Effects of ozone exposure on the xerophilic fungus, Eurotium amstelodami IS-SAB-01, isolated from naan bread☆
Introduction
Xerophilic moulds are common food spoilage organisms of low moisture food such as bakery products (Abellana et al., 1999, Hocking and Pitt, 1980, Samson, 1989, Vega et al., 1998), dried fruit (Pitt and Christian, 1968) and grain flours (Beuchat and Hwang, 1996). Eurotium, a well known xerophilic fungal genus, can account for 1 to 5% loss of bakery products along with Aspergillus and Penicillium depending on the season, type of product and method of processing (Malkki and Rauha, 1978). Xerophilic organisms are well known to be tolerant to several external stresses such as desiccation (Vaamonde and Fernández, 1993), salt/sugar concentrations, pH (Guynot et al., 2002), osmotic pressure (Christensen et al., 1965, Garg and Yadav, 2007) and heat (Splittstoesser et al., 1989). Therefore preventing spoilage caused by these organisms poses a major challenge in the food industry.
Increased awareness of the harmful effects caused by use of pesticides in food has led to interests in cleaner residue free technologies (Eom, 1994, Schafer and Kegley, 2002, Wilson and Otsuki, 2004). Ozone is well known as a strong oxidising agent and can cause elevation of reactive oxygen species in living cells leading to oxidative stress in the cells. This effect has been harnessed for preservation of food, especially fresh produce (Barth et al., 1995, Liew and Prange, 1994, Tzortzakis et al., 2008). Recent work has been carried out to explore the efficiency of ozone treatment in preservation of low moisture foods (Al-Ahmadi et al., 2009, Najafi and Khodaparast, 2009). Nevertheless very little is known on the direct effect of ozone fumigation on fungal survival and development (Antony-Babu and Singleton, 2009, Tzortzakis et al., 2008) and to our knowledge no work has examined the effect of ozone exposure on xerophilic fungi. The work presented in this paper aimed to explore the effects of ozone on Eurotium amstelodami IS-SAB-01 isolated from naan bread. The potential use of ozone fumigation in food storage facilities was assessed using a low-level long term (21 days) exposure study on media containing different sucrose levels, whereas the ability of ozone to kill the fungus was examined using high level ozone exposure for short periods (0 to 120 min).
Section snippets
Source of isolate
The fungal isolate used in this study was obtained from mouldy naan breads (after the normal shelf-life period). The surface of naan bread was scraped using a sterile inoculation loop and the fungal propagules transferred onto the surface of Rose-Bengal agar (Fluka, UK) plate. This isolation media was used initially to isolate a wide variety of spoilage moulds. The plates were incubated at 28 °C for 21 days. An isolate which was morphologically identified as Eurotium sp. was used in this study.
Confirmation of taxa
Confirmation of taxa
The Eurotium isolate was slow growing on Rose-Bengal agar showing little growth even after 7 days of incubation in comparison to the other isolates which were identified as Penicillium sp. The micromorphology of the isolate showed distinctive aspergillus-like conidiation. Along with the asexual spores comparatively larger cleistothecia and ascospores with conspicuous equatorial furrows were also observed. The 28S rRNA gene sequence analysis showed maximum identity (99.1%) with E. amstelodami.
Discussion
Members of the genus Eurotium are well known agents of spoilage in food with low moisture, such as bakery products (Abellana et al., 1999, Lund et al., 1996, Vinaixa et al., 2004) and dried fruit (Gherbawy, 2001, Romero et al., 2005, Yildiz and Çoksöyler, 2002, Zohri and Abdel-Gawad, 1993). The strain of Eurotium isolated in this study was assigned to belong to the genus initially by morphological characteristics and then confirmed to the species E. amstelodami based on 28S rRNA gene phylogeny.
Concluding remarks
Due to the known heat resistance of Eurotium sp. ozone treatment could be useful to reduce spoilage by this organism. The growth of the isolate and its ability to produce conidia were found to be sensitive to ozone fumigation but effects varied depending on the concentration of sucrose added to the growth medium and duration of exposure to ozone. It was also observed that ozone induced oxidative stress appeared to reduce formation of cleistothecia, an effect not observed previously. Conidia of
Acknowledgements
The authors like to thank Prof. J D Barnes for providing access to the ozone fumigation facility in School of Biology, Newcastle University, UK. The authors would also like to thank the anonymous reviewers for their helpful comments toward improving this paper.
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GenBank accession number related to the paper: GU808560.