Sugar cane pigments are mostly chlorophylls, carotenes, xanthophylls and flavonoids. The first three pigmnets are not water soluble, being therefore easily separated during cane juices clarification (Farber and Carpenter, 1972).
Flavonoids, being soluble and with a low anionic charge, are not easily separated during extraction and refining processes. Their general chemical structure can be represented by: C6 – C3 – C6 (Figure 6).
Figure 6: Chemical structure of flavonoids
These compounds play an important role in plant growth and their presence in cane juices is a main factor for colour formation during sugar processing.
Five classes of flavonoids were found in sugar cane: flavones, calchones, catechines and anthocianins (Smith and Paton, 1985). Each one of these flavonoids has a characteristic oxidation degree of the C3 central chain.
Many flavonoids have been found in sugar cane being their majority derived from flavones (Figure 7). These compounds are present in all sugar cane varieties (Paton, 1992).
Figure 7: Chemical structure of cane sugar flavones
Flavones are weak acids almost completely ionized at pH 9 (Smith and Paton, 1985). Generally associated to glucosydes they are stable at milling conditions and following processes (Paton, 1992). Actually, these compounds present stable anionc forms at high pH being also stable at moderate acid conditions (Smith and Paton, 1985). They present a light colour in acid solutions but, in alkaline solutions are yellow or greenish yellow (Paton, 1992). The IV (Indicator Value - quotient between absorbancies at pH 9 and pH 4, at 420nm) of theses compounds is high, attaining values above 30 (Paton, 1992).
More than twenty flavones were identified in sugar cane, all of them derived from apigenine, tricine and luteoline (Smith and Paton, 1985). Tricine is a flavone that characterizes the graminea family (Smith and Paton, 1985), in which the sugar cane is included.
Flavones, being associated to glucosides, have a great tendency to be included in sugar crystals, as there is a great chemical affinity between sucrose and other glucosides.
Anotherr kind of cane flavonoids is the anthocianins. These compounds are responsible for the red, blue and purple colours in plants. In sugar cane the red colour that can appear in cane surface is due to these compounds. As anthocianins are coloured at acid conditions and colourless at high pH they have an IV inferior to the unity. These compounds are unstable at neutral or alkaline conditions and are decomposed by heat (Paton, 1992). Heating the anthocianins at pH 7 they are partially decomposed, with loss of a ring, originating colourless compounds (Smith and Paton, 1985).
The action of sulphites provokes antocianins decolourization being the colour regenerated through acidification (Smith and Paton,1985).
As the cane juice Clarification (“5.3. Juice Clarification”) is made in hot and alkaline conditions, anthocianins do not overpass this stage. é feita a quente e em condições de alcalinidade elevada, as antocianinas não ultrapassam esta etapa.
The association of phenolic compounds to polysaccharides is important for sugar industry due to their chemical affinity with sucrose.
These associations may occur:
- by formation of ester bonds between phenolic acids and polysaccharides;
- by sequestring the poliphenols in polisaccharidse cavities (Figure 8) (Haslam and Lilley, 1985).
Figure 8 – Polyphenol sequestring by polissaccharides (Haslam and Lilley, 1985)
As phenolic compounds or their oxidation products can present intense colours, their association to polissaccharides is a very important aspect for sugar colour. Polyssaccharides might be considered the "Trojan horse" for the colourants entrance into sugar crystals.