Associative phenomena in galactomannan-deacetylated xanthan systems

The interaction between mesquite seed galactomannan (MSG; D-mannose to D-galactose ratio (M/G) approximately 1.1) and deacetylated xanthan (DX) in 5 mM NaCl leading to synergistic gel formation at 25 degrees C was investigated and compared with the far more studied system made of xanthan and locust bean gum (LBG; M/G approximately 3.5). Rheology and differential scanning calorimetry were used to measure temperatures of gel formation and transition enthalpy as a function of polymer composition, while circular dichroism was used to probe the conformation of DX in the LBG-DX system. MSG and DX associate at 25 degrees C with a well defined stoichiometry of 0.6:1.0 (w/w) at low ionic strength favouring the disordered coil state of DX. When LBG was used in place of MSG in water or 5 mM NaCl, two types of mechanisms of interpolymeric association are envisaged.     

Effect of galactomannan addition on the thermal behaviour of κ-carrageenan gels

Gelation/melting cycles of κ-carrageenan/galactomannan (guar, tara and locust bean gums) binary systems have been studied by measuring dynamic rheological parameters. Two experimental conditions were used, (i) the total polysaccharide concentration was kept at 1% and the κ-carrageenan/galactomannan ratio fixed at 4:1 and (ii) the κ-carrageenan concentration was fixed at 0·75% and the galactomannan content varied from 0% to 1·2%. A thermal hysteresis was observed for all mixed systems and was found to depend on the galactomannan used. From a comparison of the gelation temperature (T_g) and melting temperature (T_m) to values obtained with κ-carrageenan alone, it was suggested that galactomannan interferes with gel structure by the formation of a secondary network provided that the M/G ratio is high enough.     

Thermal properties of corn gluten meal and its proteic components

Thermal properties of corn gluten meal (CGM) and of its extracted proteic components (zein and glutelin) at 0% moisture content, is studied by dynamic mechanical thermal analysis (DMTA) and modulated differential scanning calorimetry (MDSC). The glass transition temperature (T_g) on first heating, is measured at 176 and 174°C, respectively, for hot-air-dried and native CGM. For zein and glutelin isolated fractions, the measured T_g values are 164 and 209°C, respectively. The calculated T_g from using Matveev’s method (Matveev YI. Spec Publ R Soc Chem 1995;156;552) is in good agreement with experimental data for zein, a well defined protein. MDSC allows the measurement of change in heat capacity at T_g (ΔCp) with a single heating scan, avoiding sample alteration, and ΔCp values are 0.365 J/g per K for zein and 0.184 J/g per K for glutelin. The differences observed in T_g, relaxation temperatures, ΔCp and tan δ peak height are related to differences in the structure of the proteins, through the cross-linkages and hydrogen or van der Waals interactions. Experimental data from DMTA and MDSC, and the Couchman–Karasz thermodynamic approach indicate that CGM behaves as a miscible blend of its components, with high non-polar interactions between zein and glutelin proteins.     

Role of galactomannan composition on the binary gel formation with xanthan

The influence of the galactomannan characteristic ratios (M/G) on the temperature of gelation (Tg) and the gel strength of mixtures of galactomannan with xanthan is reported. Two galactomannans were investigated: one highly substituted from the seeds of Mimosa scabrella (M/G = 11), and the other, less substituted, from the endosperm of Schizolobium parahybae, with (M/G = 30) [Ganter JLMS, Zawadzki-Baggio SF, Leitner SC, Sierakowski MR, Reicher F. J Carbohydr Chem 1993;12:753]. The xanthan:galactomannan systems (4:2 g l(-1), in 5 mM NaCl) showed a temperature of gel formation (Tg) of 24 degrees C for that of S. parahybae [Bresolin TMB, Milas M, Rinaudo M and Ganter JLMS. Int J Biol Macromol 1998;23:263] and 20 degrees C for the galactomannan of M. scabrella, determined by viscoelastic measurements and microcalorimetry. A Tg of 40-50 degrees C was found by Shatwell et al. [Shatwell KP, Sutherland IW, Ross-Murphy SB, Dea ICM. Carbohydr Polym 1991;14:29] for locust bean gum-LBG (M/G = 43). Lundin and Hermansson [Lundin L, Hermansson AM. Carbohydr Polym 1995;26:129] reported a difference of 13 degrees C for Tg of two LBG samples with M/G = 3 (40 degrees C) and 5 (53 degrees C), in mixtures with xanthan. It appears that the more substituted galactomannans have lower temperatures of gelation in the presence of xanthan. The mechanism of gelation depends also on the M/G ratio. For the lower values it involves only disordered xanthan chains in contrast to M/G ratios higher than 3. In addition, the presence of the galactomannan from M. scabrella increased slightly the temperature of the conformational change (Tm) of xanthan probably due to the ionic strength contribution of proteins (3.9%) present in the galactomannan. On the other hand, the galactomannans from S. parahybae, with 1.5% of proteins and M. scabrella, with 2.4% of protein, did not show this effect, the Tm of xanthan alone or in a mixture being practically unchanged.     

Synthesis and properties of pullulan acetate. Thermal properties, biodegradability, and a semi-clear gel formation in organic solvents

Pullulan acetate (AcPL) with various degree of substitution (DS: 1.0–3.0) was synthesized by the reaction of pullulan with acetyl chloride in the presence of pyridine. The product was characterized by gel permeation chromatography (GPC), infra-red (IR) and ¹H NMR spectroscopy. The weight average molecular weights of the products did not decrease less than 190,000 (GPC) in the acetylation reaction. Thermogravimetric analysis (TGA) revealed that AcPL has a higher decomposition temperature (306–363 °C) than unmodified pullulan (295 °C). Differential scanning calorimetry analysis (DSC) revealed that all the AcPLs exhibit a clear T_g, which decreased with increasing DS value in the range of DS 1.0–2.4. The AcPL with DS 2.4 showed the lowest T_g (153 °C), and the AcPL with DS 3.0 had a slightly higher T_g (163 °C). Tensile modulus of AcPL films was comparable to that of a popular cellulose acetate film. The biodegration rate of AcPL decreased with increasing degree of acetylation. The AcPL with DS 3.0 was found to form a semi-clear gel in organic solvents such as dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and 1,4-dioxane.     

Muscle temperature and muscle metabolism during short-term exercise in the goat

1. 1.|External heat exchangers acting on lower aortal blood temperature were used to dissociate hindleg muscle temperature (T_hlm) from general internal temperature (T_int) during short-term exercise of moderate intensity.

2. 2.|In series 1 39°C T_hlm was combined with 40.6°C T_int, and in series II 42°C T_hlm was combined with 39.8°C T_int.

3. 3.|At constant work rates, the 3°C difference in muscle temperature did not result in significantly different concentrations of muscle metabolites.

4. 4.|It is concluded that high local muscle temperature without general hyperthermia does not influence muscle metabolism during short-term moderate excercise.

Phase equilibria and gelation in gelatin/maltodextrin systems — Part I: gelation of individual components

As a prelude to studies of co-gelation with galatin, the gelation behaviour of Paselli maltodextrins SA-6 and SA-2 (DE ≈ 6 and 2, respectively) was mapped out over the experimentally-accessible range of temperature (T) and concentration (c), using a simple visual method to determine the time required for formation of a self-supporting network (t_g). For both samples, log t_g decreased linearly with log c and increased linearly with T. At equivalent temperatures and concentrations, SA-2 gelled between 20 and 60 times faster than SA-6.

Selected samples were monitored more rigorously by mechanical spectroscopy, taking t_g as the time at which elastic response (G′) became greater than viscous response (G″). In all cases the values of t_g obtained by this procedure were lower than those from visual inspection, by a constant factor of about 3·4.

The concentration-dependence of gel moduli (G′) for SA-2 and for gelatin (second-extract limed ossein; LO-2) fitted accurately to the form anticipated from cascade theory for normal polymer networks. For SA-6, by contrast, log G′ varied linearly with log c over the entire range at which measurements could be made, indicating a different mechanism of structure-formation (such as the agglomeration of short, aggregated helices).     

Thermal stability and chain conformational studies of xanthan at different ionic strengths

The aim of the present study was to determine the influence of the ionic strength on the thermal stability of xanthan, i.e. xanthan resistance to chain breaking at high temperatures. Xanthan solutions of various ionic strengths were kept at 80, 90 and 95°C for periods up to 95 h. The thermal stability was determined by measuring the intrinsic viscosity after the heating periods. The experiments showed a critical ionic strength for the thermal stability of xanthan between 10 and 100 m NaCl or KCl in this temperature range. Below the critical ionic strength the intrinsic viscosity was rapidly reduced, whereas above the critical ionic strength the intrinsic viscosity was virtually unaffected by heating.

We then looked for a possible correlation between thermal stability and secondary structure of xanthan. The transition ionic strength (I_m) of xanthan solutions, i.e. where xanthan is midway between an ordered and a disordered structure, was determined by NMR at constant temperatures. I_m was found to be in the range of 24 m at 80°C to 60 m NaCl at 95°C, thus lying in the range of the critical ionic strength of the thermal stability. This suggests a close relationship between thermal stability and secondary structure of xanthan, indicated by the enhanced thermal stability in the ordered state. We believe this enhanced thermostability arises from a double-stranded conformation in the ordered state, as in DNA. The presence of double-stranded xanthan is also indicated by electron micrographs taken at both high and low ionic strengths.

The transition temperature (T_m) of xanthan was determined by NMR and optical rotation measurements. At the ionic strength of 7·5 m the two methods resulted in T_m values of 67 and 52°C respectively. This difference in T_m can possibly be due to the fact that the observed NMR and optical rotation (OR) effects are caused by different molecular phenomena.     

Ambient temperature limits and stability of temperature regulation in telemetered male and female rats

Knowing the ambient temperature (T_a) limits of normothermia in laboratory rodents is important because their thermoregulatory responses are useful in studies of physiology, pharmacology and toxicology. The present study assessed the T_a limits of normothermia using radiotelemetry to monitor core temperature (T_c), heart rate (HR), and motor activity (MA) in unrestrained, male and female Long-Evans rats over a 24 h period. Rats were housed individually in acrylic cages with wire-screen tops and bottoms and maintained at T'_as ranging from 12 to 33.5°C for 24 h with food and water provided ad libitum on a 12:12 L:D photoperiod. The limits of normothermia (i.e. where there was no significant change in T_c) were <12–29.5°C for females and 14.5–29.5°C for males. T_c of males at T'_as of 12, 32, and 33.5°C increased significantly above the baseline T_c. Female rats had a lower T_c than males at the warmest and coldest T'_as. HR and MA were generally higher in females at all T'_as. Males appeared to be poorly adapted to thermoregulate at T'_as above 30°C as based on their excessively high T'_cs, low MA, and marked weight loss compared to that of the females. Within the limits of normothermia the stability of T_c regulation (i.e. [(ΔT_c/ΔT_a) × 100]/2) was ±1.3 and ± 0.9% for males and females, respectively, over a 24 h period. These data on the stability of T_c in the male and female rat provide a valuable framework to study the acute and chronic effects of drugs, chemicals and other agents that affect temperature regulation.     

High sugar/polysaccharide glasses: resolving the role of water molecules in structure formation

Small-deformation dynamic oscillation and differential scanning calorimetry were used to ascertain the role of water molecules in high sugar/polysaccharide glasses. Increasing replacement of water with sugar affects adversely tahe degree of order in the polysaccharide network to such an extent that at level of solids >90% structure formation is no longer possible. Depending of the polymeric ability to form a network, the rheological T_g can be up to 30° higher than the calorimetric T_g.     

Metabolism and thermoregulation in Maximowiczi's voles (Microtus maximowiczii) and Djungarian hamsters (Phodopus campbelli)

1 Metabolic rates (Vo₂), body temperature (T_b), and thermal conductance (C) were first determined in newly captured Maximowiczi's voles (Microtus maximowiczii) and Djungarian hamsters (Phodopus campbelli) from the Inner Mongolian grasslands at a temperature range from 5 to 35 °C.

2 The thermal neutral zone (TNZ) was between 25 and 32.5 °C for Maximowiczi's voles and between 25 and 30 °C for Djungarian hamsters. Mean T_b was 37.0±0.1 °C for voles and 36.2±0.1 °C for hamsters. Minimum thermal conductance was 0.172±0.004 ml O₂/g h °C for voles and 0.148±0.003 ml O₂/g h °C for hamsters.

3 The mean resting metabolic rate within TNZ was 2.21±0.05 ml O₂/g h in voles and 2.01±0.07 ml O₂/g h in hamsters. Nonshivering thermogenesis was 5.36±0.30 ml O₂/g h for voles and 6.30±0.18 ml O₂/g h for hamsters.

4 All these thermal physiological properties are adaptive for each species and are shaped by both macroenvironmental and microenvironmental conditions, food habits, phylogeny and other factors.

Intramuscular temperatures during exercise in the heat following pre-cooling and pre-heating

Pre-cooling improves heat tolerance and time to exhaustion in the heat. We tested the possibility that reduced tissue temperatures may explain this phenomenon, using three whole-body treatments: pre-cooling, thermoneutral (control) and pre-heating. Pre-cooling reduced muscle temperature (T_m) by 6.3 °C while pre-heating increased T_m 3.4 °C, relative to control. Despite this offset, T_m climbed towards a common asymptote, with pre-cooling offering no thermal protection beyond 40 min. Following pre-cooling, exercising oesophageal temperature (T_es) initially increased at 0.09 °C min⁻¹, being significantly faster than control (0.05 °C min⁻¹) and pre-heated conditions (0.03 °C min⁻¹). Pre-cooling lowered the sweat threshold and also resulted in a reduced cardiac frequency across the exercise-heat exposure. Our observations do not support the hypothesis that pre-cooling reduces T_m at the end of an exercise-heat exposure, thereby delaying the development of fatigue.     

Early age cold conditioning in broiler chickens (Gallus domesticus): thermotolerance and growth responses

The influence of short repetitive cold exposure at an early age (cold conditioning—exposure to 15°C for 3 h at 3 and 4 days of age) on chickens’ thermotolerance during cold challenge (15°C) at 21 days of age was examined. The first cold exposure elicited a dramatic decline in body temperature (T_b) and a significant elevation in stress response (plasma corticosterone concentration); the second cold exposure resulted in moderate T_b and stress responses. Thereafter, the corticosterone concentration remained at a significantly lower level. Acute cold challenge of conditioned broilers at 21 days of age revealed better T_b and stress recovery during the first 24 h, and a significantly lower mortality rate thereafter. Conditioned chickens exposed to optimal conditions (22°C) achieved significantly higher body weights than others. It may be concluded that early cold conditioning improves thermotolerance in broiler chickens in later life.     

Development of autonomic and behavioural thermoregulation in turkeys (Meleagris gallopavo)

1. 1.|Heat production (HP) and body temperature (T_b) measurements were conducted at ambient temperatures (T_a) between 10 and 40°C. In addition preference temperatures (PT) were determined in a temperature channel and T_b was measured at preferred T_a

2. 2.|The influence of age on T_b at constant, as well as at PT, was proved. Increasing age was accompanied by an elevation of T_b whereas HP remained constant in the mid-range of T_a

3. 3.|The lower T_b in the first days of life is suggested to result from a lower thermoregulatory set point during the postnatal period.

4. 4.|The PT were different for the observed types of behaviour. The PT at rest was higher than the PT during locomotion, food intake and drinking.

Thermal biology of the fossorial rodent Ctenomys fulvus from the Atacama desert, northern Chile

The Andean tuco-tuco, Ctenomys fulvus (Rodentia: Ctenomyidae) inhabits one of the most arid regions of the world, the Salar de Atacama, Northeast of Antofagasta, Chile (23°17′06″S, 68°05′43″W; 2.240 m.a.s.l). We found that a stable microclimate in burrows, a low evaporative water loss (EWL), and a diet of roots (59% water content) are the main factors that permit the survival of this fossorial species in harsh desert conditions. Large circadian variation in T_a was observed above ground. Daily ΔT_a (T_a max − T_a min) = 37.9±0.2°C in summer and in winter. In contrast, circadian variation of T_a inside the burrows was only 5.8±0.5°C in the same seasons. Relative humidity (RH) was 1.9–3.1% during the day, increasing to maximum values of 27% at night and early morning. Inside the burrows RH was higher and quite stable, ranging between 53.1 and 65%, independent of the time of day and season. EWL, measured between 10 and 25°C, was low (1.26 mg/g h), and a moderate increase of 13–20% was observed at higher temperatures. The low EWL may prevent dehydration. However, because of the low heat loss capability, animals became hyperthermic (0.8–1.6°C) in dry air at T_a=30–35°C. As T_a during afternoon normally exceeded 35°C, the microclimate of burrows provided the only way to avoid the lethal effects of hyperthermia.     

A central excitatory serotonergic or serotonergic-like synapse on the pathway to heat production in the sheep

1. 1.|Intraventricular injections of serotonergic agonists and receptor blockers were given to sheep to determine whether the central nervous pathway mediating the drive to heat production involves serotonergic synapses.

2. 2.|At 15°C ambient temperature (T_a), 5-hydroxytryptamine (5-HT) at all doses tested, and norfenfluramine (NF) in low doses increased heat loss and decreased rectal temperature (T_re); lysergic acid diethylamide (LSD-25) and methysergide prevented these effects.

3. 3.|AT 0°C T_a, 5-HT, even in high doses failed to increase heat production but NF increased heat production and T_re.

4. 4.|The results suggest the effects of NF and LSD-25 on heat production may be related to synapses activated by an indoleamine other than 5-HT.

Perinatal epigenetic temperature adaptation in avian species: comparison of turkey and Muscovy duck

1. Heat production (HP) and body core temperature (CT) where measured in 1- to 10-day old Muscovy ducklings and turkey chick, incubated during the last week before hatching at a lower (34.5 °C, LT-group) or at higher (38.5 °C, HT-group), than the normal temperature of 37.5 °C (control C-group).

2. In Muscovy ducklings, on the 1st day post-hatching HP was affected by exposure to low T_a of 10 °C T_a 28.2±3.9 W kg⁻¹ in the LT-group vs. 18.1±2.4 W kg⁻¹ in normal controls. On the same day, CT was higher (39.5±1.1 °C) in the HT- than in the CT-group (37.5±2.9 °C).

3. In turkeys, the relationships between T_a and HP could be described by parabola-like functions. Apart from the first day of life, the HP of the LT-group and the HT-group was higher than of the CT-group.

4. The low prenatal temperature of incubation resulted in a decrease of the preferred temperature in the LH-group and in an increase in the HT-group.

5. It is concluded that changes in incubation temperature at the end of embryonic development may induce an epigenetic temperature adaptation, which results in a long-lasting cold- and warm-adaptation in ducks but not in turkeys.

Behavioural and autonomic thermoregulation in hamsters during microwave-induced heat exposure

1. 1.|Preferred ambient temperature (T_a) and ventilatory frequency were measured in free-moving hamsters exposed to 2450 MHz microwaves. A waveguide exposure system which permits continuous monitoring of the absorbed heat load accrued from microwave exposure was imposed with a longitudinal temperature gradient which allowed hamsters to select their preferred T_a. Ventillatory frequency was monitored remotely by analysing the rhythmic shifts in unabsorbed microwave energy passing down the waveguide.

2. 2.|Without microwave exposure hamsters selected an average T_a of 30.2°C. This preferred T_a did not change until the rate of heat absorption (SAR) from microwave exposure exceeded approx. 2 W kg⁻¹. In a separate experiment, a SAR of 2.0 W kg⁻¹ at a T_a of 30°C was shown to promote an average 0.5°C increase in colonic temperature. Hamsters maintained their ventilatory frequency at baseline levels by selecting a cooler T_a during microwave exposure. In contrast, hamsters maintained at a T_a of 30°C (without a temperature gradient) underwent a sharp increase in ventilatory frequency compared to animals allowed to select their own T_a.

3. 3.|These data support previous studies suggesting that during thermal stress behavioural thermoregulation (i.e. preferred T_a) takes prescedence over autonomic thermoregulation (i.e. ventilatory frequency). It is apparent that selecting a cooler T_a is a more efficient and/or effective than autonomic thermoregulation for dissipating a heat load accrued from microwave exposure.

Optical rotation behavior of xanthan in mixtures of water and cadoxen

Optical rotatory dispersion (ORD) data were obtained for a sample of the sodium salt of xanthan gum dissolved in water-cadoxen mixtures at 25°C. The double-helical dimer of the polysaccharide was previously found to dissociate directly to single chains when w_cad (the weight fraction of cadoxen in the mixed solvent) increases from 0·3 to 0·8. ORD data were also obtained for solutions prepared by diluting xathan solutions at low concentration in cadoxen with water to different w_cad values. From previous work these were taken as those for single dissociated chains in the mixed solvent. These sets of data led to the finding that the specific rotation at 300 nm wavelength does not reflect the dissociation of the xanthan double helix in water-cadoxen mixtures, but the Moffitt parameter does. This parameter gave evidence that single coiling chains in cadoxen become intramolecularly ordered to a conformation similar to that of the individual chains in the double helix when the solution is diluted with water to w_cad below 0·4.     

Structure and thermal stability of the extracellular fragment of human transferrin receptor at extracellular and endosomal pH

Fourier transform infrared spectroscopy has been used to study the solution structure and thermal stability of the extracellular fragment of human transferrin receptor (tfRt) at extracellular and endosomal pH. At extracellular pH tfRt is composed of 56% -helix, 19% β-sheet and 14% turns. Upon acidification to endosomal pH the -helical content of the protein is reduced and the β-sheet content increased by nearly 10%. At extracellular pH, the midpoint temperature of thermal denaturation (T_m) for the loss of secondary and tertiary structure, and the formation of aggregated structures, is 71°C. At endosomal pH this temperature is reduced by ≈ 15°C. The apparent entropies of thermal denaturation indicate that the native structure of tfRt at endosomal pH is far more flexible than at extracellular pH.