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Dextran - (Luz Stella Polanco)

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By Luz Stella Polanco
Posted on 2011-02-22    Last edited on 2011-02-22

Dextran is the name given to a group of high molecular weight polymers composed of D-glucose units connected by α -1,6 linkages and with various of side branches linked with α -1,2, α -1,3, or α -1,4 to the main chain (Khalikova,2005). Dextran is produced by the enzyme (dextransucrase) secreted by bacteria of the genera Lactobacillus, Leuconostoc and Streptococcus, of which the most commonly found in deteriorated or sour sugar cane is Leuconostoc mesenteroides. The enzyme (dextransucrase) converts a molecule of sucrose to fructose that can be used by the microorganism; dextran is the by-product of this enzymatic reaction (Day 1984).

Sugarcane deterioration, with the formation of dextran and its final presence in the raw sugar, begins in the field and continues throughout the processing of the sugarcane. Sugarcane varieties, soil, weather conditions (storms, freeze), field harvesting practices, shipping time, storage time (in the yard), and handling are factors that affect the magnitude of dextran entering the mill. At the factory, shutdowns and lack of sanitation in some processes enhance dextran formation, mostly in sugarcane juices with low concentrations of sucrose, an optimal condition for microorganisms. The main problem from dextran is that its formation involves a loss of sucrose. Cerutti de Guglielmone et al. (2000) isolated a strain of Leuconostoc mesenteroides from sugar cane in Argentina, showing that this microorganism consumes sucrose very quickly during the first 6 hours of culture, 8.05 g/(l·hr) at 25 oC and 8.46 g/(l·hr) at 30 oC, after that time the microorganism stops sucrose consumption (Cuddihy 2001). Moderate and severe   dextran presence, more than 1000 mg/kg DS in mixed juice, causes several problems at the factory such as an increase in the viscosity, which implies lower evaporation rate, longer boiling times and lower purging in centrifuges (Eggleston 2005). Day (1984), points out that for low dextran concentration in the mixed juice, 10% of the dextran ends up in the raw sugar, exponentially increasing to over 30% for a concentration in the juice higher than 5000 mg/kg juice, which affects the quality of the raw sugar and is penalized by the refineries. Rauh et al. (2003) shows dextran concentration in sugar remelts from 300 to 1300 mg/kg DS (dry substance). In addition, Atkins (1984), states that for every 300 mg/kg of dextran in the syrup, the purity of the final molasses increases by 1%.

The structure of dextran isolated from cane sugar has been analyzed by several authors. Covacevich (1977) and Foster (1980) found that cane dextrans have a very similar structure to the dextrans produced by the Leuconostoc Mesenteroides strain NRRL B-512, which are predominantly linear,  95%  α -1,6 linkages with 5% branching probably α -1,3 and have high polydispersity. Side chains are attached to the α -1,6 backbone by α -1,3 linkages, see Figure . Branched side chains are one glucose unit, 45%; two glucose units linked α -1,6, 40%; and more than two glucose units linked α -1,6, 15% (Larm 1971, Parrish 1982), see Figure 2. A molecule with α -1,6 linkages is more flexible than e.g. a molecule with α -1,4 linkages and it can adopt a configuration that matches the hydrogen bond pattern on the crystal surface; then, a high proportion of  α -1,6 links can be expected to be potentially surface active (Day 1971).

Figure 2. Structure of B-512 dextran. Parrish (1982)
    40%            45%         15%

                  G
                   |
                     G              G
                      |                |
       G           G              G
        ¦              ¦               ¦
G – G – G – G – G – G – G – G – G

 ––  α -1,6 linkages
- -    α -1,3 linkages

Figure 1. Structure of a fragment of a dextran molecule. Pharmacosmos (2006)

 
     

However, Imrie and Tilbury (1972) in the linkage analysis found a different distribution presented in Table 1 (Cuddihy 2001). Edye (1997) reports that native dextran can contain another kind of branch linkage, α -1,2, and that the lower the molecular weight (103 Da) the higher is the branching.

Table 1. Linkage analysis of native dextran (Imrie and Tilbury 1972)
 
Samples    % α -1,6    % α -1,4    % α -1,3      
Stored cane I    32    66    2      
Stored cane II    33    64    4      
Deteriorated juice    79    7    14      
Refractory mill syrup    83    11    6      
Refractory refinery syrup    60    22    18     

The molecular mass of the dextran can vary from few thousand to several million. Inkerman (1980) found an average molecular mass of 5 x 106 Da with some degree of polydispersity. Atkins (1984) reports that both dextran in deteriorated cane and in sugars produced from deteriorated cane are of high molecular mass (25 x 106 Da and 20 x 106 Da were found by Foster 1977 and Covacevich 1975); furthermore, the effect of different molecular mass dextran is not the same, hence it is necessary to know what range of molecular mass is affecting process and quality of the raw sugar.

Some work has been done about molecular mass distribution of polysaccharides for different process streams and for the raw cane sugar. Vercellotti (1996) says that “regular patterns can be seen for the high (M ~900 000 Da), medium (M ~38 000 Da), a low (M ~2 600 Da) molecular mass polymers as temperature, concentration, and removal of sugars progress”; a summary is shown in Table 2. Peaks 1 and 2 contain soluble polysaccharides such as dextran, arabinogalactan (soluble cell wall of the indigenous sugarcane polysaccharide, ISP), and starch among others. Peak 3 contains low Mw colored materials, sucrose, glucose, fructose, and other compounds (Edye 1996).

Table 2. Gel permeation chromatography. Average abundance ratio. Louisiana 1995. Vercellotti (1996)
 
    PEAK 1    PEAK 2    PEAK 3      
Sample    >1 200 000 Da    38 000 – 130 000 Da    <7 000 Da      
Syrup    1.04    0.78    277      
A-molasses    3.05    2.35    495      
B-molasses    3.39    2.62    521      
C-molasses    5.66    3.87    733     

Godshall et al. (2001) show that raw cane sugar contents about 0.24 % (2412 mg/kg cane) of high molecular mass (HM) components (> 12 000 Da) of which an average of 55.8 % (1346 mg/kg cane) are polysaccharides and the rest are colorant polymers with traces of proteins (Roberts and Martin 1959) and colloidal silicate (Clarke 1975). Respect to the polysaccharides, starch is 20.5 % (276 mg/kg cane) and dextran is 32.0 % (431 mg/kg cane). The GPC analysis of the HM components in raw cane sugar samples from Australia, Louisiana, Peru and Ivory Coast gives three molecular mass peaks at 2 x 106 Da (P1), 8 x 105 Da (P2) and 3 x 105 Da (P3) while, for the samples from Brazil, Nicaragua, Dominican Republic, Florida and Panama the GPC analysis gives two molecular mass peaks at 2 x 106 Da (P1) and 8 x 105 Da. (P2). Godshall states that a high percentage of P2 is arabinogalactan (ISP) and that P2 and P3 are responsible of the dark brown color. Due to the acidic character of these colorants they can be easily removed by an anionic exchange resin; on the other hand, P1 which has a light yellow color and hazy turbidity is not significantly removed with these resins indicating a nonionic character (Godshall 1993). P1 remains in refined white sugar and presents a high content of glucose, representing the dextran and starch in the raw sugar.

Roberts et al. (1988) suggest that dextran in sugar cane and in raw sugar contains between 2 % to 3 % of mannose residues, probably linked to the glucose chain, which is not found in the dextran produced by the classified organisms from pure sucrose media or sugar cane juice. Mochtar (1985) mentions that in many polysaccharides such as dextran, a small amount of fructose is found in addition to the main component, glucose.  Additionally, he says that the elongation of the c-axis was related to the content of α -1,6 linkages of  dextran.
Hidi (1975) states that for the viscosity of the syrup, dextran with a molecular mass, M, above 2 x 106 Da has an effect about 10 times higher than a dextran with a M below 40 000 Da. High M cause problems on filtration and clarification, but problems with low M have not been observed in these factory processes. Problems in crystallization have been demonstrated for high M; in contrast, low M dextran gives problems only at very high concentrations. Finally, for refined sugar quality low M affects less the distortion of hard candy than the high M (Curtin 1986). Chou (1996) reports that occluded dextran in refined sugar causes a cloudy appearance in alcoholic solutions, so this sugar can not be used to produce cordials.




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