High Molecular Weight Colourants - (Godshall M. A.)

The molecular weight of colorant was not considered very much in the earlier literature, as the techniques for determining such were not well developed. The focus of this article will be on the colorants with a high molecular weight.

What constitutes a high molecular weight? Different authors have had different interpretations, ranging from as low as 1000 Daltons (Da) to about 10,000 or 20,000 Da for the starting point to define an HMW colorant.

At Sugar Processing Research Institute (SPRI), we have defined high molecular weight colorant to begin at around 10-12,000 Da, as obtained by dialysis through reconstituted cellulose bags with a 10-12,000 Da molecular weight cut-off.

In a seminal work by C. C. Tu and co-workers in 1977, two important observations were made: (1) the majority of colorants found in raw sugar crystals were those originally present in the cane juice and not formed during processing and (2) lower MW colorants tended to be excluded from the crystal, whereas HMW colorants preferentially remained inside the crystal. (Tu, et al, 1977) Tu described HMW colorants as above 5,000 Da.

DIFFERENCE BETWEEN CANE AND BEET HMW COLORANT

Significant differences exist between cane and beet HMW colorant, as demonstrated in Figures 1 and 2 showing gel permeation profiles of colorant. It is possible to compare beet thick juice and cane clarified juice because the colorants in cane undergo little change between clarified juice and evaporator syrup, the equivalent of beet thick juice.

Comparing the profile of cane colorants to beet colorants, the beet colorant profile shows much less color intensity than that of cane (indicating less color is present); and the molecular weight range of the beet colorants is lower over-all. Note that the higher MW colorants elute first from the column. Compared to beet colorant, the major cane colorants are higher MW than those of beet and also higher in intensity. (Godshall and McKee, 2004)

Figure 1. Colorants in beet thick juice.


Figure 2. Colorants in cane clarified juice.

BEET COLORANT IN PROCESSING

Figure 3 shows the progression of beet colorant during processing. The diagram tracks HMW material (≥12,000 Da) in beet processing samples showing the changes that occur in colorant during processing. The red line is HMW from raw beet juice: This shows a very HMW peak that elutes early. This is most likely the pectin material that is in beet juice. Thin juice (the blue line) shows that the pectinaceous material has been removed and some HMW material is becoming evident. The green line represents thick juice, which shows development of more HMW colorant. The navy blue line shows standard liquor, which represents several sources of input, and can differ slightly from thick juice. The lavender line represents HMW occluded in the white sugar. This shows that, while much of the colorant has been removed by crystallization, the material remaining in the crystal is proportionately higher MW than color that was removed. (Note: Higher MW peaks elute first.) (Godshall, et al., 2002)

Figure 3. HMW beet colorant changes during processing.

It is therefore concluded that the majority of HMW colorant in beet processing is developed as a result of the processing colorant, and is process-derived colorant, such as alkaline degradation products, melanoidins, and melanins.

CANE COLORANT IN PROCESSING

In contrast to beet colorants, cane colorants do not change very much during processing. Studies indicate that this is because most of the colorant is associated with polysaccharides, which also causes the colorant to be very high MW. The chromatogram in Figure 4 demonstrates the HMW colorant in cane juice. Comparing this to the profile of raw sugar colorant (Figure 5) one notes that the colorant does not change significantly during processing. Basically, the major HMW colorants in cane come into the process with the juice and remain little changed throughout. The refined sugar profile (Figure 5) illustrates how the HMW colorants remain preferentially in the refined sugar, although at a much lesser intensity of color. (Godshall, et al, 2002)

Figure 4. HMW colorant of raw cane juice.

Figure 5. Chromatograms of HMW colorant in raw sugar and refined sugar.

It is also of interest to note that much of the color in a raw cane sugar solution can be precipitated by bringing the solution to an 80% ethanol concentration, as shown in Figure 6. By contrast, beet sugar colorant is not precipitated by 80% ethanol, indicating a lower MW as well as a different chemical composition (no polysaccharide present).

80% alcohol removes all HMW materials (polysaccharides and color)
Note how little color remains.
Figure 6. Alcohol precipitation of colorant in raw sugar.

In summary, several basic observations, shown in the box below, can be made about high molecular weight (HMW) colorant.

Bibliography
Godshall, M.A., and McKee, M., 2004, The effect of ozone, hydrogen peroxide and sulfite on cane and beet
macromolecules, Proc. Conf. Sugar Processing Research, 2004, pp 111-127.
Godshall, M.A., Vercellotti, J.R., and Triche, R., 2002, Comparison of cane and beet macromolecules in
processing, International Sugar Journal, 2002, Vol. 104, pp. 228-233.
Tu, C.C., Kondo, A., and Sloan, G., 1977, The role of high and low molecular weight colorants in sugar
color,1977, The Sugar Journal, Vol. 40, pp.9-12.

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