Table 1.

Variables and their levels employed in the central composite design.

Variables	Units	Coded level of variables
		−1.75	−1	0	1	1.75
X 1: Enzyme amount	w/w%	1.5	3	5	7	8.5
X 2: Reaction time	h	2	8	16	24	30
X 3: Reaction temperature	°C	51.25	55	60	65	68.75
X 4: Molar ratio of substrates	OA : TEA (mole : mole)	0.25 : 1	1 : 1	2 : 1	3 : 1	3.75 : 1
X 5: Agitation speed	r.p.m.	137.5	250	400	550	662.5