RESUMEN Introducción: Con el propósito de justificar la decisión diplomática de los Estados Unidos, se publicaron dos artículos científicos en revistas médicas que intentan sostener la idea de que en ...La Habana hubo un ataque dirigido a personal de la embajada estadounidense. Objetivo: Demostrar la falta de rigor científico en dos investigaciones de series de casos no independientes sobre los síntomas de salud de diplomáticos de los Estados Unidos en La Habana. Método: Se realiza un análisis documental de dos publicaciones de series de casos. Se evalúan hipótesis diagnósticas. Resultados: Existe superposición amplia entre las dos series en cuanto a pacientes compartidos y en contraste se presentan algunas diferencias en los datos clínicos que superan lo esperado. Conclusiones: En ambas publicaciones se desaprovecha la riqueza semiográfica de síntomas y la información psicosocial. Se enfatiza más en argumentos asociados al fetichismo de la tecnología expresado en la interpretación de hallazgos inespecíficos. El análisis de datos clínicos permitió ver que se trata de un grupo heterogéneo de personas cuyas quejas de salud han sido reunidas por la interacción de otros factores psicosociales contextuales.
The use of renewable resources to develop food contact materials, such as proteins or polysaccharides, and the use of industrial residues for alternative applications are trending topics for ...researchers and the industry. Yeast cell wall (YCW) is a very rich waste from the yeast extract industry. Due to this, the aim of this work is to develop new biodegradable films based on residual YCW and the study of the effect of plasticization on films properties. Residual YCW was used as base matrix and different concentrations of glycerol (0, 15, 25 and 35 wt%) were tested to obtain casted films. Homogeneous and yellow-brownish films, which allow seeing through them, were obtained from the YCW. Total soluble matter demonstrated that glycerol enhanced solubility of films but glycerol was retained in the polymer matrix. TGA studies indicated that YCW films exhibited substantial degradation at temperatures above 180 °C. FTIR spectra of the casted films were representative of yeast cell wall material and SEM photographs showed that cell wall maintained their shape after film formation. As expected, Young’s modulus and tensile strength values were decreased with the increasing amount of glycerol. However, elongation at break was not increased further with higher concentration of plasticizer and the addition of 15 wt% of glycerol seemed to be enough to improve mechanical properties. The linear increment of water vapour permeability with glycerol concentration was produced by the increase in water solubility in the film. Therefore, based on solubility in water, mechanical, and barrier properties, it is possible to propose yeast cells residues as film-forming material for biodegradable film developments.
In the last decades, cellulose nanoparticles have been widely used to reinforce polymeric materials due to their strength and their wide availability in nature. Cellulose nanoparticles are entirely ...compatible to reinforce natural polymers, as performed in this study. The effect of rice husk cellulose nanofibers (RHCNF) and bacterial nanocellulose (BNC) on water vapor transfer and mechanical properties was studied in yeast biomass films, made from dispersions at pH 6 and 11. Water vapor permeability was reduced at 5 wt% in both reinforcements in films prepared at pH 11, although their influence was negligible at pH 6. Despite both cellulose nanofibres have similar aspect ratios, Young’s modulus, tensile strength, elongation and tensile toughness of films reinforced with BNC were significantly increased with respect to RHCNF films, and both were better than the pure matrix. In this way, the present study provides an interesting contribution regarding the effect of the mentioned nanocellulosic reinforcements on yeast biomass films used for packaging purposes.
Graphic Abstract
Due to the structure and the composition of Paecilomyces variotii, the mycelia of this fungus could have potential applications as ingredients in wettable foods. For this use, drying could be ...employed, justifying the study of thermal behavior of P. variotii. The objectives of this work were to perform a study of thermal behavior of P. variotii isolates, to evaluate the hydration properties of these mycelia and to analyze the effect of different technological parameters on the latter properties. Wet cultures exhibited a wide endothermic transition, with mean values of peak temperature of 61°C and denaturation enthalpy of 4 J/g dry matter. Initial (50°C) and final (80°C) temperatures of the endothermic transition were used to dry the mycelia. Freeze-drying was also assayed. For all dried mycelia, a decrease in denaturation enthalpy between 40 and 50% was observed for drying at 50°C and freeze-drying, and a drastic decrease of almost 100% for drying at 80°C. According to the hydration properties, wet mycelia exhibited water holding capacity (WHC) value of 45 g water/g dry matter. Significant differences among dried mycelia, resulting WHC values in order: 50°C > freeze-dried > 80°C (p < 0.05) were revealed for each P. variotii strain. Fungi obtained by drying at 50 C and by freeze-drying, showed a rapid water absorption (t ₁/₂ < 0.1 min). Ionic strength, pH and particle size of dried mycelia influenced the hydration properties.
Gravimetric and dielectric measurements have been used to monitor water uptake in composites with glass, jute and washed jute fibres. The results showed that the dielectric constant of jute fibre ...composites is higher than that of the composites based on glass fibres. Also, when the frequency increases the permittivity decreases. A Fickian diffusion model was applied and the effective diffusion coefficient was calculated. The results obtained show that when the sizing is removed in fibres washed with hot water, the effective coefficient diffusion increases. Changes in the dielectric permittivity were measured at different frequencies and at different times during water absorption at room temperature. A non-linear behaviour has been found in the changes of the permittivity with water absorption in the frequency range of 200
Hz to 1
MHz.
The cyclic water absorption behavior of vinylester (VE) and epoxy matrices and their composites with mat glass fiber is studied. The water absorption increases with the cycle. Degradation of the ...matrix occurs during fast drying and this leads to a very different behavior of the materials in both cycles. Epoxy-based materials absorb more water than VE-based ones due to their different hydrophilicity. The diffusion coefficient increases with temperature and it is higher in the second cycle than in the first one. The diffusion coefficients of VE composites are similar to that of the matrix, but the epoxy composites absorb water at a higher rate than the matrix. The mechanical properties decrease with immersion but the change in strength is higher than the modulus loss. The fiber—matrix interface presents good adhesion at low temperature but it decreases as temperature increases and there are more voids with the worst interface for the last cycle.
The relationship between the structure and the viscoelastic properties of resol resins was studied. Six phenolic resins (resol) were synthesized with different molar ratios of formaldehyde to phenol. ...These resols were cured by means of temperature and without catalyst. The characterization of the resol was done by means of infrared spectroscopy and chemical methods. From the viscoelastic properties of fully cured resins, characteristic properties such as: storage modulus (
E′), tan
δ, width of the tan
δ(Δ
T) and damping peak (tan
δ) were obtained. The compression modulus and the void content were also determined. A maximum in the methylene bridge and the result of the viscoelastic properties allow us to say that the resol with F/Ph between 1.3 and 1.4 has the highest crosslinking density.
The effects of hygrothermal aging on the mechanical and viscoelastic properties of vinylester (VE)/unidirectional glass-fiber composites are reported. The studies are done by means of a dynamometer ...(INSTRON) and a dynamic mechanical thermal analyzer (Perkin Elmer). The studies are carried out at three different temperatures: 20, 40, and 60 C. The adhesion between the fibers and matrix is studied by scanning electron microscopy (SEM) microphotographs. Flexural strength, flexural modulus, storage modulus in the glass state (Tg - 50 K), in rubber state (Tg + 50 K), and the glass transition temperatures are also investigated. These parameters are determined in the case of the matrix and composites materials, for the initial and final states. The diffusion coefficients are calculated by using the simplest Fick's equation.
Water uptake at 25°C is measured for unsaturated polyester matrix and glass fiber composites as a function of time. Dielectric measurement in the frequency range of 1 MHz-1 GHz has been performed on ...the specimens during the water absorption process. The dielectric constant decreases as the frequency increases for matrix and composites. The dielectric constant of unsaturated polyester is lower than their composites based on glass fibers. A Fickian diffusion model is applied for water absorption studies, and the effective diffusion coefficient is calculated for each sample. An empirical model describing the increase in dielectric constant with water absorption is proposed.
The effect of water absorption on the mechanical properties of the composites has been studied. The matrix used was vinyl-ester. Different arrangements of glass fibers were used to reinforce ...vinyl-ester matrix: mat, unidirectional, and bidirectional cloth. The samples were immersed in distillated water at 80 C. The flexural modulus was measured as a function of immersion time. The decrease in the glass transition temperature, Tg, was correlated with the water absorption and flexural modulus. The composite flexural modulus was fitted by means of a modified rule of mixture, which included an interfacial coefficient. The relationship between the directionality of the fiber in the composite material and the water absorption was also analyzed.
