Effect of sugar mixtures on specific protein production rate. Specific protein production rate was measured during cellulase production with fed-batch process in fed-flask protocol for both mixture designs with lactose inducer (left) and cellobiose inducer (right) then modelled using a quadratic model (Additional file 1). Mixture compositions are shown in triangles according to Figure 1. Model values are in colour scale from blue (low values) to red (high values) according to the legend. Intermediate values are shown by contour lines (black lines). Red dots are experimental points used for the models.