ABSTRACT
INTRODUCTION The traditional division of filamentous fungi has been between the food spoiling fungi, which were considered opportunistic fungi with no substrate preferences and animal or plant pathogenic fungi with tight associations to their hosts. Most food spoiling fungi have been regarded as saprophytic organisms thriving on any substrate they could encounter and they can indeed be isolated on many different laboratory media. They can grow and differentiate on minimal media containing only nitrate and sucrose as nitrogen and carbon source, as well as on complex media based on cereal, vegetable, fruit and meat, such as peptones, corn steep liquor, malt and yeast extract. (Raper and Thom, 1949; Smith, 1960; Samson et al., 2004c). However, as early as 1949 Westerdijk suggested that certain Penicillia were associated to certain food substrates, such as P. italicum and P. digitatum to citrus fruits and P. expansum to pomaceous and stone fruits. These associations were regarded as the exceptions rather than the rule for filamentous fungi. Several authorities (Thom, 1930; Raper and Thom, 1949; Pitt, 1979; Ramírez, 1982) were of the opinion that the many Penicillium species they treated were saprophytic generalists rather than species associated to specific natural habitats or processed foods and feedstuff. Difficult taxonomy and lack of data may have obscured the less obvious associations between Penicillium species and habitats or food products (Frisvad, 1988; Frisvad and Filtenborg, 1988). Furthermore, using inadequate techniques and methods would often make it impossible to distinguish between simple surface
contamination of a food product and true infection resulting from the fungal-substrate association. Fungi isolated from surface disinfected products are with some probability thriving on the product and therefore associated with it, whereas fungi growing from nonsurface-disinfected products could be accidental contaminations from other materials, storage facilities or the air (King et al., 1986; Frisvad and Samson, 1991; Samson et al., 1992; Filtenborg et al., 1996; Hocking et al., 2006). This applies not only to Penicillium, but also to all other major food spoiling genera, such as Aspergillus, Alternaria and Fusarium. The associated mycobiota of a food product can be defined as all the fungal species that are able to infect and actively grow on the product under harvest, storage or processing conditions. In connection with food mycology and safety, the fact that each fungal species found in a food product produces a species specific profile of extrolites is of particular importance. An extrolite can be a volatile or non-volatile secondary metabolite, an organic acid, an extracellular enzyme or other outwards directed biochemical compounds (Frisvad et al., 1998, 2004; Frisvad and Samson, 2004; Larsen et al., 2005). For example, only three of the approximately 90 food spoiling Penicillium species are able to produce penicillin (Samson et al., 2004c). Among the extrolites, mycotoxins and other bioactive compounds, such as ochratoxin or patulin, are of direct health concern. The production of mycotoxins is highest and most diverse under optimal conditions in a laboratory and mycotoxins are only produced in the food products during storage or processing when conditions change to the advantage
of the fungi. Therefore, knowledge of production of mycotoxins and other extrolites by individual fungal species under controlled conditions in the laboratory is important in order to relate production to specific food products. Furthermore, extrolites that once were regarded as non-toxic or at best of little importance in known mycotoxicoses have now shown to be highly toxic if they are inhaled rather than ingested. For example, brevianamide A, mycophenolic acid and roquefortine C have been shown to be cytotoxic and inflammatory in mouse lungs (Rand et al., 2005). This could be of consequence for employees in food factories, which may inhale mycotoxins via fungal spored in the air. Another reason for determining the mycobiota is that secondary metabolites which are now considered nontoxic may later turn out to be mycotoxins, and retrospective analysis may then help explain certain mycotoxicosis, where no known mycotoxins could be found. A third reason for examining foods for the mycobiota is that some mycotoxins may act synergistically. Knowing the mycobiota on a particular food product, and thereby which mycotoxins are theoretically possible to encounter in the product, can give an indication on which mycotoxins the product should analyze for. However, a negative result for a mycotoxin analysis in a food product may give a false sense of security, as it is now known that mycotoxins produced in a crop plant may be masked by glucosylation or by reaction with β-D-glucans by the crop plant (Berthiller et al., 2005; Yiannikouris et al., 2006). These masked mycotoxins are a good reason for finding the mycobiota of foods before a mycotoxin analysis, to ensure that all mycotoxins and their derivatives are recovered in an analysis. This chapter gives an overview of the associated mycobiota of different raw materials and food products together with the mycotoxins and other bioactive compounds to consider in a chemical food product analysis. A detailed encyclopaedic list on the occurrence of mycotoxins in different foods, spices and other edible substrata has been given by Weidenbörner (2001a), but this list is only backed up by a few references.
