Advances in Food Mycology

High Pressure Inactivation of Fungi 245
4. DISCUSSION
Inactivation of yeasts by ultra high pressure has been reported previously
(Ogawa et al., 1990; Palou et al., 1997; Parish, 1998; Zook
et al., 1999; Basak et al., 2002). Most studies have investigated the
effects of HPP on the common spoilage yeasts Saccharomyces
cerevisiae and Zygosaccharomyces bailii. The responses of Pichia
anomala to HPP treatment have not previously been reported. As
observed in this study, other workers have shown that treatment at 400
MPa or higher will result in inactivation of yeasts provided that the
pressure is applied for several minutes. However, the time required for
inactivation also depends on the composition of the menstruum in
which the yeasts are suspended. Yeasts are less susceptible to pressure
treatment when suspended in juice concentrate than in single strength
juice (Ogawa et al., 1990; Basak et al., 2002). Although ascospores of
yeasts are more resistant to pressure treatment than vegetative cells,
they are relatively quickly inactivated at 500 MPa, with D values of
less than 0.2 min in single strength juice (Zook et al., 1999).
The conidia of Penicillium expansum were more sensitive to pressure
treatment than yeast cells. Our study showed a 6 log 10 reduction in
viability of P. expansum conidia pressure treated at 400 MPa for 60 sec.
A suspension of microconidia and chlamydoconidia of F. oxysporum
was even more sensitive to pressure treatment than P. expansum. These
results are in agreement with results reported by Ogawa et al. (1990)
who found that treatment at 400 MPa at room temperature resulted in
at least 4 log 10 reduction in viability of conidia of Aspergillus awamori
and sporangiospores of Mucor plumbeus. Voldrich et al. (2004) also
reported that conidia of Talaromyces avellaneus exhibited comparable
pressure sensitivity to vegetative cells of other yeast and mould species.
Ascospores of the two heat resistant mould species were relatively
resistant to the pressure treatments received, as observed by other workers
(Butz et al., 1996; Reyns et al., 2003; Voldrich et al., 2004).
Blanching at 95°C for 5 min did not affect the pressure resistance, and
in fact, ascospores of N. fischeri became more resistant to pressure after
this mild heat treatment. The ascospores used in these studies were relatively
young (from 14 day old cultures), and it could be expected that
older ascospores would be more pressure resistant, as heat resistance in
N. fischeri ascospores increases with age (Conner at al., 1987).
The results of this study indicate that high pressure processing (600
MPa for several minutes) should be sufficient to inactivate vegetative
cells of yeasts and moulds and also the ascospores of yeasts, providing
a process that is equivalent to pasteurisation. However, treatment