Investigations on the influence of dextran during beet sugar production with special focus on crystal growth and morphology

El-Sayed Ali Abdel-Rahman

Abstract

In the present study, the sensitivity and accuracy of the common methods of dextran determination were investigated. Concentrations and progressions of dextran during different sugar production processes and industrial periods were determined and for the first time an evaluation of sugar and molasses quality was provided based on the measurement results. Regarding the dextran determination, the results had shown a significant effect of dextran concentration and molecular fractions on the Haze and Robert’s methods. The data demonstrate that the Robert’s method was more accurate for different dextran molecular mass fractions than Haze method. Furthermore, the Haze method was too inaccurate to be genuinely useful as a dextran analysis method especially considering low dextran molecular mass fractions. The isolation and identification of Leuconostoc mesenteroides as an important source of dextran in sugar beet factories was performed. It was observed, that the microorganisms grow during the storage of sugar beet and also during the extraction process. The mean counts of colonies were reduced after washing process from 2.5*103 cfu/g of the sugar beet sample to 6*102 cfu/g of cossettes samples. However, the number of microorganisms increased in raw juice after extraction process to reach almost 4*106. In addition, these numbers depend on the degree of deterioration of the sugar beets. Dextran content increased during the extraction process by 4 % to 20 % for fresh and deteriorated sugar beet, respectively. In contrast, the lime-carbon dioxide juice purification reduced the dextran content in raw juice by 40 % to 60 %. The obtained results from Egyptian sugar beet factories showed a correlation between the increase of sugar loss and the presence of dextran during different industrial periods as well as during the production process itself.. As a main focus in this work the negative effects of the presence of dextran during (isothermal) crystallization were investigated. The growth rate of sucrose crystals and the quality of the sugar production in pure sucrose solution at different temperatures were studied. To elucidate the influence of dextran on the growth rate of sucrose crystals, different molecular mass fractions of dextran from 40,000 g/mol (T40) to 2,000,000 g/mol (T2000) were admixed in different concentrations to pure sucrose solution. The most pronounced effect of dextran was found with T2000 at 60°C. On the other hand, negligible influences of dextran T40 and T500 were observed at the same temperature and supersaturation. The high adsorption ability and the increase of solution viscosity particularly caused by high molecular fractions of dextran were identified as the main reasons for the reduction of crystal growth rate. The application of lower temperatures during crystallization enhances the problems deriving from dextran. Regarding the final crystallization steps (e.g. after product crystallization), levels of dextrans and impurities are enhanced and the mentioned problems will be amplified in these production steps. From the morphological studies, it was found that the presence of dextran in sugar mother liquor leads to elongation of the sucrose crystal shape (elongated along the c-axis). In particular, the lower molecular mass fractions of dextran are involved in this effect. A scanning electron microscope (SEM) was utilized for the evaluation of crystal surfaces and crystal morphology. The surface topography of sucrose crystals was affected by the presence of higher dextran molecular mass fractions (T2000), which causes gaps, rough areas and strong adsorption of micro-particles (micro-crystals) on the crystal surface. Again, the described influences are enhanced by the application of lower temperatures. In addition, in this work it could be confirmed that there is a correlation between the dextran content in syrup and quality of sugar produced, in particular the turbidity of the sugar. So, a relation which has already been described in recent literature for organic macromolecules in general could be specified for dextran molecular fractions.