Advances in Food Mycology

56 M. H. Taniwaki et al.
3.2. Growth of Fungi as Assessed by Colony Diameters
Most of the fungi grew on all of the media used, though with varying
vigour, reflecting their water relations (Table 3). Best growth of
most species occurred on CY20S, 0.98 a w except for B. fulva and F.
oxysporum which grew faster at 25°C on CYA, 0.997 a w . Along with
M. plumbeus, these species produced little or no growth on MY50G at
0.89 a w , an a w near their lower limit for growth (Pitt and Hocking,
1997). A. flavus and P. commune grew strongly at all a w tested. Growth
of E. chevalieri, a xerophilic species, was slow on CYA, and faster on
CY20S and MY50G. X. bisporus, an extreme xerophile, grew only on
MY50G.
3.3. Influence of Colony Age on Growth Parameters
The influence of colony age on some of the data obtained by the
four methods used for measuring fungal growth is shown in Table 4,
for four representative species. Ratios were calculated for colony
diameter over hyphal length, mycelium dry weight and ergosterol content
over hyphal length, and ergosterol over mycelium dry weight. The
ratio of colony diameter over hyphal length showed a general downward
trend, indicating a greater rate of hyphal extension than colony
diameter increase as colonies aged. Colony diameters are therefore not
a good measure of fungal biomass production in aging colonies. The
other ratios were reasonably constant, indicating a general correspondence
between mycelial dry weight, ergosterol content and hyphal
length.
Table 3. Colony diameters of fungi grown on media of various water activities at
25°C
Species Colony diameter (mm) at 7 days on
CYA 0.997 a w CY20S 0.98 a w MY50G 0.89 a w
Aspergillus flavus 68 84 16
Byssochlamys fulva 78 58 0
Byssochlamys nivea 43 - a -
Eurotium chevalieri 19 50 36
Fusarium oxysporum 90 78 6
Mucor plumbeus 63 85 4
Penicillium commune 32 44 18
Penicillium roqueforti 52 - -
Xeromyces bisporus 0 0 12
a not tested