2.3. Ohmic heating
2.3.1. Experimental protocol
Experiments were performed in a batch stirred reactor with
ohmic heating at 60 Hz. The ohmic heating apparatus consists of:
a manual transformer (0e240 V); a data acquisition system that
recorded temperature, current and voltage data (data logger); and
an ohmic heating cell containing platinum electrodes and a water
jacket. The cell was built in a Pyrex glass shape with a diameter of
8 cm. The set-up used is shown in Fig. 1 where VT and A represent
the voltage and current transducers, respectively, and T the
temperature sensors. To homogenize the pulp, the ohmic cell was
placed above a magnetic stirrer, and to ensure a uniform temperature
profile, the temperature was monitored in two different
locations inside the ohmic cell, near the electrode and near the cell
wall. For these measurements, stainless steel Pt-100 m coated with
a nickelephosphorous alloy were used.
For the ohmic heating treatments, the pulp temperature was
raised applying the voltage determined by the experimental design
until a temperature of 90 C was reached. The voltage was then
lowered to maintain the pulp at this temperature for 2 min. This
time/temperature condition was chosen because it is suggested in
literature to inactivate anthocyanin-degrading enzymes (Fennema,
2010). When the thermal treatment was complete, the product was
rapidly cooled by passing cold water (4 C) through the jacket.