Influence of adjunct use andcheese microenvironment on nonstarter lactic acid bacteria populations in Cheddar-type cheese

This study investigated population dynamics of starter, adjunct, and nonstarter lactic acid bacteria (NSLAB) in reduced-fat Cheddar and Colby cheese made with or without a Lactobacillus casei adjunct. Duplicate vats of cheese were manufactured and ripened at 7°C. Bacterial populations were monitored periodically by plate counts and by DNA fingerprinting of cheese isolates with the random amplified polymorphic DNA technique. Isolates that displayed a unique DNA fingerprint were identified to the species level by partial nucleotide sequence analysis of the 16S rRNA gene. Nonstarter biota in both cheese types changed over time, but populations in the Colby cheese showed a greater degree of species heterogeneity. The addition of the L. casei adjunct to cheese milk at 104 cfu/ml did not completely suppress “wild” NSLAB populations, but it did appear to reduce nonstarter species and strain diversity in Colby and young Cheddar cheese. Nonetheless, nonstarter populations in all 6-mo-old cheeses were dominated by wild L. casei. Interestingly, the dominant strains of L. casei in each 6-mo-old cheese appeared to be affected more by adjunct treatment and not cheese variety

The c2d Spitzer spectroscopy survey of ices around low-mass young stellar objects, III: CH4

CH4 is proposed to be the starting point of a rich organic chemistry. Solid CH4 abundances have previously been determined mostly toward high mass star forming regions. Spitzer/IRS now provides a unique opportunity to probe solid CH4 toward low mass star forming regions as well. Infrared spectra from the Spitzer Space Telescope are presented to determine the solid CH4 abundance toward a large sample of low mass young stellar objects. 25 out of 52 ice sources in the $c2d$ (cores to disks) legacy have an absorption feature at 7.7 um, attributed to the bending mode of solid CH4. The solid CH4 / H2O abundances are 2-8%, except for three sources with abundances as high as 11-13%. These latter sources have relatively large uncertainties due to small total ice column densities. Toward sources with H2O column densities above 2E18 cm-2, the CH4 abundances (20 out of 25) are nearly constant at 4.7+/-1.6%. Correlation plots with solid H2O, CH3OH, CO2 and CO column densities and abundances relative to H2O reveal a closer relationship of solid CH4 with CO2 and H2O than with solid CO and CH3OH. The inferred solid CH4 abundances are consistent with models where CH4 is formed through sequential hydrogenation of C on grain surfaces. Finally the equal or higher abundances toward low mass young stellar objects compared with high mass objects and the correlation studies support this formation pathway as well, but not the two competing theories: formation from CH3OH and formation in gas phase with subsequent freeze-out.Comment: 27 pages, 7 figures, accepted by Ap

USING BEM TO PREDICT THE EFFECTIVE THERMAL CONDUCTIVITY FOR HETEROGENEOUS MATERIALS

This work presents a study on the effective thermal conductivity in material with heterogeneous composition in two dimensions. The Boundary Elements Method (BEM) is used to solve the steady state potential equations. The sub regions technique was implemented in order to take into account the effects of these inclusions inside the domain. In the numerical implementation, the inclusions are randomly generated in a Representative Volume Element (RVE) domain. The Average Field Theory is used to predict the effective properties (macroscopic) of the material with heterogeneous composition. The material is characterized by a specified volume fraction as well as the inclusion’s size. The samples are composed of square domains with defined number of randomly distributed inclusions and submitted to a condition of unidirectional heat conduction. Each set of samples is analyzed several times in order to guarantee statistical stability of the result

Reaction Networks For Interstellar Chemical Modelling: Improvements and Challenges

We survey the current situation regarding chemical modelling of the synthesis of molecules in the interstellar medium. The present state of knowledge concerning the rate coefficients and their uncertainties for the major gas-phase processes -- ion-neutral reactions, neutral-neutral reactions, radiative association, and dissociative recombination -- is reviewed. Emphasis is placed on those reactions that have been identified, by sensitivity analyses, as 'crucial' in determining the predicted abundances of the species observed in the interstellar medium. These sensitivity analyses have been carried out for gas-phase models of three representative, molecule-rich, astronomical sources: the cold dense molecular clouds TMC-1 and L134N, and the expanding circumstellar envelope IRC +10216. Our review has led to the proposal of new values and uncertainties for the rate coefficients of many of the key reactions. The impact of these new data on the predicted abundances in TMC-1 and L134N is reported. Interstellar dust particles also influence the observed abundances of molecules in the interstellar medium. Their role is included in gas-grain, as distinct from gas-phase only, models. We review the methods for incorporating both accretion onto, and reactions on, the surfaces of grains in such models, as well as describing some recent experimental efforts to simulate and examine relevant processes in the laboratory. These efforts include experiments on the surface-catalysed recombination of hydrogen atoms, on chemical processing on and in the ices that are known to exist on the surface of interstellar grains, and on desorption processes, which may enable species formed on grains to return to the gas-phase.Comment: Accepted for publication in Space Science Review

The Distribution and Excitation of CH₃CN in a Solar Nebula Analog

Cometary studies suggest that the organic composition of the early Solar Nebula was rich in complex nitrile species such CH3CN. Recent ALMA detections in protoplanetary disks suggest that these species may be common during planet and comet formation, but connecting gas-phase measurements to cometary abundances first requires constraints on formation chemistry and distributions of these species. We present here the detection of seven spatially resolved transitions of CH3CN in the protoplanetary disk around the T-Tauri star TW Hya. Using a rotational diagram analysis, we find a disk-averaged column density of ${N}_{T}={1.45}_{-0.15}^{+0.19}\times {10}^{12}$ cm−2 and a rotational temperature of ${T}_{\mathrm{rot}}={32.7}_{-3.4}^{+3.9}$ K. A radially resolved rotational diagram shows the rotational temperature to be constant across the disk, suggesting that the CH3CN emission originates from a layer at z/r ~ 0.3. Through comparison of the observations with predictions from a disk chemistry model, we find that grain-surface reactions likely dominate CH3CN formation and that in situ disk chemistry is sufficient to explain the observed CH3CN column density profile without invoking inheritance from the protostellar phase. However, the same model fails to reproduce a solar system cometary abundance of CH3CN relative to H2O in the midplane, suggesting that either vigorous vertical mixing or some degree of inheritance from interstellar ices occurred in the Solar Nebula

Wavelength-Dependent UV Photodesorption of Pure N2N_2 and O2O_2 Ices

Context: Ultraviolet photodesorption of molecules from icy interstellar grains can explain observations of cold gas in regions where thermal desorption is negligible. This non-thermal desorption mechanism should be especially important where UV fluxes are high. Aims: $N_2$ and $O_2$ are expected to play key roles in astrochemical reaction networks, both in the solid state and in the gas phase. Measurements of the wavelength-dependent photodesorption rates of these two infrared-inactive molecules provide astronomical and physical-chemical insights into the conditions required for their photodesorption. Methods: Tunable radiation from the DESIRS beamline at the SOLEIL synchrotron in the astrophysically relevant 7 to 13.6 eV range is used to irradiate pure $N_2$ and $O_2$ thin ice films. Photodesorption of molecules is monitored through quadrupole mass spectrometry. Absolute rates are calculated by using the well-calibrated CO photodesorption rates. Strategic $N_2$ and $O_2$ isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. Results: $N_2$ photodesorption mainly occurs through excitation of the $b^1\sqcap_u$ state and subsequent desorption of surface molecules. The observed vibronic structure in the $N_2$ photodesorption spectrum, together with the absence of $N_3$ formation, supports that the photodesorption mechanism of $N_2$ is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule. In contrast, $O_2$ photodesorption in the 7−13.6 eV range occurs through dissociation and presents no vibrational structure. Conclusions: Photodesorption rates of $N_2$ and $O_2$ integrated over the far-UV field from various star-forming environments are lower than for CO. Rates vary between $10^{-3}$ and $10^{-2}$ photodesorbed molecules per incoming photon.Astronom