David Aldred

Post-harvest fungal ecology: Impact of fungal growth and mycotoxin accumulation in stored grain

Grain quality after harvest is influenced by a wide variety of abiotic and biotic factors and has been studied as a stored grain ecosystem. Important factors include grain and contaminant mould respiration, insects and mites, and the key environmental factors of water availability and temperature. Interactions between these factors influence the dominance of fungi, particularly mycotoxigenic species. Studies have shown that growth, mycotoxin production, competitiveness and niche occupation by mycotoxigenic species are influenced by the presence of other contaminant moulds and environmental factors. This has been demonstrated for both Fusarium culmorum and deoxynivalenol production, Aspergillus ochraceus/Penicillium verruscosum and ochratoxin production and Fusarium section Liseola and fumonisin production. Interactions between mycotoxigenic spoilage fungi and insects do occur but have not been studied thoroughly. Some insects disseminate mycotoxigenic species, others are known to use spoilage moulds as a food source, while others avoid certain fungal species. Thus, a more holistic ecological view is needed when considering management approaches to long-term-safe storage of cereal grains after harvest.

Water and temperature relations of growth and ochratoxin A production by Aspergillus carbonarius strains from grapes in Europe and Israel

Aims:  This study investigated the in vitro effects of water activity (aw; 0·85–0·987) and temperature (10–40°C) on growth and ochratoxin A (OTA) production by two strains of Aspergillus carbonarius isolated from wine grapes from three different European countries and Israel on a synthetic grape juice medium representative of mid‐veraison (total of eight strains).

Comparison of environmental profiles for growth and deoxynivalenol production by Fusarium culmorum and F. graminearum on wheat grain

Aims:  Comparisons were made of the effect of water activity (aw 0·99–0·85), temperature (15 and 25°C) and time (40 days) on growth/production of the trichothecene mycotoxin deoxynivalenol (DON) by Fusarium culmorum and Fusarium graminearum on wheat grain.

Conditions of formation of ochratoxin A in drying, transport and in different commodities

The major species of fungi responsible for ochratoxin production (OTA) in a range of commodities are Penicillium verrucosum, Aspergillus ochraceus and Aspergillus section Nigri, especially A. carbonarius. P. verrucosum is particularly important in northern Europe where damp cooler conditions occur and where drying regimes need to be efficient and effective for preventing post-harvest contamination with OTA. A. ochraceus can infect cereals including barley, maize, coffee, cocoa and edible nuts. A. carbonarius has been identified as the key species responsible for OTA contamination of grapes, wine and vine fruits. Recent studies have identified the environmental regimes, especially of temperature and humidity, which are conducive to growth and OTA production by these species pre- and post-harvest and during transport. The optimum and marginal interacting conditions for growth and OTA contamination often vary considerably. This has to be borne in mind when effective preventative management strategies are being implemented. Recent studies with cereals have suggested that isolation frequency can be related to OTA contamination of cereals. A logistical model has been developed and identified that 1000 CFUs g−1 grain of P. verrucosum (CFUs) is the threshold limit at which the probability of exceeding the EU legislative limit of 5 µg kg−1 in cereal grain can be predicted under different storage regimes. Controlled atmospheres >50% C02 is required to effectively prevent OTA accumulation in damp cereals. With regard to grapes, preharvest contamination with A. carbonarius has been shown to be a good indicator for regional risk in southern Europe from OTA contamination. The ecological conditions for optimum growth and OTA production have been shown to differ with the optima being 30–35°C and 15–25°C and 0.98–0.99 and 0.93–0.95 water activity, respectively. Studies on vine fruits (drying currants) suggest that OTA contamination and increase contamination levels occur during this drying process of 7–14 days. This will be influenced by prevailing weather conditions and drying rates. Minimizing OTA contamination in these and other commodities including coffee and cocoa require clear guidelines on safe moisture and temperature regimes pre- and post-harvest for the development of effective management strategies based on ecological criteria.

Water, temperature and gas composition interactions affect growth and ochratoxin A production by isolates of Penicillium verrucosum on wheat grain

Aims:  To examine the effect of interactions between water, temperature and gas composition on growth and ochratoxin A (OTA) production by isolates of Penicillium verrucosum in vitro and in situ on grain‐based media and wheat grain.

Limiting mycotoxins in stored wheat

The quality of harvested wheat grain can deteriorate markedly during the post-harvest management stages. Biotic factors, such as grain type and ripeness, coupled with the prevailing abiotic factors, such as water content and temperature, and also preservative concentration will influence the safe storage life and the level of contamination with mycotoxins. These mycotoxins include deoxynivalenol (DON) produced pre-harvest and zearalenone (ZEA) produced post-harvest by Fusarium graminearum and Fusarium poae, respectively, ochratoxin (OTA) produced by Penicillium verrucosum post-harvest in cool damp northern European climates, and perhaps T-2 and HT-2 toxins produced by Fusarium langsethiae. This review presents recent data on the relationship between dry matter losses caused by F. graminearum under different environmental regimes (water activities, temperatures) and the level of contamination with DON. This is important as poor post-harvest drying and storage management may exacerbate DON contamination already present pre-harvest. It is thus critical to relate the environmental factors in stored wheat grain during storage, especially of intergranular relative humidity (RH) and temperature, to safe storage periods without spoilage or risk from increased DON contamination. The growth/no growth and DON/no DON (F. graminearum) and OTA/no toxin production (P. verrucosum) have been used to build a model with a simple interface to link temperature and RH values to the potential risk level which may allow growth or toxin production. This paper also considers the use of modified atmospheres, preservatives and biocontrol to minimise DON and OTA in moist wheat grain. These approaches together with clear monitoring criteria and hygiene could contribute to better post-harvest management of stored temperate cereals and ensure that mycotoxin contamination is minimised during this key phase in the food/feed chain.

Environmental Factors and Interactions with Mycobiota of Grain and Grapes: Effects on Growth, Deoxynivalenol and Ochratoxin Production by Fusarium culmorum and Aspergillus carbonarius

Mycotoxigenic fungi colonizing food matrices are inevitably competing with a wide range of other resident fungi. The outcomes of these interactions are influenced by the prevailing environmental conditions and the competing species. We have evaluated the competitiveness of F. culmorum and A. carbonarius in the grain and grape food chain for their in vitro and in situ dominance in the presence of other fungi, and the effect that such interactions have on colony interactions, growth and deoxynivalenol (DON) and ochratoxin A (OTA) production. The Index of Dominance shows that changes in water activity (aw) and temperature affect the competitiveness of F. culmorum and A. carbonarius against up to nine different fungi. Growth of both mycotoxigenic species was sometimes inhibited by the presence of other competing fungi. For example, A. niger uniseriate and biseriate species decreased growth of A. carbonarius, while Aureobasidium pullulans and Cladosporium species stimulated growth. Similar changes were observed when F. graminearum was interacting with other grain fungi such as Alternaria alternata, Cladopsorium herbarum and Epicoccum nigrum. The impact on DON and OTA production was very different. For F. culmorum, the presence of other species often inhibited DON production over a range of environmental conditions. For A. carbonarius, on a grape-based medium, the presence of certain species resulted in a significant stimulation of OTA production. However, this was influenced by both temperature and aw level. This suggests that the final mycotoxin concentrations observed in food matrices may be due to complex interactions between species and the environmental history of the samples analyzed.

Influence of water activity and nutrients on growth and production of squalestatin S1 by a Phoma sp

D. ALDRED, N. MAGAN and B.S. LANE.1999.This study investigated the effects of temperature, nutrient status and water activity (aW) on the production of squalestatin S1 by a Phoma sp. The fungus was grown on malt extract (MEA), wheat extract (WEA), oat extract (OEA) and oil seed rape extract (OSREA) agars at 15, 20 and 25 °C and 0·998, 0·995, 0·990, 0·980 and 0·960 aW levels. The growth rate and secondary metabolite formation were followed over a total of 30 d. The maximum growth rate was observed at 25 °C and 0·998–0·990 aW for all media types, which was significantly reduced (P= 0·05) for most media at 0·96 aw. The growth rate was greatest for WEA and OEA but the growth form was an effuse exploitative type compared with the dense assimilative type on the richer MEA. The lipid‐based OSREA appeared to be a poor growth substrate for this fungus. In contrast to the growth rate data, squalestatin S1 production was maximal for all media types at slightly reduced aw in the range 0·990–0·980. There was greater production of the secondary metabolite under significant water stress (0·960 aW) compared with that with freely available water (0·998 aW). Maximum production was observed in WEA. Production began earlier in WEA and OEA compared with MEA. Squalestatin S1 production was not significantly affected by incubation temperature (P= 0·05). This study has shown that nutritionally depleted substrates may be usefully employed in the production of squalestatin S1 and perhaps also for other secondary metabolites.

Mould prevention in bread

Abstract: Bread is considered an intermediate-moisture food product that is prone to mould spoilage. Normally bread is eaten fresh or preserved using additives or modified atmosphere packaging. Demands for preservative-free food affects bread production and associated mould spoilage. This chapter considers the important mould species that can cause spoilage depending on the bread. We review the current systems and new methods for preserving bread. Using mixtures of physical methods and natural compounds and the use of nanotechnology in the formulation of products are also discussed in terms of their future use.

Ecophysiology of Fusarium culmorum and mycotoxin production

Fusarium ear blight is a cereal disease responsible for significant reduction in yield and quality of wheat grain throughout the world. In addition to degradation in grain quality, Fusarium species produce an array of mycotoxins which may contaminate the grain. This mycotoxin production occurs preharvest and during the early stages of drying (Botallico and Perrone, 2002; Magan et al., 2002). F. culmorum is the most common cause of Fusarium ear blight in the United Kingdom and some other countries and can produce trichothecenes including deoxynivalenol (DON) and nivalenol (NIV). DON and NIV are harmful to both animals and humans, causing a wide range of symptoms of varying severity, including immunosuppression. Germination of macroconidia of F. culmorum can occur over a wide range of temperatures (5-35°C) with a minimum aw near 0.86 at 20-25°C based on an incubation period of about 40 days (Magan and Lacey, 1984a). Longer term incubations on other media have suggested limits for germination of about 0.85 aw (Snow, 1949). The ecological strategies used by F. culmorum to occupy and dominate in the grain niche are not understood. Fungi can have combative (C-selected), stress (S-selected) or ruderal (R-selected) strategies or