ABSTRACT
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.
Subject(s)
Bread/microbiology , Fermentation , Food Contamination/analysis , Saccharomyces cerevisiae/metabolism , Biodiversity , Colony Count, Microbial , DNA, Bacterial/genetics , Denaturing Gradient Gel Electrophoresis , Enterococcus/isolation & purification , Enterococcus faecalis/isolation & purification , Food Microbiology , Hydrogen-Ion Concentration , Lactobacillaceae/growth & development , Lactobacillaceae/isolation & purification , Lactobacillus plantarum/isolation & purification , Leuconostoc/isolation & purification , Pediococcus/isolation & purification , Polymerase Chain Reaction , RNA, Ribosomal, 16S/genetics , Saccharomyces cerevisiae/isolation & purification , Sequence Analysis, DNAABSTRACT
AIMS: To evaluate interactions between Lactobacillus sakei and coagulase negative cocci (CNC) (Staphylococcus xylosus and Kocuria varians) and to investigate the influence of these interactions on their own proteolytic activity. METHODS AND RESULTS: Interactions occurring between strains of Lact. sakei and CNC were assessed by spectrophotometric analysis. The growth of 35 strains of Lact. sakei, used as indicators, was compared to that obtained combining the same strains with growing cells or cell-free supernatants of 20 CNC (18 Staph. xylosus and 2 K. varians). The proteolytic activity expressed by single strains or by their combinations was assessed on sarcoplasmic protein extracts by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The results evidenced that interactions are able to affect not only the growth but also the in vitro proteolytic activity of Lact. sakei and CNC used in combination. CONCLUSIONS: A relationship between the presence of interactions among useful strains and the strength of technological characteristics, such as proteolysis, was defined. SIGNIFICANCE AND IMPACT OF THE STUDY: The study highlighted that CNC are able to stimulate the growth of some Lact. sakei strains. At the same time, this interaction positively influences the proteolytic activity of strains used in combination. Given the importance of proteolysis during the ripening of fermented meats, this phenomenon should be taken into account to select meat starter cultures.
Subject(s)
Bacterial Proteins/metabolism , Lactobacillus/enzymology , Lactobacillus/growth & development , Staphylococcus/enzymology , Staphylococcus/growth & development , Animals , Electrophoresis, Polyacrylamide Gel , Fermentation , Lactobacillus/chemistry , Lactobacillus/metabolism , Meat Products/microbiology , Staphylococcus/chemistry , Staphylococcus/metabolismABSTRACT
AIMS: To evaluate the interactions of Staphylococcus xylosus on Kocuria varians strains isolated from fermented meat products. METHODS AND RESULTS: Interactions were assessed in vitro by agar spot test, agar well diffusion assay and spectrophotometric assay. The growth of K. varians (five strains) alone was compared with that in the presence of growing cells of S. xylosus (50 strains) or in the presence of heat-treated or untreated supernatants of S. xylosus. Sixteen strains stimulated the growth of K. varians K4, while four strains inhibited the K4 strain. Heated cell-free supernatants of S. xylosus did not have any effect on K. varians. The proteolytic activity of single strains or their combinations was assessed in vitro and in vivo by sodiumdodecylsulfate-polyacrylamide gel electrophoresis of sarcoplasmic protein extracts. Combinations of stimulatory strains of S. xylosus and K. varians showed a higher proteolytic activity compared with that of S. xylosus or K. varians alone. CONCLUSIONS: The interactions between strains may influence both the growth of the co-cultured strains and proteolysis, technologically relevant characteristics. SIGNIFICANCE AND IMPACT OF THE STUDY: The study of interactions between coagulase-negative cocci may guide the formulation of mixed strain starters for the production of fermented sausages.
