Yeast viability during fermentation and sur lie ageing of a defined medium and subsequent growth of Oenococcus oeni

Interactions between the yeast strain used for primary oenological fermentation and the bacterium used to conduct subsequent malolactic fermentation were studied under model winemaking conditions. A commercial Saccharomyces cerevisiae wine yeast (strains, EC 1118, AWRI 835 and CY-3079) was grown in a defined medium whose composition approximated grape juice. Fermentations by all strains reached dryness, and retained a cell viability of greater than 90% upon completion of fermentation. Highest total viable cell number and percentage of viable cells were recorded for EC 1118. A sur lie ageing of the fermented medium over a 12 week period revealed a bi-phasic decay of culture viability for all strains. Thus 99% of cells had died within 2 weeks post-fermentation. Viabilities were then stable for the subsequent 4–6 week period before a second decline phase ensued and ended in either a minimal ( ca 100 CFU/mL, EC 1118) or no viable cells being detected at 12 weeks of ageing. The growth response of an Oenococcus oeni inoculum to yeast culture supernatants, previously aged for up to 12 weeks in the presence or absence of yeast lees, was evaluated in a bio-assay. In this way, yeast strains could be designated as being either inhibitory, neutral or stimulatory to the growth of O. oeni (strain Lc5p). Inhibition by supernatants of strain EC 1118 was evident, but found to be reduced by ageing the supernatant (with or without lees). Conversely, longer ageing on yeast lees increased the magnitude of the stimulatory response in O. oeni (strain Lc5p) to the supernatant from the wine yeast (strain CY-3079).     

Grain-Boundary Diffusion of 51Cr in MgO and Cr-Doped MgO

The grain-boundary diffusion product, D'δ , of ⁵¹Cr in MgO and Cr-doped MgO as a function of grain-boundary orientation and point-defect concentration was determined at T =1200° to 1450°C. A large degree of anisotropy was found in the grain-boundary diffusion behavior in MgO. The ratio of D'δ_|| parallel to D'δ_‖ perpendicular to the growth direction, D'_||/D'_‖ , is 10² for a 5° (100) tilt boundary, decreased to ∼2 in boundaries with tilt angles > 10°. The decrease in D'_||/D'_‖ is due to a large increase in D'_‖ with increasing tilt angle. The results indicate that grain-boundary diffusion in MgO is connected to the orientation of dislocations and the mechanism is one of dislocation pipe diffusion. The grain-boundary diffusion product D'δ increases with increasing Cr concentration in MgO and is ∼4 times larger for MgO containing 0.56 at. % Cr than for the undoped MgO. For all bicrystals studied, the activation energies are within 180 ± 20 kJ/mol which is 60% of the activation energy for ⁵¹Cr diffusion in undoped MgO.     

Phase Relations and Ordering in the Systems MgO-Y2O3-ZrO2 and CaO-MgO-ZrO2

The phase relations in the systems MgO-Y₂O₃-ZrO₂ and CaO-MgO-ZrO₂ were established at 1220° and 1420°C. The system MgO-Y₂O₃-ZrO₂ possesses a much-larger cubic ZrO₂ solid solution phase field than the system CaO-MgO-ZrO₂ at both temperatures. The ordered δ phase (Zr₃Y₄O₁₂) was found to be stable in the system ZrO₂-Y₂O₃ at 1220°C. Two ordered phases φ₁ (CaZr₄O₉) and φ₂ (Ca₆Zr₁₉O₄₄) were stable at 1220°C in the system ZrO₂-CaO. At 1420°C no ordered phase appears in either system, in agreement with the previously determined temperature limits of the stability for the δ, φ₁, and φ₂ phases. The existence of the compound Mg₃Y_zO₆ could not be confirmed.     

Precipitation in LiF-MgF2 Solid Solutions

Precipitation was studied for LiF single crystals containing up to 4 wt% MgF₂. Replica electron microscopy observations, combined with hardness and lattice parameter measurements, showed that, irrespective of the level of MgF₂, precipitation consistently occurs during quenching. Rapidly cooled crystals with lower MgF₂ concentrations (<1 wt%) develop large cruciform particles of a metastable phase with approximate composition MgF₂-6LiF. Lower cooling rates or higher MgF₂ contents caused transformation of these metastable particles to rod-shaped stable MgF₂ phase. This occurs via nucleation on the metastable crosses. The large misfit between the LiF matrix and the MgF₂ phase causes development of an incoherent interface and an associated disturbance of a propagating crack was observed near each MgF₂ particle.     

Phase Equilibria and Ordering in the System Zirconia-Hafnia-Yttria

The phase equilibria in the system zirconia-hafnia-yttria were investigated at 1300° and 1600°C by precise lattice parameter measurements and the direct phase-inspection method. Large fluorite solid solution regions were dominant at both investigated temperatures. The extent of ordering within the ternary system was studied in the samples containing 40 mol% yttria at 1300°C. The δ-phase (Zr,Hf)³Y₄O₁₂ formation was established only in those samples with less than 15 mol% hafnia.     

Phase Relations and Ordering in the System ZrO2-MgO

The Phase relationships in the system ZrO₂-MgO were reinvestigated over a wide range of temperatures and compositions. The extent of the cubic solid solution field was determined with precise lattice parameter measurements and a high-temperature X-ray furnace using analyzed samples. DTA results show that the addition of MgO to ZrO₂ decreases the transition temperature for monoclinic ⇌ tetragonal ZrO₂ and 1 mol% of MgO is soluble in the monoclinic zirconia at ∼1070°C.The invariant eutectiod point is at 13.5 ± 0.3 mol% MgO at 1406°± 7°C, which is in fair agreement with previous results by Grain. The ordered phase Mg₂Zr₅O₁₂ (δ-phase) can form metastably in cubic solid solutions at temperatures as low as 800°C after prolonged annealing. Evidence for the existence of the ordered phase MgZr₆O₁₃(γ-phase) was obtained by electron diffraction technique. Conditions for the formation of this phase are described. The ordered phases in this system are metastable and their formation is an intermediate step in the eutectoid decomposition of the cubic phase.     

Theorems on the bounds of solutions of semi-state models

Theorems are proved that give necessary and sufficient conditions for solutions of semi-state models to be appropriately bounded on finite and infinite time intervals. For a non-linear subclass of models considered, as well as for general time-varying linear semi-state models, necessary and sufficient conditions reduce to the test of semi-definiteness or definiteness of a time-dependent matrix.