Table 3. ANOVA for the surface tension of the biosurfactant produced by Streptomyces sp. DPUA1559 in mineral medium containing 1% residual soybean oil after 96 h of fermentation.

Factor	Quadratic sum	Degrees of freedom	Quadratic mean	F	P
X1	2.22539	1	2.22539	4.59995	0.121308
${\text{X}}_1^2$	31.74989	1	31.74989	65.62788	0.003931
X2	5.67020	1	5.67020	11.72046	0.041735
${\text{X}}_2^2$	9.85354	1	9.85354	20.36753	0.020332
X1 * X2	0.05978	1	0.05978	0.12357	0.748437
Lack of fit	0.58603	3	0.19534	0.40378	0.762023
Experimental error	1.45136	3	0.48379
Total	45.95987	11

X₁: pH; ${\text{X}}_1^2$: quadratic term of pH; X₂: temperature (°C); ${\text{X}}_2^2$: quadratic term of temperature °C; X₁ * X₂: quadratic term of pH and temperature °C; R2 : 0.9557; R: 0.9187; F and P value in ANOVA test, where F_critical = 10.12 ${\text{X}}_1^2$.