Fatty acid composition and rheological behaviour of prickly pear seed oils

Abstract

Prickly pear fruits constituted valuable foodstuff for humans and animals in arid and semi-arid regions. Two species of prickly pear from Tunisia, Opuntia ficus indica and Opuntia stricta, were investigated for fatty acid composition and physicochemical parameters of the seed oil. No significant difference in either physical or chemical parameters was found between the species. The main fatty acids of prickly pear seed oil were C16:0, C18:0, C18:1, C18:2. With an exceptional level of linoleic acid, up to 70%, the content of unsaturated fatty acids was high, at 88.5% and 88.0% for O. ficus indica and O. stricta, respectively.Rheological properties were analysed with changes of temperature and shear stress. Variations of viscosity were measured and the viscoelastic parameters were determined during heating and cooling cycles between 20 and 70 °C. Curves of flow were established with up and down cycles of shear stress at different temperatures. These measures highlighted the presence of large aggregates of crystal fatty acids in both Opuntia crude oils. Shearing and temperature destroyed this structural state and gave birth to an homogeneous stable suspension.The structural state of crude oil was confirmed using a contrast phase microscope, and the particle size distribution was obtained by laser granulometry.

Full text

Fatty acid composition and rheological behaviour
of prickly pear seed oils
Monia Ennouri
a,b,*
, Bourret Evelyne
b
, Mondolot Laurence
c
, Attia Hamadi
a
a
Unite
´dÕAnalyses Alimentaires, Ecole Nationale dÕInge
´nieurs, BPW 3038 Sfax, Tunisie
b
Laboratoire de Physique Mole
´culaire et Structurale, Faculte
´de Pharmacie de Montpellier, BP 14491, 34093 Montpellier Cedex 5, France
c
Laboratoire de Botanique, Phytochimie et Mycologie, UMR 5175, Faculte
´de Pharmacie de Montpellier, BP 14491, 34093 Montpellier
Cedex 5, France
Received 22 December 2003; received in revised form 13 October 2004; accepted 13 October 2004
Abstract
Prickly pear fruits constituted valuable foodstuff for humans and animals in arid and semi-arid regions. Two species of prickly
pear from Tunisia, Opuntia ficus indica and Opuntia stricta, were investigated for fatty acid composition and physicochemical
parameters of the seed oil. No significant difference in either physical or chemical parameters was found between the species.
The main fatty acids of prickly pear seed oil were C16:0, C18:0, C18:1, C18:2. With an exceptional level of linoleic acid, up to
70%, the content of unsaturated fatty acids was high, at 88.5% and 88.0% for O. ficus indica and O. stricta, respectively.
Rheological properties were analysed with changes of temperature and shear stress. Variations of viscosity were measured and
the viscoelastic parameters were determined during heating and cooling cycles between 20 and 70 C. Curves of flow were established
with up and down cycles of shear stress at different temperatures. These measures highlighted the presence of large aggregates of
crystal fatty acids in both Opuntia crude oils. Shearing and temperature destroyed this structural state and gave birth to an homo-
geneous stable suspension.
The structural state of crude oil was confirmed using a contrast phase microscope, and the particle size distribution was obtained
by laser granulometry.
2004 Elsevier Ltd. All rights reserved.
Keywords: Prickly pear; Fatty acid; Seed oil; Rheology; Microscopy; Opuntia
1. Introduction
About 1500 species of cactus belong to the genus
Opuntia and are distributed mainly in Africa, Mediterra-
nean countries, southwestern United States, northern
Mexico and other areas (Hegwood, 1990). The main
studies on the Opuntia fruits were the chemical analysis
of pulp, skin and seeds (El Kossori, Villaume, El Bou-
stani, Sauvaire, & Mejean, 1998), analysis of volatile
constituents of pulp (Di Cesare & Nani, 1992; Flath &
Takahashi, 1978), use of pulp in juice production
(Espinosa, Borrocal, Jara, Zorilla, & Medina, 1973),
production of alcoholic beverage (Bustos, 1981), jam
production (Sawaya, Khatchadorian, Safi, & Al-
Mohammad, 1983) and the production of cocoa butter
equivalents from prickly pear juice fermentation by an
unsaturated fatty acid auxotroph (Hassan, Blanc,
Pareilleux, & Goma, 1995). An overview of processing
technologies concerning the fruits and cladodes of cac-
tus pear has recently been published by Saenz (2000).
Other authors have studied the nutritional significance
of Opuntia sp. (Stintzing, Schieber, & Carle, 2001).
0308-8146/$ - see front matter 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.foodchem.2004.10.020
*
Corresponding author. Tel.: +216 98 278 684; fax: +216 74 221
160.
E-mail address: monia.ennouri@enis.rnu.tn (M. Ennouri).
www.elsevier.com/locate/foodchem
Food Chemistry 93 (2005) 431–437
Food
Chemistry

Crude young ÔnopalÕhave been studied as a source of
fibres (Majdoub et al., 2001), proteins and amino acids
(Teles, Whiting, Price, & Borges, 1997), and processed
in Taifi prickly pear sheets (Ewaidah & Hassan, 1992).
The extraction and the characterization of the prickly
pear mucilage from sheets was optimized by several
authors (McGarvie & Parolis, 1979;Medina-Torres,
Brito-De La Fuente, Torrestiana-Sanchez, & Katthain,
2000; Trachtenberg & Mayer, 1981, 1982).
Prickly pear seeds were first characterized by Sa-
waya, Khalil, & Al-Mohammad (1983), who demon-
strated that the seeds of Opuntia ficus indica are rich
in minerals and sulphur amino acids. A reserve protein
from the seeds has been isolated and characterized by
Uchoa, Souza, Zarate, Gomez-Filho, & Campos
(1998). The prickly pear seed oil composition and its
chemical characteristics were investigated by Sawaya
& Khan (1982), and then by Salvo, Galati, Lo Curto,
& Tripodo (2002).
Coskuner & Tekin (2003) studied the seed composi-
tion of prickly pear fruits during the maturation
period. Ramadan & Morsel (2003) compared the seed
and pulp oil compositions. However, the physical char-
acteristics of prickly pear oil are up to now unknown.
The major objective of the present work was to study
the physicochemical properties of the seed oil of the
O. ficus indica and O. stricta fruits which are the most
abundant species in Tunisia. Rheological behaviour
and microscopic structure have been studied and spe-
cial attention was paid to the effect of temperature,
since every food technological operation requires a
thermal treatment.
2. Materials and methods
2.1. Prickly pear seed
Mature Prickly pear fruits, O. ficus indica and
O. stricta, were collected, respectively, in August and
February from the same area (Sfax, Tunisia). The fruits
were immediately sorted, washed with running water,
air-dried and hand-peeled. A pulper finisher was used
to separate the seeds from the pulp. The seeds were
washed with distilled water several times, air-dried at
ambient temperature and then ground with a Diez
crusher.
2.2. Oil extraction
The seed powders oils of O. ficus indica and O. stricta
were extracted with hexane in a Soxhlet extractor for
9 h. The organic phase was then removed using a rotary
evaporator under reduced pressure; the oil was flushed
with a stream of nitrogen and stored at 20 C in sealed
tubes prior to analyses.
2.3. Physicochemical analyses
The seed weight was appraized, at random, on one
hundred seeds. Moisture content was determined by
the AOAC method (AOAC, 1984). The oil yield was
determined on a seed powder of 5 g. Nitrogen was
determined by the Kjeldahl procedure and crude pro-
tein was calculated as N·6.25 (Balogun & Fetuga,
1986). The ash content was determined according to
the AOAC method (AOAC, 1990). Refractive index
was determined at 20 C with an Abbe refractometer
with temperature adjustment. The density was mea-
sured with a densimeter PAAR DMA 60. Saponifica-
tion number and iodine value were determined using
the official method (American Oil ChemistsÕSociety,
AOCS, 1993).
2.4. Fatty acid analysis
The fatty acid compositions of both oil samples were
analyzed by GC–MS after transesterification. Fatty acid
methyl esters were prepared in the presence of 2 N
potassium hydroxide in methanol and analyzed on a
Hewlett-Packard model 5890 series II gas chromato-
graph equipped with a flame ionization detector and a
polar capillary column: HP Innowax cross-linked
PEG, Carbowax 20 M (0.32 mm internal diameter,
30 m length and 0.25 lm film thickness). The opera-
tional conditions were: injector temperature 220 C;
detector temperature 275 C; column temperature 50
C for 5 min then a gradient of 10 C/min to 240 C; car-
rier gas was nitrogen at a flow of 1.47 ml/min. Three
injections were done.
2.5. Rheological measurements
2.5.1. Instrument
The rheological properties were measured using a
controlled stress Haake rheometer (Rheostress RS
100). All the rheological studies were conducted using
cone-plate geometry: 35 mm diameter and 2cone.
The volume of the sample was 0.4 ml. That equipment
is able to control temperature on a plate sensor system
to within ±0.1 C.
For the measures at variable temperature, the varia-
tion was at the rate of 5 C/min.
2.5.2. Flow curves
In order to determine the influence of temperature
and the shear rate, measures of viscosity were conducted
at constant shear stress (10 Pa) across increasing temper-
atures from 20 to 70 C and immediately decreasing
temperature to 20 C.
To follow the influence of shear rate on the viscosity,
increasing and immediately decreasing cycles of shear
stress at constant temperature were performed.
432 M. Ennouri et al. / Food Chemistry 93 (2005) 431–437

2.5.3. Dynamic tests
The rheological measurements, in oscillatory mode,
were performed in the linear viscoelasticity range (1
Hz in frequency and 0.3 Pa in shear stress) with temper-
ature increased from 20 to 70 C and followed immedi-
ately by a decrease of temperature to 20 C.
2.6. Microscopy
The oils of O. ficus indica and O. stricta were ob-
served under a contrast phase microscope equipped with
a Nikon F301 camera. A scale graduation, introduced
on photo, indicated the magnification.
2.7. Particle size distribution
The particle size distribution of crude oil was estab-
lished by laser light scattering at room temperature,
using a Malvern Mastersizer hydro 2000S (Malvern
Instruments Ltd, Malvern, UK). All analyses were per-
formed in triplicate.
3. Results and discussion
3.1. General composition (Table 1)
The oil contents for O. ficus indica and O. stricta,
were similar (10.90% and 11.05%, respectively). These
values were higher than the crude oil yield in the work
of Coskuner & Tekin (2003) (6.91%). The difference ob-
served is probably due to the origin of the fruit. Seeds
contain 5.4–3.9% protein and 1.1–1.64% ash, less than
values found by Sawaya et al. (1983) on the saudian
variety of O. ficus indica.
The value of viscosity at 20 CofO. ficus indica
(53 ·10
3
Pa s) is close to the value reported by Oomah,
Ladet, Godfrey, Liang, & Girard (2000) for grape seed
oil (49.4 ·10
3
Pa s), whereas the value for O. stricta
(76 ·10
3
Pa s) is comparable to that of rapeseed oil
(72–82 ·10
3
Pa s) (Karlesind & Wolff, 1992).
The refractive and iodine indices are comparable with
those of rapeseed oil (Karlesind & Wolff, 1992) (respec-
tively, 1.473 vs. 1.475 and 100 vs. 101.5). The saponifica-
tion value is lower than grape seed oil (188–194) but
compared favourably with native rapeseed oil (170–
175) (Karlesind & Wolff, 1992).
The density of the seed oil at 20 C compared favour-
ably with native rapeseed oil and soybean oil (0.910 and
0.921, respectively) (Noureddini, Teoh, & Davis Cle-
ments, 1992).
The study of the density of the Opuntia seed oil as a
function of temperature revealed a linear relationship
in accordance with results reported for others oils. It is
reported that the densities of fatty acids and triglycer-
ides are linear with temperature, according to the equa-
tion q=b+mT (Fisher, 1995). The linear coefficients b
and mfor the Opuntia seed oil are shown in Table 2. The
square correlation coefficient highlighted a linear corre-
lation between density and temperature.
For triglycerides, the change in density per degree
Celsius was in the range of 0.00067–0.00073 (Formo,
Jungermann, Norris, & Sontag, 1979). The results on
Opuntia oils were in accordance with these previous data
and with the published density data for coconut, corn
and rapeseed oils (Noureddini et al., 1992;Valeri &
Meirelles, 1997).
3.2. Fatty acid composition of prickly pear seed oil
The FAME compositions of seed lipids are listed in
Table 3. Linoleic acid is the major component (74%),
followed by oleic (12.8%) and palmitic acids (7.2%).
Both lauric and myristic acids were detected in O. srticta
oil in low amounts. Prickly pear seed oil was found to be
highly unsaturated: 88.5% and 88.0% for O. ficus indica
and O. srticta,, respectively. Besides the linoleic acid
(>70%), there is oleic acid (>12%). These results are in
agreement with those of Sawaya & Khan (1982) who
previously reported the contents of the four most impor-
tant fatty acids. The lipid pattern of prickly pear is com-
parable with that of sunflower and grapeseed oils (Tan
& Che Man, 2000).
Table 1
Physicochemical characteristics of the two species of Opuntia seed oil
Characteristics
a
Species
O. ficus indica O. stricta
Weight of 100 seeds (g) 1.38 ± 0.08 1.69 ± 0.04
Dry matter (%) 93.00 ± 0.45 95.00 ± 0.30
Oil (%) 10.90 ± 0.10 11.05 ± 0.09
Protein (%) 5.40 ± 0.40 3.90 ± 0.20
Ash (%) 1.10 ± 0.10 1.64 ± 0.15
Viscosity
b
(Pa s) 0.0531 ± 0.0005 0.076 ± 0.001
Refractive index
c
1.475 ± 0.002 1.469 ± 0.001
Density
c
0.903 ± 0.002 0.919 ± 0.001
Saponification number 169.0 ± 0.1 174.0 ± 0.3
Iodine value 101.5 ± 1.0 91.6 ± 0.5
a
Means of three determinations.
b
At 20 C and s> 2 Pa.
c
At 20 C.
Table 2
Linear correlation
a
between densities and temperature
b
for the two
varieties of Opuntia seed oil
Compound Intercept bSlope mR
2
O. ficus indica oil 0.91703 6.833E04 0.99
O. stricta oil 0.9336 6.9121E04 0.99
a
Density = b+mT.
b
Temperature range: 10–70 C.
M. Ennouri et al. / Food Chemistry 93 (2005) 431–437 433

Recently, Coskuner & Tekin (2003) reported a pal-
mitic acid content higher than our results (12% vs.
9.32%) and a content of linoleic acid very much lower
(52% vs. 70.29%). The observed difference is possibly
due to the degree of maturity of the fruit; indeed, these
authors suggested that there was an increase in saturated
fatty acid content towards the end of fruit maturation.
3.3. Rheological behaviour
3.3.1. Viscosity variations with the temperature
These measurement were carried out at constant
shear stress 10 Pa and two consecutive cycles of increas-
ing and decreasing temperature between 20 and 70 C.
As expected, the viscosity decreases strongly when the
temperature increases. The curve of viscosity was higher
on the curve segment corresponding to the decreasing
temperature of 70–20 C(Fig. 1). A weak hysteresis
was observed. When a second cycle of temperature
was applied, the viscosity curve was totally superposed
on the curve obtained in the phase 70–20 C of the first
temperature cycle (data not shown). At that time, the
structural state was stabilized.
The area of hysteresis observed was due not only to
the effect of the temperature but also to the effect of
the shearing. That result might be related to the chemi-
cal composition of the oil that revealed a significant
amount of fatty acids in the solid state at 20 C. Indeed,
palmitic acid and stearic acid have melting points of 62.9
and 69.6 C, respectively. The initial crude oil was a sus-
pension. During the first cooling, both shearing and
temperature destroyed the structural state, especially
the aggregates. Because this phenomenon was not
immediately reversible, hysteresis became visible when
temperature decreased to 20 C. The smaller size of par-
ticles explained the greater strength of resistance to flow
in the resulting suspension than in the initial suspension.
So the curve of viscosity during cooling was above the
curve of viscosity during heating. At that time, the sec-
ond cycle temperature, up 70 C and down to 20 C,
showed that there was no influence of shearing. A
homogeneous suspension took shape during the cooling
phase of the first temperature cycle.
3.3.2. Viscosity curves at different temperatures
This experiment aimed to explore variations of the vis-
cosity at constant temperature with increasing and imme-
diately decreasing shear stress. When the shear stress
increases, the viscosity increases from 11% to 24% for
all temperatures, 0.3 Pa. A weak decrease of viscosity
followed this rise and a stabilization was then observed
from 2 Pa. So the flow of oil became Newtonian when
the shear stress was above 2 Pa (Fig. 2). When the shear
stress immediately decreased, the behaviour remained
Newtonian in the same zone of shearing. This behaviour
was reproduced at every temperature but a weakening
of phenomenon was observed at 50 C and a near disap-
pearance at 60 C. These temperatures were in the same
range as the melting points of palmitic (62.9 C) and stea-
ric (69.6 C) acids. This flow characterized the physical
Table 3
Fatty acid composition of prickly pear seed oil (g/100 g of total fatty
acid)
Fatty acid Species
O. ficus indica O. stricta
Lauric C12:0 –
a
0.19 ± 0.01
Myristic C14:0 –
a
0.32 ± 0.01
Palmitic C16:0 9.32 ± 0.19 7.21 ± 0.09
Palmitoleic C16:1 1.42 ± 0.01 0.38 ± 0.02
Stearic C18:0 3.11 ± 0.04 3.83 ± 0.01
Oleic C18:1 16.8 ± 0.47 12.8 ± 0.09
Linoleic C18:2 70.3 ± 0.60 74.8 ± 0.26
U/S
b
7.11 7.61
a
Not detected.
b
Unsaturation ratio = (16:1 + 18:1 + 18:2)/(12:0 + 14:0 + 16:0 + 18:0).
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
20 30 40 50 60 70
Temperature (˚C)
Apparent viscosity (Pa.s)
Fig. 1. Effect of temperature increasing (n) and decreasing (m) on the
apparent viscosity at shear stress 10 Pa.
0.01
0.1
0.1 1 10 100
shear stress (Pa)
Apparent viscosity (Pa.s)
40˚C
20˚C
60˚C
50˚C
30˚C
Fig. 2. Effect of shear stress increasing (Hollow symbols) and
decreasing (filled symbols) on the apparent viscosity of O. ficus indica
at various temperatures.
434 M. Ennouri et al. / Food Chemistry 93 (2005) 431–437

state of the oil. The crude oil before shearing was a suspen-
sion in which dispersed phase was constituted of aggre-
gated particles of fatty acids with a high melting point,
as described above. These aggregates resisted the flow
and this explained the initial increase of viscosity. They
were destroyed quickly by the shearing. Consequently, a
small decrease of viscosity and a stable suspension consti-
tuted of smallest particles were observed. So the flow be-
came Newtonian when shear stress was above 2 Pa.
These results ratified the hypothesis of the experi-
ments described in the previous paragraph and were in
accordance with previous works. Matveenko, Kirsanov,
& Remizov (1995) identified two different parts of each
flow curve as the high shear rate region and the low
shear rate region, separated by a break point. Moreover,
Geller & Goodrum (2000) showed that dynamic viscos-
ity of vegetable oils was shear-independent at high shear
rates (above 6 s
1
). Our measures displayed a Newto-
nian flow in that shear rate range. The same effect was
observed for both prickly pear seed oils. The values of
Newtonian viscosity are given in Fig. 3.
3.3.3. Dynamic tests
To eliminate the influence of the shearing and to ana-
lyze only the effect of the temperature, rheological mea-
sures were conducted in oscillatory mode with up and
down cycles of temperature. The reason for oscillatory
rheological tests was that the very low shear stress did
not destroy the structural state of sample. Curves pre-
sented in Fig. 4 showed that the loss modulus G00 was
superior to the storage modulus G0. As expected the val-
ues for G0and G00 characterized rheological behaviour of
a disorganized state at all experimental temperatures.
The effect of the temperature was visible until 55 C.
Rheological dynamic parameters, G0and G00 , remained
steady between 55 and 70 C because both palmitic
and stearic acids melted; consequently, no more change
was observed. At 70 C, there were no more aggregates
in the oil. As a consequence, this medium was totally li-
quid. During cooling, a recrystallization occurred. The
recrystallization led to the formation of fat crystals hav-
ing similar size and caused a homogeneous organization
in the whole volume. The result was that the storage
modulus G0was more raised in cooling than in heating
due to greater organization of medium than in the initial
state. The variations of loss tangent were consecutive to
changes of G0and G00.
In previous studies on fat crystal networks, workers
observed that a logarithmic linear relationship existed
between elastic modulus G0and the solid fat content
(Narine & Marangoni, 1999). This result was not ob-
served but in the studied samples of prickly pear seed
oil, which were very different from a fat crystal network.
The content of solid fats in these samples was significant
but very low. So the values of dynamic parameters were
very weak and did not permit more observation. This re-
sult is confirmed by the measurements from laser light
scattering.
3.4. Microscopy
The analysis of rheological behaviour was confirmed
by light microscopic observations. The crude oil of O.
stricta showed large aggregates of fat crystal particles
of nearly 60 lm(Fig. 5(a)). The same oil, observed after
shearing (Fig. 5(b)), revealed small particles dispersed
over all the ranges of vision; the aggregates, which were
visible in crude oil, disappeared. The same result oc-
curred with oil treated by heating to 70 C and then
shearing. This structural state of suspension was in
agreement with the Newtonian behaviour observed after
shearing on the flow curves.
The crude oil of O. ficus indica displayed the same
microscopic behaviour but the size of aggregates was
smaller (30 lm, data not shown).
0
0.02
0.04
0.06
0.08
0.1
20 30 40 50 60
Temperature (˚C)
Apparent viscosity (Pa.s)
Fig. 3. Viscosity of prickly pear seed oil at different temperatures in
the stable linear zone (s> 2 Pa). j=O. stricta seed oil, h=O. ficus
indica seed oil.
0.01
0.1
1
20 30 40 50 60 70
Temperature (˚C)
G' G" (Pa)
0
5
10
15
20
25
30
35
40
45
50
Tan (G" /G’)
Fig. 4. Changes in the storage modulus GÕ(nm), in the loss modulus
G00 (hj) and in tangent loss angle (sd) with increasing (nsh) and
decreasing temperatures (md n) for O. ficus indica at 1 Hz in
frequency and 0.3 Pa in shear stress.
M. Ennouri et al. / Food Chemistry 93 (2005) 431–437 435

3.5. Particle size distribution
With the object of analysing the particles in suspen-
sion, the crude oil of O. stricta was tested with a Mal-
vern Mastersizer. The particle size distribution
highlighted a main peak in which the mean diameter
of particles, d
4,3
, was 70 lm(Fig. 6). The peak centred
at 7 lm only represented 2.87% of total volume, so is
of minor importance.
4. Conclusions
The purpose of this research was to determine the phys-
icochemical properties, fatty acid composition and rheo-
logical behaviour of seed oils from two species of prickly
pear growing in Tunisia: O. ficus indica and O. stricta.
Seeds represented about 18–20% of peeled fruits. The
oil was extracted from ground seeds with hexane (yield
nearly 11%). The refractive index and densities were sim-
ilar for both species. Iodine values and saponification
numbers indicated little difference in qualitative compo-
sition and quantitative fatty acid content between both
oil samples.
Fatty acid analysis by GC–MS revealed (in both Opun-
tia oils) four major fatty acids: palmitic, stearic, oleic and
linoleic acid, previously reported in vegetable oils. Palmit-
oleic acid was present in low quantities. O. stricta oil con-
tained also traces of lauric and myristic acids. Both oils
were exceptionally rich in linoleic acid, (up to 70%) and
their contents of unsaturated fatty acids were high, (about
88%). The fatty acid composition of prickly pear oil was
close to those of sunflower and grapeseed oils. These char-
acteristics illustrated the interest of prickly pear as a nat-
ural source of edible oil containing essential fatty acids
and as an economic utility for Tunisia.
The rheological properties were analysed. Variations
of viscosity and viscoelasticity parameters were studied
across heating and cooling cycles from 20 to 70 C. For
the investigation on viscosity, a consecutive cycle of tem-
perature was applied. The flow curves were established
with up and down cycles of shear stress at different tem-
peratures. Simultaneously, a crude oil sample, a shearing
oil sample and an oil sample submitted to a treatment by
heat at 70 C were observed with a contrast phase micro-
scope. Lastly, a crude oil was investigated by laser light
scattering to obtain the particle size distribution.
The convergence of rheological measurements and
microscopic observations highlighted the structural state
of prickly pear seed oil: the crude oil contained large
aggregates of unmelted fatty acids. The mean size of
aggregates (by microscopy) was 60 and 30 lm for O.
stricta and O. ficus indica, respectively.
The measurment by laser light scattering showed a
peak formed by particles with mean diameter of 71.8
lm, corresponding to 14% of the total volume of the
O. stricta crude oil.
The crude oil of prickly pear seed contains aggregated
fatty acid crystals. Shearing and temperature destroyed
the aggregates and homogeneous suspension developed:
this suspension, with a stable structural state, had New-
tonian flow at high shear stress.
Fig. 5. Contrast phase microscopy of crude (a) and sheared oil (b) of O. stricta. Magnification is 400·, scale bar = 15 lm.
0
2
4
6
8
10
12
14
16
0.01 0.1 1 10 100 1000
particle size (
µ
m)
Volume (%)
Fig. 6. Particle size distribution of the O. stricta seed oil, obtained by
laser light scattering.
436 M. Ennouri et al. / Food Chemistry 93 (2005) 431–437

The findings shown in this work raise the nutritional
value of these under-exploited plants, especially in semi-
arid regions of Tunisia, where conventional crops are
difficult.
Acknowledgement
We thank Mr. Hammami Mohamed responsible for
U.S.C.R. spectrometry for chromatographic analyses.
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