| With a hydrophilic outside and a hydrophobic inside,cyclodextrins (CDs) can be widely used in food, pharmaceutical, cosmetics and agricultural industries. CDs are produced from cyclization reaction through starch by cyclodextrin glycosyltransferase (CGTase). This enzyme also catalyzes three other transferase reactions: the coupling reaction, disproportionation reaction and the hydrolysis reaction. The most common substrate used for CDs production is starch. In the practical production procedure, the use of high concentration of starch has obvious economic benefits and is conducive to save equipment investment and reduce energy consumption. After pasting, the high viscosity of starch solution makes it can hardly be stirred. In order to reduce the viscosity, it has to be hydrolyzed to an optimum degree by liquefying enzymes at the beginning of the CD production. However, the low molecular weight sugars, which are produced during the liquefaction processe, can strongly reduce the yield of the formed CDs due to the acceleration of others CGTase reaction through coupling and degradation reaction. It can be inferred that the starch degradation, which was degraded to the same DE by different methods, would have different effect on synthesis of CDs. This research used maltodextrins as model substrates to study the influence of the liquefaction degree and molecular weight distribution of starch degradations on the synthesis ofβ-CD. The effects of starch liquefaction processe on the optimum process conditions ofβ-CD production and the selection of organic solvents were also be studied.Studying on the influence of the liquefaction degree of starch onβ-CD production shows that the conversion ofα-,β-,γ-CD and the total conversion of CDs decreased with dextrose equivalent value (DE) increasing. The percentage ofβ-CD in the total products could be significantly increased by using maltodextrins as a substrate. The change of DE had little influence on the relative proportions of three types of CDs. The optimum DE of maltodextrins forβ-CD production was about 5. Theβ-CD conversion rate of samples with DE of 5.01 and 5.84 was 24.37% and 30.73% respectively.On the basis of the effects of starch liquefaction processe on the process conditions ofβ-CD production and the selection of organic solvents, a maltodextrin with DE of 5.84 was used as a model substartes. Maltodextrin (20%, w/v) mixed with CGTase (5U/g) and cyclohexane (10%,v/v) was incubated at 40℃and pH 8.5 in a shaking waterbath for 12h. The conversion rate ofβ-CD reached 62.83%.Mechanism of the effect of liquefaction processe onβ-CD production was investigated。Maltodextrins with different DE had different molecular weight distribution. With the increasing of DE, the multiple branching degree of maltodextrins increased, the chain length decreased and more low molecular weight sugars were produced. Maltodextrins with a similar DE, which were prepared by different methods, would have significantly different molecular weight distribution and conversion rate ofβ-CD. The molecular weight distribution of maltodextrins had strong influence on the synthesis ofβ-CD. Maltodextrins starch degradation with appropriate molecular weight (about MW 43,053-18,793 g·mol-1 or so) and lower multiple branching degree should be the suitable substrate for the cyclization reaction. |
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