The batch reaction system consisted of a glass beaker with magnetic stirrer and
control instruments for temperature and pH. Simultaneous conversion of mixtures
of glucose and fructose (“model solutions”, shown in Table 1) and pineapple juice
were carried out at 25 ◦C using 1.2 U GFOR per mL at a constant pH maintained by
the controlled addition of 1 M potassium hydroxide via a tube pump (Gamma/4-I;
Prominent, Heidelberg, Germany). At specific time intervals 0.5 mL samples were
taken, heated for 10 min at 99 ◦C, centrifuged at 13,000 rpm (16,000 × g) and then
analyzed by HPLC.