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Colourants - Formation

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By Bento, Luis San Miguel
Posted on 2006-09-20    Last edited on 2009-09-23

In sugar production we must, primarily, avoid colourants formation. Neverthless, because of physico-chemical and biochemical conditions occurring during processing, colour formation is inevitable, in actual process conditions. This colour formation is conditioned by temperature, by the presence of some compounds as invert sugars, iron, amino compounds, and certain enzymatic complexes, by pH and solutions concentration.  
These adverse situations occur during sugar extraction and refining processes.

During processing sugar solutions are subjected to conditions of high temperature during heating of juices, liquors, syrups and massecuites, in Clarification, Evaporation and Crystallization. This heating is necessary to dissolve sugar with sweet waters, to decrease syrups  and massecuites viscosity, to increase diffusion during decolourization, to increase filtration and crystallization rates.
Heating is normally made through steam at a pressure of 1kg/cm2 and a temperature until 150ºC, or using vapor from evaporation station. Equipment used for heating includes plate or tubular heaters, heating calandrias and steam injectors for direct heating. Although an effort is made to minimize contact time of sugar solutions with heating surfaces, using agitation systems and appying a correct equipment design, there is always formation of a surface layer were sugar is subjected to high temperature and where their degradation and colour formation can occur.

Another factor influencing colour formation is excess alkalinity occurring in certain process points. This can happen during: 
  -      juice and calcium hydroxide mixing in Clarification;
-       liquor and calcium hydroxide mixture in Carbonatation;
-       in syrups or masses neutralization processes using calcium hydroxide.

If, in these operations, occurs zones of high alkalinity, there may be formation of colourants as HADP.

The presence of iron compounds in sugar solutions is another factor for colour formation. The majority of sugar mills equipment is made of iron or iron based materials. In process conditions, mainly in the mills, with low density and slightly acid solutions, corrosion will occur. Iron salts, in solution, may react with phenolic compounds originating high colour compounds (Riffer, 1988). In refineries this can be avoided by using corrosion resistant materials as stainless steel, glass fiber or synthetic polymers.

Solutions and masses at high sucrose concentration, occurring in the final extraction phase and during refining, may also be a factor to enhance colour formation reactions. As a matter of fact, in solutions with high sucrose concentrations, water not linked to sucrose is minimal, and colour precursors concentration will be high. It was demonstrated that reactions kinetics increases in solutions with high sucrose content (Lubineau, 1995), allowing more conditions for colour formation.

To avoid formation of colour compounds during processing it will be important:
-     to choose a cane variety in which phenolics composition and enzymatic activity do not provoke a
      great quantity of browning reaction products during milling process;
-     to avoid the prsesnce of a great quantity of amino compounds in juices, by using fertilizers with low
      nitrogen compounds;
-     to avoid high alkalinity conditions;
-     to avoid high temperature conditions;
-     to avoid corrosion or erosion of iron materials;
-    to design equipment in such a way that residence time is minimal, specially at high pH and
      temperature:
-     to avoid sucrose inversion;
-     to avoid an excess of return products back into the process;
-     to minimize storage volumes of sugar products;
-     to minimize the quantity of sugar products to heat;
-     to cool massecuites of low purity;
-     to minimize calcium hydroxide consumption;
-     to use heating temperatures as low as possible.

Bibliography

Lubineau A., 1995, Les solutions concentrees de sucre comme milieu reactionel, Zuckerindustrie, 120, 9,
          804-807 
Riffer R., 1988, The nature of colorants in sugarcane and cane sugar manufacture, Chemistry and
          processing of sugarbeet and sugarcane
, Ed. M.A. Clarke, M.A. Godshal, Pub. Elsevier, Amesterdam,
          186-207

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