Table 1 Experimental deign levels. Values of independent variables and corresponding coded levels were used in a central composite design
| Symbols | Variables | Real values of coded levels | ||
|---|---|---|---|---|
| − 1 | 0 | + 1 | ||
| A | Tris (mM) | 50 | 125 | 200 |
| B | EDTA (mM) | 0.1 | 1.05 | 2 |
| C | pH | 7.2 | 8.1 | 9 |
| D | Time (min) | 15 | 37.5 | 60 |