Table 1.

Factors and design levels for the RSM-CCD independent variables.

Factor	Code	Level
		−α	−1	0	+1	+α
Reaction temperature (°C)	X1	55	60	65	70	75
pH	X2	7.5	8.5	9.5	10.5	11.5
Reaction time (h)	X3	4.5	5	5.5	6	6.5
Ratio of E/S (%, w/w)	X4	0.1	0.2	0.3	0.4	0.5

Abbreviations: CCD, central composite design; E/S, enzyme to substrate; RSM, response surface methodology.