Effects of ozone exposure on the xerophilic fungus, Eurotium amstelodami IS-SAB-01, isolated from naan bread

https://doi.org/10.1016/j.ijfoodmicro.2010.09.019Get rights and content

Abstract

Xerophilic moulds cause contamination and spoilage of low moisture foods. This study examined the effect of ozone fumigation on growth of a Eurotium species isolated from naan bread. Two ozone treatments were used — a low-level long-term exposure (0.4 μmol/mol for 21 days) and high-level short-term exposure (300 μmol/mol for 5 to 120 min). For the low level exposure the combination of different media sucrose concentrations (0, 5, 10 and 20% w/v) with ozone treatment was also assessed. The growth of the isolate was found to be sensitive to low-level ozone fumigation depending on the media sucrose concentration and duration of the exposure. Low-level ozone exposure significantly (p < 0.05) reduced the number of asexual spores formed in media with no added sucrose, an effect not observed in media with higher sucrose levels. Electron microscope observations of colonies indicated that ozone exposed cultures produced lower numbers of cleistothecia. High-level ozone exposure for short durations reduced spore viability although 100% reduction in viability was achieved only after 120 min exposure. This work demonstrates that ozone may be used to reduce spore production in Eurotium but that the ozone effect can be mediated by sucrose levels in the growth medium.

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.

References (57)

  • S.M. Romero et al.

    Toxigenic fungi isolated from dried vine fruits in Argentina

    International Journal of Food Microbiology

    (2005)
  • B.C.M. Salomao et al.

    Heat resistance of Neosartorya fischeri in various juices

    LWT Food Science and Technology

    (2007)
  • A.E.H. Shearer et al.

    Heat resistance of juice spoilage microorganisms

    Journal of Food Protection

    (2002)
  • M. Thön et al.

    The thioredoxin system of the filamentous fungus Aspergillus nidulans: impact on development and oxidative stress response

    The Journal of Biological Chemistry

    (2007)
  • N. Tzortzakis et al.

    Impact of atmospheric ozone-enrichment on quality-related attributes of tomato fruit

    Postharvest Biology and Technology

    (2007)
  • N. Tzortzakis et al.

    Deployment of low-level ozone-enrichment for the preservation of chilled fresh produce

    Postharvest Biology and Technology

    (2007)
  • N. Tzortzakis et al.

    Impact of low-level atmospheric ozone-enrichment on black spot and anthracnose rot of tomato fruit

    Postharvest Biology and Technology

    (2008)
  • J.S. Wilson et al.

    To spray or not to spray: pesticides, banana exports, and food safety

    Food Policy

    (2004)
  • M. Abellana et al.

    Water activity and temperature effects on germination and growth of Eurotium amstelodami, E. chevalieri and E. herbariorum isolates from bakery products

    Journal of Applied Microbiology

    (1999)
  • M.R. Adams et al.

    Micro-organisms and food materials

  • S.S. Al-Ahmadi et al.

    Possible control of fungal and insect infestation of date fruits using ozone

    Biosciences, Biotechnology Research Asia

    (2009)
  • S. Antony-Babu et al.

    Effect of ozone on spore germination, spore production and biomass production in two Aspergillus species

    Antonie van Leeuwenhoek, International Journal of General and Molecular Microbiology

    (2009)
  • M.M. Barth et al.

    Ozone storage effects on anthocyanin content and fungal growth in blackberries

    Journal of Food Science

    (1995)
  • C.M. Christensen et al.

    A new Eurotium from rough rice stored in Mexico

    Mycologia

    (1965)
  • M. Eisendle et al.

    The intracellular siderophore ferricrocin is involved in iron storage, oxidative-stress resistance, germination, and sexual development in Aspergillus nidulans

    Eukaryotic Cell

    (2006)
  • G. Emiliani et al.

    A horizontal gene transfer at the origin of phenylpropanoid metabolism: a key adaptation of plants to land

    Biology Direct

    (2009)
  • Y.S. Eom

    Pesticide residue risk and food safety valuation: a random utility approach

    American Journal of Agricultural Economics

    (1994)
  • N. Garg et al.

    Control of osmophilic microorganism induced spoilage of aonla (Embellica officinalis) segments and murabba

    Journal of Food Science and Technology (Mysore)

    (2007)
  • Cited by (0)

    GenBank accession number related to the paper: GU808560.

    View full text