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1 Introduction

White sugar quality was defined nearly 40 years ago by European legislation in terms of points (“European Points”) specified by the EU sugar market regime. According to legislation 1265/69 of 1 July 1969, the criteria adopted in order to determine points for white sugar quality are three: color type of sugar as compared to standards established by the Braunschweig Sugar Institute, color in solution (50% sucrose content) and conductivity ash. To these cri-teria, other complementary analyses are added such as polarimetric sugar, invert sugar or water content.However, it often happens that customers ask for further controls. It is the case for grain size distribution (for baked goods or cham-pagne wine), microbiology of sugar (for dairy or canned food). Soft drink bottlers, who are important clients of the sugar factories, often require non-foaming sugar without turbidity and insoluble matter. The predominant industrials among these beverage proces-sors have developed their own methods which they have imposed on sugar suppliers. Other analyses such as odor, taste, apparent

Effectofcalciumonwhitesugarturbidity

EinflussvonCalciumaufdieTrübungvonWeißzuckerinLösung

BarbaraRogé,AbdelfattahBensouissiandMohamedMathlouthi

Sugar turbidity is routinely analyzed in French sugar factories. It accounts for the presence of suspended particles in sugar solution which display light diffusion in the analytical conditions. However, the nature of such particles is not accurately known. It has been described as high molecular mass colloidal particles associated with micro-crystals of calcium salts (oxalate, sulfate, phosphate and citrate).Turbidity measurement is linked to that of solution color. Choice of wavelength, filtration membrane pore diameter, and even the type of spectrometer may affect the result. On the other hand, the ori-gins of sugar turbidity are not easily correlated with the processing conditions. It rather seems to be a multi-parameter origin in which calcium salts play a preponderant role.Recent work carried out at the ‘Laboratoire de Chimie Physique In-dustrielle’ (Faculté des Sciences, Université de Reims, France) on the evolution of stored beet syrup during the inter-campaign peri-od, has made it possible to show the complexity of the mechanism of turbidity formation. In all cases, an increase in syrup turbidity yields an augmentation of sugar turbidity. Likewise, sugar color in-creases as turbidity increases. Adding anti-scaling agents or filter-ing the syrup does not seem to help. Besides calcium salts, there is a need for macromolecules and colorants to ensure the repartition of turbidity inside the sugar crystal.

Key words: sugar turbidity, calcium salts, color in solution, sugar color

In französischen Zuckerfabriken wird die Trübung von Zucker in Lösung routinemäßig untersucht. Die Trübung wird durch das Vor-handensein von Partikeln in der Suspension, die eine Lichtstreu-ung unter den Analysebedingungen zeigen, hervorgerufen. Die Zusammensetzung dieser Partikel ist nicht genau bekannt. Wahr-scheinlich sind es hochmolekulare kolloidale Substanen, an die Calciumsalz-Mikrokristalle (Oxalat, Sulfat, Phosphat und Citrat) angelagert sind. Die Messung der Trübung ist verbunden mit der der Farbe in Lö-sung. Die Wahl der Wellenlänge, des Porendurchmessers der Fil-trationsmembran sowie des Spektrometertyps können das Resultat beeinflussen. Die Trübung kann nicht einfach mit den Prozessbe-dingungen korreliert werden – sie scheint eher einen vielschich-tigen Ursprung zu haben, bei dem Calciumsalze die wesentliche Rolle spielen. Eine an der Faculté des Sciences, Université de Reims (Frankreich), durchgeführte Untersuchung zur Veränderung von außerhalb der Rübenverarbeitung gelagerten Rübenzuckersi-rupen zeigte die Komplexität des Mechanismus der Trübungsbil-dung. Je größer die Trübung des Sirups, desto höher ist auch die Trübung des daraus kristallisierten Zuckers (in Lösung). Gleicher-maßen nahm auch die Zuckerfarbe mit steigender Trübung des Si-rups zu. Die Zugabe von Belagverhinderern oder Filtrierung des Sirups scheint nicht zu helfen. Neben Calciumsalzen sind Makro-moleküle und Farbstoffe verantwortlich für die Trübungsverteilung innerhalb der Kristalle.

Stichwörter: Zuckertrübung, Calciumsalze, Farbe in Lösung

density, sulfur content, ... are sometimes required by some custom-ers e.g. syrup and liquor processors.Measurement of turbidity is usually associated with that of solu-tion color. This type of analysis has been found important for a long time as evidenced by the review published in ‘Advances in Carbohydrate Chemistry’ in 1954 [1]. Questions posed by this type of analysis dealt with the nature of the quantity measured. Was it the color or the visual aspect of sample as may be compared to a standard which is measured and how to account for all types of colorants which have their maxima of absorbance at different wavelengths from that retained for the method (caramel at 228 nm, HADP (hexose alkaline degradation products) at 280 nm and mela-noidins at 330 nm)? Does the presence of solid impurities affect the result of color and turbidity? Is the settling of particles during measurement important for the final result? Is it necessary to filter? What effect has the filtration on the color measurement? What is the optimal wavelength to adopt? Observation of the general shape of curve as a function of wavelength shows a decrease of absorb-ance between 300 and 750 nm. The wavelength widely used and

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adopted in ICUMSA is 420 nm, situated in the middle of absorb-ance curve.Apart from the questions on the nature of the quantity measured, the validity of the method for turbidity measurement was also dis-cussed. This was the case at the annual meeting of G.T.S. (Groupe-ment Technique de Sucreries) in 1963 [2]. Discussion concerned the most appropriate type of signal, extinction or attenuation. If filtration is needed, what should be the diameter of filter pore? In the US, after using a method without filtration, based on the differ-ence in attenuation between 420 and 720 nm, a harmonization is now accepted by ISBT (International Society for Beverage Tech-nologists) and the method agreed involves a filtration [3]. Other parameters such as cell length or type of spectrophotometer may have an influence on the result. Specific tests established by soft drink processors such as filtration with an 8 µm filter are possible. Nevertheless, the most widely used method for color and turbidity is the ICUMSA (GS 2/3-10) method which involves a filtration on 0.45 µm filter and the measurement of absorbance of a 50% sucrose content unfiltered solution at 420 nm.

2 Turbidityandcrystallization

Even though the measurement of turbidity is questionable, it is however an important criteria of white sugar quality, worthy of interest. This is the aim of the present work which consisted in controlling the effect of turbidity on white sugar quality using a pilot evaporating crystallizer. Results of this work were partially published previously [4]. Turbidity used in that work came from the stored syrup during the 1996 and 1997 campaigns. Analysis of calcium salts in stored syrups show (Fig. 1) a good correlation between turbidity and calcium concentration.

Other constituents from stored syrup were tested for their effect on sugar turbidity. Hence, high molecular mass macromolecules obtained by ultrafiltration were added to pure sucrose syrup and their effect on turbidity of white sugar crystals determined after centrifuging and washing of massecuite. It was found that the frac-tions of ultrafiltration retentates which were the most harmful for crystals were the fractions with molar masses above 10,000 amu (1 amu [atomic mass unit] = 1 Dalton) and above 300,000 amu. Therefore, apart from calcium salts, turbidity seems to depend on the size of the macromolecular impurity.Moreover, analysis of the composition of the turbidity fraction in a sugar shows a good correlation between the calcium ion concentra-tion and white sugar turbidity (Fig. 2).What was the form of calcium in the syrup yielding by crystalliza-tion a turbid sugar? To answer this question, the crystallization of stored syrup from 1996 campaign was preceded by the addition of milk of lime. Likewise, calcium saccharates or milk of lime were added to pure sugar syrup. Such an addition of calcium ions re-sulted in an increase of sugar turbidity after crystallization of the supplemented syrup (Table 1).If calcium is bound to sucrose to form saccharates, this might be identified from the structure of the sucrose molecule. This is what was observed on the analysis of crystalline sucrose contain-

Table1: Influence of the addition of calcium (free or bound) on sugar turbidity

Sample Mean aperture Sugar turbidity [mm] [points]

Syrup 96 0.62 1.50Syrup 96 + 250 mg/L CaO 0.64 1.50Pure syrup (Sugar Q1*) – 0.30Pure syrup + 0.75% saccharate 0.62 3.30Pure syrup + 0.5% milk of lime 0.61 3.30

* European Points Grade 1

* Turbidity in European Points is obtained as the ratio of difference in optical density before and after filtration on 0.45 mm membrane by 7.5 (Turbidity (E.P.) = (D OD.)/7.5). The difference between ICUMSA and European Points is that in E.P. determination ICUMSA points are divided by 7.5.

Fig. 2: Relationship between calcium in sugar (mg/kg ds) and turbidity in European Points*

Fig.1: Relationship between turbidity (European Points*) and calcium salt content in stored syrups during the 1996 and 1997 campaigns

Fig.3: FTIR spectra of sugars containing different amounts of turbidity in Euro-pean Points*

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ing different proportions of turbidity. The sample of sucrose with the highest amount of turbidity shows an FTIR (Fourier Transform Infrared) spectrum different from other spectra (Fig. 3) very likely because of a modification of sucrose structure after chelating of calcium to form a saccharate complex or because of the interfer-ence of a calcium oxalate spectrum.Another important parameter in crystallization is the rate of growth of crystals. This rate was measured using the “end-to-end” method [4] for crystals having an initial size of 0.86 mm. It was observed that turbidity in syrup provokes a decrease in growth rate which increases with the turbidity value. Likewise, determination of the crystal dimensions after growth in the presence of turbid impurities shows a change in the sucrose crystal morphology. Elongation of face c is significant and such a phenomenon increases with syrup turbidity.The method of successive washing of sucrose crystals was applied to localize the turbidity and determine its origin in turbid sugar. While a first wash, decreasing the average size of crystals from 0.62 to 0.52 mm, shows a decrease in turbidity and color in sugar, the second wash shows an increase in turbidity and a decrease in color. Turbidity does not seem to be localized at the surface of crys-tals. As is shown later, part of the turbidity (micro-crystals of cal-cium oxalates) is fixed at the surface of the sucrose crystals while macromolecules complexing calcium are very likely present as in-clusions inside the crystals (Table 2).A recent study of the localization of turbidity in crystals with sizes above 0.8 mm shows that most of turbidity is localized at the sur-face of crystals. However, after 4 washings, the turbidity was ho-mogeneously distributed inside the crystals, very likely in the form of macromolecular inclusions (Fig. 4).

Table2: Localization of turbidity in sugar crystals grown in stored turbid syrup after 2 washings

Sugar sample Mean Color Color in Turbidity aperture type solution [mm] [points]

Initial crystals 0.60 1.60 2.31 7.70After a first wash 0.52 1.60 2.04 5.40After a second wash 0.44 1.60 1.78 8.50

Fig.4: Changes in turbidity as a function of the number of wash-ings in an undersaturated sucrose solution (s = 0.95) for crystals > 0.8 mm

Fig.5: Differences in turbidity according to crystal size for a turbid white sugar

Fig.6 A: SEM picture of the surface of a single sucrose crystal before washingFig.6 B: SEM picture of the surface of a single sucrose crystal after washing in an undersaturated sucrose solution (s = 0.95)

Depending on crystal size, analysis of turbidity in fractions of sug-ar retained by screens between 0.1 and 1 mm shows that the finer the sugar crystals, the higher their turbidity (Fig. 5). The observed augmentation of turbidity for large crystals is a sign of the pres-ence of micro-crystals of calcium oxalates on the surface of the

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sucrose crystals. This is clearly evidenced by the observation of SEM (Scanning Electron Microscopy) pictures of sucrose crystals before and after washing (Fig. 6 A before and Fig. 6 B after).

3 Originofwhitesugarturbidity

White sugar turbidity is constituted of solid particles associated in some cases with macromolecules. The evidence of the solid state of turbid particles is given by the good correlation between the clogging index of a 0.45 µm membrane and the turbidity of sugar (Fig. 7). The clogging index is obtained by application of a method established by the French Sugar Union (SNFS) which defines this index as the ratio of filtrate volume difference between the start and the end of filtration to the final volume.The nature the of solid particles at the origin of turbidity was ana-lyzed using the comparison of FTIR spectra of a turbid sugar and the spectrum of calcium oxalate monohydrate (COM). This com-parison showed that turbid particles are comparable to calcium oxalate dihydrate (COD) especially as two characteristic infrared absorption bands (1643 cm–1 and 1322 cm–1) are present (Fig. 8) in both the turbid sugar and COD spectra. These COD spectra very

likely correspond to that of particles adhering at sucrose crystals surface as suggested from the SEM pictures.The presence of COD in concentrated sucrose solution was ob-served by an Australian team [5] from a comparison of the FTIR spectra (Fig. 9) of pure COD and oxalate crystals obtained in the presence of 40% sucrose in solution.

4 Reductionofturbidity

As calcium oxalates seem to be the major constituents of turbidity in white sugar, addition of anti-scaling agents in the evaporating crystallizer is sought to be the solution to prevent the precipitation of calcium oxalate crystals. This was tested in a micro-evaporating crystallizer for syrups with high turbidity [4]. Such an addition of anti-scaling agent was found to be inefficient. There was no de-crease in sugar turbidity when crystallization was achieved in pres-ence of the anti-scaling agent.Another attempt to reduce sugar turbidity consisted in syrup filtra-tion at the level of a sugar factory. Using filters with pore diam-eters varying between 1 and 25 µm, it was impossible to achieve filtration because membranes clogged rapidly. Even after dilution of syrup and filtration on a 1.3 µm membrane in the laboratory, the reduction of turbidity was only 15%. This solution consisting in

Fig.7: Relationship between membrane clogging index and sugar turbidity

Fig.8: 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 a 0.45 µm membrane filter from an aqueous solution (50% sucrose content) of turbid sugar

Fig.9: FTIR spectra of pure COD and oxalate crystals obtained in the presence of sucrose (according to Yu et al. [2004]): FTIR spec-tra of calcium oxalate obtained at initial calcium oxalate concen-tration of 130 mg/kg in sugar solution at 10% ds (A); 25% ds (B); 40% ds (C) and composites obtained from sugar solutions at initial concentration of calcium oxalate 44 mg/kg (D) and 130 mg/kg (E)

Fig.10: Relationship between calcium salt content (mg/100 g ds) and turbidity (2004/05 campaign)–––––– average; -------- upper and lower limits

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the filtration of syrup to reduce sugar turbidity seems difficult to achieve and should be rejected.As already remarked, turbidity is linked to the presence of calcium ions in the syrup. Therefore, decalcification of thin juice seems one of the best means for the reduction of white sugar turbidity. This can be deduced from Figure 10 which shows a relatively good cor-relation between thin juice calcium salt content and white sugar turbidity. For turbidity as for other problems, prevention seems more efficient than healing. In this case, decalcification of thin juice appears as a good means of turbidity prevention.

5 Conclusion

– Turbidity in white sugar seems related to the presence of cal-cium ions in the syrup as well as to high molecular mass macro-molecules, especially fractions with molar masses above 10,000 and above 300,000 amu. Turbidity is localized at the surface of sucrose crystals in the form of micro-crystals of calcium oxalates. Inside sugar crystals, complexes of calcium oxalates and macromolecules might be at the origin of turbidity. The preponderant polymorph of calcium oxalate present in turbidity particles is calcium oxalate dihydrate.

– Syrup turbidity leads to a decrease in the sucrose crystallization rate and a change in crystal morphology. The smaller the sugar crystals the more the calcium oxalate is concentrated and the turbidity is high.

– To reduce white sugar turbidity, decalcification of thin juice seems to be the most efficient procedure. Adding anti-scaling agents during the evaporation step protects the evaporator tube from scale formation, but delays the problem as the turbidity is concentrated in the syrup, especially in the case of storage. Other methods such as syrup filtration or thin juice filtration were proposed with more or less efficiency.

References1 Liggett, R.W.; Deitz, V.R. (1954): Color and turbidity of sugar products. In:

Wolfrom, M.L. (ed.): Advances in Carbohydrate Chemistry, 247–2842 Compte Rendu Réunion annuelle du G.T.S. (1963) pp E1 – E12 et F1 – F73 Godshall, M.A. (2006): The quality of white sugar required for the beverage

industry. Proceedings of the XIIIth AVH Symposium, 44–494 Cosmeur, A.; Mathlouthi, M. (1999): Etude en microcuite de l’effet de cer-

taines impuretés sur la qualité du sucre blanc de betteraves. Comptes rendus du 6ème Symposium AVH, 45–57

5 Yu, H.; Sheikholeslami, R.; Doherty, W.O.S. (2004): The effect of silica and sugar on the crystallographic and morphological properties of calcium oxalate. J. Crystal Growth 265, 592–603

Effetducalciumsurletroubledusucreblanc(Résu-mé)Le trouble du sucre est une analyse de routine dans les sucreries françaises. Elle vise à rendre compte de la présence de particules en suspension dans la solution de sucre qui diffuseraient la lumière. Cependant, la nature de ces particules n’est pas connue avec pré-cision. Il s’agit de «colloïdes» de haut poids moléculaire souvent associés à des microcristaux de sels de calcium (oxalate, sulfate, phosphate, citrate).La mesure du trouble est effectuée en même temps que la colo-ration en solution. Le choix de la longueur d’onde, du diamètre

des pores de la membrane de filtration de la solution, et même du spectrophotomètre peut influencer le résultat. Par ailleurs, les cau-ses d’apparition ou d’augmentation du trouble ne sont pas toujours expliquées par des variations de paramètres du process. Cela sem-ble être plutôt un phénomène multifactoriel dans lequel les sels de calcium jouent un rôle prépondérant.Une étude que nous avions menée, il y a quelques années sur l’évo-lution de la qualité du sirop stocké pendant l’inter-campagne nous a permis de montrer la complexité du phénomène de formation du trouble. Dans tous les cas, l’augmentation du trouble dans le sirop entraîne une augmentation du trouble dans le sucre. De même, la coloration augmente parallèlement au trouble. L’addition d’anti-tartre ou la filtration du sirop ne semblent pas avoir d’effet sur le trouble du sucre qui nécessite en plus des sels de calcium la pré-sence de macromolécules et de colorants qui favorisent sa réparti-tion dans le cristal.Dans cette présentation, nous nous proposons de rappeler les travaux antérieurs sur le trouble dans le sucre notamment ceux concernant la nature des particules le constituant. De même, nous évoquerons les problèmes d’analyse de trouble et d’insolubles et leur utilité.

Elefectodecalciosobrelaturbidezdeazúcarblanco(Resumen)En fábricas de azúcar francesas se analiza con regularidad la tur-bidez de azúcar blanco en solución. La turbidez se debe a la pre-sencia de partículas en la suspensión que, bajo las condiciones del análisis, muestran una difusión de la luz. Actualmente no se cono-ce exactamente la composición de las partículas - parece que son sustancias coloidales de alto peso molecular con microcristales de sales de calcio asociados (oxalato, sulfato, fosfato y citrato).La medida de la turbidez está acoplada a la medida del color en la solución. La selección de la longitud de onda, del diámetro de los poros de la membrana de filtración y del espectrómetro pueden influir el resultado. Entre la turbidez y las condiciones del proceso no hay una correlación simple – la turbidez más bien parece ser de origen múltiple en el que las sales de alcio tienen un papel muy importante.En la Facultad de Ciencias de la Universidad de Reims (Francia) se estudió la transformación de jarabes de remolachas almacenados con toda la complejidad del mecanismo de la formación de turbi-dez. Cuanto más turbio es el jarabe tanto más turbio también es el azúcar cristalizado (en solución). Con una mayor turbidez del jara-be también aumenta el color del azúcar. La filtración del jarabe o la adición de agentes contra incrustaciones no parecen tener mucho efecto. Las sales de calcio junto con macromoléculas y colorantes son responsables para la distribución de la turbidez dentro de los cristales.

Authors’ address: Barbara Rogé, Abdelfattah Bensouissi, Mohamed Mathlouthi, Laboratoire de Chimie Physique Indus- trielle, UMR A 614, Fractionnement des Agroressources et Emballage, INRA, Université de Reims Champagne-Ardennes, B.P. 1039, F-51687 Reims Cedex, France: e-mail: mohamed. mathlouthi@univ-reims.fr