Sucrose crystallization

Fig. 2. Normal sucrose crystal showing (a) a combination of the eight most important and frequent forms (aopp drqo), and (b) the 15 simple forms...

Cane sugar is generally available ia one of two forms crystalline solid or aqueous solution, and occasionally ia an amorphous or microcrystalline glassy form. Microcrystalline is here defined as crystals too small to show stmcture on x-ray diffraction. The melting poiat of sucrose (anhydrous) is usually stated as 186°C, although, because this property depends on the purity of the sucrose crystal, values up to 192°C have been reported. Sucrose crystallines as an anhydrous, monoclinic crystal, belonging to space group P2 (2). 

In the confectionery industry, com symps are used extensively in nearly every type of confection, ranging from hard candy to marshmallows. In hard candies, which are essentially soHd solutions of nearly pure carbohydrates, com symp contributes resistance to heat discoloration, prevents sucrose crystallization, and controls hygroscopicity, viscosity, texture, and sweetness. Maltose symps, high conversion symps, and acid-converted symps (36 and 42 DE) are used for this appHcation. 

Doucet, J. and Giddey, C., 1966. Automatic control of sucrose crystallization. International Sugar Journal, 68, 131-136. 

Mantovani, G., Vaccari, G., Accorsi, C.A., Aquilano, D. and Rubbo, M., 1983. Twin growth of sucrose crystals. Journal of Crystal Growth, 62, 595-602. 

Randolph, A.D. and Tan, C., 1978. Numerical design techniques for staged classified recycle crystallizers Examples of continuous alumina and sucrose crystallizers. Industrial and Engineering Chemistry Process Design and Development, 17(2), 189. 

Surface crustation is caused by sucrose crystallization. It is characterized by hard white spots on the surface. It is remedied by the use of corn sirup solids and larger levels of hydrocolloids. During freezing in a continuous freezer there is ice separation caused by centrifugal separation of small ice crystals. Increase in mix viscosity by use of hydrocolloids inhibits this action. 

As an attempt to quantify the effect of solvent on individual faces of sucrose crystal we have ertqrloyed a solvent accessibility of Van der Waals sized surface atoms for a spheric probe representing the solvent molecule flSI. The smooth surface generated by rolling such a probe along the crystal surface consists of... 

The carbon backbone of the sucrose structure (Figure 3) is almost completely shielded by the o gen and hydrogen atoms and is expected to play only a minor role, if any, in solvent interactions. The relative contact areas of oj gens and hydrogens are orientation dependent and may impart a specific character to the different faces of the sucrose crystal. 

Figure 4. A nine-molecule space-filling model of the (100) sucrose crystal. Atoms of one of the molecules are labeled. The sub-layer of the screw-axis related molecules is completely shielded from possible solvent effects.

Figure 5. The relative solvent accessible areas (SAA) of the five most common faces on the sucrose crystal. Solvent radius 0 A.

Even though still in a prelinainaiy stage, it is hoped that this approach will result in a better solvent - effect corrector to the attachment energy calculations (IS) than the broken hydrogen bond model and a better fit of the predicted sucrose crystal habits with the observed ones. It is already clear that the present model can, at least qualitatively, distinguish between the fast growing ri t pole of the crystal and its slow left pole. 

The face-by-face (R,a ) Isotherms on sucrose crystals growing from pure solutions allow us to determine the activation energies and, to some degree, the growth mechanisms for each of the F faces. 

Figure 5. Morphology of sucrose crystal grown in pure solution (b) and in the presence of raffinose (a) 100 projection.

Figure 6. Morphology of sucrose crystals grown in the presence of different amount of glucose ( a = 150 b 100 c = 50 d 0 grams per 100 grams of water). Dashed lines represent the initial stage.

Water molecules pull the sucrose molecules in a sucrose crystal away from one another. This pulling away from the crystal does not, however, affect the covalent bonds widiin each sucrose molecule, which is why each dissolved sucrose molecule remains intact as a single molecule. 

Figure 16.5. Supersaturation behavior, (a) Schematic plot of the Gibbs energy of a solid solute and solvent mixture at a fixed temperature. The true equilibrium compositions are given by points b and e, the limits of metastability by the inflection points c and d. For a salt-water system, point d virtually coincides with the 100% salt point e, with water contents of the order of 10-6 mol fraction with common salts, (b) Effects of supersaturation and temperature on the linear growth rate of sucrose crystals [data of Smythe (1967) analyzed by Ohara and Reid, 1973],...

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