J.I. Pitt

Influence of Solute and Hydrogen Ion Concentration on the Water Relations of some Xerophilic Fungi

Germination and growth of six xerophilic fungi, Aspergillus flavus, Aspergillus ochraceus, Eurotium chevalieri, Chrysosphorium fastidium, Wallemia sebi and Xeromyces bisporus were examined on media of a wide range of water activities (aw). The influence of three solutes, NaCl, glycerol and a glucose/fructose mixture, was studied at pH 4-0 and pH 6-5 using a plate-slide technique. Germination times and growth rates were affected by solute type, but the influence of pH was less marked. Except for Wallemia sebi, the fungi grew most strongly on glucose/fructose and were partially or completely inhibited by NaCl. The results showed that a universal isolation medium for xerophilic fungi could be based on glycerol or glucose/fructose but not on NaCl as aw-limiting solute.

Growth of fungi and mycotoxin production on cheese under modified atmospheres.

The use of modified atmospheres to prevent fungal growth and mycotoxin production in cheese was evaluated. Eight fungal species: Mucor plumbeus, Fusarium oxysporum, Byssochlamys fulva, B. nivea, Penicillium commune, P. roqueforti, Aspergillus flatus and Eurotium chevalieri were inoculated onto cheese and incubated under conditions of decreasing concentrations of O2 (5% to < 0.5%) and increasing concentrations of CO2 (20-40%). Fungal growth was measured by colony diameter and ergosterol content. All fungi examined grew in atmospheres containing 20% and 40% CO2 with 1% or 5% O2, but growth was reduced by 20-80%, depending on species, compared with growth in air. The formation of aflatoxins B1 and B2, roquerfortine C and cyclopiazonic acid was greatly decreased but not totally inhibited in these atmospheres. At 20% or 40% CO2 with < 0.5% O2, only B. nivea exhibited growth, which was very slow. Growth of F. oxysporum, B. fulca, P. commune and A. flavus showed good correlations between colony diameter and ergosterol content. However, for the other species correlations were inconsistent.

Mycotoxins in Australia: biocontrol of aflatoxin in peanuts

The major mycotoxin problem in Australia is the formation of aflatoxins in peanuts by Aspergillus flavus and A. parasiticus. This is controlled by good farm management practice, segregation into grades on aflatoxin content at intake to shelling facilities, colour sorting and aflatoxin assays. A second problem is the potential presence of ochratoxin A in grapes and grape products, resulting from infection by Aspergillus carbonarius. Good quality control before and during wine making ensures ochratoxin A is kept to very low levels, but in dried vine fruit, ochratoxin A levels may be higher. Biocontrol by competitive exclusion has been developed as the most promising means of controlling aflatoxins in peanuts. Some details of the process are given, including some basic laboratory experiments.

Growth and mycotoxin production by food spoilage fungi under high carbon dioxide and low oxygen atmospheres

The influence of high carbon dioxide and low oxygen concentrations on growth by the foodborne fungal species, Mucor plumbeus, Fusarium oxysporum, Byssochlamys fulva, Byssochlamys nivea, Penicillium commune, Penicillium roqueforti, Aspergillus flavus, Eurotium chevalieri and Xeromyces bisporus was investigated. Production of aflatoxin by A. flavus, patulin by B. nivea and roquefortine C by P. roqueforti was also studied. Fungal growth was evaluated under atmospheres consisting of 20, 40 and 60% CO(2) plus <0.5% O(2), on two media, Czapek Yeast Extract agar and Potato Dextrose agar. Several methods for measuring fungal growth were used: colony diameter, ergosterol content, hyphal length and/or mycelium dry weight. Among the nine species, three groups were distinguished with respect to their growth responses under modified atmospheres: (i) species which did not grow in 20% CO(2) <0.5% O(2) (P. commune, E. chevalieri and X. bisporus); (ii) species which grew in 20% CO(2) <0.5% O(2), but not 40% CO(2) <0.5% O(2) (P. roqueforti and A. flavus); (iii) species which grew in 20%, 40% and 60% CO(2) <0.5% O(2) (M. plumbeus, F. oxysporum, B. fulva and B. nivea). Facultatively anaerobic behaviour was observed in these last four species, which grew under the same conditions as the obligate anaerobe, Clostridium sporogenes. The production of aflatoxin, patulin, and roquefortine C was greatly reduced under all of the atmospheres tested.

Water relations of Alternaria alternata, Cladosporium cladosporioides, Cladosporium sphaerospermum, Curvularia lunata and Curvularia pallescens

The effects of water activity (aw) on the growth rates of two isolates of Alternaria alternata and a single isolate each of Cladosporium cladosporioides, Clad. sphaerospermum, Curvularia lunata and Curv. pallescens were examined at 25 °C. The aw of the growth media was controlled by glucose/fructose. The minimum aw for germination of A. alternata, Curvularia lunata and Curv. pallescens was 0·855. Cladosporium cladosporioides and Clad. sphaerospermum germinated at a minimum aw of 0·815. Clad. sphaerospermum also grew at this aw, but germinating conidia of Clad. cladosporioides did not produce microcolonies.

Growth and mycotoxin production by fungi in atmospheres containing 80% carbon dioxide and 20% oxygen.

The effect of atmosphere containing 80% CO(2) and 20% O(2) on growth of Mucor plumbeus, Fusarium oxysporum, Byssochlamys fulva, Byssochlamys nivea, Penicillium commune, Penicillium roqueforti, Aspergillus flavus, Eurotium chevalieri and Xeromyces bisporus was investigated. Production of aflatoxin by A. flavus, patulin by B. nivea, roquefortine C by P. roqueforti, and cyclopiazonic acid by P. commune was also studied. Fungal growth was evaluated by three methods: colony diameter, hyphal length or mycelium dry weight and ergosterol content. Among the nine fungal species examined, two E. chevalieri and X. bisporus, did not grow under these conditions. In this study, fungi differed in their response to modified atmospheres in biomass, ergosterol content, mycotoxin production and morphology. Reductions of 57.8-96.9%, 73.7-99.6% and 91.5-99.9% were obtained in colony diameter, hyphal length and ergosterol content, respectively, under this atmosphere compared to air. Ergosterol content was more affected in most species than other measurements. Patulin, cyclopiazonic acid and roquefortine C were produced in this atmosphere, although levels were very low and aflatoxin was not produced at all. Growth was quite extensive as measured by colony diameters, but hyphal lengths were low and ergosterol production was also affected in all species of this study.

Corrections to species names in physiological studies on Aspergillus flavus and Aspergillus parasiticus

A detailed review of papers reporting physiological studies on the aflatoxin producing fungi Aspergillus flavus and Aspergillus parasiticus has been undertaken. Critical examination has shown that work reportedly on A. flavus has frequently used isolates correctly identified as A. parasiticus . Several studies reportedly on A. flavus have also actually examined the physiology of the recently described species Aspergillus nomius . Correct identifications are provided where possible. In light of this report, further physiological studies on A. flavus , using correctly identified isolates, are urgently needed.

Influence of Temperature on the Water Relations of Polypaecilum pisce and Basipetospora halophila, Two Halophilic Fungi

SUMMARY: Growth Characteristics Of Polypaecilum Pisce And Basipetospora Halophila, Two Fungi Isolated From Dried Salt Fish, Were Examined At 20, 30, 34 And 37°C Over A Range Of Water Activity (AW) Values (0.99-0.70) On Media Containing Various Concentrations Of Nacl Or Glucose/Fructose. At High Aw Growth Of P. Pisce Was Similar On Media Containing Either Solute, But Below 0.90 AW Growth Was More Rapid On Nacl Media, Indicating That P. Pisce Can Be Classified As A Halophilic Fungus. The Minimum AW For Growth On Nac1-Based Media Was 0.746; On Glucose/Fructose Media The Minimum Was 0.700 AW. Optimal Growth, I.E. Growth At 80% Or More Of The Maximum Growth, Occurred Between 0.96 And 0.91 Aw at 30°C. Over the wide range of temperatures studied, B. halophila exhibited more rapid growth on NaCl media than on glucose/fructose media over the entire range of aw at which growth occurred. The optimal growth rate on NaCl- based media occurred between 0.89 and 0.86 aw, an exceptionally low value. B. halophila can be classified as an extreme halophile, markedly intolerant of high aw, particularly on glucose/fructose-based media at temperatures above 30°C.

Biocontrol of aflatoxins in peanuts

Aflatoxins remain the most important mycotoxin problem in the world, and peanuts are a major source of these toxins. Control of aflatoxin levels in peanuts on a commercial scale is possible by colour sorting and aflatoxin assays on shelled peanuts, and this is widely practised in developed countries. However, this procedure is expensive.

Current mycotoxin issues in Australia and Southeast Asia

Problems associated with the potential or actual occurrence of mycotoxins in food are a significant element in food safety worldwide and our local region is no exception.