Safety assessment of dairy microorganisms: the hemiascomycetous yeasts

Hemiascomycetous yeasts constitute a class of unicellular fungi often associated with the food and drink processing industries. A number of species including Kluveromyces lactis, Debaryomyces hansenii, Yarrowia lipolytica, play a key role in the cheese-making process by providing aroma, affecting texture and/or permitting the growth of other microorgansisms. The large majority of yeast infections are due to a few opportunistic species presently classified within the genus Candida, and occur in immunocompromised patients. Recent advances in taxonomy have provided evidence for the incorrect classification of a number of yeasts and suggest that their association with the genus Candida should be reconsidered. Indeed, none of the most common pathogenic Candida species are found in cheese. Improved techniques, combined with more advanced analytical methods have brought to light several emerging pathogens, some of which are involved in cheese-making, for example D. hansenii and Y. lipolytica. Other emerging pathogens may also be found as rare occurrences in cheese. Problems in designation of these isolates are due in part to the still limited range of specific methods of identification and are exacerbated by lack of consensus concerning yeast taxonomy. These organisms cause rare infections in immunocompromised and hospitalized patients, which are generally mild and either self-limiting or easily treated. From studies with Saccharomyces cerevisiae, it seems that it is more the exposure to high doses of yeast than the identity of the species or strain that is associated with infection. As such yeasts in cheese cannot be considered to constitute a risk for healthy individuals.