Mould and yeast flora in fresh berries, grapes and citrus fruits
Introduction
Fruits contain high levels of sugars and other nutrients, and they possess an ideal water activity for microbial growth; their low pH makes them particularly susceptible to fungal spoilage, because a big part of the bacterial competition is eliminated since most bacteria prefer near neutral pH. Some fungi are plant pathogens and can start the spoilage from the field while others, although they could contaminate the fruits in the field, actually proliferate and cause substantial spoilage only after harvest when the main plant defenses are reduced or eliminated. Fungal spoilage of fruits will depend on cultivation, harvesting, handling, transport, and post-harvest storage and marketing conditions. Various means of controlling post-harvest microbial spoilage of fruits such as careful culling, storage at low temperatures or under controlled atmospheres and application of fungicides have been used (Beuchat, 1987, Ryall and Pentzer, 1982, Eckert and Ogawa, 1988). Refrigeration slows down fungal growth dramatically and prolongs the shelf life of fruits. Some fruits, however, are sensitive to low temperatures and they could suffer chilling injuries, therefore, become very susceptible to microbial spoilage (Ryall and Pentzer, 1982). On the other hand, many fungi can grow at low temperatures and cause substantial damage especially if the fruits are stored for extended periods of time. The use of synthetic fungicides could prevent spoilage to some degree but some fungi could become resistant to commonly used pesticides (Spotts and Cervantes, 1986). Also, attempts to reduce chemical contamination of the environment as well as health hazards associated with consumption of pesticide residues dictate the reduction of the use of such chemicals.
Post-harvest fruit spoilage results in significant economic losses. Additionally, if the spoiling fungi are toxigenic or pathogenic, they could pose a health risk for the consumer. Toxigenic fungi have been isolated from spoiling fruits in the past (Ryall and Pentzer, 1982, Stinson et al., 1981). Some of these moulds could produce mycotoxins while grown on fruits (Stinson et al., 1980, Stinson et al., 1981) even during refrigeration (Tournas and Stack, 2001). Pathogenic fungi, on the other hand, could cause infections or allergies in susceptible individuals (Kurup, 2003, Lewis et al., 1975, Monso, 2004).
Restrictions or ban of certain fungicides and use of new ones in recent years may have changed the post-harvest fungal profiles of fruits. This study investigates the current fungal profiles of various fresh fruits sold in the Washington, DC Metro area in order to find out if toxigenic moulds and pathogenic moulds and yeasts are present and likely to grow on these commodities.
Section snippets
Materials and methods
Ripe, sound fruits including strawberries, blueberries, raspberries, blackberries, various types of grapes (red seedless, red seeded, green seedless, black seedless, and black seeded), navel, temple and Minneola oranges, tangerines, Sunkist and citron lemons, and limes were purchased from local supermarkets in the Washington, DC area. All berry samples were purchased in their individual intact packages weighing 6 oz (for raspberries, blackberries and blueberries) and 16 oz (for strawberries),
Berries
The overall fungal contamination of tested fruits is summarized in Table 1. One hundred percent of blackberry and raspberry, 97% of strawberry and 95% of blueberry samples showed some sort of fungal contamination. The contamination level (percent of contaminated berries per sample) differ among the various types of berries; the highest mean contamination level of 82% was observed in raspberries, closely followed by blackberries and strawberries, whereas the lowest of 38% occurred in blueberries
Conclusions
Several fungi including some from the mycotoxin-producing genera of Penicillium, Alternaria, Fusarium and Aspergillus were present and capable of growing on fresh fruits at room temperature. Among all fruits tested, berries had the highest levels of contamination; strawberries, raspberries and blackberries were the most susceptible probably due to the fact that their skins are soft, easily ruptured with numerous indentation and hair-like protuberances which allow most organisms to attach and
Acknowledgments
This project was part of the National Food Safety Initiative and was solely supported by FDA funds.
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