Reactor optimization for alpha-1,2 glucooligosaccharide synthesis by immobilized dextransucrase

The immobilization of dextransucrase in Caalginate beads relies on the clos e association between dextran polymer and dextransucrase. However, high amo unts of dextran in the enzyme preparation drastically limit the specific ac tivity of the immobilized enzyme (4 U/mL of alginate beads). Moreover, even in the absence of diffusion limitation at the batch conditions used, the e nzyme behavior is modified by entrapment so that the dextran yield increase s and the alpha -1,2 glucooligosaccharides (GOS) are produced with a lower yield (46.6% instead of 56.7%) and have a lower mean degree of polymerizati on than with the free dextransucrase. When the immobilized catalyst is used in a continuous reaction, the reactor flow rate necessary to obtain high c onversion of the substrates is very low, leading to external diffusion resi stance. As a result, dextran synthesis is even higher than in the batch rea ction, and its accumulation within the alginate beads limits the operationa l stability of the catalyst and decreases glucooligosaccharide yield and pr oductivity. This effect can be limited by using reactor columns with length to diameter ratio greater than or equal to 20, and by optimizing the subst rate concentrations in the feed solution: the best productivity obtained wa s 3.74 g.U-1.h(-1), with an alpha -1,2 GOS yield of 36%. (C) 2001 John Wile y & Sons, Inc.