RESUMO
The smear of surface-ripened cheese harbors complex microbiota mainly composed of typical Gram-positive aerobic bacteria and yeast. Gram-negative bacteria are usually classified as un-wanted contaminants. In order to investigate the abundance and impact of Gram-negative bacte-ria naturally occurring in the smear of surface-ripened cheese, we performed a culture-based analysis of smear samples from 15 semi-hard surface-ripened cheese varieties. The quantity, di-versity and species distribution of Proteobacteria in the surface smear of the analyzed cheese vari-eties were unexpectedly high, and comprised a total of 22 different species. Proteus and Morganella predominated most of the analyzed cheese varieties, while Enterobacter, Citrobacter, Hafnia and Serratia were also found frequently. Further physiological characterization of Proteus isolates re-vealed strong proteolytic activity, and the analysis of volatiles in the smear cheese surface head-space suggested that Enterobacterales produce volatile organic flavor compounds that contribute to the organoleptic properties of surface-ripened cheese. Autochthonous members of Enterobac-terales were found in 12 of the 15 smear samples from surface-ripened cheeses, suggesting that they are part of the typical house microbiota that shape the organoleptic properties of the cheese rather than represent unwanted contaminants. However, further investigation on safety issues of the individual species should be performed in order to manage the health risk for consumers.
RESUMO
Fresh produce is frequently contaminated by microorganisms, which may lead to spoilage or even pose a threat to human health. In particular sprouts are considered to be among the most risky foods sold at retail since they are grown in an environment practically ideal for growth of bacteria and usually consumed raw. Because heat treatment has a detrimental effect on the germination abilities of sprout seeds, alternative treatment technologies need to be developed for microbial inactivation purposes. In this study, non-thermal plasma decontamination of sprout seeds is evaluated as a promising option to enhance food safety while maintaining the seed germination capabilities. In detail, investigations focus on understanding the efficiency of non-thermal plasma inactivation of microorganisms as influenced by the type of microbial contamination, substrate surface properties and moisture content, as well as variations in the power input to the plasma device. To evaluate the impact of these parameters, we studied the reduction of native microbiota or artificially applied E. coli on alfalfa, onion, radish and cress seeds exposed to non-thermal plasma in an atmospheric pressure pulsed dielectric barrier discharge streamed with argon. Plasma treatment resulted in a maximum reduction of 3.4 logarithmic units for E. coli on cress seeds. A major challenge in plasma decontamination of granular food products turned out to be the complex surface topology, where the rough surface with cracks and crevices can shield microorganisms from plasma-generated reactive species, thus reducing the treatment efficiency. However, improvement of the inactivation efficiency was possible by optimizing substrate characteristics such as the moisture level and by tuning the power supply settings (voltage, frequency) to increase the production of reactive species. While the germination ability of alfalfa seeds was considerably decreased by harsh plasma treatment, enhanced germination was observed under mild conditions. In conclusion, the results from this study indicate that cold plasma treatment represents a promising technology for inactivation of bacteria on seeds used for sprout production while preserving their germination properties.
