2.2.2. Experimental design and data analysis
A central composite rotatable design was used to design the
tests for the ohmic heating process, considering two variables: the
solids content of the pulp (2e8 g/100 g) and the heating voltage
(120e200 V). The statistical design consisted of a 22 factorial with
four axial points and four center points, giving a total of twelve
combinations. The experimental design is shown in Table 1, where
X1 and X2 are the real values of the heating voltage and the solids
content of the pulp, respectively. The dependent variables were the
ascorbic acid degradation (DAA) and total vitamin C degradation
(DVTC).
The solids content was chosen as independent variable because
it affects the electrical conductivity of the product. The rate of
ohmic heating is directly proportional to the square of the electric
field strength and the electrical conductivity. Changing the rate of
the ohmic heating results in different times of heating and this may
influence on vitamin C degradation.
The statistical analyses were carried out using Statistica 5.0
(Statsoft Inc., Tulsa, OK, USA).