Range and levels Symbol Independent Variables -2 -1 0 +1 +2 X1 Temperature oC 30 35 40 45 50 X2 Immersion time (min) 30 60 90 120 150 X3 Sugar concentration oB 40 45 50 55 60                             (1)                       (2)                (3)

Table 1: Process variables and their levels of experimental design.