.2
0.15
0.1
B
0.05
-2
0
-0.05 0
2
4
6
8
10
12
Strain (%)
Fig. 3. Stress-strain curves of tensile tests on fresh (A) and shelf life apple tissue.
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Comportement Micromécanique des Tissus de la Pomme dans les Tests de
Tension et de Compression.
Mots clés : Malus domestica, élasticité, pression de turgescence, paramètres de la
structure de la cellule
Résumé
Le comportement micromécanique des tissus de la pomme a été étudié en utilisant une
lamelle déformable miniature positionnée sous un microscope autorisant une acquisition
simultanée du couple force-déplacement pendant laquelle la déformation des cellules a
été suivie et enregistrée. Les tests de tension et de compression ont été réalisés sur de
petits échantillons de parenchyme provenant de deux cultivars différents (Jonagored and
Braeburn) et deux catégories de fermeté (molle et ferme). Les pressions de turgescence de
tous les échantillons de chaque lot ont été égalisées en les trempant toute une nuit dans
une solution de mannitol isotonique. Des paramètres micromécaniques ont été mesurés,
comme le module de Young (E), la force à la rupture (εmax) et la contrainte au stress
maximal (σmax). Nous avons mis en évidence une grande différence dans la forme des
courbes de contraintes. Alors que des courbes de contraintes sygmoïdales ont été
obtenues à partir d’échantillons sous compression, des courbes quasi-linéaires ont été
obtenues sous tension. De plus, les pommes Braeburn ont montré un module d’élasticité
plus important que les pommes Jonagored dans les tests d’étirement, indiquant un
matériau plus rigide. Ces différences n’ont pas été retrouvées lors des tests de
compression. Avant conservation, les pommes Jonagored ont montré une plus forte
compressibilité dans les tests de compression, et dans une certaine mesure, une plus forte
extensibilité que les pommes Braeburn. Nous avons trouvé aussi de manière inattendue
des valeurs du test de compression plus élevées pour les pommes Jonagored. Les
déformations individuelles des cellules ont été plus importantes dans les tests de tension
que dans les tests de compression, alors que c’est l’inverse qui apparaît à l’échelle
titulaire.
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