Computerized apparatus for measuring dynamic flavor release from liquid food matrices     

Rabe S  Krings U  Banavara DS  Berger RG 《Journal of agricultural and food chemistry》2002,50(22):6440-6447

A fully computer-controlled apparatus was designed. It combines a glass reactor with a temperature-controlled hood, in which headspace volatiles are captured. Flavored liquids can be introduced into the reactor and exposed to conditions of temperature, air flow, shear rate, and saliva flow as they occur in the mouth. As the reactor is completely filled before measurements are started, creation of headspace just before sampling start prevents untimely flavor release resulting in real time data. In the first 30 s of flavor release the concentrations of the volatiles can be measured up to four times by on-line sampling of the dynamic headspace, followed by off-line trapping of the samples on corresponding Tenax traps and analysis using GC-TDS-FID. Flavor compounds from different chemical classes were dissolved in water to achieve concentrations typically present in food (micrograms to milligrams per liter). Most of the compounds showed constant release rates, and the summed quantities of each volatile of three 10 s time intervals correlated linearly with time. The entire method of measurement including sample preparation, release, sampling, trapping, thermodesorption, and GC analysis showed good sensitivity [nanograms (10 s)(-1)] and reproducibility (mean coefficient of variation = 7.2%).  相似文献   

Decomposition of atmospheric hydrogen by soil microorganisms and soil enzymes     

Ralf Conrad  Wolfgang Seiler 《Soil biology & biochemistry》1981,13(1):43-49

The decomposition of atmospheric hydrogen in different types of soil was measured. The decomposition of H₂ was apparently a first-order reaction. H₂ decomposition activity was proportional to the amount of soil with maximum activities at soil water contents of approx. 6–11% (w/w). The activity was lower under anaerobic conditions, but was constant between 1–20% O₂. It was destroyed by autoclaving and was partially inactivated by fumigation with NH₃, CHC1₃ or acetone, by u.v. irradiation and by treatment with NaCN or NaN₃, indicating that biological processes in the soil were responsible for the observed H₂ decomposition. Treatment of soil with toluene or CHCl₃ caused only a partial inactivation. Incubation of soil in the presence of streptomycin or actidione reduced H₂ decomposition by less than 50%, whereas CO consumption was abolished. The H₂ decomposition rates showed H₂ saturation curves with apparent Michaelis-Menten kinetics. Cooperative effects were not observed. V_max was reached at approx. 200 μl1^?1. The K_m values for H₂ were in the range of 30μl 1^?1, but increased to higher values, when the soil had been pretreated with high H₂ mixing ratios. Apparently, the observed H₂ decomposition by soil is not only due to the activity of viable microorganisms, but soil enzymes as well.  相似文献   

Changes of Al and Heavy Metal Concentrations in Slovak Soils During the Last 25 Years     

Ralf Berghofer  Wolfgang Wilcke  Vladimír Linke&#x;  Otmar Nestroy  Wolfgang Zech 《植物养料与土壤学杂志》1997,160(4):469-474

The objective of this study was to investigate changes of total concentrations and various extract-defined Al and heavy metal fractions in Slovak agricultural soils during the last 25 years. We compared 7 stored soil samples collected between 1966 and 1970 with samples collected in 1994 at the same sites. Seven fractions of Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn were determined with a sequential extraction procedure in all samples. Total concentrations of Cd, Zn, Mn, Ni, and Cu were lower in the 1994 samples; those of Al, Fe, Pb, and Cr were higher. Based on the initial concentrations, the average total concentration changes were: Cd(-10,3%)<Zn(-7,2%)<Mn(-4,8%)<Ni(-2,3%)<Cu(-1,4%)<Al(+2,1%)<Fe(+2,9%)<Cr(+7,4%)<Pb(+8,3%). This row is consistent with the decrease in metal mobility. The differences in salt-extractable metals showed the same pattern; however, changes were more pronounced than for total concentrations. The results suggest that decreases during the last 25 years are caused by higher leaching than deposition rates and increases vice versa. The highest increase in Cr and Pb concentrations is observed in the EDTA-extractable fraction, which mainly characterizes organically bound metals.  相似文献   

Competition for electron donors among nitrate reducers,ferric iron reducers,sulfate reducers,and methanogens in anoxic paddy soil   总被引：17，自引：0，他引：17  

Christof Achtnich  Friedhelm Bak  Ralf Conrad 《Biology and Fertility of Soils》1995,19(1):65-72

Slurries of anoxic paddy soil were either freshly prepared or were partially depleted in endogenous electron donors by prolonged incubation under anaerobic conditions. Endogenous NO ₃ ^– was reduced within 4 h, followed by reduction of Fe³⁺ and SO ₄ ^2– , and later by production of CH₄. Addition of NO ₃ ^– slightly inhibited the production of Fe²⁺ in the depleted but not in the fresh paddy soil. Inhibition was overcome by the addition of H₂, acetate, or a mixture of fatty acids (and other compounds), indicating that these compounds served as electron donors for the bacteria reducing NO ₃ ^– and/or ferric iron. Addition on NO ₃ ^– also inhibited the reduction of SO ₄ ^2– in the depleted paddy soil. This inhibition was only overcome by H₂, but not by acetate or a mixture of compounds, indicating that H₂ was the predominant electron donor for the bacteria involved in NO ₃ ^– and/or SO ₄ ^2– reduction. SO ₄ ^2– reduction was also inhibited by exogenous Fe³⁺, but only in the depleted paddy soil. This inhibition was overcome by either H₂, acetate, or a mixture of compounds, suggesting that they served as electron donors for reduction of Fe³⁺ and/or SO ₄ ²⁺ . CH₄ production was inhibited by NO ₃ ^– both in depleted and in fresh paddy soil. Fe³⁺ and SO ₄ ^2– also inhibited methanogenesis, but the inhibition was stronger in the depleted than in the fresh paddy soil. Inhibition of CH₄ production was paralleled by a decrease in the steady state concentration of H₂ to a level which provided a free enthalpy of less than G=–17 kJ mol^-1 CH₄ compared to more than G=–32 kJ mol^-1 CH₄ in the control. The results indicate that in the presence of exogenous fe³⁺ or SO ₄ ²⁺ , methanogenic bacteria were outcompeted for H₂ by bacteria reducing Fe³⁺ or SO ₄ ²⁺ .Deceased on 27 December 1992  相似文献   

The formation and fragmentation of disks around primordial protostars     

Clark PC  Glover SC  Smith RJ  Greif TH  Klessen RS  Bromm V 《Science (New York, N.Y.)》2011,331(6020):1040-1042

The very first stars to form in the universe heralded an end to the cosmic dark ages and introduced new physical processes that shaped early cosmic evolution. Until now, it was thought that these stars lived short, solitary lives, with only one extremely massive star, or possibly a very wide binary system, forming in each dark-matter minihalo. Here we describe numerical simulations that show that these stars were, to the contrary, often members of tight multiple systems. Our results show that the disks that formed around the first young stars were unstable to gravitational fragmentation, possibly producing small binary and higher-order systems that had separations as small as the distance between Earth and the Sun.  相似文献   

A molecular basis for natural selection at the timeless locus in Drosophila melanogaster     

Sandrelli F  Tauber E  Pegoraro M  Mazzotta G  Cisotto P  Landskron J  Stanewsky R  Piccin A  Rosato E  Zordan M  Costa R  Kyriacou CP 《Science (New York, N.Y.)》2007,316(5833):1898-1900

Diapause is a protective response to unfavorable environments that results in a suspension of insect development and is most often associated with the onset of winter. The ls-tim mutation in the Drosophila melanogaster clock gene timeless has spread in Europe over the past 10,000 years, possibly because it enhances diapause. We show that the mutant allele attenuates the photosensitivity of the circadian clock and causes decreased dimerization of the mutant TIMELESS protein isoform to CRYPTOCHROME, the circadian photoreceptor. This interaction results in a more stable TIMELESS product. These findings reveal a molecular link between diapause and circadian photoreception.  相似文献   

In situ stable isotope probing of methanogenic archaea in the rice rhizosphere   总被引：1，自引：0，他引：1  

Lu Y  Conrad R 《Science (New York, N.Y.)》2005,309(5737):1088-1090

Microorganisms living in anoxic rice soils contribute 10 to 25% of global methane emissions. The most important carbon source for CH4 production is plant-derived carbon that enters soil as root exudates and debris. Pulse labeling of rice plants with 13CO2 resulted in incorporation of 13C into the ribosomal RNA of Rice Cluster I Archaea in the soil, indicating that this archaeal group plays a key role in CH4 production from plant-derived carbon. This group of microorganisms has not yet been isolated but appears to be of global environmental importance.  相似文献   

Revisiting the taxonomic impediment     

de Carvalho MR  Bockmann FA  Amorim DS  de Vivo M  de Toledo-Piza M  Menezes NA  de Figueiredo JL  Castro RM  Gill AC  McEachran JD  Compagno LJ  Schelly RC  Britz R  Lundberg JG  Vari RP  Nelson G 《Science (New York, N.Y.)》2005,307(5708):353

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Effects of the aglycone of ascaulitoxin on amino acid metabolism in Lemna paucicostata     

Stephen O. Duke  Antonio EvidenteMichele Fiore  Agnes M. RimandoFranck E. Dayan  Maurizio VurroNicole Christiansen  Ralf LooserJohannes Hutzler  Klaus Grossmann 《Pesticide biochemistry and physiology》2011,100(1):41-50

Ascaulitoxin and its aglycone (2,4,7-triamino-5-hydroxyoctanoic acid, CAS 212268-55-8) are potent phytotoxins produced by Ascochyta caulina, a plant pathogen being developed for biocontrol of weeds. The mode of action of this non-protein amino acid was studied on Lemna paucicostata. Ascaulitoxin is a potent growth inhibitor, with an I₅₀ for growth of less than 1 μM, almost completely inhibiting growth at about 3 μM. Its action is slow, starting with growth inhibition, followed by darker green fronds, and then chlorosis and death. Most amino acids, including non-toxic non-protein amino acids, reversed the effect of the toxin when supplemented in the same medium. Supplemental sucrose slightly increased the activity. d-Amino acids were equally good inhibitors of ascaulitoxin activity, indicating the amino acid effects may not be due to inhibition of amino acid synthesis. Oxaloacetate, the immediate precursor of aspartate, also reversed the activity. LC-MS did not detect interaction of the compound with lysine, an amino acid that strongly reversed the effect of the phytotoxin. Metabolite profiling revealed that the toxin caused distinct changes in amino acids. Reduction in alanine, paralleled by enhanced levels of the branched chain amino acids valine, leucine and isoleucine and nearly unchanged levels of pyruvate, might indicate that the conversion of pyruvate to alanine is affected by ascaulitoxin aglycone. In addition, reduced levels of glutamate/glutamine and aspartate/asparagine might suggest that synthesis and interconversion reactions of these amino group donors are affected. However, neither alanine aminotransferase nor alanine: glyoxylate aminotransferase were inhibited by the toxin in vitro. Our observations might be explained by three hypotheses: (1) the toxin inhibits one or more aminotransferases not examined, (2) ascaulitoxin aglycone affects amino acid transporters, (3) ascaulitoxin aglycone is a protoxin that is converted in vivo to an aminotransferase inhibitor.  相似文献   

Effect of potassium phosphate fertilization on production and emission of methane and its C-stable isotope composition in rice microcosms     

Ralf Conrad  Melanie Klose 《Soil biology & biochemistry》2005,37(11):2099-2108

Rice fields are an important source for atmospheric CH₄, but the effects of fertilization are not well known. We studied the turnover of CH₄ in rice soil microcosms without and with addition of potassium phosphate. Height and tiller number of rice plants were higher in the fertilized than in the unfertilized microcosms. Emission rates of CH₄ were also higher, but porewater concentrations of CH₄ were lower. The δ¹³C values of the emitted CH₄ and of the CH₄ in the porewater were both 2-6% higher in the fertilized microcosms than in the control. Potassium phosphate did not affect rate and isotopic signature of CH₄ production in anoxic soil slurries. On the other hand, roots retrieved from fertilized microcosms at the end of incubation exhibited slightly higher CH₄ production rates and slightly higher CH₄-δ¹³C values compared to roots from unfertilized plants. Addition of potassium phosphate to excised rice roots generally inhibited CH₄ production and resulted in increasingly lower δ¹³C values of the produced CH₄. Fractionation of ¹³C during plant ventilation (i.e. δ¹³C in pore water CH₄ versus CH₄ emitted) was larger in the fertilized microcosms than in the control. Besides plant ventilation, this difference may also have been caused by CH₄ oxidation in the rhizosphere. However, calculation from the isotopic data showed that less than 27% of the produced CH₄ was oxidized. Collectively, our results indicate that potassium phosphate fertilization stimulated CH₄ emission by enhancing root methanogenesis, plant ventilation and/or CH₄ oxidation, resulting in residence times of CH₄ in the porewater in the order of hours.  相似文献