Advances in Food Mycology

High Pressure Inactivation of Fungi 241
2.3. High pressure treatment
Cell suspensions (1.2 ml) were transferred to sterile high pressure
processing vials (1.5 ml). High pressure treatments were applied using
a U-111 High Pressure Multi-vessel Apparatus (High Pressure
Research Centre, Polish Academy of Sciences, Warsaw, Poland), comprising
five separate vessels which can be pressurised simultaneously,
but depressurised individually. The equipment also has the capacity
for temperature control, but these experiments were performed at
ambient temperature (20°C).
For the two yeasts and P. expansum and F. oxysporum, the unit was
pressurised to 400 MPa, with pressure applied for 15, 30, 45, 60 and
120 sec. This pressure treatment regimen was selected to allow the
acquisition of data that would provide an inactivation response curve.
Treatment at the pressure intended for the pear product (600 MPa)
would have resulted in inactivation times that were too short to measure
with the equipment available.
To assess the effect of a proposed blanching process for the fruit
product, cell suspensions of the two heat resistant moulds, B. fulva
and N. fischeri, were heated at 95°C for five minutes before they were
subjected to high pressure treatment. Cell suspensions (approx. 10 ml)
were filled into sterile plastic vials which were then immersed in a
water bath at 95°C. A thermocouple attached to a data logger was
placed in a vial containing a similar volume of suspending fluid (20
°Brix sucrose, pH 4.2). The 5 min blanching period was taken from the
time the control vial reached 95°C.
For the two heat resistant moulds, the pressure treatment was 600
MPa applied at the same time intervals as used for the yeasts and heat
sensitive moulds, using a 2 l high pressure processing unit (Flow
International Corporation, USA). Spore suspensions (approximately
4 ml) were aseptically filled into the bulb of sterile disposable plastic
Pasteur pipettes (5 ml) (Copan Italia S.P.A., Italy), and the end heat
sealed. To protect the high pressure unit from contamination in the
event of sample tube leakage, the sealed tubes were immersed in peroxyacetic
acid (250 ppm) (Proxitane Sanitiser, Solvay Interox,
Australia) within high barrier vacuum bags (Cryovac Australia Pty
Ltd, Australia), which were heat sealed. The pressure fluid was water
and pressurization was carried out at ambient temperature (18-20°C).
Come up times to the designated pressure (600 MPa) were less than 10
sec, and depressurization required less than 5 sec. The pressure run
was repeated using separate cell suspensions of each species to provide
duplicate data.