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Crystal half layer	7 + 8		7		7
				9
			4		1
		5	3		2
		6
		Step		7

Figure 10: Diagrammatic representation of the processes involved in crystal growth from solution, according to Elwell and Scheel ,(1975) as modified by Kruse and Ulrich, (1993).

The description of Figure 10 can be summed up as follows: 1 Diffusion of hydrated crystal building blocks from the solution to the crystal surface. 2 Adsorption of hydrated crystal building blocks (cluster) and partial dehydration on the crystal surface. 3 Surface diffusion of hydrated crystal building blocks to a step. 4 Adsorption of hydrated crystal building blocks and partial dehydration on the step. 5 Diffusion of hydrated crystal building blocks along a step to a semi-crystal position. 6 Insertion of crystal building blocks into a semi-crystal position and complete dehydration; 7 Diffusion of the liberated hydrate envelope into solution. 8 Liberation of latent heat on insertion of crystal building blocks into a semi-crystal position. 9 Desorption of hydrated crystal building blocks from the crystal surface into the solution.

2.6.3Parameters influencing crystallization kinetics

Whatever the origin, the polysaccharides from beet processing are found as minor constituents in white sugar and their association with nitrogen and cations can cause the coloration of the crystals. These polysaccharides generally have a high hygroscopicity and their presence at the surface of sugar crystals my affect the stability of crystalline sugar during storage and provoke caking. Moreover, occlusion and crystal elongation (c-axis) are observed in presence of dextran (Gudshall, 1992;

Parrisch and Clarke, 1983).

During processing, macromolecules (e.g. dextran) increase viscosity, slow or inhibit crystallization, and increase the loss of sucrose to molasses (i.e. they have a high melassigenic effect). Because of their carbohydrate nature and high solubility, they are difficult to remove in processing, and tend to be included in the raw sugar crystal,

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going into the refining process and causing similar problems in refining (Cuddihy et al., 2000; Godshall et al., 1994).

The viscosity is a dominating factor in the technology of the process. The growth rate is sensitive to variation in crystal size and slightly influenced by surface integration, with the diffusion step in crystal growth as a significant factor in determining the overall rates (Gumaraes et al., 1995). Frenkel, (1958) has found that the absolute viscosity of sugar solutions is related to the absolute temperature by an equation of the form:

η = A*10B / T

(2-9)

where:

ηDynamic viscosity

A and B	Constants
T	Temperature

Figure 11 shows the relationship between the viscosity in pure and impure solution and solution concentration at different temperatures (40 to 80°C) according to Schliephake and Ekelhof, (1983). They illustrated that the solution viscosity increased with a decrease of temperatures. However, it increased with the increase of solution concentration. These correlations between relative viscosity ηu /ηr and other factors were described as follows:

	nu		1− q	(4.734	−0.0379ϑ)	wDS		(2-10)
		=1−
ηr
			0.4			19 −18wDS

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η mPa.s

1 0 0 0

1 0 0

10

1

0	2 0	4 0	6 0	8 0
		W D S %

Figure 11: The viscosity of the pure and impure sucrose solution according to Ekelhof, (1997)

2.6.4Crystal morphology

The normal morphology shown by sucrose crystals grown from pure solution is depicted in Figure 12 (Vavrinecz, 1965). The three primary hydroxyl groups at the C- 1' and C-6' (Fructose) and the C-6 (glucose) belong to the three hydroxymethyl side's groups of the sucrose. The others are secondary hydroxyl groups at the heterocyclic ring. The average values of the surface angles for proportional axis were measured by different investigators as, a : b : c = 1.2543 : 1 : 0.8878 with an angle of the a to the c axis of β = 103°30 (Vavrinecz, 1965). So far 15 different surfaces were observed, which arise most frequently with the technical sucrose crystallization. Under the influences of non-sugar materials and other factors, which affect on the growth conditions such as e.g. temperature and supersaturation of solution, the surface growth and thus the habit of the sucrose crystal can vary strongly (Bubnik et al., 1992; Vaccari et al., 1999; Vaccari et al., 2002). The presence of oligosaccharides such as raffinose, which is present in sugar beets, causes an elongation of the crystal toward the b-axis to a needle form (Mantovani et al., 1967; Vaccari et al., 1986). Dextran also causes an elongation of the crystal toward the c axis due to slowing down the growth of the p and p’ surfaces (Sutherland and Paton, 1969).

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Figure 12: Schematic representation of a sucrose crystal with the surface designation according to Vavrinecz ( 1965)

Bubnik et al., (1992) studied the relationship between the c/b ratio if dextran concentration was increased (Figure 13). They observed that the effect of dextran on crystal elongation was much more striking compared to that obtained with invert sugar and 300 % higher than for pure solution, at the highest impurity concentration. This correlation between c/b and dextran concentration was described as follows:

	R	= R + a	o	c0.1	+ b c0.7		(2-11)
	d	s		d	o	d
where:
Rd	c/b ratio in the presence of dextran
Rs	c/b ratio for pure sucrose solution (= 0.70)
cd	Dextran concentration (g/100 g water)
ao	Constant (0.1363)
bo	Constant (0.1234)

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+	+
C	C

+b

+b

(A)

(A)

(A)

Figure 13: (A) Sucrose crystal growth in the presence of dextran (40g/100g water), (B) One of the crystals shown in (A) and (C) Sucrose crystal growth in the presence of glucose, fructose and dextran (50g/100g water each) (According to Bubnik et al., (1992)

Also, it was possible to calculate a two-parameter function which would permit calculation of the c/b ratio as a function of both invert sugar and dextran concentrations:

					R = R + A c0.1				+ B c0.7	(2-12)
							i	d	d
R		c/b ratio in the presence of impurities
Ri		c/b ratio in the presence of invert sugar
cd		Dextran concentration (g/100g water)
with
A = a	o	+ a	+c	i	+ a	2	c0.7
		1					i
ao		Constant (0.1363)
a1		Constant (0.102)

a2 Constant (-0.00193)

c Invert sugar concentration with

B = bo +bi * ciC

bo	Constant (0.1234)
b1	Constant (-0.0147)
C	Constant (0.270)
ci	Invert sugar concentration