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

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Abstract

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 °C were tested. Optimum temperatures for growth were between 30 and 37 °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.

Introduction

Ochratoxins are secondary metabolites produced by moulds belonging to several species of the genera Aspergillus and Penicillium. The most extensively studied compound of this group, ochratoxin A (OTA), has been shown to be a nephrotoxic, immumosuppressive, teratogenic and carcinogenic agent (JEFCA, 1991).

The presence of OTA has been reported in a number of plant products and occasionally in body fluids and kidneys of animals and humans Xiao et al., 1996, Solti et al., 1997, Burdaspal and Legarda, 2000. First reported in wines in 1996 (Zimmerli and Dick, 1996), OTA has since been found in other grape beverages. Fungi belonging to Aspergillus section Nigri are the main OTA producers in these products. In addition, recent studies have shown that the three major black species, A. carbonarius, A. niger aggregate, and A. section Nigri uniseriates, are all very common in grapes at harvest Battilani et al., 2003, Bellı́ et al., 2004.

The most used measure of the availability of water to microorganisms is water activity (aw), the ratio of the vapour pressure of the water in the substrate to that of pure water at the same temperature and pressure. Other environmental factors such as temperature, pH, nutritional factors, etc. also affect mycelial growth and mycotoxin production by moulds. Temperature and aw are the main factors influencing germination, growth and sporulation of spoilage fungi (Magan and Lacey, 1984).

The objectives of this study were to determine in vitro the effect of aw and temperature on mycelial growth of several isolates of Aspergillus section Nigri isolated from grapes, on a synthetic nutrient medium similar to grape composition.

Section snippets

Fungi

All the fungi used in this study were isolated from European wine grapes during the year 2001. Isolates were classified as A. carbonarius (W120, 93cr4, A0204, 36br4, 01UAs219, 01UAs294, A0933, CA332, Mu644), A. niger aggregate (93RJ4, 11V4, 01UAs203, 01UAs127, A1099, A1109, C432, Mu246), and Aspergillus section Nigri uniseriates (73r3, W118, 01UAs128, A0704, A0212).

Isolates were obtained from (1) the Faculty of Agriculture, Università Cattolica del Sacro Cuore, Piacenza, Italy; (2) the Institut

Effect of glycerol and glucose on the growth rates of six A. Section Nigri isolates at 25 °C on SNM

No significant differences between glycerol and glucose were found, although the growth was slightly faster and the lag phase for growth shorter with glucose, as sugars are the preferred carbon source for A. niger (Hatzinikolaou and Macris, 1995). However, glycerol was chosen for subsequent experiments as the dissolution of high amounts of glucose was more time-consuming than working with glycerol.

Significant differences were found between isolates, aw and their interaction. The differences in

Conclusions

A. section Nigri may grow at all the temperatures tested in this study, ranging from 10 to 37 °C, with an optimum between 30 and 37 °C, with the latter conditions being frequent in the field close to harvest time. However, at 10 °C the growth of all the isolates was negligible when aw levels were low (0.90 and 0.93 aw). The optimum aw for black Aspergilli to grow, seems to be at 0.98, similar to the aw of grapes in the field. Thus, field conditions are likely to be conducive to optimum growth

Acknowledgements

The authors are grateful to the Catalonian Government (Direcció General de Recerca, Generalitat de Catalunya), the Spanish Government (CICYT, Comisión Interministerial de Ciencia y Tecnologı́a, project AGL 2001 2974-C05-02), and the EC, Quality of Life Programme (QoL), Key Action 1 (KA1) on Food, Nutrition and Health (QLRT-2000-01761) for their financial support. We also thank the participants of this European project for providing us with the isolates used in this study.

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