Understanding the cause of high-colour sugar – intrinsic cane colour, extraneous matter or factory practices?

In recent years the colour of Brand 1 sugar from mills processing green cane has fluctuated near the upper limits of Queensland sugar colour expectations. Cane supply to these mills has also contained higher levels of extraneous matter (EM), which is known to produce sugar of high colour. For these cane conditions, when highly coloured raw sugars are produced, the affined sugars also tended to be highly coloured. The colourants that tend to more strongly partition into the crystal lattice are known to be of higher molecular weight. These colourants originate from the plant pigments associated with polysaccharides in the incoming cane supply and are produced during the processing of cane to sugar, particularly during pan boiling. Investigations were conducted through sampling from three mills. Sampling of process streams from juice to liquor and from magma to final shipment sugar was performed at four stages across the 2018 season. Extraneous matter was measured at two mills where the infrastructure was available. Massecuites were subjected to varying wash times at each high- grade fugal station to produce sugars of differing purity to examine the efficiency of colour and impurity removal. Colour partitioning into the crystal lattice at one mill was observed to be greater at different times through the season, suggesting that extraneous matter was a dominant contributor. Trials mainly focused on assessing the incorporation of colour and impurities in shipment sugar, but extensive sampling was also performed within the process to help identify where specific operations were contributing to colour development or incorporation into crystal. The combination of extraneous matter, sources of high-grade massecuite foundation crystal and cycle times for shipment sugar massecuites, all contribute to affined sugar colour intensity and require further investigation. Considerably more intense sampling and analysis is still required to identify and confirm which constituents in the cane supply and which aspects of factory operations most strongly influence the partitioning of colour into the crystal lattice. Key words Raw sugar colour, affined sugar colour, extraneous matter, partitioning coefficient