Exopolysaccharide production by Streptococcus thermophilus SY: production and preliminary characterization of the polymer

AIMS

To evaluate the effect of yeast extract (YE) concentration, temperature and pH on growth and exopolysaccharide (EPS) production in a whey-based medium by Streptococcus thermophilus SY and to characterize the partially purified EPS.

METHODS AND RESULTS

Factorial experiments and empirical model building were used to optimize fermentation conditions and the chemical composition, average molecular weight (MW) and rheological properties of aqueous dispersions of the EPS were determined. Exopolysaccharide production was growth associated and was higher (152 mg l(-1)) at pH 6.4 and 36 degrees C with 4 g l(-1) YE. High performance size exclusion chromatography of the partially purified EPS showed two peaks, with a weight average MW of 2 x 10(6) and 5 x 10(4), respectively. The EPS was a heteropolysaccharide, with a glucose : galactose : rhamnose ratio of 2 : 4.5 : 1. Its water dispersions had a pseudoplastic behaviour and showed a higher viscosity of xanthan solutions.

SIGNIFICANCE AND IMPACT OF THE STUDY

The fermentation conditions and some properties of an EPS produced by Strep. thermophilus, a dairy starter organism, were described.

A survey of yeasts in traditional sausages of southern Italy

The evolution of the yeast population during manufacturing and ripening of 'salsiccia sotto sugna', a typical salami of the Lucania region (southern Italy), was investigated. Four different batches, produced in four farms in Lucania, were studied. Each batch showed a specific yeast population, and the most frequently isolated yeasts belonged to Debaryomyces hansenii and its anamorph Candida famata, and Rhodotorula mucilaginosa. Yarrowia lipolytica was isolated from three sausage batches. The Y. lipolytica isolates were further characterised, in particular for their lipolytic activity on pork fat. Lipolytic activity was maximal at pH 5.5, with oleic and palmitic acids as major free fatty acids produced. The use of randomly amplified polymorphic DNA-polymerase chain reaction allowed the detection of a high genetic heterogeneity among the isolates phenotypically assigned to the species Y. lipolytica.

Phenotypic diversity of lactic acid bacteria isolated from fermented sausages produced in Basilicata (Southern Italy)

AIMS

to evaluate the evolution of lactic acid bacteria (LAB) populations in traditional fermented sausages (salsiccia and soppressata) produced in artisanal and industrial plants in Basilicata (Southern Italy).

METHODS AND RESULTS

Four hundred and fourteen lactic acid bacteria (LAB) cultures were isolated from samples of sausages at different stages of ripening. A phenotypic characterization of the isolates was carried out using a set of 28 tests, and 34 clusters were identified at the 80% similarity level using hierarchical cluster analysis. Of the isolates 50% were identified as Lactobacillus sakei (with several biotypes), 22% as Pediococcus spp. (mainly Ped. pentosaceus), 7% as Leuconostoc (Leuc. carnosum, Leuc. gelidum, Leuc. pseudomesenteroides), 6% as Lact. plantarum, 1% as Lact. curvatus. Other lactobacilli, including unidentified species, were present in lower numbers.

CONCLUSION

The phenotypic diversity and composition of the LAB flora varied as a function of the production plant, product type and ripening time. SIGNIFICANCE AND IMPACT OT THE STUDY: A new procedure based on bootstrapping and Multidimensional Scaling was successfully used to obtain a graphical representation of the evolution of the LAB populations.

Evolution of microbial populations and biogenic amine production in dry sausages produced in Southern Italy

AIMS

To evaluate the occurrence and evolution of biogenic amines during ripening of fermented sausages and their relationship with physico-chemical and microbiological properties of the product.

METHODS AND RESULTS

Salsiccia and Soppressata were obtained from artisanal and industrial plants in Basilicata and pH, aW, microbial counts and biogenic amine content were measured. A high variability in amine content was observed. 2-Phenylethylamine and histamine were rarely found, while the tyramine, putrescine and cadaverine content increased during ripening. No correlation was found between individual biogenic amine content, microbial counts or physico-chemical parameters.

CONCLUSION

Starter cultures did not necessarily prevent the production of biogenic amines whose total contents were usually higher in Soppressata, a product with a larger diameter and aW compared with Salsiccia.

SIGNIFICANCE AND IMPACT OF THE STUDY

Literature findings on biogenic amine content and the evolution of microbial populations were confirmed. Normal ranges for amine content in Salsiccia and Soppressata are reported.

Effects of pH, temperature and NaCl concentration on the growth kinetics, proteolytic activity and biogenic amine production of Enterococcus faecalis

In this work, the combined effects of temperature, pH and NaCl concentration on the growth dynamics of Enterococcus faecalis EF37, its proteolytic activity and its production of biogenic amines have been studied. The effects of the selected variables have been analysed using a Central Composite Design. The production of biogenic amines, under the adopted conditions, was found to be mainly dependent on the extent of growth of E. faecalis. Its proteolytic activity was not a limiting factor for the final amine production, because in the system studied (skim milk) an excess of precursors was guaranteed. Quantitatively, the most important biogenic amine produced was 2-phenylethylamine but substantial amounts of tyramine were detected in all the samples. This work confirms that the main biological feature influencing the biogenic amine formation is the extent of growth of microorganisms, like E. faecalis, characterised by decarboxylase activity. In the traditional and artisanal cheeses produced using raw milk, enterococci usually reach levels of 10(7) cells/g. With this perspective, it is important that the presence of biogenic amines due to the activities of these microorganisms is maintained within safe levels, without affecting the positive effects of enterococci on the final organoleptic characteristics of the cheese.

Biogenic amine formation and oxidation by Staphylococcus xylosus strains from artisanal fermented sausages

Fifty strains of Staph. xylosus, isolated from artisanal fermented sausages in Southern Italy (Lucania region) were tested to verify their potential to produce or degrade biogenic amines. Twenty-six strains analysed were not able to form amines, but seven had the potential to produce spermine and/or spermidine and, at lower levels, tryptamine and tyramine. By contrast, about 80% of the strains that did not possess amino acid decarboxylase activity, exhibited an ability to degrade histamine. The greatest histamine-oxidase activity was present in the strains S81 (100% degradation), S206 (93%), S79 (68%) and S90 (53%). The strain S142 exhibited a remarkably high potential to oxidase tyramine and histamine, reducing the initial concentrations by 63 and 47%, respectively.