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Colourants - (Godshall, M. A.)

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By Godshall, Mary An
Posted on 2008-02-21    Last edited on 2009-09-19

COLORANTS
Mary An Godshall, Sugar Processing Research Institute, Inc.

In one of the earliest reviews on color, in 1942, Zerban discussed the recognized sources of “dark coloring matters” formed during sugar processing (Zerban, 1942).  To a large extent, these categories are still valid today.  Zerban’s categories are the following:

Caramel substances, free from nitrogen and produced by overheating sucrose, glucose or fructose.  (Alkaline degradation products of fructose and glucose was added as a subset of thermal degradation color in more recent years.)

Melanoidins, derived from condensation products of reducing sugars with amino compounds (Today, these are also called Maillard reaction products or browning reaction products.)  These reactions occur more rapidly at higher pH and temperature.

Iron compounds of polyphenols.  These tend to be low molecular weight and will not be further discussed in this article.

To this list we would also add the important category of plant pigments – phenolic-based colorant complexed with polysaccharides found in cane sugar.

A category of colorants called melanins is sometimes added to the list.  These are mainly found in beet processing and are formed by enzymatic reactions of phenols, leading to very dark, nearly black colors.

Another category of colorants that has been studied are those formed by the action of polyphenol oxidase enzymes on phenolic compounds, or enzymatic colorant.  Enzymes may play a role in HMW cane colorants that have a high proportion of phenolic compounds.

Zerban stated that continued condensation (which we now call polymerization) of colorant precursors increased the color.  This is an early recognition that high molecular weight colorants are more of a problem than lower molecular weight colorants, simply because they are darker in color and may continue to increase in color as condensation/polymerization continued.  Zerban considered many colorants to be colloidal in nature, implying a very high molecular weight.  In this same monograph, Zerban discussed the effect of colored impurities on sugar crystallization, and differentiated among colorants that absorb onto the surface (easy to remove by washing) and those that crystallize in the core of the crystal (difficult to remove). 

Bibliography
Zerban, F.W., 1942, The color problem in sucrose manufacture, Technological Report Series, No. 2, 1942,
           Sugar Research Foundation, Inc., New York.

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