Grundlagen und Grenzgebiete der Lebensmittelwissenschaft

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Scattering delay time of Mie scatterers determined from steady-state and time-resolved optical spectroscopy

We have determined the scattering delay time of Mie scatterers (r = 255 nm quartz spheres in polyester resin) from a combination of steady-state (integrating-sphere) and time-resolved (frequency-domain) measurements performed in the multiple-scattering regime. The effective transport velocity of light was derived from intensity and phase measurements at four different wavelengths by using the time-integrated microscopic Beer-Lambert law. We could demonstrate a systematic underestimation of the effective transport velocity compared with the phase velocity in the medium. Assuming that this discrepancy was caused entirely by the transient nature of a single-scattering process, the data presented resulted in time delays of between 18 fs (lambda = 678 nm) and 177 fs (lambda = 1,064 nm) per scattering event. For three out of four wavelengths investigated, the measured values are in excellent agreement with values predicted by a theoretical model for the scattering delay time based on Mie theory.     

Reduced Fresnel losses in chalcogenide fibers obtained through fiber-end microstructuring

We demonstrate microstructuring of chalcogenide fiber facets in order to obtain enhanced transmission due to the antireflective properties of the microstructured surfaces. A variety of molding approaches have been investigated for As(2)S(3) and As(3)Se(3) fibers. Transmission as high as 97% per facet was obtained in the case of As(2)S(3) fiber, compared to the native, Fresnel-loss limited, transmission of 83%.     

Growth potential of Salmonella infantis and Escherichia coli in fermenting tempeh made from horsebean,pea and chickpea and their inhibition by Lactobacillus plantarum

Salmonella infantis and Escherichia coli multiplied to varying degrees during the fermentation of unacidified horsebean, pea and chickpea tempeh. Active mycelial growth on the beans resulted in a sharp increase in pH. This was always accompanied by a sharp increase in the growth rate of the test organisms. Acidification of the beans during soaking decreased the growth rate of the test organisms only until active mycelial growth started. Inoculation of cooked beans with Lactobacillus plantarum resulted in a complete inhibition of E coli in unacidified and acidified chickpea tempeh and acidified pea tempeh. Marked inhibition of E coli was also noted in unacidified pea tempeh and acidified horsebean tempeh. S infantis was also completely inhibited by L plantarum in unacidified and acidified chickpea tempeh. In pea and horsebean tempeh, counts of S infantis were always lower in Lactobacillus-inoculated fermenting beans than in the control. Beside the pH, undissociated acids and other substances produced by L plantarum may be inhibitory to the test organisms. The use of L plantarum may be considered for the control of pathogens during commercial-scale tempeh production.     

Nano‐Scale Tungsten Oxides for Metastable Intermolecular Composites

Tungsten oxides are of interest as an oxidant for metals in metastable intermolecular composites, a nano‐scale powder useful for such applications as electric matches and gun primers. Smaller particles typically lead to faster reactions and we have synthesized nano‐scale WO₃ using wet chemistry. Ammonium paratungstate was dissolved in acid. Tungstic acid was precipitated by addition to distilled water and the resulting powder was dried. Analysis by X‐ray diffraction, Raman spectroscopy and electron microscopy revealed hydrated WO₃, having approximately 100 nm×7 nm platelet morphology. Thermal treatment in air at 200 °C or 400 °C removed water, forming cubic or monoclinic WO₃, respectively. Tungsten forms other stable oxides and we accessed WO₂ by reducing WO₃ in dilute H₂ at 650 °C. Reaction performance of these WO_x‐materials, mixed with nano‐Al at various stoichiometries, is reported. Open‐tray burn velocity reached 250 m/s. Pressure cell experiments found a maximum pressure of 1.45 MPa and maximum pressurization rate of 500 GPa/s. These maximum pressures and rates occurred very near a stoichiometric mixture of the Al and WO₃.