Colourants - Crystallization

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: 

• 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 (Table).

In the case of retention, the colourants will have the same composition as the syrup colourants.  

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 bonded to glucosids, phenolic acids esterified with polysaccharides, as in CWP, 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 finalphase of crystallization. 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 the crystal if they do not diffuse quickly to the syrup. High molecular colourants, with low diffusion velocity, will be in this case. Amphyphilic colourants, with a part hydrophobic, with more tendency to dislocate to less water regions, may also be occluded in the crystal.    

Table – Integration mecanismos of colourants in sugar

 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).  
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 increasing 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).  The higher inclusion of neutral and low charge colurants were observed in tests of colourants separation by strong base anionic resins (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 14,100 IU and an IV of 6.9, it was produced a raw sugar with 2,300 IU 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.

The separation of colourants with GPC and ELS detector is a usuful tool to study sugar colourants distribution in crystal sugar. If we analyse by this method sugar colourants of raw sugar and its affined sugar (Figure 1) we observe that there is a relative increase on high molecular weights in affined sugar, compared with the raw sugar.

Figure 1

If we compare the colurants in Affined Sugar compared the ones in White Sugar (Figure 2) we observe that the ones that remains in the white sugar belongs to the group A (> 250 kD) and Group C (2.5 kD).

Figure 2

If we wash the white sugar crystal with methanol it is observed that peak A is maintained (meaning tha these compounds are in the interior of the crystal) and peak C vanish (meaning that these compoundas are in the crystal surface) (Figure 3).

Figure 3

Group A colourants will include polysaccharides and phenols associated with polysaccharides (as CWP) and their oxidation products. Group C contains colurants produced by degradation of hexoses, that I refer here as HDP (hexoses degradation products). These compounds are present in different fors of hexoses degradation: thermic (caramels), chemical (melanoidins) and alkaline (HADP). (Figure 4).

Figure 4

Bibliography
Bento L.S.M., Pereira M.E., Sa S., 1997, Gel permeation chromatography of sugar materials using
           spectrophotometric and evaporator light scattering detectors, Proc. of S.I.T. Conf., 143-160
Bento L.S.M., 2002, Separation of beet and cane sugar colourants through styrenic strong base resins,
Proc. of S.P.R.I. Conf., 311-327
Paton N.H., 1992, The origin of colour in rawsugar, Proc. of Aust. S.S.C.T. Conf., 8-17
Tu J.C., A. Kondo, E. Sloane, 1977, The role of high and low molecular weight colorants in sugar color,
Proc. of I.S.S.C.T. Conf., Maufect., 1393-1400