An ecological study of yeasts associated with fruits and vegetables
Wolmarans, Johannes Barend Bodenstein
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English: Currently, fruits and vegetables suffer significant losses from fungal diseases after harvest. The use of synthetic fungicides which pose a risk to human health and the environment, have resulted in an intensive search for safer control of postharvest diseases by the use of microorganisms, particularly yeasts, occurring naturally on the surfaces of fruits and vegetables. An ecological survey was preformed on five different fruit and vegetable commodities to determine the diversity of yeast species associated with these commodities and the cause of spoilage at different temperatures. A. THE INCIDENCE OF YEASTS ASSOCIA TED WITH FRUITS AND VEGETABLES. Different fruits and vegetables were surveyed to determine the diversity of yeasts and to differentiate between yeasts found on the surface and inner layer of the different fruit and vegetable commodities. Bacterial growth dominated on the surface of vegetables due to factors such as higher water content, more neutral pH and close proximity to the soil facilitating their growth. Yeasts represented the major component of the microflora on the surface of fruits due to a lower pH and higher carbohydrate content that encouraged their growth. The inner layer of both fruits and vegetables yielded lower microbial counts. Schizosaccharomyces pombe and Debaryomyces hansenii were the predominant species isolated on both fruits and vegetables. Spoilage of fresh fruits usually results from the fermentative activity of yeasts, but no such species were isolated during this study. B. THE GROWTH AND SURVIVAL OF YEASTS IN FRUITS AND VEGETABLES STORED AT DIFFERENT TEMPERATURES. The growth and survival of yeasts were carried out at different temperatures over a 30- day period to determine an epiphytic yeast population developing on the surfaces of fruits and vegetables. The chemical and physical composition were monitored over a 30-day period to determine if the changes in pH and organic acids contributed to the developing yeast population on the surface of fruits and vegetables. A total of 119 yeast strains were isolated from fruits and vegetables with bacteria dominating on the surface of vegetables and yeasts on the surface of fruits. The most predominating species isolated were S. pombe and D. hansenii. A new yeast species, showing cellulase activity, was isolated from potatoes on day 20 at 25°C. C. THE IDENTIFICATION OF A NEW BASIDIOMYCETOUS YEAST SPECIES ISOLATED FROM POTATOES. The new basidiomycetous related species showed interesting characteristics in assimilating lactose as a carbon source. Its sexual reproduction by teliospores verifies the characteristically difference compared to Cryptococcus and Rhodotorula spp. where no sexual reproduction is noted. Accordingly, the new species was described.