Abstract:
This study investigated the effects of interacting environmental factors on the
ecophysiology of Aspergillus carbonarius and ochratoxin A (OTA) production, to
facilitate the building of a database of knowledge for risk prevention. Spore
germination and germ tube extension was faster on grape flesh than in vitro on grape-
based media, or grape skin. Under optimum conditions (25-35°C; 0.90-0.99 water
activity (aw)) 50% of conidia germinated in < 4.5 hours. Two dimensional profiles in
relation to aw x temperature were constructed for rate of germination and germ tube
extension for the first time. Mycelial growth was also influenced by aw, temperature,
pH and modified atmospheres. Two dimensional temporal profiles of growth and OTA
production were constructed based on information on strains from four different
European countries. This showed that the optimum aw and temperature, and range were
0.98 aw, 30°C and 0.99-0.88 aw and 15-40°C, respectively. In contrast, for OTA
production these were 0.95 aw, 15-20°C, and 0.99-0.93 aw and 15-35°C respectively.
Environmental factors also influenced mycelial interactions between A. carbonarius
and nine other common grape fungi. An Index of Dominance showed that A.
carbonarius was very competitive against these species over a range of aw levels and
temperatures. OTA production was generally suppressed by interactions at 0.95 aw
with some inhibition of OTA production at 30°C at 0.99 and 0.95 aw. At 0.99 and
20°C all strains except two species produced a stimulation in OTA production.
The efficacy of fungicides (5) and anti-oxidants (2) on growth and OTA production
was limited at the concentration used. The only two fungicides that did suppress growth were Switch (cyprodinil and fludioxonil) and carbendazim. OTA production
was stimulated at sub-optimal concentrations regardless of aw level used. Anti-oxidants
were ineffective under all conditions tested. Modified atmospheres (0.03, 25 and 50%
CO2) at different aw levels showed that there was very little long term effect on spore
germination and germ tube extension, even at 50% CO2. Regardless of aW) OTA
production was influenced more by the aw itself than CO2 concentrations. The efficacy
of Sodium metabisulphite (NaMBS) at different aw levels (0.99, 0.97, 0.93 aw) on grape
juice-based media at 25°C on germination and germ tube extension showed inhibition
by >500 mg kg' 1 of NaMBS. Low concentrations (100, 250 mg kg'1) stimulated growth
and OTA production. LD50 and LD90 ranges were determined for both growth and
OTA production for the first time.
Field sampling found that black aspergill! were present on grapes from setting to
harvest with a significant increase in the population with grape ripening, and higher
temperatures. The most commonly isolated OTA species from grapes was A.
carbonarius with no Pénicillium verrucosum and only a few Aspergillus ochraceus
isolated. In all the field sampling found no OTA present in any of the grape musts.