1 *University of Ferrara - Ferrara, Italy **University of Genoa - Genoa, ltaly Key words: Crystal growth, habit modification

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1 Processing I SUGAR,CRYSTAL MORPHOLOGY * >{+,"!>"{*k',( G. Vaccari*, G. Mantovani*, G. Sgualdino* and L. Zefiro** 1 *University of Ferrara - Ferrara, Italy **University of Genoa - Genoa, ltaly Key words: Crystal growth, habit modification I ABSTRACT Since the special morphology of cane sugar crystals is caused by the effects of many impurities, the influences of glucose and fructose have been considered. Sucrose crystals were grown from solutions in the presence of glucose and/or fructose in conditions as similar as possible to those found in cane processing. In the presence of sufficiently high concentrations of glucose a total blocking of the p faces is particularly noticeable, whereas in the presence of similar amounts of fructose a marked slowing down of the growth of the right pole of the crystal can be seen. In particular, the growth of the 0' and q' faces occurs and crystals showing the characteristic D shape observed in cane processing are obtained. INTRODUCTION After research had been carried out on the morphology of cane sugar crystals originating from. some sugar factories in South Africa (Vaccari"), it was decided to investigate the causes of drastic morphological differences when comparisons are made with sugar crystals grown in pure solution. The changes concern: 1) the elongation of crystals along the c-axis, 2) the appearance of the less common faces on the crystal right pole, and 3) the appearance of the characteristic D shape of crystals (see Fig 1). Concerning the last point, it is important to note that the round portion of the D shape concerns the right pole rather than the left one. Such particular morphology is to be ascribed to the presence of impurities in industrial cane sugar solutions. To explain which substances could be responsible for this morphological modification, the investigation began with glucose and fructose, which are present in considerable amounts in sugar cane factory liquors. Although the effects of these monosaccharides have been considered previously (Kelly3, Mantovani4s5, Smythe8, VanHo~k'~-'~, VavrineczI5), the concentrations have not always been comparable with those encountered during cane sugar processing. Because glucose and fructose are present in relatively high amounts in sugar cane factory liquors, and particularly low grade niassecuites, concentrations between 10 and 150 grams per 100 grams of water were used in the experiments. Crystallization was carried out at 500C both to simulate processing conditions as closely as possible, and to take into account the high viscosity of solutions with high contents of invert sugar. The supersaturation of the growth solutions was not specified owing to the influence of high amounts of glucose and fructose on sucrose solubility, but it was maintained within limits which avoided secondary nuclation. Crystals used as seeds were placed in containers (50 ml) which were filled with the growth solution. The containers were rotated in a thermostatic bath so that the crystals could float freely. Variations in morphology of the crystals were then observed under the from the solution, or observing them directly in the RESULTS AND DISCUS To help explain which parts of sugar crystals were involve lowing down of growth

2 PROCESSING Figure 1. Characteristic D shape shown by cane sucrose crystals grown in low purity massecuites in the presence of the impurities, small coloured sugar crystals were prepared for use as seed. These were grown in solutions containing colourants of industrial origin under conditions that promoted the inclusion of coloured mother liquor droplets, as reported elsewhere (Mantovani et UP) The seed was used to grow sugar crystals in the presence of amounts of glucose from zero to 150 grams per 100 grams of water. Some of these crystals are shown in Fig 2. The view is along the a-axis. The nearly total blocking of growth along the-b-axis when the concentration of impurity is greatest can be seen quite clearly. This effect becomes less as the glucose concentration decreases. On the other hand, the advancement of the +b end with respect to the coloured seed, becomes less evident as the glucose concentration increases. This effect has been noted by Kelly3, quoting the data of VavrineczI5, and it obviously contributes to the crystal elongation along the c-axis. In Fig 3a the crystal shown in Fig 2a can be seen along the c-axis, confirming the total blocking of growth relative to the crystal left pole. This phenomenon particularly involves the p faces, thus reducing the area of the o and q faces, as clearly shown in Fig 3b. Holographic interferometry has been used successfully to study the effect of raffinose on sucrose crystal morphology (Bedarida et UP). This technique was used to confirm the observation described above. The total absence of interference fringes on the left pole of the crystal shown in the hologram of Fig 4 illustrates this phenomenon.

3 I G. VACCARI, G. MANTOVANI, G. SGUALDINO AND L. ZEFIRO Figure 2. Sucrose crystals grown in the presence of different amounts of glucose: a 150 g glucose per 100 g of water b 100 g glucose per 100 g of water c 50 g glucose per 100 g of water d - 0 g glucose per 100 g of water The dashed white lines drawn at the left of the crystals permit a better evaluation to be made of the different growth of the left end of the crystal with regard to the coloured seed. I Figure 3. The crystal shown in Fig 2 a (150 g glucose per 100 g of water): a - crystal seen along the c-axis b - crystal showing the limited sizes of the o and q faces

4 PROCESSING Figure 4. lntederometric hologram of the crystal shown in Fig 20 (150 g glucose per 100 g of water) Fructose was not observed to cause total blocking of one pole or the other of the crystal, but a considerable slowing down of the o' and q' faces on the right pole was observed, which is not normal. The great extension of the 0' and q' faces in the presence of 100 grams of fructose per 100 grams of water is shown in Fig 5. To confirm this effect on the right pole of the crystal, twins of the first type were grown in the presence of various amounts of fructose. Twins of the first type have their left poles turned towards each other while the right poles point outwards (Aquilano et all, Mantovani et ai6, Vaccari et ai9). Twins grown in the presence of different amounts of fructose, from 10 to 150 grams per 100 grams of water, are shown in Fig 6. A crystal (a), grown in the presence of 150 grams of fructose per 100 grams of water, confirms the slowing down of the o' and q' faces. This effect becomes progressively less evident as the concentration of fructose in the growth solution decreases, ie crystals (b) and (c), and disappears when the impurity concentration is 10 grams or less per 100 grams of water, ie crystal (d). By comparing the four crystals shown in Fig 6 it can be inferred that large amounts of fructose cause a certain slowing down of the p' faces because the c/b ratio of the twins tends to increase with the increase of the fructose concentration in the growth solution, as has been pointed out by Kelly3, quoting Vavrine~z'~. When the fructose concentration is increased above 10 grams per 100 grams of water, the slowing down of growth is evident on the q' face before the of face is involved. In the presence of 25 grams of fructose per 100 grams of water, only the q' face appears, as can be seen in Fig 7. Above 50 grams per 100 grams of water, both the of and q' faces appear, as can be seen in the sequence of crystals shown in Fig 8. The same twins as shown in Fig 6 are present, but viewed along the-c-axis. However, when the concentration of fructose increases, the size of the 0' face becomes progressively wider in comparison with the size of the q' face.

5 -- - G. VACCARI, G. MANTOVANI, G. SGUALDINO AND L. ZEFIRO Figure 5. Appearance of the o' and q' faces om the right end of a sucrose crystal grown in the presence of fructose, 100 g per 100 g of water 67

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7 G. VACCARI, G. MANTOVANI, G. SGUALDINO AND L. ZEFIRO Figure 8. The same twins as shown in Fig 6, but seen along the c-axis The results obtained by growing sucrose crystals in the presence of glucose and fructose separately, led to a study of their growth in the presence of both monosaccharides. Fig 9 shows crystals grown in the presence of glucose and fructose, each in a concentration of 100 grams per 100 grams of water. The concurrent effects of the two monosaccharides are clear in Fig 9A, where the crystal tends to assume the characteristic D shape. In Fig 9B, seen along the +b-axis, the effect of fructose in promoting the appearance of the 0' and q' faces is clear, whereas in Fig 9C, seen along the -b-axis, the effects of glucose in causing the disappearance of the o face, and causing the q face to have a very limited area, are evident. CONCLUSIONS The results lead to the following conclusions: e the concurrent effects of slowing down both the +b and -b poles, caused by the presence of significant amounts of glucose and fructose in the growth solution, can contribute to the elongation of cane sucrose crystals along the c-axis the blocking of the growth of p faces in the presence of high amounts of glucose promotes the disappearance of the o and q faces on the left end of the crystal the appearance of the o' and q' faces on the right end of the crystal in the presence of high amounts of fructose promotes the rounding of the right end the combined action of glucose and fructose can be assumed to be responsible for the growing of cane sucrose crystals showing the characteristic D shape (Fig I), which can be found particularly in low purity massecuites. Although other impurities present in cane processing may also be responsible for this phenomenon, it is clear that the two monosaccharides considered here are important. Following the theory of sucrose crystal structure proposed by Hartmann and Perdok (Aquilano'), it is intended to examine carefully the possible interference which molecules of the two monosaccharides can exert on the various faces of sucrose crystals, as has been done previously with raffinose (Vaccari eta1 "). In this way the experimental observations may be explained.

8 PROCESSING Figure 9. Sucrose crystals grown in the presence of glucose (100 g per 100 g of water) and fructose (100 g per 100 g of water): A - Crystal, seen along the a-axis, showing the characteristic D-shape B - Crystal seen along the +b-axis C - Crystal seen along the -b-axis REFERENCES 1. Aquilano, D, Franchini-Angela, M, Rubbo, M, Mantovani, G and Vaccari, G (1983). Growth morphology of polar crystals: a comparison between theory and experiment in sucrose. J Crystal Growth 61: Bedarida, F, Zefiro, L, Boccacci, P, Aquilano, D, Rubbo, M, Vaccari, G, Mantovani,

9 G. VACCARI, G. MANTOVANI, G. SGUALDINO AND L. ZEFIRO G and Sgualdino, G (1988). Growth of sucrose crystals and tapering effect studied by holographic interferometry. J Crystal Growth 89: Kelly, FHC and Mak, FK (1975). The sucrose crystal, Singapore University Press, Singapore, 167p. 4. Mantovani, G and Fagioli, F (1965). Modifikationen des habitus von saccharosekristallen in gegenwart einiger nichtsaccharosestoffe, Zuckerindustrie 15: Mantovani, G, Gilli, G and Fagioli, F (1967). Einfluss der kristallstruktur von nichtzuckerstoffen auf die anderung des habitus von saccharose, Zucker 20: Mantovani, G, Vaccari, G, Accorsi, CA, Aquilano, D and Rubbo, M (1983). Twin growth of sucrose crystals, J Crystal Growth 62: Mantovani, G, Vaccari, G, Sgualdino, G, Aquilano, D and Rubbo, M (1986). Colouring matter inclusions in sucrose crystals. Proc int Soc Sug Cane Technol 19: Smythe, BM (1971) Sucrose crystal growth. Sug Technol Rev 1: Vaccari, G, Accorsi, CA and Mantovani, G (1982). Twinning and crystal growth, Sug J 44(11): Vaccari, G, Mantovani, G, Morel du Boil, PG and Lionnet, GRE (1989). Cane sucrose crystal colour inclusions and habit modification. Proc int Soc Sug Cane Technol 20 (in press). 11. Vaccari, G, Mantovani, G, Sgualdino, G, Aquilano, D and Rubbo, M (1986). The raffinose effect on sucrose morphology and kinetics. Sug Technol Rev 13: VanHook, A (1961) Crystallization. Reinhold Publication, London, 245 p. 13. VanHook, A (1967). Sull'influenza dl impurezze adsorbite sulla velocita di accrescimento e sull'habitus di cristalli di saccarosio, Ind Sacc ltal 60: VanHook, A (1988). A note on the hemimorphism of sucrose. Ind Sacc ltal 81: Vavrinecz, G (1965). Atlas der Zuckerkristalle, verlag Dr A Bartens, Berlin. EFFETS DU GLUCOSE ET DU FRUCTOSE SUR LA MORPHOLOGIE DES CRISTAUX DE SUCRE DE CANNES G. Vaccari*, G. Mantovani*, G. Sgualdino* et L.Zefiro** "Universit.4 de Ferrara - Ferrara, ltalie **Universite de Genes - GBnes, ltalie Mots clefs: Accroissement des cristaux, modification de I'habitude Etant convaincus que la morphologie speciale des cristaux de sucre de cannes est originee par I'effet concomitant de Inaction de maintes impuretes, les auteurs prennent en consideration I'influence du glucose et du fructose. Des cristaux de saccharose ont Bte formes a partir des solutions en presence du glucose et/or du fructose dans des conditions les plus similaires possibles 6 celles rencontrees dans le traitement de la canne. En presence des concentrations suffisarnment fortes de glucose, on observe particulierement un blocage total des pans p, tandis qu'en presence des quantites analogues de fructose, on peut noter un effet macroscopique de ralentissement du p81e droit du cristal. En particulier, l'accroissement des pans 0' y q' se produit, obtenant ainsi des cristaux avec la forme D caractbristique observee dans le traitement des cannes.

10 EFECTOS DE LA GLUCOSA Y DE LA FRUCTOSA SOBRE LA MORFOLOG~A DE LOS CRISTALES DE AZ~CAR DE CANA G. Vaccari*, G. Mantovani*, G. Sgualdino* y L.Zefiro** *Universidad de Ferrara - Ferrara, ltalia **Universidad de G6nova - Genova, ltalia PROCESSING Palabras claves: crecimiento de 10s cristales, modificaci6n del habito RESUMEN Estando convencidos de que la morfologia especial del cristal de azljcar de catia es causada por el efecto concornitante de la accibn de muchas irnpurezas, 10s autores toman en consideracibn la influencia de la glucosa y de la fructosa. Se desarrollaron cristales de sacarosa a partir de soluciones en presencia de glucosa y/o fructosa en condiciones lo rn6s sirnilares posibles a las halladas en la elaboraci6n de la catia. En presencia de concentraciones suficienternente elevadas de glucosa, se observa particularrnente un bloqueo total de las caras p, en tanto en presencia de cantidades andlogas de fructosa, puede observarse un efecto rnacrosc6pico de desaceleracibn del polo derecho del cristal. En particular, el crecirniento de las caras o' y q' se produce, obteniendo asi cristales que rnuestran la forrna D caracteristicas observada en la elaboraci6n de la catia.