Advances in Food Mycology

240 Ailsa D. Hocking et al.
2. MATERIALS AND METHODS
2.1. Yeast and mould cultures
A single strain of each of two yeasts and four filamentous fungi
was chosen for inclusion in this study. The species/strains examined
were Saccharomyces cerevisiae FRR 1813, beer fermentation strain;
Pichia anomala FRR 5220 isolated from fermenting vanilla-blueberry
yoghurt; Penicillium expansum FRR 1536 isolated from mouldy pears;
Fusarium oxysporum FRR 5610, isolated from spoiled UHT treated
fruit juice; Byssochlamys fulva FRR 3792 from heat treated strawberry
puree; and Neosartorya fischeri FRR 4595 from heat treated strawberries.
FRR is the acronym of the fungal culture collection of Food
Science Australia, CSIRO, North Ryde, NSW. The two yeast species
were selected because of their propensity for spoilage of fruit products
and their ability to form ascospores, which may be more resistant to
high pressure treatment than vegetative cells. P. expansum was
included because of its significance in post harvest spoilage of apple
and pears and the consequent possibility of high numbers of spores
on pears before processing. F. oxysporum has recently been causing
spoilage problems in UHT processed juice products, indicating that it
may be heat resistant and, therefore, possibly pressure resistant also.
The two heat resistant moulds were included because it was considered
important to be able to control these species if a shelf stable fruit
product were to be developed.
2.2. Preparation of cell suspensions
The two yeasts were grown on Malt Extract agar (MEA) at 25°C
for 10 days. P. expansum was grown on Czapek Yeast Extract agar
(CYA) at 25°C for 7 days. F. oxysporum was grown on Tap Water Agar
with carnation leaf pieces at 25°C for 14 days under a light bank providing
a 12 hour photoperiod to induce formation of chlamydoconidia.
The formulae for these media are from Pitt and Hocking (1997).
The heat resistant moulds B. fulva and N. fischeri were grown on Malt
Extract Agar (MEA) at 30°C for 14 days. Ascospore production in the
yeasts and the heat resistant fungi was confirmed by microscopy. Cells
from culture plates were suspended in sucrose solution (20° Brix)
adjusted to pH 4.2 with citric acid, to yield ca 10 4 ascospores/ml for
heat resistant moulds, and 10 7 cfu/ml for other microorganisms.
Suspensions of cells from the two yeast species contained 20-25%
ascospores. Duplicate suspensions were prepared.