Table 1. Experimental design of fermentation conditions for black soybean

Fermentation conditions		Coded level1)
		−2	−1	0	1	2
X1	Enzyme concentration (%)	0.1	0.2	0.3	0.4	0.5
X2	Fermentation time (min)	20	40	60	80	100

The experiment was designed based of five-level central composite design.