147
Effect of selected impurities on sucrose crystal
growth rate and granulated sugar quality
Abdelfattah Bensouissi1, Céline Rousse2, Barbara Rogé1 and Mohamed Mathlouthi1
Laboratoire de Chimie Physique Industrielle – UMR 614-FARE; 2 Laboratoire de Dynamique
des Transferts aux Interfaces-EA 3083, Université de Reims Champagne-Ardenne, Reims, France
1
Abstract
The use of antiscale products is very common in
the evaporation station of sugar factories. These
products are generally water-soluble polymers
like polyacrylates. Their role seems to be the
prevention of formation of calcium oxalate
scale. However the stability of calcium-acrylate
complexes and their behaviour after evaporation
are not well known.
Effects of antiscale on calcium oxalate solubility on white sugar turbidity and on sucrose
crystal growth rate were studied. It was demonstrated that antiscale protects evaporator from
abundant calcium oxalate scale formation. Yet,
they delay the problem of oxalate precipitation
and cannot prevent turbidity of final sugar. The
phenomenon is especially emphasized by decrease of temperature which affects both calcium oxalate solubility and antiscale sequestering
efficacy.
Effect of antiscales on growth rate and on
morphology of sucrose crystals was determined
by end-to-end laboratory crystallization and microboiler pilot methods. It was shown that antiscales inhibit sucrose crystal growth especially
in b crystallographic direction. The inhibition
of sucrose clusters formation needed for crystal
growth was proposed as a possible explanation.
Keywords: Antiscale, Calcium oxalate, Crystal
Growth Rate and White Sugar Turbidity.
Introduction
A distinctive property of white sugar is its extremely high purity. At an industrial scale, sugar
purity generally exceeds 99.8% and rarely falls
below 99.7%.
For a long time, the industrial users considered sugar as a raw material without problems.
Its quality was judged simply by visual comparison and few quality criteria such as ash content
and color of dissolved sugar. However, due to
the progress realized in food industry and analytical chemistry it was revealed that, small fraction of less than 0.2% of non-sucrose substances
are at the origin of numerous problems not only
for sugar industry but also for food industry customers.
As a consequence, rigorous criteria were adopted for the determination of sugar quality.
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Abdelfattah Bensouissi, Céline Rousse, Barbara Rogé and Mohamed Mathlouthi
In European Union, white sugar quality has
been defined nearly four decades ago in terms
of European Points. According to legislation
1265/69 of July 1st, 1969, the criteria adopted
in order to determine the European Points are
three: aspect (brightness), coloration in solution
(50% D.S.) and conductimetric ash. To these criteria, other complementary ones are added like
polarization, invert sugar and water content.
Specifications recommended by this legislation are regarded as minimum quality requirements for industrial customers who ask for further analysis. It is the case for grain size distribution (for baked goods or Champagne wine), microbiology of sugar (for dairy or canned food).
Soft drink bottlers, who are important clients
of sugar factories, often require non-foaming
sugar without turbidity, odor, sulphur and insoluble matter.
To satisfy the increasing customer requirements, sugar industrialists must improve quality
by considering all factors that can affect it. In
particular, effect of some minor impurities like
technical aids should be more thoroughly studied. In fact, the quality of white sugar can be impacted by processing aids, usually for the better,
but some times for the worse, in particular when
they are over-used or improperly used.
Material and Methods
1
Turbidity measurements
Turbidity was determined according to ICUMSA method. Syrup solution with a concentration
of 50% D.S. was prepared and filtered through
a 0.45 µm-membrane filter. Absorbance was
measured at a wavelength of 420 nm before and
after filtration using a UV-2101 shimadzu spectrometer.
Concentration was controlled using refractometer from Euromex (Euromex microscopes,
Netherlands).
2
Turbid particles characterization
2.1 Scanning Electron Microscopy
Turbid particles were collected from an industrial high turbid sugar (100 I.U.) using filtration.
Sample preparation and filtration conditions
are similar to those used for turbidity measurements. A Joel JSM-6460 LA microscope operating at 0–30 kV was used for samples imaging.
Particle samples were coated with a conductive
layer of gold in a sputter coater to avoid charging effects.
2.2 Differential Scanning Calorimetry
A SETARAM differential scanning calorimeter
(Model DSC 92, France) was used for thermal
analysis. An empty aluminum sample pan hermetically sealed was used as a reference. Sample
of approximately 10 mg was sealed in sample pan
and analyzed over the temperature range 20 to
450 °C with a heating rate of 10 °C min–1. Calorimeter was monitored by CS92-G11 software
(SETARAM, France), which allowed heat capacity monitoring and thermal curves processing.
2.3 Fourier Transform Infrared Spectroscopy
Infrared spectra of calcium oxalate monohydrate (COM) and of retained fraction on 0.45
µm-nitrocellulose membrane filter from turbid
sugar solution were recorded. A Nicolet (Impact
410, USA) spectrometer equipped with a thunderdome attenuated total reflectance accessory
(Spectra Tech, USA) was used. 200 scans were
recorded with a spectral resolution of 2 cm–1 and
then averaged using the OMNIC software.
2.4 X-Ray Diffraction
Sample characterization and crystal structure
were determined over the range of 2q angles
from 3 to 100 deg. A Bruker diffractometer
Effect of selected impurities on sucrose crystal growth rate and granulated sugar quality 149
(Bruker D8 Advance) equipped with a copper
anticathode (lcuKa = 1.54056 Å) was used. Analysis was performed at a scanning speed of 0.06
deg 2q · min–1 and a step size of 0.06 deg 2q.
Determination of soluble oxalate
concentration
3
Antiscale efficacy was determined at 85 °C and
10 µL/L concentration. Efficacy was expressed
in terms of soluble oxalate concentration determined by HPLC. A thin juice was sampled in a
French sugar beet factory not practicing decalcification during the end of 2005 campaign. Juice
was firstly preheated to desired temperature then
Table 1: Gradient used for anions separation.
Eluent (1) NaOH 100 mM; eluent (2) deionized water
Time (min)
0
3
10
20
35
% (1)
% (2)
1
1
3
3
15
99
99
97
97
85
Fig. 1:End-to-end crystallization system (Vaccari and al., 1996)
mixed with antiscale and concentrated using a
Büchi vacuum system (Switzerland). Aliquots
of 5 mL were sampled during concentration, immediately filtered through a nitrocellulose membrane filter (Ø = 0.22 µm, Millipore, Bellerica)
and diluted five times with deionised water and
finally analyzed. The chromatograph used is a
Dionex (USA) fitted with AS11 column (4·25
mm; ion exchange capacity = 45 µeq). Column
was used with an AG11 guard column and an
ion trap column ATC-1 (4 mm). Separation was
performed with the elution gradient shown in
Table 1 with a flow rate of 0.5 ml min–1. A conductivity detector (Dionex, USA) was used and
eluent neutralization was performed with an anion self-regenerating suppressor ASRS-1. Calibration was external and was done with dipotassium oxalate (99.995, Merck, France) standards.
The AI-450 software (Dionex) was used for
chromatograph monitoring and chromatograms
processing.
4
Crystallization by the «End-toEnd» method (Vaccari et al., 1991)
To control growth rates and study sucrose morphology changes due to the presence of antiscale, the «End-To-End» method was used
(Figure 1). The growing crystals were selected
under camera microscope and heated at work
temperature (30 ± 0.1 °C) prior to introduction
in tubes. Growing crystals were selected from a
highly pure sugar obtained by three successive
crystallizations of an industrial sugar (quality
N° 1). Selection was based on crystal dimensions along the b and c crystallographic axis
(Lb / Lc = 1.65 ± 0.06). Growth rates were determined at supersaturations between 1 and 1.1
Supersaturated solutions were preheated at 65 °C
to erase traces of nuclei and inhibit further spontaneous nucleation and then mixed with 100 µL/
L of antiscale. 10 crystals were introduced in
each tube filled with supersaturated solutions.
Tubes were fixed on a disc rotating at 6 rpm
placed in a thermostated box (30 ± 0.1 °C). Di-
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Abdelfattah Bensouissi, Céline Rousse, Barbara Rogé and Mohamed Mathlouthi
mensions along the crystallographic axes b and
c, before and after growth, were determined
by image analysis using a camera microscope
(Nikon) connected to a computer equipped with
the «Lucia» software (Hinkova et al., 2003). The
determination of the third dimension (La) was
performed with a high precision (± 1 µm) digital
micrometer.
The measurement of the dimensions La, Lb
and Lc before and after growth gives an average
growth rate in the units of length/time. In order
to express the rate in unit of weight/area · time,
it is necessary to know volume (weight) and
surface of each crystal before and after growth.
To evaluate these two parameters, two formulas
were established and given by equation (1) and
equation (2). Determination of these formulas
was based on the constancy of angles between
crystallographic axes and faces of sugar crystals.
Checking of the two formulas was made with
more than 1600 crystals with different shapes
and methods of preparation.
V = La·Lb·Lc – La2·Lc / 2.434 – La2·Lb / 3.92 + La3 / 7.174
(1)
S = 2·Lb·Lc + 1.226·La·Lb + 0.958·La·Lc – 0.748·La2
(2)
where V is the volume, S is the surface and La,
Lb and Lc are respectively the crystal dimensions
along the crystallographic axis a, b, and c.
5
Micro boiler crystallization
Crystallization in microboiler (Figure 2) was
conducted at constant pressure (absolute pressure = 0.28 kPa) and increasing temperature
(70–80 °C). Sucrose solution (1.5 L; 68% D.S.)
introduced in the crystallizer was obtained by
dissolving industrial sugar (Quality N° 1) in ultra pure water. Seeding was practiced at a supersaturation of 1.1 with 5 g of slurry (MA =
0.396 mm; CV = 20.6) dispersed in 50 mL of
saturated sucrose solution.
6
Crystal size control
Crystal size was controlled using a CAMSIZER
device (Retsch, Germany). Before analysis,
crystals obtained by micro boiler crystallization
were sifted to eliminate grains with sizes smaller
than 0.250 mm and larger than 1 mm. For analysis, samples were introduced in a funnel positioned at 2.5 mm above a stirred table that permits the crystals to disperse. Dispersed crystals
were then canalised with a feeder (16 mm) and
flexible guidance sheet (1.6 mm) until its vertical falling in a field photographed by two high
resolution cameras. The cameras allow taking of
100 images each second and to transmit them in
real time to a personal computer equipped with
appropriate image analysis software (CAMSIZER, Retsch, Germany).
Fig. 2: Laboratory evaporating crystallizer
Effect of selected impurities on sucrose crystal growth rate and granulated sugar quality 151
For each particle, the dimensions along the
crystallographic axes b and c were determined.
The high number of analyzed particles (several
millions) guarantees the reproducibility and the
validity of measured parameters.
7
Surface tension measurements
Surface tension measurements were performed
using the Wilhelmy method with a semiautomatic SIGMA 70 tensiometer (KSV, Finland). Measurements were carried out at 20 ± 0.2 °C using
a platinum plate (width = 19.6 mm; Thickness =
0.1 mm and wetted length = 39.4 mm). Before
each measurement, platinum plate was tested
with ultra pure water (0.055 µS cm–1 conductivity), which was used for preparing the samples.
Sucrose was used at the concentration of (10%
D.S.) and was purified by tripled recrystallization of industrial sugar (quality N° 1). Antiscale
samples are commercial grade and were used
at the concentration of 100 µL/L. The only information available on its compositions is that
they are polyacrylate chemicals with molecular
weight between 1500 amu and 2500 amu (1 amu:
atomic mass unit = 1 Dalton). All solutions were
prepared 24 hours before measurement and all
glass vessels were cleaned with pure ethanol
(99%), rinsed thoroughly with ultra pure water
and then dried at 40 °C.
Results and Discussion
1
Nature and localization of particles at the origin of sugar turbidity
1.1 Turbidity nature
Sugar turbidity is routinely analyzed in French
sugar factories. The most widely used method for
its determination is the ICUMSA one. The validity of the method is questionable because of the
unknown nature of the quantity measured. The
aim of this section is to characterize the nature of
particles retained when white sugar turbidity is
measured according to ICUMSA method.
Various instrumental techniques were used
to unveil the nature of turbid particles:
SEM image (Figure 3A) shows that they had
small sizes ranging from 0.1 µm to several micrometers. With such sizes, particles can infiltrate across the membrane pores (Figure 3B) and
eventually clogs it (Figures 3C and 3D). This
can explain the proportionality between membrane clogging index and turbidity described by
Rogé et al. (2007). In addition the retained particles have crystal shapes and we can distinguish
two different types of crystals: needle crystals
(Figure 3E) and tetragonal bi-pyramidal crystals
(Figure 3F). Such crystalline nature can explain
the lightness of retentive fraction on 0.45 µmmembrane filter from raw sugars reported by
Vianna, (2000) and Godshall et al. (2006). The
tetragonal bi-pyramidal and the needle shapes
were described to be characteristic of calcium
oxalate hydrates (Yu et al., 2004). According to
these authors, the abundance of a form or of the
other is mainly dependent on the hydration level
acquired: as hexagonal monohydrate (COM),
tetragonal bi-pyramidal dihydrate (COD) or triclinic and needle-shaped trihydrate (COT). From
the above observations COD and COT are much
suspected to be the main components of turbidity not only because of similarities of forms with
COD and COM but also because turbidity of
white sugar was recently correlated to the presence of calcium (Rogé and al., 2007).
FTIR spectrum of turbid particles was compared to the spectra of COM (Figure 4) and
COD (Figure 5) and the comparison has shown
that turbid particles are comparable to COD especially based on two characteristic infrared absorption bands at 1643 cm–1 and 1322 cm–1.
DSC curves corresponding to turbid particles and to pure sucrose from Merck (Figure
6) present two endothermic peaks: At 176 °C
and 185 °C for turbid particles and at 188 °C
and 215 °C for pure sucrose. Peaks obtained
in the case of turbid particles take the form of
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Abdelfattah Bensouissi, Céline Rousse, Barbara Rogé and Mohamed Mathlouthi
Fig, 3: SEM micrographs of retained fraction on 0.45 µm-membrane filter from a turbid sugar
(100 I.U.)
Fig. 4:FTIR spectra of calcium oxalate monohydrate obtained from equimolar reaction between
calcium chloride and sodium oxalate at 85 °C and of the retained fraction on 0.45 µm
membrane filter from an aqueous solution of turbid sugar (50% D.S.)
Effect of selected impurities on sucrose crystal growth rate and granulated sugar quality 153
Fig. 5: FTIR spectra of calcium oxalate dihydrate crystals obtained at initial concentration of
130 µL/L from sugar solutions of: (A) 10%, (B) 25% and (C) 40% and composites Calcium
oxalate dihydrate-SiO2 obtained from sugar solutions at an intial calcium oxalate concentration of (D) 44 µL/L and (E) 130 µL/L (Yu et al., 2004)
Fig. 6:Thermal curves of retained fraction on 0.45 µm-membrane filter from an aqueous turbid sugar
solution (50% D.S.) and of pure sugar from Merck
154
Abdelfattah Bensouissi, Céline Rousse, Barbara Rogé and Mohamed Mathlouthi
thermal accidents specific of
polymorphs whereas those of
pure sucrose show a distinct
fusion followed by sample
thermal degradation. This
clearly points out that endothermic peaks displayed by
turbid particles don’t correspond to a contamination
with sucrose (especially the
first one at 176 °C) and correspond probably to the loss
of the first and the second
COD water molecules to give
respectively COM (164 °C)
and anhydrous calcium oxalate (187 °C) melting points
(Kaloustian et al., 2003).
XRD pattern of turbid particles (Figure 7) clearly point
out that turbidity is mainly
composed of COD with peaks
at 2q = 10, 14, 20, 22, 24, 28,
31, 32, 37, 37.3, 40, 42, 46, 47
and 49° as indicated in XRD
standard data base (PDF 01075-1314).
Fig. 7:XRD spectra of retained fraction on 0.45 µm-membrane
filter from an aqueous turbid sugar solution (50% D.S)
1.2 Turbidity localization
To localize turbidity it seems
important to note that calcium
oxalte, main component of
turbidity, does not crystallize
in the same crystallographic
system as sucrose. As a consequence it seems more logical
Fig. 8:Differences in turbidity according to crystal size for a turto find turbidity at the surface
bid white sugar
of sugar crystals rathen than
inside. Two methods were applied for this localization:the first one consists in turbid particles are mainly localized at the suranalyzing turbidity in fractions of sugar (quality face and can be concentrated in fine sugar fracN° 2, turbidity = 100 I.U.) retained by screens tions which have larger surface area.
between 0.1 and 1 mm and the second one is the
In addition the slight augmentation of turbidmethod of successive washings.
ity of large fractions is a sign of presence of turFigure 8 shows that the finer the sugar crys- bidity at the surface. This is clearly evidenced by
tals, the higher their turbidity. This means that the observation of SEM pictures of the surface
Effect of selected impurities on sucrose crystal growth rate and granulated sugar quality 155
Fig. 9: SEM picture of the surface of a single
sucrose crystal before washing
Fig. 10: SEM picture of the surface of a single
sucrose crystal after washing
of sucrose crystals (0.8 mm) before and after
washing.
Figure 9 shows that sucrose crystal surface is
rough and it seems that some particles are hidden
under a thin layer of sucrose. Particles initially
covered become visible and do not dissolve with
washing (Figure 10). In addition, its imprint can
be shown when the surface of the sugar crystal is
well washed (Figure 11).
From successive washing results (Figure 12),
we can show that elimination of 12 µm of sucrose
at crystals surface (1 washing) decreases turbidity from 22 to 12 I.U. However, after elimination of 62 µm of crystal thickness (4 successive
washings), the turbidity was stabilized at 12 I.U.
This means that turbidity is mainly localized at
the surface but also homogenously distributed
inside the crystals. The role of macromolecules
in promoting turbidity of final sugar was reported by Cosmeur and Mathlouthi (1999). The
authors found that the increase of syrup turbidity due to the presence of high molecular mass
macromolecules leads to an increase of final
sugar turbidity. Since, turbid sugar particles are
not macromolecules but thin crystals of calcium
oxalate we can say that the role of macromolecules is limited to the transport and the inclusion
of calcium oxalate crystals. As demonstrated by
Vaccari (1996), the incorporation of inclusions
in sugar crystals depends mostly on the crystallization rate and on impurities size. The higher the
crystal growth rate and impurity size, the higher
the incorporation level.
2
Fig. 11: SEM picture of the imprint of turbid
particles at the surface of a single sucrose crystal well washed
Contribution of antiscales in promoting sugar turbidity
Calcium oxalate formation is not only harmful for the quality of final sugar but also for the
heating capacity of evaporation station. Calcium
oxalates (COD and COM) represent about the
half (51% of the weight) of scale encountered
in multi-effect evaporator (Doherty, 2000). Because of its low thermal conductivity (20 times
lower than that of stainless steel and 60 times
156
Abdelfattah Bensouissi, Céline Rousse, Barbara Rogé and Mohamed Mathlouthi
Fig. 12: Changes in turbidity as a function of thickness of sugar layer eliminated
by a washing of crystals with an undersaturated pure sucrose solution (s = 0.95)
Fig. 13: XRD spectra of scale samples scratched at evaporation station wall surfaces
in a French sugar beet factory not practicing decalcification
Effect of selected impurities on sucrose crystal growth rate and granulated sugar quality 157
lower than that of brass (Helmut and Joachim,
2003)) calcium oxalate can reduce the coefficient
of heat transmission to almost half of its value
when it forms a thin scale layer of 0.2 mm.
To improve the evaporator performance and
avoid stopping the whole evaporation system
during campaign to clean tubes, most of sugar industrialists use antiscales. The use of such products is until now considered a solution without
disadvantages. In fact, this practice is not only
beneficial but it seems to have some drawbacks.
The increase of sugar turbidity due to the use of
antiscales will be demonstrated in this section.
Calcium oxalate is the result of combination
between calcium and oxalate. This combination
occurs when concentrations of cationic calcium
and anionic oxalate exceeds the solubility limit
of calcium oxalate. This limit was measured in
water and does not exceed 13.8 mg·L–1 at 85 °C
([C2O42–] = 9.49 mg·L–1). It is lower at the same
temperature in sugar solution with 45% sucrose content 8 mg/L ([C2O42–] = 5.50 mg·L–1).
Calcium and oxalate concentrations were also
measured in different thin juices sampled in a
French sugar beet factory not practicing decalcification. These concentrations vary respectively
in the ranges of 20–80 g·L–1 and 8–15 mg·L–1.
With such levels of concentrations, calcium oxalate easily crystallizes and precipitates since the
first effect of evaporation station. As shown in
Figures 13 and 14 calcium oxalate begins to be
present only at the fourth effect of evaporation.
In fact, scale encountered at the earlier steps
of the evaporation (from last heater before evap-
Fig. 14: Scanning electron micrographs of scale samples scratched at evaporation station wall surfaces in a French sugar beet factory not practicing decalcification
158
Abdelfattah Bensouissi, Céline Rousse, Barbara Rogé and Mohamed Mathlouthi
Fig. 15: Soluble oxalate concentration versus dry matter in a thin juice free from antiscales during
evapo-concentration under vacuum at 85 °C
Fig. 16: Soluble oxalate concentration versus dry matter in a thin juice mixed or not with 10 µL/L
of antiscale during evapo-concentration under vacuum at 85 °C
Effect of selected impurities on sucrose crystal growth rate and granulated sugar quality 159
oration to third effect) is characterized by XRD
patternswith a diffraction peaks at 2q = 23, 29.4,
35.9, 39.4, 43.2, 47.1, 47.5, 48.5, 57.4, 60.7,
64.7, 83.8° and a broad peak at 2q ≈ 12°. The
first peaks are typical of calcium carbonate (calcite) (PDF 01-083-0578) whereas the broad one
(2q ≈ 12°) is characteristic of amorphous silica
(Nakamura et al., 1989). In contrast to what can
be shown in the earlier evaporation steps, scale
of the last ones (4th effect-concentrator) is characterized by XRD spectra typical of COD.
The role of antiscales in delaying COD precipitation until evaporator fourth effect was examined under conditions similar to that encountered in sugar factories. Soluble oxalate concentrations were determined during evaporation of a
thin juice and compared to theoretical ones (Figure 15). Experimental concentrations were determined using HPLC whereas the theoretical ones
were determined by multiplying thin juice initial
oxalate content by the coefficient of dry matter
concentration. For example, an initial thin juice
oxalate content of 6 mg·L–1 is multiplied by 3
when concentration of dry matter increases from
15% D.S. to 45% D.S. Theoretical concentration
increases from 6 (mg·L–1) to 18 (mg·L–1).
In contrast to what can be envisaged, the experimental soluble oxalate concentration found
is much lower than the theoretical one (only
12.6 mg·L–1 at 45% D.S). This value is higher
than the solubility limit (5.50 mg·L–1), which
means that observed loss is probably caused by
the conjugation of oxalate with calcium to form
insoluble calcium oxalate. Oppositely to what
can be shown for pure juice, the presence of
10 µL/L of antiscale permits to concentrate oxalate without further crystallization (Figure 16).
This is probably due to the chelating of calcium
susceptible to react with oxalate. The chelating
of calcium by the polyacrylate polymers was described in many papers (Manning (1981); Kuila
and al., (1999)) and three coordination types can
be obtained between calcium and polyacrylate
carboxylate group (COO–): the unidentate, the
bidentate and the bridging. In the unidentate
coordination, only 1 oxygen atom is bound to
1 atom of calcium; in the bidentate, both oxygen
atoms of the COO– are coordinated to 1 atom of
calcium; in the bridging type, the 2 oxygen atoms of the COO– are coordinated to 2 different
calcium atoms (Fantinel et al., 2004). In addition
to calcium chelating, which represents the principal way of solubilization, antiscale can also
increase oxalate solubility via other ways: by
hindering clusters at the origin of nucleus needed for crystallization; by adsorption on nucleus;
and by increasing the interfacial nucleus-solution surface tension.
Figure 15 and Figure 16 show that at a given
dry matter content, the experimental oxalate
concentrations found in samples mixed with antiscale are slightly higher than that determined
theoretically in juice free from antiscales. This
means that the values found in juice mixed with
antiscale are not only the result of concentrating
but allow suggesting that soluble oxalate is produced during evaporation. In fact it was reported
that under evaporation conditions, oxamic acid
(Buchholz, 1998) and glyoxilic acid (Bohn,
1998) could be transformed into oxalic acid.
Oxalate can also be produced during measurement as a result of dissolution of thin calcium
oxalate crystals unfiltered through the 0.22 µmmembrane filter.
From the above results, it seems that adding
antiscale agents during the evaporation step protects the evaporator tubes from abundant scale
formation, but delays the problem to the following steps. In fact, Figure 13 and Figure 14 show
that COD is present in the syrup tank, in the preconcentrator and in the concentrator and there is
no reason to explain its absence in stored syrup.
Increase of turbidity in stored syrups was shown
by Cosmeur (1999) and was correlated to the decrease of temperature. In order to investigate the
contribution of calcium oxalate in the promotion
of this turbidity, a syrup (65% D.S.) was prepared in laboratory by concentrating a thin juice
(15% D.S.; 6 mg·L–1 soluble oxalate content)
mixed with 10 µL/L of antiscale under vacuum
at 85 °C. Syrup was stored at room temperature
(25 ± 2 °C) during 70 days. Along the period of
160
Abdelfattah Bensouissi, Céline Rousse, Barbara Rogé and Mohamed Mathlouthi
Fig. 17: Soluble oxalate concentration versus time in a syrup (65% D.S.) obtained in laboratory
by concentrating under vacuum a thin juice (15% D.S.; 6 mg·L–1 soluble oxalate contents)
mixed with 10 µL/L of antiscale at 85 °C
Fig. 18: Sucrose crystal growth rate in pure solution and in presence of antiscales impurities
Effect of selected impurities on sucrose crystal growth rate and granulated sugar quality 161
storage aliquots of 5 mL were sampled at different time intervals and analyzed using HPLC to
determine soluble oxalate concentration.
Obtained results are given in Figure 17,
which shows a rapid decrease of soluble oxalate
concentration during the first days of storage followed by stabilization. This agrees with Cosmeur
(1999) and means that oxalate is responsible for
syrup turbidity in the form of insoluble calcium
oxalate probably combined with macromolecules.
The calcium oxalate formation is promoted by the
decrease of temperature (from 85 °C to 25 ± 2 °C).
Decrease of temperature was described to affect
both antiscale sequestering efficacy and calcium
oxalate solubility (Bensouissi et al., 2007).
3
Antiscales effects on sucrose crystal growth rate and morphology
Sucrose crystal growth rate is a subject that attracted the interest of researchers both for its
theoretical and practical importance. The effect
of impurities like raffinose, glucose, fructose, invert sugar and dextran on sucrose crystal growth
was extensively investigated. The interest given
for such impurities can be attributed to its abundance in syrups and to the molecular similarities
that they had with sucrose molecule. Oppositely,
the effect of minor and hydrophilic impurities is
not well studied despite the affinity they have for
water and for sucrose itself.
Hydrophilic impurities can exist naturally in
beet and cane plant or can be intentionally added
to juices in order to facilitate some processing
unit operations. Antiscales are a good example
of hydrophilic impurities added in sugar factory.
As mentioned above, its addition prevents calcium oxalate crystallization at the evaporation
step. Following the evaporation, they have no
special role and certainly are present at the level
of several mg/L in stored syrup. As far as we are
aware, the effect of such impurities on growth
was never studied before. The aim of this section is to discuss their effect on sucrose crystal
growth and morphology.
The effect of 3 antiscales on the overall sucrose crystal growth rate and on the «a», «p/p'»
and «r/c» faces growth rates were studied in
the range of supersaturation (1–1.105). As may
be observed in Figure 18, crystal growth rate
is decreased by the presence of antiscales. For
example at a supersaturation of s = 1.084 the
growth rate decreases by about 12.5% in presence of antiscale C. This decrease can be considered as non-negligible with regard to the amount
of antiscale used (100 µL/L). We can also show
that growth rate as a function of supersaturation
follows a 2nd order kinetics law expectable from
a BCF type of growth. From face by face growth
rates results (Figure 19a; 19b and 19c), it can be
shown that antiscales inhibit the sucrose crystal
growth elongation in the direction of the crystallographic «b» axis. In fact, comparison of surface growth rates in presence of antiscales with
those in absence of antiscale show that «p/p'»
had the most affected growth. For example, at
a supersaturation of s = 1.084 and in presence
of 100 µL/L of antiscale C, the growth rate of
the face «p/p'» is decreased by about 10.29%
whereas those of the faces «r/c» and «a» are respectively decreased by 6.29% and 6.21%.
Inhibition of crystal elongation in the b direction is clearly evidenced by the comparison
of Lb / Lc ratios of a great number of crystals obtained in a pilot crystallizer from pure sucrose
solutions mixed or not with 100 µL/L of antiscale (Figure 20).
4
Mechanism involved in crystal
growth inhibition
In order to understand the mechanism by which
antiscales inhibit sucrose crystal growth, a thermodynamic approach can be proposed. Before
integrating crystal lattice, sucrose molecules
must aggregate to form clusters. The total free
energy variation DG involved in this process is
determined by the contribution of the surface
(DGs) and the volume (DGv) free energies. The
DGs represents the energy needed to create the
162
Abdelfattah Bensouissi, Céline Rousse, Barbara Rogé and Mohamed Mathlouthi
Fig. 19 a: Growth rate isotherm of the face
«a» in pure solution
and in presence of
antiscales impurities
Fig. 19 b: Growth rate isotherm of the «r/c»
faces in pure solution and in presence
of antiscales impurities
Fig. 19 c: Growth rate isotherm of the «p/p'»
faces in pure solution and in presence
of antiscales impurities
Effect of selected impurities on sucrose crystal growth rate and granulated sugar quality 163
surface of cluster whereas DGv represents the
energy released by molecules when they transit
from solution to the cluster (DGs > 0 and DGv < 0).
If we consider that a cluster is formed by N sucrose molecules and has a spherical shape (to facilitate reasoning) with a radius r, we can write:
∆Gs = 4 p r 2 g
(3)
and
∆Gv = – N ∆μ
∆G = – (4 p / 3 VM) r 3 ∆μ + 4 p r 2 g
(7)
To be stable, a cluster should contain a defined
number of sucrose molecules and have a radius
r and a free energy ∆G. As a consequence, to the
ratio ∆G /r constancy we can write:
(4)
Where ∆μ represents the difference of chemical
potential that a molecule undergoes when passing from solution to cluster and γ is the specific
surface energy at the cluster-solution interface.
It follows that the total energy involved in cluster formation is:
∆G = – N ∆μ + 4 p r 2 g
Where VM is the molecular volume, we can deduce the following expression for clusters total
free energy:
(5)
d∆G / dr = 0 = – (4 p /VM) ∆μ r 2 + 8 p g r (8)
From eq. (8), we can deduce the expression of
cluster radius:
r = 2 g VM / ∆μ
(9)
Substituting r by its expression in eq. (7), we
can deduce the free energy needed for a cluster
formation:
Remembering that the spherical volume occupied by N molecules is:
∆G = (16/3) p V 2M · g 3 / (∆μ)2
N VM = (4 p r 3 / 3) · N
This means that sucrose solution should reach a
(6)
(10)
Fig. 20: Lb / Lc ratio as a function of crystals size classes (Mean Aperture) obtained in pilot microboiler from a pure sucrose solution mixed or not with 100 µL/L of antiscale
164
Abdelfattah Bensouissi, Céline Rousse, Barbara Rogé and Mohamed Mathlouthi
Table 2: Surface tension of pure sucrose solutions (10% D.S.) mixed or not with 100 µL/L of antiscale
Sample
Ultra pure water
Sucrose solution without antiscale
Sucrose solution mixed with 100 µL/L of antiscale A
Sucrose solution mixed with 100 µL/L of antiscale C
Sucrose solution mixed with 100 µL/L of antiscale D
defined surface tension g to easily give clusters
needed for crystal growth. In order to explain
growth inhibition shown in the last section, surface tensions of aqueous sucrose solution (10%
D.S.) and of aqueous sucrose solutions mixed
with 100 µL/L of antiscale were measured
(Table 2).
Without antiscale, the surface tension obtained for pure sucrose solution is (73.53 ± 0.13
mN·m–1). Surface tension of ultra pure water used
for its preparation is (72.78 ± 0.11 mN·m–1). The
difference Dg = 0.75 mN·m–1 obtained between
water and sucrose solution is close to values reported in literature (Docoslis et al., 2000; Aroulmoji et al., 2004). Increase in surface tension results in one hand from the elevation of cohesion
free energy between the electron-acceptor and
electron-donor sites of molecularly dissolved
sugar molecules as a result of hydrogen bonding and on the other hand from the decrease of
surface tension as a result of the occurrence of
depletion layer (Docoslis et al., 2000).
In presence of 100 µL/L of antiscale, surface
tension of sucrose solution (10% D.S.) decreases as shown in Table 2. This decrease probably
results from the hydrogen bonding established
between sucrose and antiscale molecules. Establishment of such bonding can lower free energy
cohesion between dissolved sucrose molecules
and consequently lower the probability of clusters formation needed for crystal growth in supersaturated sugar solutions. Establishment of
sucrose-antiscale hydrogen bonding can also
affect shape properties of the sucrose molecule
and consequently modify the crystal habit (Mantovani, 1996). This argument seems to be con-
Surface tension mean value
Standard deviation
72.78
73.53
72.63
72.68
70.67
0.11
0.13
0.15
0.13
0.46
vincing to explain the inhibition effect of antiscale on the elongation of sugar crystals along
b axis.
Conclusions
From the experimental work presented in this
paper, it can be concluded that:
– Particles at the origin of beet white sugar turbidity are mainly constituted of thin crystals
of calcium oxalate dihydrate. These crystals
are located at the surface as well as inside
of sugar crystals. Inside the sugar crystals it
seems that macromolecules play the role of
carrier for the calcium oxalate inclusion.
– The adding of antiscale agents during the
evaporation step protects the evaporator
tubes from abundant scale formation, but delays the problem as the turbidity is concentrated in the syrup and final sugar especially
in case of storage.
– To reduce sugar turbidity, decalcification
of thin juices seems to be the most efficient
method compared to the syrup filtration and
to the crystals washing methods.
– Antiscale can decrease the overall sucrose
crystal growth rate probably by inhibiting
clusters formation needed to crystal growth.
– Antiscales inhibit sucrose crystal elongation
in the b crystallographic direction probably
by a mechanism of specific surface adsorption as was observed by Mantovani (1996).
Effect of selected impurities on sucrose crystal growth rate and granulated sugar quality 165
Acknowledgements
The authors wish to thank the French sugar
profession for funding this work and Mr. Jean
GENOTELLE for the fruitful discussion of the
crystal growth section. Mr. James O'DONNELL
(Horiba Jobin-Yvon) is also thanked for making
grain size distribution measurements with Camsizer device possible.
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