Ecophysiology of Penicillium expansum and patulin production in synthetic and olive-based media

Highlights

• Penicillium expansum produced patulin on olive-based medium at 15 °C but not at 4 °C.

• Olive matrix permits patulin accumulation under abusive storage temperatures.

• Contamination of olives with patulin must be considered a potential safety risk.

Abstract

Olives and their derivatives, in particular olive oil, represent one of the most significant agricultural products in the Mediterranean basin. Storage under inadequate conditions poses serious problems concerning fungal contamination, with consequent defects and potential mycotoxin production in olives and olive oils. Penicillium expansum represents one of the most significant postharvest pathogens in several fruits, including olives. Not only it causes blue mold but also is one of the most relevant patulin producing species of the genus Penicillium.

The aim of this research was to evaluate the ecophysiological conditions governing growth and PAT production by P. expansum strains previously isolated from Tunisian olives. For this purpose, four P. expansum isolates were tested in a synthetic medium (Czapek Yeast Autolysate, CYA) and in olive-based medium (OM) for their ability to grow and produce PAT under different temperatures (4 °C, 15 °C and 25 °C) for 10 and 20 d. The mycotoxin was analysed by HPLC-UV.

Results showed that all isolates were able to grow on tested media at different temperatures. Different PAT production profiles were found, showing that at 25 °C P. expansum isolates were able to produce PAT on CYA and OM medium. At 15 °C the production of PAT was only detected on CYA medium, while no PAT production was detected at 4 °C for the two media.

Introduction

Olives and their derivatives, in particular olive oil, represent one of the most significant agricultural products in the Mediterranean basin. Tunisia is considered the fourth largest producer of olive oil in the world (Abdelhamid et al., 2013). Tunisian olive oil production has an important position in the olive oil market; it exports about 75 % of its production and is considered as the second largest exporter after the European Union with an average of 115,000 tons per y over the last five years (ONH, 2015). In Tunisia, the olive sector contributes directly or indirectly to more than one million people and provides 34 million d of work per y, which is equivalent to more than 20 % of agricultural employment (Gharbi et al., 2015).

Olives are often stored for a long time under conditions that promote the growth of molds, such as prolonged contact with the ground, in bags of jute and in little ventilated places with high relative humidity. Contamination of olive fruits by hazardous microorganisms may also occur through insect pest infestation (Al-Ameiri et al., 2015). Such conditions favor a moldy taste and appearance and thus reduce the acceptable quality of olives and olive oils (Heperkan et al., 2006). Inappropriate storage or fermentation conditions favor the proliferation of hazardous fungi such as Penicillium sp. (Heperkan et al., 2009, Heperkan, 2013).

According to Roussos et al. (2006), olives can support the growth of mold and mycotoxins production. Many fungal strains, in particular from Aspergillus, Penicillium and Alternaria species, are able to grow on olives and produce several mycotoxins, and these genera have been reported as the dominant fungi on olives and olive products (El Adlouni et al., 2006, Roussos et al., 2006, Bavaro et al., 2017). Mycotoxin contamination of olives has also been studied, mostly for aflatoxins, ochratoxin A and citrinin (Leontopoulos et al., 2003, Ghitakou et al., 2006, Heperkan et al., 2009). Still, compared with other agricultural commodities, studies concerning contamination of olives with toxigenic fungi and the ability of these fungi to produce mycotoxins in this matrix are scarce.

Several Penicillium species were found on olives, including P. expansum, Penicillium citrinum and Penicillium crustosum (Arici, 2000, Heperkan et al., 2006, Heperkan et al., 2009, Bavaro et al., 2017). P. expansum is a wound parasite fungus that enters fruits via injuries potentially caused by inadequate conditions before harvest or by rough handling, harvesting, and transport (Sanderson and Spotts, 1995). It is one of the best-known and most studied molds of the genus Penicillium, occurring frequently at fruit harvest or postharvest stages (Andersen et al., 2004). P. expansum is known to contaminate a wide range of foods including apples, pears, cherries, walnuts, pecans, olives, hazelnuts, and acorns (Filtenborg et al., 1996, Zouaoui et al., 2015). It's defined as an important producer of PAT and citrinin, and is the major mycotoxin menace in apples and apple-derived products such as apple juice, purée, jam and cider (Frisvad and Samson, 2004). Studies of this species have been generally devoted PAT contamination and production in apple, apple juice, pears and cereals. However, only few studies report the contamination of olives with this species (Arici, 2000, Bavaro et al., 2017). To our knowledge, no studies have been developed on the detection of PAT in olives or other olive-derived products.

PAT is considered to have potential mutagenic, carcinogenic and embryotoxic effects on humans (Puel et al., 2010). The World Health Organization and the Food and Agriculture Organization have set a Maximum Tolerable Daily Intake for PAT of 0.4 μg/kg body weight/day (Leggott and Shephard, 2001). The European Commission has set limits of allowable PAT content in various foodstuffs: 50 μg/L in fruit juices and spirit drinks, 25 μg/L in solid apple products and 10 μg/L in infant food (EC, 2006).

It is thus necessary to acknowledge the importance of P. expansum in olives and to determine its ability to produce PAT in olive-based substrates under different conditions, by determining the ecophysiological conditions governing PAT production by the fungus. This knowledge is of major importance to make a risk assessment and to develop preventive measures especially during storage.

The objectives of this work were to study the effect of temperature and matrix on P. expansum strains isolated from Tunisian olives in terms of growth and PAT production.