An easy procedure for cell free biotransformation of pineapple juice sugars into dietetic derivatives
was accomplished using a commercial invertase and an oxidoreductase from Zymomonas mobilis. First,
pineapple juice sucrose was quantitatively converted into glucose and fructose by invertase, thus increasing
the concentration of each monosaccharide in the original juice to almost twice. In a second step,
glucose–fructose oxidoreductase (GFOR) transformed glucose into gluconolactone, and fructose into the
low calorie sweetener sorbitol. The advantage of using GFOR is simultaneous reduction of fructose and
oxidation of glucose, allowing the continuous regeneration of the essential coenzyme NADP(H), that is
tightly bound to the enzyme. The yield of GFOR catalyzed sugar conversion depends on initial pH and
control of pH during the reaction. At optimal conditions (pH control at 6.2) a maximum of 80% (w/v) sugar
conversion was obtained. Without pH control, GFOR is inactivated rapidly due to gluconic acid formation.
Therefore, conversion yields are relatively low at the natural pH of pineapple juice. The application of
this process might be more advantageous on juices of other tropical fruits (papaya, jackfruit, mango) due
to their naturally given higher pH.