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Crystallization - Sucrose and Water Molecules

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By Bento, Luis San Miguel
Posted on 2011-01-19    Last edited on 2011-01-19

Introduction
Sucrose molecule has eight hydroxyls that can form hydrogen bonds with water. At high concentrations three hydroxyls are involved in intramolecular hydrogen bonds. At these conditions only five sucrose hydroxyls are completely available to form hydrogen bonds with water. Saturated sucrose solutions, between 81.9 oC and 11.7 oC, contains 5 to 10 water molecules per sucrose molecule, respectively (Fig.1). At these concentrations and in presence of a hydrophilic compounds as sucrose, water is highly structured (Fennema). As in saturated solutions all water molecules are directly or indirectly bonded to sucrose molecules, the possible distribution of water molecules, in mono, dimer and trimers, can be determined. This distribution, for saturated sucrose solutions, is presented in Table 1 (Bento, 2009).

From sucrose molecules to nuclei
In order to form hydrogen bonds between sucrose, hydroxyls must be dewatered (Mathlouthi, 2008). Increasing saturation coefficient, y, to values higher than 1, by evaporation and/or by cooling, sucrose hydroxyls become dewatered. Consequently, sucrose form intermolecular hydrogen bonds and sucrose aggregates will be formed. Hydrogen bonds involved in inter sucrose bonds are listed in Table 2 (Perez S.,1995).  The three first hydrogen bonds, with lower energy, are involved in inter sucrose hydrogen bonds, in the pathway from sucrose to nuclei here proposed. In these hydrogen bonds two hydroxyls are common to two bonds: C3(f) in bonds type 3 and 2, and C3(g) in bonds 2 and 1. This fact makes the sucrose aggregation simpler and can explain the easiness of sucrose crystallization in comparison with other sugars.

The pathway from sucrose molecules to nuclei, proposed here, is presented in five steps.
In the first step,  a sucrose dimer is formed using hydrogen bond type 3 (Table 1, Fig.3). One hydroxyl per sucrose molecule is dewatered (n = 1). Variation of sucrose concentration with temperature at n =1 was calculated using equation (3) (Table 1).This equation was derived from the variation of water dimers in the saturated solution. The molar fraction of water associated in dimers in relation to total water, Dmf,  at different temperatures in a saturated solution is given in Table 1 (Bento, 2009). Due to the formation of inter sucrose hydrogen bonds, the number of hydroxyls bonded to water decreases and distribution of water molecules by these hydroxyls change, compared with distribution at saturation. Determining the Dmf of the solution after the sucrose-sucrose bond, the solution temperature, at a given value of n, can be calculated with equation (1), assuming that the average distribution of water molecules in mono, dimer and trimer form, in concentrated sucrose solutions, statistically do not vary with temperature.

Results are presented in Table (1) and Fig. 1. Saturation coefficients, y, at n = 1, were calculated and their variation with temperature are presented in Table 1, equation (4) and in Fig. 2. These same calculations were executed at other values of n and results presented in the same way.
 In the second step, for nuclei formation, two dimmers are associate by dewatering one hydroxyl and using hydroxyl bond type 2. A tetramer is formed and the value of n is 1.25. Hydrogen bonds in tetramer are involved in crystal periodic bond chain, PBC, type –3-2-3-2-, forming face (011).
In the third step, two tetramers are bond by dewatering one hydroxyl and using hydrogen bond type 1 (Table 2) forming a octamer (Fig.3). Value of n in this step is 1.375.The new bond is involved in PBC type–1-2-1-2- forming faces (100) (1-10) (110) and PBC type –3-1-3-1- building the faces (101) (1-11) (111). The octamer contains six molecules, a hexamer, described by  Kelly and Mak, 1975, that represents the minimum aggregate of sucrose molecules needed to form a basic unit of crystal (Mathlouthi, 2008).
In the fourth step, here proposed, octomers will bond by hydrogen bonds type 1 or 2 using the dewatered hydroxyls as indicated by the arrows in Fig.3.To form these bonds the complementary hydroxyl must be dewatered: C3(g) or C6(g) hydroxyl for bonds (a) or (b) respectively (Fig.3). Octomers aggregation starts with only one extra dewatered hydroxyl, for bond (a) or (b). At this step 12 hydroxyls are dewatered per octamer, that is, n = 1.5.
In the fifth step, increasing hydroxyls dewatering beyond n = 1.5, octomers aggregates will be bonded by more hydrogen bonds. One hypothesis is the dewatering of C3(g) and C6(g), (c) and (d) in Fig.3, allowing the formation of hydrogen bond type 1 between two octomers aggregates. This bond, type 1, in conjunction of bond (1) used in tetramers, is part of the bond chain of two fold screw axis parallel to b axis (Perez, 1995).

Considering an aggregation of four octomers in step 4, and further three aggregations in step 5, the final aggregate will contain 96 sucrose molecules and the critical size is reached. Therefore, nuclei are formed. Van Hook, 1961, consider a critical size as 80 – 100 molecules. Therefore, sucrose concentration equivalent to n = 1.5 will correspond to the limit of the Labile zone. In Fig 4 is presented is presented the variation of sucrose concentration with temperature, at different values of n. In the same graphic are presented experimental values obtained by Gharsallaoui et al., 2008, for the Labile zone limit using a saturoscopic method. The results obtained in this study are in agreement with the experimental results.


Conclusions
Nucleation is an important step in sugar crystallization. Studying the water-sucrose molecules interaction in this stage it will be possible to determine parameters as saturated coefficients and labile zone limit at different temperatures. These parameters are important for the control of both nucleation and crystallization.
The knowledge of the steps involved in nucleation may be helpful to make a better control during this phase of crystallization.

References
Bento L.S.M., 2010, Sucrose Crystallization - Influence of water molecules, S.I.T.
          Conference, 2010
;
Bento L.S.M., Sugar Industry / Zuckerindustrie 134 (2009) No 12. 743-746
Fennema O., Food Chemistry, CRC Press, 4th ed.;
Gharsallaoui A.  et al.., Food Chemistry, 106 (2008) 1329 – 1339;
Mathlouthi M., Genotelle J., Carbohydrate Polymers, 37 (1998) 335 – 342;
Perez S., 1995, Sucrose Properties and Applications, Blackie Ac. Press, 11-32;
Van Hook, Crystallization: Theory and Practice, Reinhold Pub.Corp.,Chapman &
         Hall Ltd

 

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