ABSTRACT
INTRODUCTION Most people have experienced the smell of fungal spoiled foods such as bread. The typical reaction is “this really smells bad” and most people will associate this with a mouldy smell. It has been shown that the odour thresholds of some off-flavour-related fungal volatile compounds are very low. The odour threshold for the earthy smelling terpene alcohol geosmin (Figure 1) in water is between 0.0082-0.018 ppb and the musty smelling 2-methyl-isoborneol (Figure 1) has an odour threshold of 0.1 ppb in water (Medsker et al., 1969). Traditionally, specific fungal species have been used as starter cultures in certain fermented foods such as blue and white mould cheeses (Penicillium roqueforti; and Penicillium camemberti and Geotrichum candidum, respectively) and soy sauce (Aspergillus oryzae and Aspergillus sojae). It is also well known that various mycotoxin-producing fungi occur as contaminants in foods and feed stuffs, which is a considerable problem in terms of food and feed quality and safety and hence economically, as in 1985 FAO (the Food and Agriculture Organization) estimated that 25% of world crops (Pitt and Hocking, 1985) and as much as 50% of crops in developing countries, are contaminated with mycotoxins (Waller and Brayford, 1990; Pohland, 1993). It is therefore important that it is ensured that starter cultures are pure. Fungal detection is not only of importance in terms of food safety. For centuries, truffles, a
most valuable fungal commodity, have been found by use of pigs, which can smell the truffles odorous compounds very well and find them covered under soil. In the field of medicine, it is also important to be able to detect fungal infections, such as aspergillosis, as soon as possible. Food quality has, traditionally, often been assessed by sensory panel evaluation, for instance in quality control of cereals (Börjesson et al., 1996). Sensory panel analysis is a very laborious process as it requires a panel of sensory judges which is very expensive and time consuming to train to a proficient level. Even a well-trained sensory judge will give an at least partially subjective score in sensory panel analysis. There are further restrictions in using sensory panel analysis, as potentially toxic samples cannot be analyzed in this manner. Of the traditional analytical methods, GCMS analysis is time consuming, but somewhat less expensive than sensory panel analysis or HPLC-DAD and LC-MS analysis. Data analysis
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Geosmin 2-Methylisoborneol
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Figure 1. Chemical structure of the mouldy smelling volatile fungal biomarkers geosmin and 2methylisoborneol, two volatile compounds with extremely low odour-threshold values.
