Stevia-based sweeteners as a promising alternative to table sugar: The effect on Listeria monocytogenes and Salmonella Typhimurium growth dynamics.

Sugar is commonly substituted with stevia-based products in food industry and in our daily-life. This substitution results in a change in food product characteristic formula and properties that may affect the growth dynamics of food pathogenic and spoilage bacteria. This work studies the effect of table sugar (TS), laboratory sucrose (LS), commercial stevia (St) and steviol glycosides (SG) on the growth dynamics of Salmonella Typhimurium and Listeria monocytogenes. Experiments were carried out in general and minimal culture media at 3 equivalent concentration levels in terms of sweetness intensity (TS and LS at 3, 9 and 15% (w/v); St at 0.3, 0.9 and 1.5% (w/v); and SG at 0.01, 0.03 and 0.05% (w/v)). Incubation temperatures were: 4, 8 and 20°C for general media, and for minimal media 20°C. To decipher the role of these sweeteners, their concentration evolution in minimal media was determined via HPLC analysis. The results revealed slow maximum specific growth rates (µmax) of S. Typhimurium in general media with increasing concentrations of TS and LS at 20°C; and reduced maximum cell population (Nmax) at 8°C. The growth of L. monocytogenes in general culture media remains invariable independently of the sweetener added, except at 4°C. At this critical temperature, the presence of TS, LS and St seems to facilitate the growth of L. monocytogenes, presenting higher µmax values in comparison to SG and the control. Varying bacterial response to changes in media formulation suggests that further research is required, focusing on revealing the microbial dynamics in structured media, as well as in real food products.

Stability of steviol glycosides in several food matrices.

As steviol glycosides are now allowed as a food additive in the European market, it is important to assess the stability of these steviol glycosides after they have been added to different food matrices. We analyzed and tested the stability of steviol glycosides in semiskimmed milk, soy drink, fermented milk drink, ice cream, full-fat and skimmed set yogurt, dry biscuits, and jam. The fat was removed by centrifugation from the dairy and soy drink samples. Proteins were precipitated by the addition of acetonitrile and also removed by centrifugation. Samples of jam were extracted with water. Dry biscuits were extracted with ethanol. The resulting samples were concentrated with solid-phase extraction and analyzed by high-performance liquid chromatography on a C18 stationary phase and a gradient of acetonitrile/aqueous 25 mM phosphoric acid. The accuracy was checked using a standard addition on some samples. For assessing the stability of the steviol glycosides, samples were stored in conditions relevant to each food matrix and analyzed periodically. The results indicate that steviol glycosides can be analyzed with good precision and accuracy in these food categories. The recovery was between 96 and 103%. The method was also validated by standard addition, which showed excellent agreement with the external calibration curve. No sign of decomposition of steviol glycosides was found in any of the samples.

Performance analysis and optimization of autotrophic nitrogen removal in different reactor configurations: a modelling study.

The autotrophic nitrogen removal process (partial nitritation combined with the Anammox process) is a new and sustainable nitrogen removal technique for nitrogen-rich streams. A modelling study has been performed to define optimal process conditions (temperature, oxygen supply, pH and biomass retention) and to investigate the influence of chemical oxygen demand, nitrogen loading rate and hydraulic retention time on three alternative reactor configurations: a single oxygen-limited partial nitritation reactor, a single Anammox reactor, and a combination of partial nitritation and Anammox in a single reactor. The model applied was compared to experimental data from the literature and gave good agreement for all three reactor configurations. The simulations revealed that a system with separated partial nitritation and Anammox offered a wider range of optimal process conditions than a one-reactor system. The key factors in the successful operation of partial nitritation were found to be control of aeration, ammonium loading rate and temperature. Heterotrophs remained present in all three reactor systems and it was confirmed that interaction between heterotrophs and Anammox and between heterotrophs and ammonium oxidizers was possible.