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Crystallization - Cane Sugar Mill

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By Bento, Luis San Miguel
Posted on 2006-11-30    Last edited on 2009-09-24

Syrup resulting from clarified juice concentration is crystallized under vacuum. This operation is done in vacuum pans, single effect evaporators, normally with agitation. Sugar is produced in a system of three crystallization steps (boiling) in series.
Massecuite resulting from first boiling (A) origins, by centrifugation, a sugar, named raw sugar. This sugar is the final product of sugar factories and is refineries raw material.
Resulting syrup from this centrifugation (A) is crystallized in a second boiling (B), producing a massecuite B, a sugar B and a syrup B. Sugar B is mixed with syrup A and will be used as footing for A boiling. Therefore in this boiling B sugar crystals are growing, by adding syrup to the pan. No crystal formation is then necessary.

Syrup B is crystallized in the third boiling (C). Resulting massecuite (C) is cooled in cooling crystallizers where crystallization proceeds. At the same time sucrose content in syrup decreases. Massecuite after cooling crystallization is heated before continual centrifugation.
C syrup obtained from this operation, due to its minimal content of sucrose, is separated for commercialization, being a sub-product of cane sugar factories, and is named molasses.
Resulting sugar (C) is mixed with B syrup and is used as footing for B boiling. C sugar can also be melted and resulting liquor mixed to syrup coming from evaporation. 


Three boiling crystallization scheme

Crystallization is an efficient separation system between sucrose and impurities in solution. Among these impurities are sugar colourants that came in the syrup. However certain colourants are not efficiently separated and stay preferentially in the crystal.

The integration of colourants in sugar crystals can occur in different ways (Figure 28):
-     by inclusion in the sucrose crystalline mesh;
-     by occlusion on the crystal surface layer;
-     by retention of syrup between crystals inconglomerates.

The composition of colourants integrated into sugar crystals vary according to the mechanism of integration that has occurred.
In the case of retention, the colourants will have the same composition as the syrup colourants. This case will be not included in this study as conglomerates occur due to bad crystallization practices. We will consider only the production of perfect crystals without conglomerates.

Integration of colourants in sugar

Inclusion of colourants in crystals may occur in the crystal formation and growth phases. In these phases, colourants with higher chemical affinities to sucrose, or colourants associated with polysaccharides, will be prone to be included into crystals.  In these cases are the phenols bond to glucosids, phenolic acids esterified with polysaccharides, as in ISP, and the association phenols/polysaccharides. The products of enzymatic browning involving these compounds are also prone to inclusion.

 Occlusion of colourants in crystals may occur in the crystallization final phase. In this phase massecuite brix is increased, before pan discharge, in order to increase boiling yield. Due to this, sobresaturation and crystallization velocity increase. In these conditions, molecules in the vicinity of crystal growing zones may be integrated in crystal mesh if they do not diffuse quickly to the syrup. colourants will be trapped in the crystal surface layer. High molecular colourants, with low diffusion velocity, will be in this case. Amphyphilic colourants, with a part hydrophobic, with more propensity to dislocate to less water regions, may also be occluded in the crystal.    

Integration mecanismos of colourants in sugar

Kind of integration integração
Mechanism
Colourant
Inclusion
Chemical affinity
ISP
Polysaccharides - phenols
Enzymatic browning products mechanismnzimática
Glucophenols
Occlusion
Diffusion
High molecular weight  colourants bcolcnnnncolocolourantsCorantes de alto peso molecular
Neutral phenols
Retention
Malfunction
All



In order to explain colourants integration into crystals, we will define the affinity coefficient as the quotient between sugar colour and the colour of liquor or syrup from where sugar was crystallized:
Affinity coefficient = Sugar colour / Liquor or syrup colour

Some authors refer to the partition coefficient as the inverse of the affinity coefficient (Paton, 1992). Therefore, a high partition coefficient corresponds to low colourants integration into crystals. The inverse value, as the affinity coefficient, will turn clearer the explanation that follows.

In crystallization studies, it was observed that HADP and melanoidins present a higher affinity coefficient than cane juice natural colourants (Paton, 1992). Phenolic colourants can be ordered by rising affinity coefficients as follows: phenolic acids, as chlorogenic acid, then flavonoids, and finally, neutral phenolics. Colourants originated from enzymatic browning reactions present a high affinity coefficient (Paton, 1992). In the referred experiments it was observed that 18% of juice neutral phenolics were integrated in sugar crystals (23ppm from 130ppm) and only 8% of cinnamic acids enter the crystals (9ppm from 110ppm). In the case of chlorogenic acid, this value is 10% (16ppm from 160ppm) (Paton, 1992). The higher inclusion of neutral and low charge colurants were observed in tests of colourants separation by strong base anionic resins and UIV (Bento, 2002).

The fact that cinnamic acids present a low affinity coefficient in relation to neutral phenols and enzymatic browning products, explains the IV value decrease between clarified juice and sugar obtained from it. From a juice with a colour of 14100IU and an IV of 6.9, it was produced a raw sugar with 2300IU and a IV of 4.6 (Paton, 1992). Affinity coefficient was, in this case of 0.16.

Enzymatic browning colourants can polymerize, forming high molecular and high coloured compounds, with high affinity coefficient for crystals. Tu et al., 1977, showed that although  high molecular colourants are present in a lower quantity in juices, they have a high influence in crystal colour. Low molecular compounds are not removed during processing but present a low affinity coefficient.

In Crystallization section colour formation can occur during boilings or during syrups and massecuites storage. Munday et al., 1968, report colour formation higher than 30% in storage of C massecuites. This colour increase is mainly due to Maillard reaction compounds.

Bibliography

Bento L.S.M., 2002, Separation of beet and cane colourants through styrenic strong base resins, Proc. of
          S.P.R.I. Conf.
, 311-327
Munday B.M., I.R. Bugess, R.V. Ames, C.W. Davis, 1968, Colour in rawsugar manufacture, Proc. of
          I.S.S.C.T. Conf
., 395-404
Paton N.H., 1992, The origin of colour in raw sugar, Proc. of. Aust.S.S.C.T. Conf, 8-17

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