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At least 217 records · Page 12Linked to original sources
PubMed2003

Evolution of chemical and sensory properties during aging of top-fermented beer.

The aging and consequent changes in flavor molecules of a top-fermented beer were studied. Different aging conditions were imposed on freshly bottled beer. After 6 months of aging, the concentration changes were recorded for acetate esters, ethyl esters, carbonyls, Maillard compounds, dioxolanes, and furanic ethers. For some flavor compounds, the changes with time of storage were monitored at different temperatures, either with CO(2) or with air in the headspace of the bottles. For some molecules a relationship was determined between concentration changes and sensory evaluation results. A decrease in volatile esters was responsible for a reduced fruity flavor during aging. On the contrary, various carbonyl compounds, some ethyl esters, Maillard compounds, dioxolanes, and furanic ethers showed a marked increase, due to oxidative and nonoxidative reactions. A very high increase was found for furfural, 2-furanmethanol, and especially the furanic ether, 2-furfuryl ethyl ether (FEE). For FEE a flavor threshold in beer of 6 mug/L was determined. In the aged top-fermented beer, FEE concentrations multiple times the flavor threshold were observed. This was associated with the appearance of a typical solvent-like flavor. As the FEE concentration increased with time at an almost constant rate, with or without air in the headspace, FEE (and probably other furanic ethers) is proposed as a good candidate to evaluate the thermal stress imposed on beer.

Beer↗
PubMed1995

The evolution of chemical patterns in reactive liquids, driven by hydrodynamic instabilities.

We summarize our activity in unveiling a very wide phenomenon: When a chemical reaction takes place at a liquid interface, spectacular patterns of product form (see Plate 1). The pattern formation phenomenon is general, and is observed in reactions between liquids separated by a membrane, in liquids subjected to gaseous reactants, and in photoreactive liquids. We have demonstrated the phenomenon on over 100 different reactions of all types, thus discovering what we believe to be one of the widest macroscopic pattern formation processes known to chemistry. As can be seen in the accompanying pictures, the richness, beauty, and variations in types of patterns can be breathtaking. Two important aspects of these patterns are noted: First, the patterns are true far-from-equilibrium structures, which are maintained only as long as reactants are available, or only as long as light energy is supplied to the system; and second, the chemical products that form the patterns are not precipitates, but are entirely soluble in the liquid in which they form. Thus, if the containers in which the patterns form are shaken or stirred, a homogeneous solution results. Our research of this phenomenon concentrated on three main aspects. The first one was phenomenological. Here we explored the scope and generality of the phenomenon, motivated both by the aesthetic appeal of the phenomenon, and by the puzzle of how is it that such a wide-scope, experimentally simple phenomenon, has by and large, escaped the attention of the scientific community.The second aspect was devoted to the understanding of the underlying general mechanism. Of the many mechanisms we analyzed and tested, some very complex, others quite trivial, the one that fits the majority of the physical and chemical observations is the following: By performing a reaction through a liquid interface, a concentration gradient of the product forms near the interface. We have shown that in many cases, these gradients lead to hydrodynamic instabilities, which then break nonlinearly into a pattern which onsets slow convections. In other words, we found that these patterns mark the route along which a chemical instability relaxes. The third aspect of our research was theoretical. Here we concentrated in depth on one of the reactions (the Fe(+2)/Fe(+3) photoredox reaction), determined all its important physical parameters, and modeled its behavior theoretically. Our model, which was based on the instability buildup described above, was solved numerically, and its results compared with computerized image analysis of the evolving patterns; very good agreement between theory and experiment, was obtained. (c) 1995 American Institute of Physics.

Journal Article↗
PubMed2005

Chemical mixtures: considering the evolution of toxicology and chemical assessment.

The assessment of chemical mixtures is a complex topic for toxicologists, regulators, and the public. In this article the linkage between the science of toxicology and the needs of governmental regulatory agencies in the United States is explored through an overview of environmental regulations enacted over the past century and a brief history of modern toxicology. One of the goals of this overview is to encourage both regulators and scientists to consider the benefits and limitations of this science-regulatory relationship as they tackle existing issues such as chemical mixtures. It is clear that a) over the past 100 years chemical regulation and toxicologic research, have in large part, shared a common emphasis on characterization and regulation of individual chemicals. But chemical mixtures have been, and continue to be, evaluated at hazardous waste sites around the United States. For this reason the current U.S. Environmental Protection Agency guidelines for chemical mixtures assessment are also reviewed. These guidelines highlight the current practice of mixtures assessment, which relies primarily on the existing single-chemical database. It is also clear that b) the science and assessment of chemical mixtures are moving forward through the combined efforts of regulatory agencies and scientists from a broad range of disciplines, including toxicology. Because toxicology is at this exciting crossroads, particular attention should be paid to the forces (e.g., public demands, regulatory needs, funding, academic interests) that both promote and limit the growth of this expanding discipline.

Environmental Exposure↗
PubMedDate not supplied

[Abiogenic nucleoside synthesis in the presence of phosphorus salts].

By means of UV-spectroscopy, gel-filtration, ion-exchange and thin layer chromatography it has been shown that the action of ionizing radiation on the mixture of dry preparations of adenine and deoxyribose in the presence of the K, Na and Ca phosphates results in the formation of nucleoside-like substances. The phosphates catalyze or inhibit the nucleoside synthesis but they are not phosphorylating agents. The data obtained indicate the reality of abiogenic synthesis of nucleoside-like substances from the mixture of dry preparations of adenine and deoxyribose in the lithosphere during chemical evolution.

Adenine↗
PubMed2005

Organic matter in carbonaceous meteorites: past, present and future research.

Carbonaceous meteorites are fragments of ancient asteroids that have remained relatively unprocessed since the formation of the Solar System. These carbon-rich objects provide a record of prebiotic chemical evolution and a window on the early Solar System. Many compound classes are present reflecting a rich organic chemical environment during the formation of the planets. Recent theories suggest that similar extraterrestrial organic mixtures may have acted as the starting materials for life on Earth.

Carbon↗
PubMed2003

Structural degradation of acrylic bone cements due to in vivo and simulated aging.

Acrylic bone cement is the primary load-bearing material used for the attachment of orthopedic devices to adjoining bone. Degradation of acrylic-based cements in vivo results in a loss of structural integrity of the bone-cement-prosthesis interface and limits the longevity of cemented orthopedic implants. The purpose of this study is to investigate the effect of in vivo aging on the structure of the acrylic bone cement and to develop an in vitro artificial aging protocol that mimics the observed degradation. Three sets of retrievals are examined in this study: Palacos brand cement retrieved from hip replacements, and Simplex brand cement retrieved from both hip and knee replacement surgeries. In vitro aging is performed using oxidative and acidic environments on three acrylic-based cements: Palacos, Simplex, and CORE. Gel permeation chromatography (GPC) and Fourier transform infrared spectroscopy (FTIR) are used to examine the evolution of molecular weight and chemical species within the acrylic cements due to both in vivo and simulated aging. GPC analysis indicates that molecular weight is degraded in the hip retrievals but not in the knee retrievals. Artificial aging in an oxidative environment best reproduces this degradation mechanism. FTIR analysis indicates that there exists a chemical evolution within the cement due to in vivo and in vitro aging. These findings are consistent with scission-based degradation schemes in the cement. Based on the results of this study, a pathway for structural degradation of acrylic bone cement is proposed. The findings from this investigation have broad applicability to acrylic-based cements and may provide guidance for the development of new bone cements that resist degradation in the body.

Acrylates↗
PubMed1995

Exobiological exploration of Mars.

Of all the other planets in the solar system, Mars remains the most promising for further elucidating concepts about chemical evolution and the origin of life. Strategies were developed to pursue three exobiological objectives for Mars exploration: determining the abundance and distribution of the biogenic elements and organic compounds, detecting evidence of an ancient biota on Mars, and determining whether indigenous organisms exist anywhere on the planet. The three strategies are quite similar and, in fact, share the same sequence of phases. In the first phase, each requires global reconnaissance and remote sensing by orbiters to select sites of interest for detailed in situ analyses. In the second phase, lander missions are conducted to characterize the chemical and physical properties of the selected sites. The third phase involves conducting "critical" experiments at sites whose properties make them particularly attractive for exobiology. These critical experiments would include, for example, identification of organics, detection of fossils, and detection of extant life. The fourth phase is the detailed analysis of samples returned from these sites in Earth-based laboratories to confirm and extend previous discoveries. Finally, in the fifth phase, human exploration is needed to establish the geological settings for the earlier findings or to discover and explore sites that are not accessible to robotic spacecraft.

Evolution, Chemical↗
PubMed2000

Prebiotic methylation and the evolution of methyl transfer reactions in living cells.

An hypothesis is presented for the prebiotic origin of methyl groups and the evolution of methyl transfer reactions in living cells. This hypothesis, described in terms of prebiotic and early biotic chemical evolution, is based on experimental observations in our lab and in those of others, and on the mechanisms of enzymatic methyl transfer reactions that occur in living cells. Of particular interest is our demonstration of the reductive methylation of ethanolamine and glycine in aqueous solution by excess formaldehyde. These reactions, involving prebiotic compounds and conditions, are mechanistically analogous to the de novo origin of methyl groups in modern cells by reduction of methylene tetrahydrofolate. Furthermore, modern cellular methyl transfers from S-adenosylmethionine to amine nitrogen may involve formaldehyde as an intermediate and subsequent reductive methylation, analogous to the prebiotic chemistry observed herein.

Betaine↗
PubMed1980

Clays in prebiological chemistry.

In this review an attempt is made to highlight the structures and properties of clay that may contribute to a better understanding of the role of clays in chemical evolution. The adsorption of organic molecules on clays has been demonstrated, as has the synthesis of bioorganic monomers in the presence of clays. For instance, amino acids (glycine, aspartic acid, threonine, alanine and others) as well as purines and pyrimidines, have been obtained from CO and NH3 in the presence of clays at relatively high temperatures (250-325 degrees C). Carbohydrates are also easily derived from formaldehyde at relatively low temperatures (approximately equal to 80 degrees C). The oligomerization of biochemical monomers, mediated by clays has also been shown to result in the formation of polymer molecules basic to life. For instance the condensation of amino acyl adenylates at room temperature in the presence of montmorillonite is known to yield polypeptides in discrete ranges of molecular weights with degrees of polymerization up to 56. Clays have also been found to affect the condensation of mononucleotides to oligonucleotides. Although the role of clays in the origin or metabolic pathways has not been demonstrated, it is possible that clays may have played a cooperative role with catalytic peptides in an intermediate stage of prebiological chemistry preceding the emergence of life on this planet.

Adsorption↗
PubMed2006

The possible influence of L-histidine on the origin of the first peptides on the primordial Earth.

One of the most unsettled problems of prebiotic evolution and the origin of life is the explanation why one enantiomeric form of biomolecules prevailed. In the experiments presented in this paper, the influence of L-histidine on the peptide formation in the Salt-Induced Peptide Formation (SIPF) reaction of the enantiomeric forms of valine, proline, serine, lysine, and tryptophan, and the catalytic effects in this first step toward the first building blocks of proteins on the primordial earth were investigated. In the majority of the produced dipeptides, a remarkable increase of yields was shown, and the preference of the L-amino acids in the peptide formation in most cases cannot be denied. In summary, our data provide further experimental evidence for the plausibility of the SIPF reaction and point at a possible important role of L-histidine in the chemical evolution on the primordial Earth.

Catalysis↗
PubMed2002

[Frontier fields of plant chemical ecology in the 21st century].

It has focused on chemical interactions between plant and other organisms, which mediated by secondary plant metabolites in recent years. Induced chemical defense in plant, plant chemical communication, relationships between secondary plant metabolites and evolution, chemical relationships between plant and human and chemical ecology of marine plant are the frontier fields needed attention of plant chemical ecology in the 21st century. Progress in these frontier fields of plant chemical ecology will play an important role in sustainable development, particular in increasing agricultural production and effective control of pest, disease and weed under ecological security in the 21st century.

Agrochemicals↗
PubMed2004

A new type of floral oil from Malpighia coccigera (Malpighiaceae) and chemical considerations on the evolution of oil flowers.

New, partially acetylated dihydroxy fatty acids could be identified in the floral oil of Malpighia coccigera (Malpighiaceae): 7-OAc,3-OH 20:0, 7-OAc,3-OH 22:0, 9-OAc,3-OH 22:0, 9-OAc,5-OH 22:0, 3,9-diOAc 22:0, 9-OAc,3-OH 24:0, and 11-OAc,5-OH 24:0. The substitution patterns of all hitherto undescribed dihydroxylated and additionally identified monohydroxylated fatty acids are in agreement with a polyketide analogous biosynthesis. Intermediates may be 3-acetoxy fatty acids (C16, C18, and C20), known from flower secretions of other phylogenetically unrelated plant families. A possible relationship between plant epicuticular wax and floral oil biosynthesis is discussed. It may explain why an independent but convergent development of oil flowers and flower oils in unrelated plant families was possible.

Evolution, Chemical↗
PubMed1977

Selective formation of certain amino acids from formaldehyde and hydroxylamine in a modified sea medium enriched with molybdate.

Amino acids produced from formaldehyde and hydroxylamine in modified sea mediums with different concentrations of molybdate were analyzed. The modified sea mediums contained lower concentration of sodium chloride and higher concentrations of transition metal ions (Zn2+, Fe3+, Cu2+, Co2+, Mn2+ each 10(-4)m, and Mo O4(2-)10(-6), 10(-4), or 10(-2)m) than sea water. The concentration of molybdate had apparently no remarkable effect on the total yields of primary amino groups, but a remarkable effect on the nature of amino acids produced. The formation of alanine, aspartic acid, beta-alanine and, in particular, proline was increased, and that of glycine and serine was decreased with the enrichment of molybdate. The results suggest the possibility of a natural selection of prebiotic organic molecules based on the nature of environmental catalysers in the course of chemical evolution.

Amino Acids↗