Effect of preharvest fungicides and interacting fungi on Aspergillus carbonarius growth and ochratoxin A synthesis in dehydrating grapes

AIMS

To evaluate the effect of preharvest grape pesticides in Aspergillus section Nigri infection in dehydrating grapes and the final ochratoxin A (OTA) content. Additionally, the effect of coinoculation of moulds frequently isolated from grapes and raisins on Aspergillus section Nigri infection was studied.

METHODS AND RESULTS

Fungicide-treated grapes were inoculated with Aspergillus carbonarius, Aspergillus niger aggregate, Eurotium amstelodami and Penicillium janthinellum in different combinations, then dehydrated by reducing a(w) for 20 days. The percentages of colonized grapes treated with fungicides were, in general, lower, but no differences were observed among fungicides. The untreated grapes always showed higher concentrations of OTA, regardless of the inoculum applied. In general, Chorus was the most effective antifungal treatment in reducing OTA accumulation in grapes during dehydration. Penicillium janthinellum reduced Aspergillus section Nigri colonization and OTA accumulation in grapes during dehydration.

CONCLUSIONS

The four preharvest fungicides studied reduced the Aspergillus section Nigri growth and OTA production by A. carbonarius during dehydration of grapes.

SIGNIFICANCE AND IMPACT OF THE STUDY

The success of these chemical treatments might depend on the mycobiota composition of grapes.

Effect of germicidal UVC light on fungi isolated from grapes and raisins

AIMS

To examine how UVC affects the different genera of fungi commonly isolated from grapes, with the aim of understanding changes in mycobiota during grape ripening and possible applications for preventing grape decay during storage.

METHODS AND RESULTS

Spores of Aspergillus carbonarius, Aspergillus niger, Cladosporium herbarum, Penicillium janthinellum and Alternaria alternata (between 100-250 spores/plate agar) were UVC irradiated for 0 (control), 10, 20, 30, 60, 300 and 600 s. Plates were incubated at 25 degrees C and colonies were counted daily up to 7 days. Alternaria alternata and Aspergillus carbonarius were the most resistant fungi. Conidial germination in these species was reduced by approx. 25% after 10 s of exposure, compared with greater than 70% reduction for the remaining species tested. Penicillium janthinellum spores were the most susceptible at this wavelength. UVC exposures of 300 s prevented growth of all isolates studied, except for Alternaria alternata.

CONCLUSIONS

UVC irradiation plays a major role in selecting for particular fungi that dominate the mycobiota of drying grapes.

SIGNIFICANCE AND IMPACT OF THE STUDY

The UVC irradiation of harvested grapes could prevent germination of contaminant fungi during storage or further dehydration.

Patulin distribution in Fuji and Golden apples contaminated with Penicillium expansum

This work assesses the extent of patulin contamination in Penicillium expansum-infected apples stored at room temperature for short periods of time and its relationship with apple variety (Golden or Fuji), degree of ripeness and size of lesions. Inoculated apples were incubated at 20 degrees C. Patulin was determined in both sound and decayed tissue from cylindrical samples taken around the lesions and cut into 0.5-cm thick sections. Higher accumulation of patulin occurred in Golden apples, with less ripened apples showing higher concentrations. Total accumulated patulin was similar or higher in 4-cm compared to 2-cm lesioned apples, although a decrease in patulin concentration was observed in older lesion sections. Patulin accumulation occurred over a short period of time at room temperature, thus the stand-by period before processing should be minimised. Of total patulin, 2-6% migrated to the surrounding sound tissue, thus trimming tissue around the rotten part may be a good preventive practice for apple derivative production.

Impact of fungicides on Aspergillus carbonarius growth and ochratoxin A production on synthetic grape-like medium and on grapes

A study was undertaken to evaluate the impact of the application of several fungicide treatments used in Spanish vines on Aspergillus carbonarius growth and ochratoxin A production. Three trials were designed in order: (1) to screen 26 fungicides at the doses recommended by manufacturers on grape-like synthetic medium at 20 and 30 degrees C; (2) to find out the minimum inhibitory concentration of each fungicide for A. carbonarius growth on synthetic medium; and (3) to investigate the effect of several fungicides on A. carbonarius-inoculated grapes. In synthetic medium nine fungicides significantly reduced A. carbonarius growth rate. Meanwhile, 13 fungicides completely inhibited its growth. In general, growth was faster at 30 degrees C than at 20 degrees C, contrary to ochratoxin A production. Fungicides that stopped fungal growth also inhibited ochratoxin A production, but not all the fungicides that reduced growth reduced the ochratoxin A synthesis. In general, fungicides that contained copper or strobilurins reduced both growth and ochratoxin A production, contrary to sulphur fungicides. At the optimum temperature for A. carbonarius growth of 30 degrees C, higher amounts of fungicide were needed to prevent fungal growth than at 20 degrees C. Among the fungicides that inhibited A. carbonarius growth on synthetic medium at the initial doses, cyprodinil seemed to be the active ingredient more effective at stopping fungal growth when testing reduced doses. The fungicide effect on grapes was similar to that on synthetic medium. Both infection and ochratoxin A production were reduced when using cyprodinil (37.5%) plus fludioxonil (25%) and azoxystrobin (25%). Penconazole (10%) also showed a clear reduction in ochratoxin A production at both temperatures, although infection was only reduced at 20 degrees C. Ochratoxin A reduction was strain and temperature-dependent. In general, fenhexamid (50%), mancozeb (80%) and copper hydroxide (80%) plus copper (50%) enhanced infection and ochratoxin A production.

Patulin accumulation in apples by Penicillium expansum during postharvest stages

AIMS

The aim of this study was to assess the opportunities of Penicillium expansum to develop and produce patulin in apples during cold storage and in the steps prior to processing of apple products.

METHODS AND RESULTS

Two lots of apples var. Golden with different ripeness degree were used. Half of each lot was fungicide treated. Apples were inoculated with P. expansum and stored at 1 degrees C for 6 weeks. The extent of lesions and patulin accumulation both at the end of cold storage and after 3 days at 20 degrees C were assessed. Short storage at 20 degrees C aimed to simulate the transport and storage steps at room temperature before processing. Lesion size significantly increased during the storage at 20 degrees C. An interaction between fungicide treatment and ripeness degree was found; efficiency of fungicide treatment was higher for ripe apples. Although lesions were evident after cold storage, no patulin was detected. Patulin was detected only when fruits were further stored at 20 degrees C. Neither ripeness degree nor fungicide treatment affected patulin accumulation.

CONCLUSIONS

Cold storage periods of 6 weeks do not lead to patulin accumulation.

SIGNIFICANCE AND IMPACT OF THE STUDY

Shortening preprocessing times at warm temperatures would result into a reduction in patulin content at initial steps of fruits entering the processing plants.

Use of a MS-electronic nose for prediction of early fungal spoilage of bakery products

A MS-based electronic nose was used to detect fungal spoilage (measured as ergosterol concentration) in samples of bakery products. Bakery products were inoculated with different Eurotium, Aspergillus and Penicillium species, incubated in sealed vials and their headspace sampled after 2, 4 and 7 days. Once the headspace was sampled, ergosterol content was determined in each sample. Different electronic nose signals were recorded depending on incubation time. Both the e-nose signals and ergosterol levels were used to build models for prediction of ergosterol content using e-nose measurements. Accuracy on prediction of those models was between 87 and 96%, except for samples inoculated with Penicillium corylophilum where the best predictions only reached 46%.

Contamination of pine nuts by fumonisin produced by strains of Fusarium proliferatum isolated from Pinus pinea

AIMS

To test the ability of Fusarium proliferatum strains isolated from Pinus pinea to synthesize fumonisin B(1) (FB(1)) in pine nuts.

METHOD AND RESULTS

Eleven strains were inoculated in moist sterile shelled pine nuts and whole pine nuts, and incubated for 28 days. Moist sterile maize was inoculated in parallel as an optimum substrate for FB(1) production by Fusarium species.

CONCLUSIONS

Six of the strains produced FB(1) in shelled pine nuts, and two of them did so in whole pine nuts. IMPACT AND SIGNIFICANCE OF THE STUDY: F. proliferatum is able to grow and produce FB(1) in the husk of pine nuts and the mycotoxin can be found in the nut once shelled. Consequently, FB(1) may be an important hazard in pine nuts.

Effect of photoperiod and day–night temperatures simulating field conditions on growth and ochratoxin A production of Aspergillus carbonarius strains isolated from grapes

The effect of light and temperature regimes simulating day and night in the field in the months preceding grape harvest on Aspergillus carbonarius growth and ochratoxin A (OTA) production were investigated. Twelve-hour photoperiod affected positively A. carbonarius growth with no differences between incubating the mould at day temperature (28 degrees C) or alternating day/night temperatures (28 degrees C/20 degrees C). Slower growth, however, was observed with constant incubation at 20 degrees C. Under 12h-alternation periods of day and night temperatures, growth was faster at continuous darkness than under continuous light conditions. Light did not cause any morphological changes in the aspect of the colonies. No significant differences on OTA production were detected due to either fluctuating temperature or photoperiod. However, as photoperiod enhanced the growth of colonies, it also enhanced OTA accumulation. The ability of A. carbonarius to produce OTA reported in previous laboratory studies has been demonstrated to be stimulated in field conditions.

Mycobiota and ochratoxin A producing fungi from Spanish wine grapes

Grapes from three different regions with a long winemaking tradition in Spain were analysed at different growth stages in order to identify the ochratoxigenic mycobiota during three consecutive seasons. The correlation between meteorological parameters and ochratoxigenic fungi was studied and revealed a significant positive correlation between black aspergilli infection and temperatures in the month preceding each sampling date. No significant correlation was found with either relative humidity or rainfall. Biodiversity indexes were also calculated in this study. Black aspergilli species were the most abundant in grapes before harvest, and among them, Aspergillus carbonarius was the main ochratoxin A (OTA) producer species and represented 78-100% of the isolates tested. The results obtained support the key role of A. carbonarius as the main source of OTA contamination in grapes.

Non-specificity of nutritional substrate for ochratoxin A production by isolates of Aspergillus ochraceus

Aspergillus ochraceus is an important contaminant of diverse substrates, such as cereals, coffee, grapes and derivates. This fungus produce a nephrotoxic metabolite, ochratoxin A (OTA), whose presence on food and feeds may be an important risk for animal and human health. The aim of this work was to evaluate the significance of the origin of A. ochraceus isolates on their OTA production patterns on different substrates (yeast extract sucrose (YES) broth, irradiated barley grains, irradiated green coffee beans and sterilized grapes) and under different environmental conditions. Results did not show a significant influence of the isolation source on OTA-production profiles by A. ochraceus isolates on several substrates, since the isolates which produced the highest OTA amounts in vitro (YES medium) were also the isolates with the highest OTA yields on the other substrates. Abiotic factors assayed (water activity, temperature and substrate) affected significantly OTA productions by A. ochraceus. Maximum OTA amounts were detected at 25 degrees C and 0.98 a(w) on all substrates tested. The highest OTA accumulations found on the different substrates were: green coffee beans (> 2 mg g(-1)), barley grains (approximately 1 mg g(-1)), YES medium (13.9 microg ml(-1)) and grape (approximately 3 ng g(-1)).

Ecophysiology of ochratoxigenicAspergillus ochraceusandPenicillium verrucosumisolates. Predictive models for fungal spoilage prevention – a review

Ochratoxin A (OTA) is a secondary metabolite produced by several species of Aspergillus and Penicillium; among them Aspergillus ochraceus and Penicillium verrucosum are two ochratoxigenic species capable of growing in different climates and thus contamination of food crops with OTA can occur worldwide. OTA can be found in a wide range of foods such as cereals, coffee, cocoa, spices, beer, wine, dried vine fruit, grapes and meat products. OTA is toxic to animals, it presents neurotoxic, immunotoxic and nephrotoxic effects. It has been implicated in a human kidney disorder known as Balkan Endemic Nephropathy. This review focuses on the ecophysiology of ochratoxin-producing Aspergillus ochraceus and Penicillium verrucosum, the effect of environmental factors on their germination, mycelial growth, and OTA production. Knowledge of environmental conditions required for sucessive stages of fungal development represent the first step towards preventing mycotoxin formation. Predictive models for different stages of fungal development are presented, which allow prediction of the time before spoilage as a function of the abiotic factors. Finally, the implications of these studies in management of barley, coffee and grapes are described. This can help to identify the critical control points in their production, storage and distribution processes.

Effects of water activity and temperature on germination and growth profiles of ochratoxigenic Penicillium verrucosum isolates on barley meal extract agar

The aim of this work was to study the combined effects of water activity (aw, 0.75-0.99), temperature (10-30 degrees C) and isolate on spore germination and mycelial growth of ochratoxigenic Penicillium verrucosum isolates on barley meal extract agar (BMEA). Lag phases prior to germination (h), germination rates (h(-1)) and growth rates (mm day(-1)) were obtained at each set of conditions and for all the isolates. Analysis of variance showed a significant influence of the abiotic factors assayed on fungal development. Minimum aw levels found for germination and mycelial growth were 0.80 and 0.85, respectively, while maximum germination and growth rates were obtained at 0.95-0.99 aw level. Although maximum germination was obtained at 20 degrees C, significant differences were not found between growth rates at 20 and 30 degrees C. Data were modelled by MLR regressions and response surface models were obtained. Spore germination and mycelial growth of ochratoxigenic P. verrucosum isolates are significantly affected by water activity and temperature. Moreover, different isolates may have slightly different response to environmental factors. Predictive models obtained may allow a rough prediction of germination/growth as a function of the storage temperature and moisture content of barley grains.

Ochratoxin A-producing species in grapes and sun-dried grapes and their relation to ecophysiological factors

AIMS

To explain the dominance of OTA-producing fungal species in sun-dried grapes for special wine production through an ecophysiological approach.

METHODS AND RESULTS

Grapes at different ripening stages, sun-dried grapes and raisins were analysed for fungal presence, and isolates identified. Aspergillus section Nigri incidence in grapes increased with grape maturation. In the ecophysiological study five isolates (Alternaria alternata, Cladosporium herbarum, Penicillium decumbens, Aspergillus carbonarius, A. niger aggregate and A. section Nigri uniseriate) were inoculated in SNM medium at four a(w) (0.82-0.97) and incubated at 20, 30 and 40 degrees C for 18 d. Isolates were also inoculated in pairs to evaluate fungal interactions recording their growth rates and indexes of dominance. Aspergillus section Nigri grew in a wider range of temperature and a(w), had higher growth rates than the others under most of the conditions tested and showed behaviour usually dominant.

CONCLUSIONS

The presence of A. section Nigri is predominant in grapes during harvesting and sun-drying period likely because of a better adaptation to hot and humid environment.

SIGNIFICANCE AND IMPACT OF THE STUDY

The duration of the drying period should be reduced as much as possible without compromising the quality of the final product, or drying the grapes in controlled chambers with dry hot air flow.

Fate of mycotoxins in cereals during extrusion cooking: a review

Extrusion cooking is one of the fastest growing food-processing operations in recent years due to several advantages over traditional methods. Apart from its main goal of improving the quality of intermediate and final processed products, it may incidentally also improve safety because of the potential to reduce mycotoxin levels in cereals. This review is focused on extrusion cooking and aims to give a general overview of its impact in reducing mycotoxin levels in cereals. Extrusion cooking generally decreases the mycotoxins levels at rates depending on different factors such as the type of extruder, the type of screw, the die configuration, the initial mycotoxin concentration, the barrel temperature, the screw speed, the moisture content of the raw material and the use of additives. Reductions of 100, 95 and 83% for fumonisins, aflatoxins and zearalenone, respectively, have been reported during extrusion cooking of cereals, while lower reductions were observed for deoxynivalenol, ochratoxin A and moniliformin, where maximum reductions did not exceed 55, 40 and 30%, respectively.

Study of benzoate, propionate, and sorbate salts as mould spoilage inhibitors on intermediate moisture bakery products of low pH (4.5–5.5)

A hurdle technology approach has been applied to control common mold species causing spoilage of intermediate moisture bakery products (Eurotium spp., Aspergillus spp., and Penicillium corylophilum), growing on a fermented bakery product analogue (FBPA). The factors studied included a combination of different levels of weak acid preservatives (potassium sorbate, calcium propionate, and sodium benzoate; 0-0.3%), pH (4.5-5.5), and water activity (a(w); 0.80-0.90). Potassium sorbate was found to be the most effective in preventing fungal spoilage of this kind of products at the maximum concentration tested (0.3%) regardless of a(w). The same concentration of calcium propionate and sodium benzoate was effective only at low a(w) levels. On the other hand, potassium sorbate activity was slightly reduced at pH 5.5, the 0.3% being only effective at 0.80 a(w). These findings indicate that potassium sorbate may be a suitable preserving agent to inhibit deterioration of a FBPA of slightly acidic pH (near 4.5) by xerophilic fungi. Further studies have to be done in order to adjust the minimal inhibitory concentration necessary to obtain a product with the required shelf life.

Aspergillus carbonarius growth and ochratoxin A production on a synthetic grape medium in relation to environmental factors

AIMS

The effects of water activity (0.90-0.99 a(w)), temperature (15-37 degrees C), and their interaction on growth and ochratoxin A (OTA) production by eight isolates of Aspergillus carbonarius were investigated on synthetic nutrient medium (SNM) with composition similar to grapes.

METHODS AND RESULTS

Growth data were modelled by an multiple linear regression and response surface models were obtained. Aspergillus carbonarius grew much faster at 30 degrees C than at the other temperature levels tested and its growth rate increased with increasing a(w), maximum growth rate being between 0.95 and 0.99 a(w). In general, isolates grew faster at 35-37 degrees C than at 20 degrees C, although no significant differences were found between these temperatures. OTA accumulation was also favoured by high a(w) levels, and although it was observed in the whole range of temperatures, maximum amounts were detected at 20 degrees C. No OTA was found at the most unfavourable growth conditions.

CONCLUSIONS

Optimum a(w) level for growth seems to correspond with optimum for OTA production, meanwhile the most propitious temperature for the toxin production was below the best one for growth.

SIGNIFICANCE AND IMPACT OF THE STUDY

Prediction of A. carbonarius growth would allow estimating their presence and therefore, the OTA production, as it was found that conditions for the toxin production were more limited than those permitting growth.

Modelling of effects of water activity and temperature on germination and growth of ochratoxigenic isolates of on a green coffee-based medium

Influence of water activity (0.75-0.99 a(w)) and temperature (10, 20 and 30 degrees C) on germination and mycelial growth on green coffee extract agar medium of three ochratoxigenic isolates of Aspergillus ochraceus was studied. Optimal conditions for germination and growth were observed at 0.95-0.99 a(w) and 20-30 degrees C for the three isolates. Minimum a(w) level for germination was 0.80, and 0.85 for mycelial growth. At marginal a(w) and temperature levels assayed, the lag phases prior to germination increased and the growth rates showed a significant decrease in comparison with the optimal conditions. Data were modelled by a multiple linear regression (MLR) and response surface models were obtained. Germination and growth of A. ochraceus in green coffee beans could be prevented or at least inhibited to some extent by minimising the time that coffee beans are exposed to temperature and humidity conditions near to the optimum during processing and storage. This could be an empirical approach to predict the effects of water activity and temperature conditions on the development of ochratoxigenic isolates of A. ochraceus during handling and storage of green coffee.

Effect of water activity and temperature on mycelial growth and ochratoxin A production by isolates of Aspergillus ochraceus on irradiated green coffee beans

Aspergillus ochraceus as a fungal contaminant and ochratoxin A (OTA) producer plays an important role in coffee quality. Temperature and water activity (a(w)) significantly influence mycelial growth and OTA production by isolates of A. ochraceus on green coffee beans. Maximum mycelial growth was found at 30 degrees C and 0.95 to 0.99 a(w). A marked decrease in growth rate was observed when temperature and a(w) were reduced. At 0.80 a(w), mycelial growth occurred only at 30 and 20 degrees C for one isolate. Maximum OTA production was found at 20 degrees C and 0.99 a(w). At 10 degrees C, OTA was not produced, regardless of a(w). Similarly, no OTA was detected at 0.80 a(w). OTA production ranged from the limit of detection (40 ng g(-1) of green coffee) to 17,000 ng g(-1) of green coffee. Significant intraspecific differences in mycelial growth and OTA production were found. Primary data for lag phases prior to mycelial growth under the influence of temperature and a(w) were modelled by multiple linear regression, and the response surface plots were obtained.

Prediction of fungal growth and ochratoxin A production by Aspergillus ochraceus on irradiated barley grain as influenced by temperature and water activity

Ochratoxin A (OTA) is a secondary metabolite of Aspergillus and Penicillium species, including Aspergillus ochraceus, a species that can be found in stored cereal grains such as barley. The objective of this study was to determine the effects of water activity (a(w), 0.80-0.99), temperature (10, 20, 30 degrees C), and A. ochraceus isolate differences on radial growth and OTA production in irradiated barley grains. The three isolates showed optimal conditions for growth and ochratoxin A production at 0.99 a(w) and 30 degrees C, with a marked decrease of growth rates and OTA production at the lowest levels of a(w) and temperature assayed. The minimum a(w) level for growth, observed in this study, was 0.85 and 0.90 a(w) for OTA production. Significant differences among the isolates were found. Lag phases prior to fungal growth and OTA production values were modelled by multiple linear regression and response surface models. These models could provide an approximate prediction of growth and OTA production.

Influence of water activity and temperature on growth of isolates of Aspergillus section Nigri obtained from grapes

The effects of water activity (aw) and temperature on growth of Aspergillus section Nigri isolated from wine grapes were investigated on an agar medium with composition similar to that of grapes. Temperatures in the range of 10-37 degrees C were tested. Optimum temperatures for growth were between 30 and 37 degrees C. Water activity levels ranging from 0.90 to 0.995 were tested. Optimum aw for growth was 0.98 in most cases. Statistical differences were found among the groups tested (A. carbonarius, A. niger aggregate and A. section Nigri uniseriates). Growth rates models for the factors assayed have been obtained.

Impact of essential oils on growth rate, zearalenone and deoxynivalenol production by Fusarium graminearum under different temperature and water activity conditions in maize grain

AIMS

The effect of five essential oils (oregano, cinnamon, lemongrass, clove and palmarose) on growth rate, zearalenone (ZEA) and deoxynivalenol (DON) production by Fusarium graminearum strains was assessed.

METHODS AND RESULTS

The influence of the essential oils was tested on irradiated maize at two concentrations (500 and 1000 mg kg-1), at different water activity (aw) (0.95 and 0.995) and temperature (20 and 30 degrees C) levels. At 0.995 aw all essential oils tested had an inhibitory effect on growth rate of F. graminearum at both temperatures studied. At this aw level, DON production in general was inhibited by all essential oils at 30 degrees C and, although palmarose and clove were the only essential oils with statistically significant inhibitory effect on ZEA production, an inhibitory trend was observed when cinnamon and oregano oils were added to maize grain.

CONCLUSIONS

Antifungal and antimycotoxigenic activity of the essential oils assayed was shown to depend on environmental conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

It is apparent that essential oils should be considered as alternative preharvest natural fungicides. Further investigation on natural maize grain might be useful to study the effectiveness of these essential oils in the presence of natural mycoflora of maize grain.