2,504 research outputs found
Rheological study into the ageing process of high methoxyl pectin/sucrose aqueous gels
The ageing process of high methoxyl pectin (HMP)/sucrose gels was followed at different ageing temperatures by small amplitude oscillatory experiments. Dynamic mechanical measurements allowed the characterisation of the point at which the system undergoes the sol/gel transition. The HMP/sucrose system is extremely sensitive to temperature variation during ageing, especially in the lower temperature range. The viscoelastic behaviour through the gel point changes with the ageing temperature, probably due to variations in mobility of the pectin chains, and consequently, in the lifetime of junction zones. Weaker pectin networks are formed under thermal conditions unfavourable to the development of hydrophobic interactions. Gel time and elastic modulus have a complex dependence on temperature, which could be attributed to the different thermal behaviour of the intermolecular interactions that stabilise the nonpermanent cross links of these physical networks
Influence of temperature on the dynamic and steady-shear rheology of pectin dispersions
The influence of temperature on the dynamic and steady-shear rheology of ionic polysaccharides, high-methoxyl and low-methoxyl pectins, has been studied and compared with the behaviour of locust bean gum, a virtually neutral biopolymer. Using the time-temperature superposition principle, the rheological parameters were reduced to an arbitrary reference temperature. Activation energies were calculated and their dependence on temperature and shear rate analysed. Concerning the viscosity dependence on temperature, two approaches have been considered: one associated with the theory of absolute reaction rates leading to an Arrhenius type equation, and the other associated with the free volume theory, expressed by the Williams-Landel-Ferry equation. The difficulties encountered in the superposition of the dynamic properties of the pectin dispersions, the high activation energies, yield values and elastic plateau at low oscillatory frequency, are consistent with a macromolecular organization of these polymers dominated by important aggregation phenomena, which could be attributed to important intermolecular interactions like hydrogen bonding and hydrophobic interactions, especially in conditions of low degree of ionization of the carboxylic groups
Kinetics and thermal behaviour of the structure formation process in HMP/sucrose gelation
The concentration and temperature dependence of the gelation kinetics of high-methoxyl pectin (HMP; 60% sucrose, pH 3) was investigated using measurements of small-amplitude oscillatory shear. The rate of gelation close to the gel point can be described as a second-order rate process using the kinetic model of Ross-Murphy (Carbohydr. Polym. 1991, 14, 281) and a critical exponent close to that predicted by the percolation approach. The modulus after a long ageing time showed a power concentration dependence with an exponent around 3.1, higher than the classical square of concentration dependence, which was probably either due to the non-equilibrium state of the HMP gels even after long ageing times, or due to the proximity of the concentration range studied to the critical gelling concentration. The gelation rate of HMP/sucrose systems is strongly dependent on the temperature. An Arrhenius relationship was applied to describe this dependence. Two different processes are proposed to explain the discontinuity observed, each one having rates with different temperature dependence. The applicable kinetics at longer times are quite different, with a lower dependence on polymer concentration and ageing temperature. A non-isothermal kinetic model was used to describe the gelation process of the HMP/sucrose system during cooling
Diagnóstico e acompanhamento de propriedades leiteiras nas mesorregiões metropolitana de Belém e nordeste paraense.
bitstream/item/61281/1/CPATU-Doc127.pd
Viabilidade econômica de doses de ureia e fosfato monoamônico na produção de milho safrinha, em Ipameri, GO.
Assessment of physical vulnerability of buildings and analysis of landslide risk at the municipal scale: application to the Loures municipality, Portugal
This study offers a semi-quantitative assessment
of the physical vulnerability of buildings to landslides in a
Portuguese municipality (Loures), as well as the quantitative
landslide risk analysis computed as the product of the
landslide hazard by the vulnerability and the economic value
of the buildings. The hazard was assessed by combining the
spatiotemporal probability and the frequency–magnitude relationship
of the landslides. The physical vulnerability assessment
was based on an inquiry of a pool of European landslide
experts and a sub-pool of landslide experts who know
the study area, and the answers’ variability was assessed with
standard deviation. The average vulnerability of the basic geographic
entities was compared by changing the map unit
and applying the vulnerability to all the buildings of a test
site, the inventory of which was listed on the field. The economic
value was calculated using an adaptation of the Portuguese
Tax Services approach, and the risk was computed
for different landslide magnitudes and different spatiotemporal
probabilities. As a rule, the vulnerability values given by
the sub-pool of experts who know the study area are higher
than those given by the European experts, namely for the
high-magnitude landslides. The obtained vulnerabilities vary
from 0.2 to 1 as a function of the structural building types and
the landslide magnitude, and are maximal for 10 and 20m
landslide depths. However, the highest risk was found for the
landslides that are 3m deep, because these landslides combine
a relatively high frequency in the Loures municipality
with a substantial potential damage.info:eu-repo/semantics/publishedVersio
Optimal Controller Identification for multivariable non-minimum phase systems
This paper extends the formulation of a data-driven control method - the
Optimal Controller Identification (OCI) - to cope with non-minimum phase (NMP)
systems, without a priori knowledge of the NMP transmission zero, i.e. without
obtaining a prior model of the plant - as in any data-driven approach. The
method is based on the Model Reference paradigm, in which the desired
closed-loop performance is specified by means of a closed-loop transfer
function - the reference model. Considering a convenient parametrization of the
latter and a flexible performance criterion, it is possible to identify the NMP
transmission zeros of the plant along with the optimal controller parameters,
as it will be shown. Both diagonal and block-triangular reference model
structures are treated in detail. Simulation examples show the effectiveness of
the proposed approach
Allergen sensitization associates with worse lung function parameters
To assess the association between the number of allergen sensitizations and lung function variables in individuals with airway symptoms. Methods. Retrospective study with all individuals who performed lung function and skin-prick tests at CUF-Porto (01/2011-06/2016). Six allergen groups were considered. % predicted Pre-Bronchodilator test (BD) and % change after BD were analysed for spirometry and plethysmography parameters. Results. A total of 1293 individuals were included, 54% (n = 698) adults and 69% (n = 891) with sensitization to ≥ 1 allergen group. % FEV1 was significantly higher and % change in FEV1 significantly lower in non-sensitized individuals. % sRaw was higher in polysensitized (vs non-sensitized). Conclusions. The presence of allergen sensitizations was significantly associated with worse key lung function parameters.info:eu-repo/semantics/publishedVersio
Integration of particles of different sizes in a hydrothermal process for the pre-treatment of agro-industrial residues such as wheat straw
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