THE colour of raw sugar is important to refiners because of the high cost of its removal in the production of refined sugar. Unless the refiners’ maximum raw sugar colour
specification is met, millers’ revenue is reduced. Understanding what affects sugar colour is important if it is to be controlled. It is known that colour made in the process by
polymerisation of components of juice can be reduced by control of temperatures and pH levels and that minimising the amount of impurities in sugar reduces colour. Doing all of these does not necessarily ensure low sugar colour. BSES provided assistance to
Mulgrave Central Mill Co. Ltd to determine reasons for high raw sugar colour.
Mulgrave’s cane quality scheme had provided the mill with a significant improvement in
cane quality, yet despite this, colour in sugar was increasing. Using colour data on a rake-by-rake basis obtained from an in-line cane analysis system (CAS) that uses near infrared reflectance spectroscopy, in combination with laboratory cane payment data, an attempt was made to quantify relationships between first expressed juice (FEJ) colour at pH 7 and sugar colour. The effects of cane variety, cane age and growing district on juice colour were evaluated. Quantitative relationships between FEJ or MJ (mixed juice) colour/impurity were obtained for both the 2002 and 2003 seasons. In the best regression result found, sugar colour decreased with increasing sugar pol and increased with both mixed juice colour/impurity and percent impurities. All variables were significant statistically and not strongly correlated. For Mulgrave mill juice, colour/impurity was influenced mainly by cane variety with the two major varieties having the highest colour. Juice colour/impurity increased with age from plant to third ratoon and then decreased for older ratoons. There were statistically significant differences in the colour/impurity of juices dependent on where the cane was grown. Based on a small number of laboratory analyses on FEJ, colour for the same varieties was 50% to 100% higher at Mulgrave mill than at Racecourse mill. Juice colour can be used to evaluate cane supply relative to sugar colour. Further analysis incorporating data from other mills is required to improve confidence in predictions. Varieties have the greatest effect within the Mulgrave area but growing conditions may account for much of the difference between Mulgrave and other mills. Selection of varieties is the best option for control of juice and sugar colour.