Table 1 Central composite design (CCD) with 4 variants at 3 levels
| Tests | X1 | X2 | X3 | X4 | Tests | X1 | X2 | X3 | X4 | Tests | X1 | X2 | X3 | X4 |
| 1 | 0 | –1 | –1 | 0 | 11 | –1 | 0 | –1 | 0 | 21 | –1 | 0 | 0 | 1 |
| 2 | 0 | 1 | 0 | 1 | 12 | 0 | –1 | 1 | 0 | 22 | 0 | 0 | 0 | 0 |
| 3 | –1 | 0 | 0 | –1 | 13 | 0 | 0 | 1 | 1 | 23 | 0 | 1 | –1 | 0 |
| 4 | 0 | 1 | 1 | 0 | 14 | –1 | –1 | 0 | 0 | 24 | 0 | 0 | 0 | 0 |
| 5 | –1 | 0 | 1 | 0 | 15 | 0 | 0 | –1 | 1 | 25 | 0 | –1 | 0 | 1 |
| 6 | 0 | 0 | 0 | 0 | 16 | 1 | 1 | 0 | 0 | 26 | 1 | –1 | 0 | 0 |
| 7 | 0 | 1 | 0 | –1 | 17 | 0 | –1 | 0 | –1 | 27 | 0 | –1 | –1 | 0 |
| 8 | 1 | 0 | –1 | 0 | 18 | 0 | 0 | 0 | 0 | 28 | 0 | 1 | 0 | 1 |
| 9 | 1 | 0 | 0 | –1 | 19 | 0 | 0 | 1 | –1 | 29 | –1 | 0 | 0 | –1 |
| 10 | –1 | 1 | 0 | 0 | 20 | 1 | 0 | 0 | 1 | | | | | |
SBH, sheep bone hydrolysate.
X1, X2, X3, and X4 were four variants represented fermenting temperature, initial pH of SBH, fermenting time and inoculation amount. Each variant had 3 levels of –1, 0, and 1. The 3 levels for X1 were 35℃, 37℃, and 40℃; X2 4, 5, and 6; X3 10 h, 12 h, and 15 h; X4 3%, 4%, and 5%.