| Variable | Code | Level | ||||
| −a (−1.68)* | −1 | 0 | +1 | +a (+1.68) | ||
| Temperature (˚C) | X1 | 20 | 23 | 28 | 33 | 36 |
| Moisture content (%) | X2 | 70 | 74 | 80 | 86 | 90 |
| pH | X3 | 3.0 | 4.62 | 7.0 | 9.38 | 11.0 |