variables (factors) with coded levels +1 and 1; (ii) to account
for non-linearity, a star of 2k points, coded as +a and a on the axis
of the system at a distance of a = [2k]1/4 from the origin; and (iii)
two central points to provide an estimate of the lack of fit of the
obtained linear statistical model as well as of the pure error of
the experiments [18]. The ranges of interest of each independent
variable were: power level (PL): 10–100%; treatment time (t): 1–
19 min; and storage period (Sp): 1–15 days. Table 2 shows the
coded and uncoded matrix of independent variables.
The evaluated quality parameters (dependent variables) were:
colour, texture and total phenolic content (TPC).