Dynamic rheology of agar gels: theory and experiments. Part I. Development of a rheological model

A theoretical rheological model for agar gels is proposed, based on the bead and spring model for linear flexible random coils and the model for crosslinked polymers. The model introduces the concept of a temperature dependence of the monomeric friction coefficient, ζ₀, of the agar molecule. The model has a random coil-like behavior at high temperatures (close to 373 K), and contributions from a three-dimensional network at low temperatures (close to 273 K). A proposed temperature dependence of the net association rate allows the calculation of the fraction of associated molecules as a function of time and temperature. The proposed model predicts the gelation behavior of agar gels utilizing time–temperature data (cooling curves).     

A Simple Model for the Cold Requirement of the Tulip

A simple model is proposed that qualitatively describes someimportant features of the cold requirement of the tulip. Themodel may be used as a basis for quantitative experimental investigationsinto the response of tulip growth and development to differentperiods of bulb storage at low temperatures.     

Purification, characterization, and nucleotide sequence of the thermolabile alpha-amylase from the antarctic psychrotroph Alteromonas haloplanctis A23.

The alpha-amylase excreted by the antarctic bacterium Alteromonas haloplanctis was purified and the corresponding amy gene was cloned and sequenced. N- and C-terminal amino acid sequencing were used to establish the primary structure of the mature A. haloplanctis alpha-amylase which is composed of 453 amino acids with a predicted Mr of 49,340 and a pI of 5.5. Three Ca2+ ions are bound per molecule and its activity is modulated by chloride ions. Within the four consensus sequences, Asp-174, Glu-200, and Asp-264 are the proposed catalytic residues. The psychrotrophic A. haloplanctis alpha-amylase is characterized by a high amylolytic activity at low temperatures, a reduced apparent optimal temperature, and typical thermodynamic activation parameters A. haloplanctis alpha-amylase has also a low thermal stability as demonstrated by the temperature effect on both activity and secondary structure. It is suggested that structure flexibility and lower sensitivity of secondary structure to temperature variations in the low temperature range are the main structural adaptations of the psychrotrophic enzyme. The unusual stacking of small amino acids around the catalytic residues is proposed as a factor inducing active site flexibility and concomitant high activity of the enzyme at low temperatures.     

A new approach to enhance PCR specificity

A new approach to enhanced specificity and product yield of polymerase chain reaction is proposed. It is based on control of DNA polymerase activity during PCR by changing the magnesium ion concentration, which depends on the temperature of the reaction mixture. A slightly soluble magnesium salt, magnesium oxalate, whose solubility depends on temperature, was used as a source of magnesium ions. During PCR, magnesium oxalate was maintained at saturating concentration by the presence of an insoluble excess of this salt, and the concentration of magnesium ions depended on the salt solubility: binding of magnesium ions at lower temperatures and their release at higher temperatures was shown to affect the DNA polymerase activity and to favor the specific PCR amplification of the target DNA fragment.     

A New Approach to Enhanced PCR Specificity

A new approach to enhanced specificity and product yield of polymerase chain reaction is proposed. It is based on control of DNA polymerase activity during PCR by changing the magnesium ion concentration, which depends on the temperature of the reaction mixture. A slightly soluble magnesium salt, magnesium oxalate, whose solubility depends on temperature, was used as a source of magnesium ions. During PCR, magnesium oxalate was maintained at saturating concentration by the presence of an insoluble excess of this salt, and the concentration of magnesium ions depended on the salt solubility: binding of magnesium ions at lower temperatures and their release at higher temperatures was shown to affect the DNA polymerase activity and to favor the specific PCR amplification of the target DNA fragment.     

Vernalization in Summer/Autumn Cauliflower (Brassies oleracea var. botrytisL.)

The effect of a range of temperatures on the progress of plantstowards curd initiation was studied using data from field experimentsdesigned to produce a wide range of temperatures. A model describingthe effect of temperature on vernalization rate by linear relationshipsusing four cardinal temperatures was proposed to predict changesin the diameter of the growing point with time, and hence, thetime of curd initiation. A method was developed to estimatethe parameters of the model from continuously varying temperaturesencountered in the field. Key words: Model, cardinal temperatures, juvenility, vernalization, initiation     

Determination of membrane lipid phase transition temperature from C-NMR intensities

A new and simple approach for the determination of the temperature of gel-to-liquid crystalline phase transitions (T_C) of biological (chloroplast) membrane lipids from ¹³C-NMR resonance intensities is proposed. The variation of intensity of a temperature-sensitive NMR resonance is monitored by recording the spectra of the sample at a range of temperatures. From such a series of spectra recorded at different temperatures, a temperature-insensitive resonance is located. Then the ratio of the intensity of the temperature-sensitive to the intensity of the temperature-insensitive resonance is calculated from each spectrum to even out the procedural error, if any. The values of this ratio at different temperatures, when plotted against sample temperature, shows a break at T_C as confirmed by spin label ESR studies.     

Substrate inhibition or activation kinetics of the beta-galactosidase from the extreme thermoacidophile archaebacterium Caldariella acidophila

The kinetics of the hydrolysis of p-nitrophenyl-beta-D-galactopyranoside (pNPG) by a thermophile, beta-galactosidase, was studied at different temperatures. This enzyme was isolated from the thermophilic microorganism archaebacterium Caldariella acidophila. The hydrolysis of pNPG by beta-galactosidase does not follow Michaelis-Menten law. This enzyme is inhibited by excess substrate at low temperatures and it is activated by excess substrate at high temperatures. A minimum mechanistic model is proposed to explain the behaviour. This model assumes the binding of an additional substrate molecule on the glycosidyl enzyme intermediate. This model is in good agreement with the postulated mechanism for beta-galactosidase from Escherichia coli. The kinetic parameters are calculated at six different temperatures.     

The importance of seasonal variation in hair coloration for thermoregulation of Ctenucha virginica larvae (Lepidoptera: Arctiidae)

ABSTRACT. . A marked seasonal variation in the hair colour of Ctenucha virginica (Esp.) larvae is observed under field conditions. During the autumn and spring larvae are black and yellow, while during the summer they are predominantly yellow. Under sunny conditions in the field, or when exposed to a radiation heat source in the laboratory, black and yellow larvae have significantly higher body temperatures than yellow ones. As larvae feed over a limited range of temperatures, it is proposed that seasonal changes in hair colour may help larvae to thermoregulate, thereby maintaining body temperatures within the range where feeding is possible.     

Spin-labeling studies on the lipids of psychrophilic, psychrotrophic, and mesophilic clostridia.

Spin-labeling studies were conducted to elucidate the viscosity and phase transition temperatures of lipids isolated from psychrophilic, psychrotrophic, and mesophilic clostridia. Electron spin resonance spectroscopy indicated that the lipids, for all the growth temperatures tested, were in a fluid state and from 13 to 24 C higher than the corresponding lipid transition temperatures. When the organisms were grown at different temperatures, a psychrotropic and two mesophilic clostridia were shown to be able to adjust their lipid-phase transition temperature to the growth temperature. A psychrophilic Clostridium strain, when grown at different temperatures, synthesized lipids that had the same phase transition temperature. It is suggested that this lack of growth temperature-inducible regulation of lipid-phase transition temperature may be a molecular determinant for the psychrophily of this organism. It is proposed that the growth temperature range of an organism is dependent upon the ability of the organism to regulate its lipid fluidity within a specific range.     

Not changes in membrane fluidity but proteotoxic stress triggers heat shock protein expression in Chlamydomonas reinhardtii

A conserved reaction of all organisms exposed to heat stress is an increased expression of heat shock proteins (HSPs). Several studies have proposed that HSP expression in heat‐stressed plant cells is triggered by an increased fluidity of the plasma membrane. Among the main lines of evidence in support of this model are as follows: (a) the degree of membrane lipid saturation was higher in cells grown at elevated temperatures and correlated with a lower amplitude of HSP expression upon a temperature upshift, (b) membrane fluidizers induce HSP expression at physiological temperatures, and (c) membrane rigidifier dimethylsulfoxide dampens heat‐induced HSP expression. Here, we tested whether this holds also for Chlamydomonas reinhardtii. We show that heat‐induced HSP expression in cells grown at elevated temperatures was reduced because they already contained elevated levels of cytosolic HSP70A/90A that apparently act as negative regulators of heat shock factor 1. We find that membrane rigidifier dimethylsulfoxide impaired translation under heat stress conditions and that membrane fluidizer benzyl alcohol not only induced HSP expression but also caused protein aggregation. These findings support the classical model for the cytosolic unfolded protein response, according to which HSP expression is induced by the accumulation of unfolded proteins. Hence, the membrane fluidity model should be reconsidered.     

Is there a common chemical model for life in the universe?

A review of organic chemistry suggests that life, a chemical system capable of Darwinian evolution, may exist in a wide range of environments. These include non-aqueous solvent systems at low temperatures, or even supercritical dihydrogen-helium mixtures. The only absolute requirements may be a thermodynamic disequilibrium and temperatures consistent with chemical bonding. A solvent system, availability of elements such as carbon, hydrogen, oxygen and nitrogen, certain thermodynamic features of metabolic pathways, and the opportunity for isolation, may also define habitable environments. If we constrain life to water, more specific criteria can be proposed, including soluble metabolites, genetic materials with repeating charges, and a well defined temperature range.     

Advances,shortcomings, and recommendations for wind chill estimation

This article discusses briefly the advances made and the remaining short-comings in the "new" wind chill charts adopted in the US and Canada in 2001. A number of indicated refinements are proposed, including the use of whole body models in the computations, verification of heat exchange coefficients by human experiments, reconsideration of "calm" wind conditions, reconsideration of frostbite threshold levels, the inclusion of cold-related pain and numbness in the charts, etc. A dynamic numerical model is applied to compare the effects of wind speeds, on the one hand, and air temperatures, on the other, on the steady-state exposed facial and bare finger temperatures. An apparent asymmetry is demonstrated, favoring the effects of wind speeds over those of air temperatures for an identical final facial temperature. This asymmetry is reversed, however, when SI unit changes in these quantities are considered.     

The Effect of Temperature on Cold and Heat Resistance of Growing Plants: II. COLD RESISTANT SPECIES

The dynamics of cold and heat resistance in a number of coldresistant plant species (potato, meadow fescue, spring and winterwheat) exposed to temperatures from –13 °C to + 50°Chas been studied under controlled environmental conditions.The thermo-resistance of leaves was shown to be constant atcertain temperatures (a range of background temperatures), butit increased (ranges of heat and cold hardening) or decreased(ranges of heat and cold injury) at other temperatures. A gradationof temperatures with respect of ‘thermo-resistance’for these ranges is being proposed. The limits of the rangesvary depending on endogenous (species characteristics, phaseof development) and exogenous factors (environmental conditions).Thus, at gradually rising or falling temperatures the boundariesbetween the ranges of hardening and injury are markedly shiftedtowards more extreme temperatures. Generally, the data showuniformity of responses to extreme temperatures by cold resistantplants; the differences observed between species are quantitative. Key words: Temperature, Cold-resistant plants     

Ovotestes suggest cryptic genetic influence in a reptile model for temperature-dependent sex determination

Sex determination and differentiation in reptiles is complex. Temperature-dependent sex determination (TSD), genetic sex determination (GSD) and the interaction of both environmental and genetic cues (sex reversal) can drive the development of sexual phenotypes. The jacky dragon (Amphibolurus muricatus) is an attractive model species for the study of gene–environment interactions because it displays a form of Type II TSD, where female-biased sex ratios are observed at extreme incubation temperatures and approximately 50 : 50 sex ratios occur at intermediate temperatures. This response to temperature has been proposed to occur due to underlying sex determining loci, the influence of which is overridden at extreme temperatures. Thus, sex reversal at extreme temperatures is predicted to produce the female-biased sex ratios observed in A. muricatus. The occurrence of ovotestes during development is a cellular marker of temperature sex reversal in a closely related species Pogona vitticeps. Here, we present the first developmental data for A. muricatus, and show that ovotestes occur at frequencies consistent with a mode of sex determination that is intermediate between GSD and TSD. This is the first evidence suggestive of underlying unidentified sex determining loci in a species that has long been used as a model for TSD.     

Design and analysis of accelerated degradation tests for the stability of biological standards III. Principles of design

The principles of designing an accelerated degradation test for a biological standard are considered, with special emphasis on ensuring accuracy and precision of the predicted low temperature degradation rate. A three-stage design is proposed which initially monitors samples at the highest of the elevated temperatures, but which switches attention progressively to the lower temperatures as degradation begins to be detected. The advantages of this design over one where testing is carried out after a predetermined period are that it is (i) more efficient, (ii) more reliable and (iii) better suited to test the assumptions which underly the analysis of the results. Consideration is also given to additional checks which can be made on the stability of biological standards and to methods of monitoring long-term stability after the standard has been set up.     

New trends in cryoenzymology: I.-Supercooled aqueous solutions.

A water in oil emulsion technique is proposed to investigate enzyme catalyzed reactions at sub-zero temperatures in the supercooled liquid state to avoid some reversible effects of the usual cosolvents on kinetics. Some results are listed: potentialities and technical problems of the procedure are discussed.     

Modelling the growth,survival and death of Listeria monocytogenes

In this paper, the predictive microbiology approach has been generalized to the study of growth, survival and death of Listeria monocytogenes. As this micro-organism is involved in food poisoning, its growth, survival and death were studied as functions of low temperatures, NaCl and phenol compounds, in a synthetic medium, by a factorially designed experiment. A significant inactivation of L. monocytogenes was obtained with 20 ppm of phenol and 4% (w/v) NaCl at temperatures from 4 to 12 degrees C. An empirical model is proposed to describe, in a single step, the biomass profile vs studied factors. Thereby, the influence of temperature, NaCl and phenol concentration on L. monocytogenes biomass quantity (0.5-8 log cfu ml(-1)) are presented as a function of storage duration. The comparisons of the proposed model with existing models (Gompertz for growth, vitalistic for survival and death) were performed. The use of a single equation allows the prediction of contamination levels in all experimental conditions without knowledge a priori. The model offers considerable prospects for its use in food microbiology.     

X-ray diffraction evidence for fully interdigitated bilayers of 1-stearoyllysophosphatidylcholine

X-ray diffraction experiments have been performed on 1-stearoyllysophosphatidylcholine or C(18):C(0)PC as a function of hydration at temperatures below the order/disorder transition (Tm = 26.2 degrees C). At these temperatures, hydrated C(18):C(0)PC forms lamellae. The bilayer thickness, as determined by the saturation hydration method and electron-density profile, is 35-36 A, and the average area per C(18):C(0)PC molecule at the lipid/water interface is 45.5 A2. The packing geometry of C(18):C(0)PC in the lamella is proposed to adopt a fully interdigitated model in which the long C(18) acyl chain extends across the entire hydrocarbon width of the bilayer. Thus far, three different types of interdigitated bilayers are known for phosphatidylcholines. These various types of chain interdigitation are discussed in terms of the chain length difference between the sn-1 and sn-2 acyl chains.     

IMPACT OF MICROWAVE BLANCHING ON THE FLAVOR OF ROASTED PEANUTS

Microwave blanching of peanuts was proposed as an attractive alternative to traditional techniques of blanching, because of energy and time savings. However, the occurrence of a processing‐related off‐flavor has been reported. This study examined the effect of processing factors during microwave blanching on the MC and sensory characteristics of the peanuts. The peanuts reached a range of internal temperatures during microwave blanching treatments between 4 and 11 min. A total offnote attribute was introduced to the peanut lexicon and was used successfully to differentiate the effects of microwave treatments. The microwave‐associated off‐flavor was related (but not identical) to cardboardy/stale flavor, and was related inversely to the positive flavor attributes roasted peanutty, sweet aromatic and sweet taste. Peanuts reaching the highest internal temperatures and greatest moisture losses during blanching exhibited the most total offnote flavor; however, temperatures as high as 113C did not produce significantly increased total offnote intensity.