Colorants and Decolorization using Ion Exchange Resins

Colorants
Sugar colorants are a very complex mixture of organic compounds stemming from various sources. They can be categorized into two main types; natural colorants resulting from the cane plant and those developed during juice processing. The main characteristics of the colorant to be removed are that they are mainly hydrophobic (non-polar) and cover a large range of molecular weights. This results in exhibiting anionic behavior.

Natural Colorants

• Flavanoids: < l kDa*

• Melanins: >150 kDa

• Chlorophylls, Xanthrophylls, Carotene: < l kDa

 Colorants Developed During Juice Processing

• Melanoldins, Maillard Reaction Products: < 50 kDa

• Caramels: 25 kDa

• HADPs: 1-5 kDa

*  kDa is kiloDalton or a thousand Daltons. A Dalton is the atomic mass unit.

Decolorization with Ion Exchange Resin
The resin matrix is usually composed of one of two types of polymeric material: polystyrenic with divinylbenzene crosslinks, resulting in a hydrophobic matrix, or a polyacrylic with divinylbenzene crosslinks which results in a more hydrophilic matrix.

Some refineries, for major decolorization, use only one resin type: acrylic or styrenic. Others employ an acrylic resin followed by a styrenic resin for polishing. The acrylic resin removes large molecular weight colored compounds which tend to foul the styrenic resin, providing good protection to the polishing resin.

Acrylic Resins
Acrylic resins have a slightly lower selectivity for sugar colorants but, unlike polystyrenic resins, they can be completely regenerated with sodium chloride solutions. This can be explained by the more hydrophilic character of the acrylic resin matrix. Even dark regenerant effluents from styrenic resins can be used to regenerate acrylic resins. The use of acrylic resin is justified when the color load is high and a low level of decolorization may be expected. Also, the use of a two-pass system gives a more constant level of decolorization than when only one resin bed is used.

Styrenic Resins
Styrenic resins are more prone to fouling by organic compounds than acrylic resins and need occasional acid regeneration. Also, their decolorization capacity is higher than that of acrylic resins, but the color is not so efficiently removed during regeneration and capacity can drop rapidly if overloaded.
 
Styrenic resins have a higher decolorization power because of their ability to fix colorants both through ionic bonds to the ionic active groups and through hydrophobic interactions with the resin matrix. Hydrophobic interactions increase with salt concentration, explaining the difficulty of using salt solutions to remove colorants fixed to the resin matrix.

Polystyrenic resins with an aromatic matrix should have a higher affinity for unsaturated colorants than acrylic resins. Styrenic resin types are favored when the inlet color is on the low end (i.e., up to 800 ICUMSA) and acrylic types when it is higher. Numerous plants today operate with a combination of both types, which is advisable when the inlet feed color fluctuates during the season.