Impact of water activity and temperature on growth and ochratoxin A production of two Aspergillus carbonarius isolates from wine grapes in Greece

Climate Change and Effects on Molds and Mycotoxins

Earth’s climate is undergoing adverse global changes as an unequivocal result of anthropogenic activity. The occurring environmental changes are slowly shaping the balance between plant growth and related fungal diseases. Climate (temperature, available water, and light quality/quantity; as well as extreme drought, desertification, and fluctuations of humid/dry cycles) represents the most important agroecosystem factor influencing the life cycle stages of fungi and their ability to colonize crops, survive, and produce toxins. The ability of mycotoxigenic fungi to respond to Climate Change (CC) may induce a shift in their geographical distribution and in the pattern of mycotoxin occurrence. The present review examines the available evidence on the impact of CC factors on growth and mycotoxin production by the key mycotoxigenic fungi belonging to the genera Aspergillus, Penicillium, and Fusarium, which include several species producing mycotoxins of the greatest concern worldwide: aflatoxins (AFs), ochratoxins, and fumonisins (FUMs).

Intraspecific variability of growth and ochratoxin A production by Aspergillus carbonarius from different foods and geographical areas

Ochratoxin A (OTA) is a nephrotoxic mycotoxin naturally found in a wide range of food commodities throughout the world. Aspergillus carbonarius is the most important source of OTA in food commodities such as wine, grapes and dried vine fruits and is also responsible for the formation of OTA in coffee. The aim of this study was to determine the simultaneous effect of three culture media (Czapek Yeast Extract Broth (CYB); Synthetic Grape Juice Medium (SGM) and White grape juice (WGJ)) at three water activity (a_w) levels (0.90; 0.95 and 0.98-0.99), and three incubation temperatures (15 °C, 25 °C and 35 °C) on the growth and OTA production by 16 strains of A. carbonarius. The strains were mainly isolated from grapes from areas with a Mediterranean climate. All the strains were confirmed for identity by sequencing of the calmodulin gene. The assay was performed in microtiter plates, determining the absorbance at 530 nm and the concentration of OTA after 1, 2, 4 and 10 days of incubation. No significant differences were observed in absorbance values between the strains. The highest absorbance values were recorded in CYB at 0.99 a_w and at 0.95 a_w after 10 days of incubation at 25 °C and 35 °C. None of the strains were able to grow at 0.90 a_w and 15 °C in any culture media after 10 days of incubation. OTA concentration was statistically higher at 15 °C than at 25 °C or 35 °C. The highest significant OTA values were obtained at 0.98-0.99 a_w and the best culture media for OTA production was CYB, followed by WGJ and SGM. While strains isolated from Mediterranean climate foods had a similar behavior despite being isolated from different geographical areas, OTA concentration produced by one Robusta coffee strain from Thailand was statistically higher at 25 °C than at 15 °C. This would suggest that the type of food matrices and consequently the adaptation of A. carbonarius strains to different climatic conditions would have a greater influence on the ecophysiology of the strains than only their geographical origin.

Monitoring the Temporal Expression of Genes Involved in Ochratoxin A Production of Aspergillus carbonarius under the Influence of Temperature and Water Activity

The objective of this study was to investigate the effect of environmental factors, namely temperature and water activity, on genes involved in the regulation of ochratoxin A (OTA) production over time. For this purpose, the previously characterized toxigenic Aspergilluscarbonarius Ac29 isolate from Greek vineyards and the A. carbonarius ITEM 5010 reference strain were subjected to combined temperature and water activity (a_w) treatments to study OTA production and relative gene expression. The fungal isolates were grown on a synthetic grape juice liquid medium (SGM) under different temperature (20 °C, 25 °C and 30 °C) and a_w (0.94 and 0.98) regimes. The expression of the AcOTApks, AcOTAnrps, and laeA OTA related genes was investigated using real time PCR. Gene expression was monitored at the same time points, along with fungal biomass and OTA accumulation at three, six and nine days of incubation. In gene expression analysis, stimulation of the biosynthetic genes was observed a few days before any toxin could be detected. This fact may underline a possible early indicator of potential toxin contamination of grapes. However, the transcript levels varied with respect to the different combinations of ecophysiological conditions and time, highlighting a complex regulation of OTA related gene expression of A. carbonarius in the specific medium.

Effect of post-harvest treatments on the occurrence of ochratoxin A in raw cocoa beans

Cocoa beans are the principal raw material for chocolate manufacture. Moulds have an important place in the change in the quality of cocoa beans due to their role in the production of free fatty acids and mycotoxins, namely ochratoxin A (OTA). This study investigated the impact of the key post-harvest treatments, namely the fermentation and drying methods on OTA contamination of raw cocoa beans. Analytical methods for OTA detection were based on solid-liquid extraction, clean-up using an immunoaffinity column, and identification by reversed-phase HPLC with fluorescence detection. Of a total of 104 randomly selected cocoa samples analysed, 32% had OTA contents above 2 µg kg(-1). Cocoa sourced from pods in a bad state of health had a maximum OTA content of 39.2 µg kg(-1), while that obtained from healthy pods recorded 11.2 µg kg(-1). The production of OTA in cocoa beans increased according to the pod-opening delay and reached 39.2 µg kg(-1) after an opening delay of 7 days after harvest, while 6.1 and 11.2 µg kg(-1) were observed when pods were opened after 0 and 4 days. OTA production also seemed to depend considerably to the cocoa fermentation materials. When using plastic boxes for bean fermentation, the OTA production was enhanced and reached an average OTA content of about 4.9 µg kg(-1), while the raw cocoa treated in banana leaves and wooden boxes recorded 1.6 and 2.2 µg kg(-1) on average respectively. In parallel, the OTA production was not really influenced by either the mixing or the duration of the fermentation or the drying materials.

Effect of temperature, water activity, and pH on growth and production of ochratoxin A by Aspergillus niger and Aspergillus carbonarius from Brazilian grapes

The growth of ochratoxigenic fungus and the presence of ochratoxin A (OTA) in grapes and their derivatives can be caused by a wide range of physical, chemical, and biological factors. The determination of interactions between these factors and fungal species from different climatic regions is important in designing models for minimizing the risk of OTA in wine and grape juice. This study evaluated the influence of temperature, water activity (aw), and pH on the development and production of OTA in a semisynthetic grape culture medium by Aspergillus carbonarius and Aspergillus niger strains. To analyze the growth conditions and production of OTA, an experimental design was conducted using response surface methodology as a tool to assess the effects of these abiotic variables on fungal behavior. A. carbonarius showed the highest growth at temperatures from 20 to 33°C, aw between 0.95 and 0.98, and pH levels between 5 and 6.5. Similarly, for A. niger, temperatures between 24 and 37°C, aw greater than 0.95, and pH levels between 4 and 6.5 were optimal. The greatest toxin concentrations for A. carbonarius and A. niger (10 μg/g and 7.0 μg/g, respectively) were found at 15°C, aw 0.99, and pH 5.35. The lowest pH was found to contribute to greater OTA production. These results show that the evaluated fungi are able to grow and produce OTA in a wide range of temperature, aw, and pH. However, the optimal conditions for toxin production are generally different from those optimal for fungal growth. The knowledge of optimal conditions for fungal growth and production of OTA, and of the stages of cultivation in which these conditions are optimal, allows a more precise assessment of the potential risk to health from consumption of products derived from grapes.

Aspergillus carbonarius growth and ochratoxin A production on irradiated dried grapes under different water activity and temperature conditions

Grapes have different destinations. The most important in the national and international market is wine production, but another is dehydration to produce raisins. Dried vine fruits are at greater risk of ochratoxin A (OTA) contamination than wine grapes because the ratio of Aspergillus carbonarius to Aspergillus niger aggregate increases during drying. The growth of these species, and consequently OTA production, can be influenced by different environmental factors, the two most important being water activity (aW) and temperature. The objective of the present work was to evaluate the lag phase, growth rate and OTA production by two A. carbonarius isolates on irradiated dried grapes at different aW (0.910, 0.928, 0.955, 0.973 and 0.995), temperatures (15, 25 and 30 °C) and incubation times (7, 14 and 21 days). Growth was observed at all aW and temperature ranges assayed. No significant differences between the growth rates reached at 25 °C and 30 °C by both isolates were observed. At the assayed conditions, OTA production occurred over the whole range of temperatures (15-30 °C), with the maximum at 25 and 30 °C depending on the aW. In general, OTA concentration increased as aW increased with no statistically significant differences at the tested incubation times. This work provides information that can be used by companies for the purpose of preventing A. carbonarius and OTA contamination during storage on this and other substrates (e.g. dried prunes, figs and apricots) destined for human consumption.

Effect of temperature and water activity on growth and ochratoxin A production boundaries of two Aspergillus carbonarius isolates on a simulated grape juice medium

AIMS

To develop and validate a logistic regression model to predict the growth and ochratoxin A (OTA) production boundaries of two Aspergillus carbonarius isolates on a synthetic grape juice medium as a function of temperature and water activity (a(w)).

METHODS AND RESULTS

A full factorial design was followed between the factors considered. The a(w) levels assayed were 0.850, 0.880, 0.900, 0.920, 0.940, 0.960, 0.980 and the incubation temperatures were 10, 15, 20, 25, 30, 35 and 40 degrees C. Growth and OTA production responses were evaluated for a period of 25 days. Regarding growth boundaries, the degree of agreement between predictions and observations was >99% concordant for both isolates. The erroneously predicted growth cases were 3.4-4.1% false-positives and 0.7-1.4% false-negatives. No growth was observed at 10 degrees C and 40 degrees C for all a(w) levels assayed, with the exception of 0.980 a(w)/40 degrees C, where weak growth was observed. Similarly, OTA production was correctly predicted with a concordance rate >98% for the two isolates with 0.7-1.4% accounting for false-positives and 2.0-2.7% false-negatives. No OTA production was detected at 10 degrees C or 40 degrees C regardless of a(w), and at 0.850 a(w) at all incubation temperatures. With respect to time, the OTA production boundary shifted to lower temperatures (15-20 degrees C) as opposed to the growth boundary that shifted to higher temperature levels (25-30 degrees C). Using two literature datasets for growth and OTA production of A. carbonarius on the same growth medium, the logistic model gave one false-positive and three false-negative predictions out of 68 growth cases and 13 false-positive predictions out of 45 OTA production cases.

CONCLUSIONS

The results of this study suggest that the logistic regression model can be successfully used to predict growth and OTA production interfaces for A. carbonarius in relation to temperature and a(w).

SIGNIFICANCE AND IMPACT OF THE STUDY

The proposed modelling approach helps the understanding of fungal-food ecosystem relations and it could be employed in risk analysis implementation plans to predict the risk of contamination of grapes and grape products by A. carbonarius.