Pavels Semjonovs

The influence of stevia glycosides on the growth of Lactobacillus reuteri strains.

Use of stevia‐derived sweeteners was recently officially approved by the European Commission, and their application in the food industry has increased, especially in functional foods. However, there are scarce data about the influence of stevia on probiotic bacteria, which are important both as an inhabitant of the human gut and as a functional food additive. Taking into consideration the broad application of Lactobacillus reuteri in functional foods, the aim of the research was to evaluate the influence of stevia glycosides on its growth. Six Lact. reuteri strains were tested for their ability to grow in the presence of stevioside and rebaudioside A (0·2–2·6 g l−1). The effect of stevia glycosides on biomass concentration, cell count, pH and lactic and acetic acid synthesis was analysed. Both glycosides impaired the growth of analysed strains. However, the inhibitory effect was strain specific, and the concentration‐dependent effect was not observed for all parameters. The most pronounced concentration‐dependent effect was on lactic and acetic acid production. Taking into account the observed strain‐specific inhibitory effect of stevia glycosides, it could be suggested to evaluate the influence of them on each strain employed before their simultaneous application in functional foods.

Production of extracellular fructans by Gluconobacter nephelii P1464

Bacterial extracellular fructans, known as levans, have potential applications in food, pharmaceutical and cosmetic industries and high fructan producing strains could contribute into the cost reduction and more extensive commercial usage of them. An acetic acid bacteria (AAB) isolate P1464 was obtained from the Microbial Strain Collection of Institute of Microbiology and Biotechnology, University of Latvia and identified as Gluconobacter nephelii by DNA‐DNA hybridization and the formation of extracellular fructans by this strain was confirmed. Isolated extracellular fructose polymers were characterized using FT‐IR spectroscopy and the structural features of fructan appeared as similar to the reference sample of bacterial levan. Molecular mass estimates showed that the isolated G. nephelii P1464 fructose polymer has a relatively small molecular weight (Mw 1122·939 ± 153·453 kDa) and a sizeable polydispersity (Mw/Mn = 21·57 ± 1·60), as compared with other AAB, which could promote their physiological activity, including the prebiotic effects. Obtained at different cultivation conditions characteristics of fructan production, including the biotechnological indices such as the productivity (Qp) and yield (Yp/s) ranging from 0·774 to 1·244 g l−1 h and from 0·181 to 0·436 g g−1, respectively, confirmed, that G. nephelii P1464 could be used as promising strain for commercial production of levan.