Biodiversity of Geotrichum candidum strains isolated from traditional French cheese.

摘要

Over the centuries, cheesemakers have optimized production techniques in a manner that selected empirically for native strains of microorganisms that produced the best cheeses. The diversity of fungal-ripened cheeses is due partly to the fungi that colonize the cheese during ripening. We have documented the sequential appearance of microorganisms on the rind and in the curd of a St. Nectaire-type cheese over a 60-day ripening period using scanning electron microscopy and light microscopy of stained paraffin sections. Geotrichum candidum appears at the early stages of ripening on soft cheeses such as Camembert and semi-hard cheeses such as St. Nectaire and Reblochon. Its lipases and proteases promote flavor development, and its aminopeptidases reduce bitterness imparted by low molecular weight peptides in cheese. Native strains of G. candidum were isolated from milk, curd and cheese collected in seven major cheese-making regions of France. We used Randomly Amplified Polymorphic DNA-Polymerase Chain Reaction (RAPD-PCR) to differentiate the isolates of G. candidum to the strain level. The genetic diversity study was correlated with phenotypic tests for carbon assimilation, salt tolerance, and aminopeptidase activity. We found high genetic diversity of G. candidum even within the same cheese-making regions. Strains did not group according to region. Aminopeptidase tests showed diversity between cheese types, with the strains from St. Nectaire having high activity and strains from Reblochon having little or no activity. The results of carbon assimilation and salt tolerance tests suggest that cheese-making technologies play a role in strain selection. Our study reveals an enormous diversity of G. candidum that has been empirically selected through the centuries by the cheese-makers of France. Native strains of G. candidum could provide a rich source of cheese ripening enzymes for developing aroma and texture. Since much of the flavor development associated with cheese ripening is due to microbial activity it seems likely that the diversity of G. candidum strains that we have found may contribute to the acknowledged diversity of flavor found in French cheeses.