This study assessed the levels of microbial contaminants in liquid, compressed and dry commercial baker's yeasts used as starters in breadmaking. Eumycetes, Enterobacteriaceae, total and fecal coliforms, Bacillus spp., and lactic acid bacteria (LAB), in particular enterococci, were quantified. Results obtained in this study highlighted that baker's yeast could represent a potential vehicle of spoilage and undesirable microorganisms into the baking environment, even if these do not influence the leavening activity in the dough, as ascertained by rheofermentometer analysis. Different microbial groups, such as spore-forming bacteria and moulds, were found in baker's yeast starters. Moreover, different species of LAB, which are considered the main contaminants in large-scale yeast fermentations, were isolated and identified by Denaturing Gradient Gel Electrophoresis (DGGE) and 16S rDNA sequencing. The most recurrent species were Lactobacillus plantarum, Enterococcus faecalis, and Enterococcus durans, isolated from both compressed and dry starters, whereas strains belonging to Leuconostoc and Pediococcus genera were found only in dry ones. Nested-Polymerase Chain Reaction (Nested-PCR) and Randomly Amplified Polymorphic DNA–PCR (RAPD-PCR) were also used to highlight the biodiversity of the different commercial yeast strains, and to ascertain the culture purity.
Our study highlighted that baker's yeast starters can be considered an important source of numerous and undesirable spoilage microorganisms such as Bacillaceae, moulds, coliforms, and enterococci. Bacillus genus and different species of moulds are the main cause of substantial economic loss in the bakery industry and might also cause public health problems due to the production of cytotoxic substances. Since the microbiological quality control of baker's yeast starter in breadmaking is of crucial importance, we suggest that the bakery industry routinely performs appropriate microbiological controls throughout the entire production line to guarantee the quality, safety and acceptability of final products. At the same time, precautions against the introduction of spoilage and potentially pathogen bacteria during the fermentation process should also be taken through the observance of Good Manufacturing Practice.