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Phenols

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By Bento, Luis San Miguel
Posted on 2007-07-23    Last edited on 2009-09-25

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 – C
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 Ccentral 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). 

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.

4.1.2.phenolic acids

In cane juice exist other phenolic compounds, beside the ones described in the last section, the phenolic acids, normally linked to alcohols, to other phenols or to polysaccharides by ester bonds.The phenolic acids occurring in sugar cane are usually derived from cinamic acids, whose geral chemical formula is presented in Figure 9 (Farber e Carpenter, 1972).

Figure 9: geral chemical formula of phenolic acids derived from cinamic acid

These compounds are very difficult to remove during extraction and refining, being found in the final product. As an example, chlorogenic acid, resulting from the esterification of caféic acid and quinic acid, was detected in white sugar (Farber and Carpenter, 1972).
Another kind of esterification is the one that occurs in its bonding to polysaccharides existing in cellular walls, composed by arabinoglucan with glucoronic residues. These high molecularcomplexes, known as ISP - Indigenous Sugar Cane Polysaccharides (Clarke et al., 1988), exits in cells membranes, and have a plant protection function, against external aggressions (insects, cuttings, etc.). Being soluble and with a low anionic charge, these compounds are not removed in most factory and refining processes, appearing in the white sugar (Clarke et al., 1988). Due to their high molecular weight they cause great difficulties in sugar processing, increasing sugar products viscosity, resins poisoning, difficulties in crystallization and massecuite purging in centrifugals. The presence of ISP in raw sugars decrease refining process efficiency and increases energetic consumptions (Clarke et al., 1988) and affect sugar quality.

The bonds of ferulic acid to polysaccharides are difficult to hydrolise, and their association persists during process, even if the polisaccharidews degrades in small (Clarke et al., 1988). Nevertheless, at high alkalinity conditions, it may happen the de-esterification of these compounds. The colourants release by this reaction, being no more connected to polysaccharides, will have a lower affinity to sugar crystals. This reaction provokes a colour solution increase but a decreasing in the sugar colour crystallized from the resulting solution.  (Clarke et al., 1988). 

4.1.3. phenolic compounds oxidation

In the presence of enzymatic complexes, as PPO - polyphenol oxidase, in an oxidant environment, polyphenols are oxidized, forming quinones. These compounds may react with o-diphenols,  amino acids, proteins (Mersad et al., 2000) or other quinones, forming polymeric compounds.
The formation of enzymatic browning compounds in juices depends on the cane variety, on type and concentration of phenolic compounds, and on the activity of enzymes involved in the oxidation reaction (Paton, 1992). Some products of this reaction present an intense dark brown colour, as in the melanines.  
A characteristic of these products is their great solubility and their weak anionic charge, what causes them not to be removed during Clarification and following processing, being sometimes incorporated in white sugar. This does not happen with enzymatic browning products originate from sugar beet which are insoluble in water (Wyse, 1971; Paton, 1992).
Flavonoids derived from apigenine and lutheoline are more prone to suffer enzymatic browning than those derived from tricine (Smith and Paton, 1985).  The concentration of flavonoids in juice, especially those derived from lutheoline, with an o-phenol group, are important markers for a possible browning colour formation (Paton, 1992). The cane varieties containing these flavonoids and chlorogenic acid have potentialities for colour formation during milling process.

Phenolic acids esterified with polysaccharides, when undergoing a enzimatic browning reaction, may contribute for the formation of cross-links among the carbohydrates, resulting in very high molecular weight complexes with a high colour. An example of this kind of reaction is the enzimatic oxidation of ferulic acid, catalized  by PPO, in ISP (Clarke et al., 1988) (Figure 10).



Figure 10: Dimerization of ferulic acid in ISP by PPO

Phenols, as chlorogenic acid, may also be oxidized non enzimatically, through chemical reactions. These reactions are slower than the enzimatic ones, but their velocity may increase at high pH and temperature (Paton, 1992). This kind of reaction can occur during sugar solutions storage, even at low temperatures (Cilliers and Singleton, 1989; Paton, 1992). 

Reactions of phenols with iron

Iron solubilized in juices may form chelates with anthocianins, as with most of phenolic compounds, forming dark colour compounds (Smith and Paton, 1985; Riffer, 1988).
This reaction may happen whenever sugar solutions are in contact with iron, particularly during sugar extraction in mills (“5.2.1. Milling”). During cane milling there is an erosion of the rollers with iron passing to the juice. The presence of a great quantity of iron compounds in cane bagasse is a proof of the occurrence of this erosion. A quantity of 11.6% of Fe2O3 (on ashes) in bagasse, was detected (Magasiner et al., 2002). 




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