2.2. Thermosonication
Thermosonication treatments were carried out using an ultrasonic
processor (UP 400S, Hielscher, Germany) working at a constant
frequency of 24 kHz. The processor was fitted with an
ultrasonic probe (Horn H22D) which had a 22 mm diameter tip.
Milk samples (200 ml) of varying fat content were transferred to
a 250 ml beaker fitted with a recirculating water jacket and equilibrated
for 5 min at a temperature of 45 C. The ultrasound probe
was immersed in the centre of the beaker to a depth of 30 mm.
Treatment time was 10 min of a continuous application of ultrasound
with a power output of 400 W for milk samples. Milk temperature
increased to 72 C over the first 6 min of sonication and
thereafter remained constant. After processing the sample was
cooled and stored at 4 C for a maximum of 6 h prior to culturing.