JJKAU: Eng. Sci., Vol. 22 No. 2, pp: 187-200 (2011 A.D./1432 A.H.)
DOI: 10.4197 / Eng. 22-2.10
187
Experimental Investigation of Particle Size Distribution in
Commercial Tahina
Nedim Turkmen
Assistant Professor, Department of Thermal Engineering and
Desalination, King Abdulaziz University, Jeddah, Saudi Arabia
Abstract. Tahina samples were obtained locally. Using a unique
approach for particle size analysis, a microscope with electronic
output was utilized, and the distributions of solid particles contained
in tahina produced by various manufacturers were studied. Available
software was employed to measure areas and perimeters of particles.
Then average equivalent diameters and circularities were computed
for samples of tahina from six manufacturers.
It was determined that there exist substantial differences in
particle size distributions, average equivalent diameters and
circularities of tahina produced by the different manufacturers.
Preliminary guidelines were proposed for the classification of
tahina in the light of these findings.
Keywords: Circularity, Equivalent diameter, Microscope, Particle size,
Tahina
1. Introduction
Tahin, tahina, taheena or sesame paste, as it is variously called, is a
suspension that is produced by grinding peeled and roasted sesame seeds.
It consists of about 45 to 50% sesame oil and the rest is fiber and other
solid ingredients. Since the density of sesame oil is somewhat less than
the density of solids, the oil in tahina tends to separate, and the solids
settle down. Therefore, tahina requires stirring before use. Moreover,
there are certain applications where coarsely ground tahina is not
acceptable.
N. Turkmen 188
Particle size distribution in powders is usually found by sieving or
sedimentation. The material to be analyzed must be carefully blended,
and the sample withdrawn using techniques that avoid size segregation.
Particular attention must be laid to avoidance of loss of fines during
manipulation of the sample.
Other common techniques are laser diffraction for sizing particles
from submicron to millimeter in size, Counter principle for sizing and
counting particles from micron to millimeter in size, and dynamic light
scattering for sizing submicron and nanometer particles. The equipment
needed for all of these techniques are all currently too expensive for
small industries to own and to operate.
The current practice in the tahina industry is to measure the particle
size in tahina by the use of a micrometer [1]
. While giving quick results, it
must be pointed out that the micrometer yields one number only to
represent the size of the largest particle entrapped between the jaws of
the micrometer. It cannot therefore indicate the actual particle size
distribution.
Most of the tahina produced in the Middle East is used for the
manufacture of various varieties of halawa, a middle-eastern delicacy.
Large quantities of tahina are also consumed by bakeries, restaurants and
households as salad dressings, sauces and for cooking. Current tahina
standards do not seem to call for any particle-size based classification of
tahina. In other words all tahina that is traded today is labeled simply
"tahina". There is no differentiation between tahina produced in different
batches or by using different grinding equipment or sequences, and
whether or not the tahina has been re-cycled during grinding. And yet
tahina experts refer to a given batch of tahina as "smooth", "medium" or
"coarse" or "unacceptably chunky".
During production of tahina, raw sesame seeds are screened and
cleaned, and then de-hulled (peeled). There is a phase of drying the
peeled seeds, after which they are roasted. The moisture content of the
seeds after roasting is normally reduced to about 0.5% [2]
. The next step
is to stone-grind the seeds, normally in several steps. The thick liquid that
emerges following the grinding operation is called tahina.
Experimental Investigation of Particle …. 189
Although most tahina in the market is produced from peeled (de-
hulled) sesame seeds, there are reports however, that there is a kind of
tahina that is made from whole sesame seeds, and sold under the name
bozkir tahini in Turkey [3]
. Another study mentions a kind of tahina and
halawa to which mushroom has been added [4]
. Yet another study [5]
discusses the utilization of sunflower seeds in tahina and halawa
processing.
There has been a flurry of research activities on tahina recently.
Thus Akbulut[6]
studied the mineral contents of hulled sesame paste
(tahin), un-hulled sesame paste (bozkir tahin) and their blends. He found
that the bozkir tahini is richer in mineral content than tahini made from
peeled seeds.
Jamará and Isa [7]
proposed a plan for the production of tahina.
Kotzekidou [8]
undertook a study to investigate the microbial stability and
salmonella resistance of halva (halawa). Being a traditional low-moisture
confectionery, halva was studied with respect to microbial stability over
prolonged storage. It was kept under refrigeration or at room temperature
in air-sealed or vacuum packaging in moisture-proof material. In all
samples tested the microorganisms were in acceptable levels, while
sulfite-reducing clostridia, Salmonella spp., and molds were not detected.
An area that has seen a particular concentration of research efforts
is physico-chemical and rheological properties of tahini [9]
. Thus, Abu-
Jdayil et al. [10]
and Abu-Jdayil [11]
studied the rheological
characterization of milled sesame (tahineh). They noted that a major
concern facing the tehineh (tahina) industry is the production and
maintenance of the product while preserving the proper consistency,
stability, color, and texture properties. They found that tehina behaves as
a non-Newtonian pseudo-plastic fluid, and that the power-law model is
the most appropriate to fit the flow curves of tahina.
Altay and Ak [12]
investigated the effects of temperature, shear rate
and constituents on rheological properties of tahin (sesame paste).
Studying tahin at temperatures between 20 and 70 °C and shear rates in
the range 0.13-500 s-1
, they reported observing hysteresis loops in the
flow curves. They concluded that the behaviour of tahin was
pseudoplastic and describable by a power law model.
N. Turkmen 190
The rheological characterization of tahin / pekmez (sesame paste /
cooked and concentrated grape juice) blends were investigated by Arslan
et al. [13]
. The authors used a concentric cylinder rotational viscometer to
study properties of tahin / pekmez blends at tahin concentrations of 20 to
32% and at temperatures ranging from 35 to 65 °C. They concluded that
the blends exhibited non-Newtonian, shear thinning behavior at all
temperatures and at all tahin concentrations. Apparent viscosity versus
shear rate data was successfully fitted to the power-law model.
In a similar study, Razavi et al. [14]
determined the time
independent rheological properties of low fat sesame paste/date syrup
blends. They used a Brookfield rotational viscometer at temperatures of
25, 35, 45, and 55 °C. They concluded that all sesame paste/date syrup
blends exhibit non-Newtonian, pseudo-plastic behavior at all
temperatures.
Akbulut and Çoklar [3]
and as well as El-Adawy and Mansour [15]
,
and Hamed and Sadek [16]
studied the physicochemical and rheological
properties of sesame pastes (tahin) processed from both hulled and
unhulled roasted sesame seeds and their blends at several levels and at
temperatures ranging from 15 to 65C and shear rates from 0.5 to 100 1/s.
They determined, like in earlier studies, that all blends of tahin exhibit
non-Newtonian, pseudoplastic behavior at all temperatures. Apparent
viscosity versus shear rate data was successfully fitted to the power law
model.
In a remarkable recent study, Çiftçi et al. [17]
studied the colloidal
stability and rheological properties of sesame paste (tahini). They
investigated particle size and temperature effects on the colloidal stability
and rheological characteristics of sesame paste, which is a kind of
protein–oil suspension. They determined multimodal particle size
distribution using a laser-scattering analyzer. It was detected that
colloidal stability of sesame paste was improved by decreasing the
median particle size below 5 μm. A decrease in the storage temperature
also caused the increase in colloidal stability.
In summary of the recent literature cited above, it is clear that
tahina is treated by researchers, as far as particle size distribution is
concerned, as if it is a substance of uniform properties. The only
exception is the study by Ciftci et al. [17]
, where reference is made to
particle sizes.
Experimental Investigation of Particle …. 191
2. Materials and Methods
Handicapped by a tiny budget, it was clear that purchasing or even
loaning expensive research equipment was not feasible. It was decided to
go through a number of alternatives. One particular microscope, the
MD828T, with electronic readout was selected, and ordered along with
its accessories. Figure 1 shows the microscope and the computer
assembly.
Fig. 1. The test setup.
To this end exploratory studies were undertaken on locally
acquired tahina. Since tahina is oil-based, searches were started for
determining those agents that can be used to process samples of tahina on
slides of the microscope. Of various chemicals experimented with,
including water, alcohol and hexane. Hexane, which is used in industrial
oil extraction processes, yielded favorable results. It was noted that the
addition of hexane to tahina caused the oil in tahina to spread on the glass
slide. Then the layer dried up to yield a crisp picture of the particle
constituents (Fig. 2).
a) b) a)
Fig. 2: a) A thin layer of tahina under the microscope,
b) Adding various quantities of hexane to tahina.
N. Turkmen 192
Figures 3-5 illustrate the process and results of treating tahina with
hexane.
In Fig. 5(b), the first number refers to the reference number of the
particle, the second number is the area of particle (in pixel2) and the third
number indicates the circumference in pixels.
Fig. 3: a) Tahina when diluted by 50% hexane,
b) Tahina when diluted by 75% hexane.
Fig. 4: a) Tahina when diluted by 83% hexane, b) Clear particles in tahina after being
washed out with hexane.
b) a)
a) b)
Experimental Investigation of Particle …. 193
Fig. 5: a) Tahina particles after being washed out with hexane and printed as black and
white, b) Clarified picture of tahina particles after being measured.
During ensuing measurements and computations, the software that
came with the microscope was utilized. To convert the software
measurement, which is in pixel, to the micron a scaled lam that has 100
lines in 1 mm is used. Since software measures the area, the equivalent
diameter De is defined as:
ADe
4 (1)
The perimeter of the particle, P, also measured by the software. We
define a shape factor (circularity), Cf , were defined as:
e
fD
PC
(2)
Thus, for a perfectly circular particle, De = D, otherwise De < D.
The Cf of a perfectly round particle would be = 1.0. For any other shape
the Cf would be expected to be less.
Before starting wide-scale experimentation, a system was devised
for thoroughly mixing the tahina before taking samples, “washing” the
samples with hexane, and taking measurements after the drying of
hexane. The proper bookkeeping of the procedure and test results were
done and any observations were recorded. Thus tahina samples were
coded to indicate production date, manufacturer and sample number.
Testing of tahina was then initiated on batches purchased from the
local market. All in all, these were nine half-kg or one-kg tahina
b) a)
N. Turkmen 194
containers from 6 different manufacturers. During testing, and after
rigorous mixing, about 10 samples were extracted from each tahina
container. This corresponded to about 100 particles of size 30 microns
and above per container. It became clear at this stage that dealing with
particles less than about 30 microns in diameter would prove to be too
laborious.
3. Results
It would be of fundamental importance to compare the results of
measurements with the current measurement system with those of a
universally recognized measuring system. To this end samples of tahina from
company H were sent for analysis to Sympatec (www.sympaTEC.com), a
laboratory specialized in particle size analysis. There the tahina was analyzed
using laser diffraction techniques. Simultaneously samples were analyzed at
KAU under the microscope, using the technique developed during the current
study. The results are displayed comparatively in Fig. 6. It may be noted in
Fig. 6 that the actual start of data at lower end from Sympatec occurs at about
5 microns. Data from KAU reading commences at 30 microns due to practical
reasons. It will be agreed that the results of the readings are by the two
techniques, as displayed in Fig. 6, are not exactly the same, but they are
reasonably close. This is especially true about the slopes of the curves.
0
10
20
30
40
50
60
70
80
90
100
0 1 10 100 1000
Par
ticl
e n
um
ber
(C
um
ula
tive,
%)
Diameter, micron
Microscope measurement (Sample H) Sympatec Measurements (Sample H)
Fig. 6. Comparison of results for essentially the same tahina by two different techniques.
Experimental Investigation of Particle …. 195
Figure 7 shows particle size (De) distributions in tahina samples from
these six manufacturers. It was noticed that particle size distributions of tahina
produced at different times but from the same manufacturer did not seem to
vary substantially. This is evident in Fig. 8 for samples that were produced 6
weeks apart. For this reason particle size distribution of only one sample from
a given manufacturer is shown in Fig. 8 in order not to clutter the graph.
Fig. 7. Particle size distributions in tahina samples from selected manufacturers.
0
10
20
30
40
50
60
70
80
90
100
10 100 1000
Par
ticl
e num
ber
(C
um
ula
tive,
%)
Diameter, micron
Sample H: (08/03/2009) Sample H: (17/09/2009)
Fig. 8. Particle size distribution in tahina that were produced 6 weeks apart by the same
manufacturer.
0
10
20
30
40
50
60
70
80
90
100
10 100 1000
Nu
mb
er o
f p
arti
cle
(cu
mu
lati
ve)
, [%
]
Diameter, micron
A B C D E H
N. Turkmen 196
Table 1 depicts the distribution of average De and Cf between the
tahina of a number of manufacturers.
Table 1. Average equivalent diameters of tahina.
Company Average Equivalent
Diameter, De, (micron)
Shape factor
(circularity), Cf
A 86 0.52
B 104 0.40
C 126 0.52
D 110 0.47
E 80 0.35
F 109 0.67
G 162 0.48
H 141 0.41
It is to be pointed out in Table 1 that specimens C and F were
produced by the same company at different times, with an average De of
about 118 microns. The same is true for G and H, which were produced
by another company, and the average De for this company comes to about
152 microns.
Curiously enough, laser spectrometers are not able to detect
circularity. The microscope however readily determines circularity by
measuring the circumference of a particle. Average circularities of the
tahina samples studied are displayed in Table 1. It is observed from this
table that sample F has the largest value of circularity of about 0.65. The
circularities of the rest of the samples are less, ranging from about 0.35 to
0.5. There does not seem to be a clear-cut relationship between
circularity and equivalent particle size. In a similar manner, the average
circularity of tahina C and F produced by one company is observed to be
about 0.6 whereas for tahina G and H it is 0.45, indicating that in the
latter company tahina is passed through a ball mill that flattens the
particles.
4. Discussion and Conclusions
Referring to Fig. 7, all six tahina samples are seen to be uniform at
the 40 micron range, indicating that only about 5 % of the particles
would pass a sieve of this size. This uniformity disappears quickly
though when the samples are viewed at a sieve opening of 70 microns.
Thus tahina C and H seem to contain relatively large particles such that
only about 30 to 40 % of their particles pass the sieve of 70 micron size
Experimental Investigation of Particle …. 197
whereas tahina A and B contain finer particles such that at the same sieve
opening 65 to 70 percent of their particles go through. A similar
distribution pattern is displayed at 100 micron sieve size. The disparity
between particle sizes does not disappear even at a sieve opening of 200
microns, where 95 % of almost all particles of tahina A, B, D and E pass
through although barely 80% of tahina C and H have gone through the
sieve. It looks like it took a sieve size of 400 microns for all particles of
all tahina to finally pass through.
The observations made on Fig. 7 are also reflected in the average
equivalent diameter distributions shown in Table 1. Here it is brought
into sharp focus that tahina A and E have an average size of around 80
microns, whereas particles in tahina G and H are nearly twice that size.
Average particle sizes in tahina B, C, D and F vary from 100 to about
120 microns.
An important point to note here has to do with what has been
defined as circularity above. This is a factor that is related to the shape of
the particle. Circularity is thought to depend on the grinding and
processing machinery utilized. Low values of circularity may also be
related to the aging of tahina and the onset of rancidity due to the
increase in surface area.
With reference to Fig. 7 and Table 1, there seem to exist wide
differences and variations between the particle size distributions and
circularities of tahina in the market. The mean equivalent diameters of tahina
produced by certain manufacturers were found to be twice those of some
others. It seems to be also understood that the smoothness of tahina used for
certain processes needs to be different from that used for other processes. It
is therefore suggested that there be some indication of particle size for
informing the consumer as to what he is purchasing. The guideline given in
Table 2 may be considered an initial step in this direction.
Table 2. Suggested classification of Tahina according to average equivalent diameter.
Average equivalent particle size Classification
Less than 50 microns Extra smooth
Larger than 50 microns and less than 100 microns Smooth
Larger than 100 microns and less than 160 microns Coarse
Larger than 160 microns Extra course or chunky
Accordingly no manufacturer currently produces tahina that can be
classified as extra smooth. Only two companies, A and E produce tahina
N. Turkmen 198
that would be classified as smooth. The tahina produced by most
companies, i.e., B, C, D and H, would be classified as coarse. The tahina
of one company, G, would be classified as extra course or chunky.
It follows from what has been presented above that it is possible to
conduct particle size analyses of tahina with indeed modest expenditures
in equipment. The current research effort required the purchasing of a
$400 microscope, a $1000 laptop computer and a few hundred dollars
worth of chemicals and dispensable items. This type of a budget would
not be imaginable with the laser equipment currently utilized for the
same purpose, which carry a tag of several hundred thousand dollars. It is
true that the manual labor requirement is a bit more with the microscope,
but not enough to upset the difference in investment.
Since the results from the two systems seem to be acceptably close,
it follows that particle size analyses can now be performed by tahina
manufacturers themselves, rather than investing in expensive laser
equipment or sending samples to specialized laboratories abroad for
analysis. It goes without saying that the classical method of measuring
the particle size in tahina by using a micrometer is unsatisfactory since it
only reveals the size of the largest particle between the jaws of the
micrometer.
Acknowledgement
The author would like to acknowledge the assistance of Drs M.
Akyurt and S. Aldousari and the financial support by KAU to this effort.
References
[1] Akyurt, M., Sesame, pp 93,TEMA - National Confectionary and Tahina Co., Jeddah,
(2007).
[2] Akyurt, M., The Sesame Line at TEMA, Technical Report, p. 83, National Confectionary
and Tahina Co., Jeddah (2007)
[3] Akbulut, M. and Çoklar, H., Physicochemical And Rheological Properties of Sesame
Pastes (Tahin) Processed From Hulled And Unhulled Roasted Sesame Seeds And Their
Blends At Various Levels. Journal of Food Process Engineering, 31: 488–502 (2008).
[4] Eissa, H.A. and Azza, Z., Quality and safety of halawa modified with mushroom, Journal
of the Science of Food and Agriculture, 86 (15): 2551 – 2559 (2006).
[5] Damir, A.A., Utilization of sunflower seeds in tahina and halawa processing, Food
Chemistry, 14(22): 83-92 (1984).
Experimental Investigation of Particle …. 199
[6] Akbulut, M., Comparative studies of mineral contents of hulled sesame paste (tahin),
unhulled sesame paste (bozkir tahin) and their blends, Asian Journal of Chemistry, 20(3):
1801-1805 (2008).
[7] Jamará, M.I. and Isa, J.К., Microbiological Quality of Tehena and Development of a
Generic HACCP Plan for its Production, World Journal of Agricultural Sciences, 2(3): 290-
297, (2006).
[8] Kotzekidou, P., Microbial Stability and Fate of Salmonella Enteritidis in Halva, a Low-
Moisture Confection, Journal of Food Protection, 61(2): 181-185 (1998).
[9] Ozcan, M. and Akgul, A., Physical and chemical properties and fatty acid composition of
tahin (sesame paste), Gida, 19: 411-416 (1994).
[10] Abu-Jdayil, B., K. Al-Malah and Asoud, H., Rheological characterization of milled
sesame (tehineh), Food Hydrocolloids, 16 (1): 55-61, (2002).
[11] Abu-Jdayil, B., Modelling the time-dependent rheological behavior of semisolid foodstuffs,
Journal of Food Engineering, 57(1): 97-102 (2003).
[12] Altay, F.L. and Ak, M.M., Effects of temperature, shear rate and constituents on
rheological properties of tahin (sesame paste), Journal of the Science of Food and
Agriculture, 85 (1): 105 – 111 (2007).
[13] Arslan, E., Yener, M.E. and Esin, A., Rheological characterization of tahin/pekmez
(sesame paste / concentrated grape juice) blends, Journal of Food Engineering, 69(2): 167-
172 (2005).
[14] Razavi, S.M.A., Najafia, M.B.H. and Alaeea, Z., The time independent rheological
properties of low fat sesame paste/date syrup blends as a function of fat substitutes and
temperature, Food Hydrocolloids, 21(2): 198-202 (2007).
[15] El-Adawy, T.A. and Mansour, E.H., Nutritional and physicochemical evaluations of
tahina (sesame butter) prepared from heat-treated, Journal of the Science of Food and
Agriculture, 80(14): 2005 – 2011 (2000).
[16] Hamed, K.E.A. and Sadek, M.A. The nutritive value and consumption of sesame products
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[17] Çiftçi, D., Kahyaoglu, T., Kapucu, S. and Kaya S., Colloidal stability and rheological
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N. Turkmen 200
حجم الجسيمات فى أنواع الطحينة لتوزيع ةتجربة تحقيقي التجارية
نديم تركمان ، جامعة الممك عبد العزيز،هالمياتحمية الهندسة الحرارية وتقنية قسم
المممكة العربية السعودية – جدة
استتتتتندت الدراستتتتة عمتتتتى أ تتتتد عينتتتتة متتتتن منتتتتت الطحينتتتتة .المستتتتت م والتتك باتستتتعانة ،المحمتتى، كمتتا استتت دمت طريقتتة فتتاة لتحميتتل العينتتة
لكترونتتتتت، وتتتتتم فحتتتت عينتتتتات متتتتن المنتتتتت متتتتن قبتتتتل إبميكروستتتتكو ،لتتتى متتتتوفرإمصتتتانع م تمبتتتة بالمنطقتتتة، وباتستتتتعانة ببرنتتتام حاستتت
جتترح حستتا المستتتاحات والمحيطتتات المحتتددة لمعينتتتة محتتل البحتتت ، لألقطتار لعينتة منتت الطحينتتة يوبعتد التك تتم حستا المتوستتط المستاو .المتحصل عميها من هاه المصانع
دة فتتت حجتتم و ن هنتتاك ا تافتتات معتبتترة موجتتألقتتد تتتم استتتنتا ،توزيع العينة، والمتوسط المساوي لألقطار والدوائر ال اصتة بالطحينتة
.التت أنتجتها المصانع الم تمبة محل التجربة
جتترح تقتتديم مقترحتتات ،اتستتتنتاجات المست مصتتة وعمتتى ءتتو .طحينة المناسبةتتعمق بإرشادات مبدئية تتعمق بتحديد مواصبات ال