1,721,235 research outputs found
Water activity in viscous heterogeneous bio-systems
Full text linkClassical Thermodynamics does not treat the effects of the medium viscosity as long as it deals with equilibrium states. Unfortunately, this has consequences on the experimental practices and on the reliable characterization of the immense variety of bio-systems and bio-products that are rather viscous and heterogeneous. Since most of them are aqueous or contain aqueous phases, one is inclined to use water as an internal probe to describe their behavior and water activity, aW, as a suitable parameter.
When the viscosity of a condensed phase is large, as in many real systems, the migration of water can be very slow. This is why any steady state can mimic the attainment of equilibrium and the detected water activity may have a value, aW,app, smaller than the true one.
An alternative approach is therefore necessary to reconcile expectations from a thermodynamic description of an aqueous (or simply moist) system with the experimental difficulties related to the low mobilityof the water molecules, uW.
Some NMR experimental evidence was provided by Brian Hills who proposed a correlation between aW and the normalized overall FID relaxation rate, which can be related to uW. At least at the microscopic level, namely, the level "seen" with a NMR investigation, uW and aW would accordingly be someway correlated to each other, in spite of the fact that uW and aW belong to different realms of physics.
In present work advantage is taken from the fact that every viscous aqueous system can be imagined as obtained from a starting poorly viscous solution either by isothermal dehydration, or by cooling (or combination of these treatments), so to approach the glass transition threshold, across which viscosity and heat capacity undergo large changes.
The formal treatment of the problems starts with an expression of the chemical potential of water that includes an extra contribution to the overall potential energy of the aqueous phase(s), namely, a fraction of the “free energy” that becomes available once the viscosity drops down.
μ_W= μ_W^*+RTlna_W- μ_visc= μ_W^*+RTlna_(W,app)
The explicit form of the visc comes from the one proposed by Gibbs and Di Marzio to describe the connection between the excess (with respect to the solid state) configuration part of the Helmholtz free energy Fcexc to the viscosity, , of a liquid close to its glass transition threshold, namely,
ln = - Fcexc /RT
(μ_W^visc)/RT=- ln a_W/a_(W,app) = α (F_c^exc)/RT= β- lnη
When approaches the viscosity of pure water, *, aW,app approaches aW, that approaches unity: thence = ln *. This allows the conclusion that, at any temperature,
a_W/a_(W,app) = (η/η^* )= ((u_W^*)/u_W )≥1
where the mobility has been replaced with the reverse of the viscosity, , experienced by water molecules that migrate from the core to the surface of the system. Similar expressions are proposed to describe the effects of physical barriers that hinders water displacements across heterogeneous systems
Phenomenological kinetics - An alternative approach
No full textTraditional phenomenological kinetics describes the process rate in terms of the progress degree (mass fraction which has already undergone the change), alpha, and with a monomial function (or combination of monomial functions for multistep processes) of (1-alpha), without any connection to the underlying mechanism at the molecular level.
The approach proposed in the present work aims at the direct treatment of the experimental data, like DSC records, without suggesting any specific reaction mechanism and excluding any Arrhenius like behaviour. Formal expressions are proposed that include the thermodynamic constraints for any spontaneous process, viz. a negative drop of the Gibbs function throughout the process, and describe the process rate as the result of the effects of a thermodynamic driving force, identified with the drop of the Gibbs function, and of the medium hindrance
Microbial growth and metabolism: Modelling and calorimetric characterization
Full text linkMicrobial growth can be qualitatively described with the empirical Gompertz function. This, however, has no specific physical meaning and gives no information about the underlying biochemical activity. When used to fit the isothermal calorimetric traces obtained from microbial cultures, it is inadequate. A more satisfactory description comes from a kinetic model that can reproduce the plate count data, the isothermal calorimetric trace and the change in the metabolite concentration
AgI-Ag oxysalt high conductivity glasses : a tentative approach to their structure
No full textAs a result of electronic microscopy observations the amorphous nature of the high conductivity AgI-Ag oxysalt electrolytes seems due to a random orientation of structurally ordered microdomains having less than 1 micron size. A tentative approach to the structure of these domains is suggested in order to account for (i) the fast silver ion migration, which would occur along `smooth' passageways tracked by iodide ions, (ii) the critical composition, viz 80 mole.% AgI, corresponding to the conductivity maximum found for most of these materials at room temperature, (iii) the low activation energy of the conductivity Arrhenius plots, (iv) the presence of some immobile Ag + which closely surround the oxyanion
Phase Transitions in Hydrophilic and Hydrophobic Food Systems
No full textFood products are multi-component heterogeneous systems which can be viewed as the result of changes related to the phase transitions that take place in the preparation process.
These transitions are substantially irreversible and therefore cannot be described with a thermodynamic approach. The reason for their irreversibility is mainly related to the metastable character of the starting and/or the final state involved in the transition. Either of them can be amorphous or vitreous and the whole system can be finely, although not definitely, dispersed. Because of the large extension of the inter-phases, the average size of the solid particles (usually crystals) or the droplets (aqueous or fatty) or the bubbles (usually air) can directly affect the rate and/or the relevant onset temperature of these transitions.
Although the driving force of these irreversible transitions is the relevant drop of the Gibbs function, the rate of their progress depends on other physical properties of the medium, like viscosity and/or permeability, which in turn can be modified by to the products formed. The overall process can therefore be referred to as an auto-catalysis (either positive or negative) of the overall progress toward the final state.
One way to overcome the complexity of these processes can be found looking at the role of some key factors.
When the food systems considered host large amounts of water shared between different phases (hydrophilic systems), the key factor that controls the overall progress of the phase transitions (and phase separations) is the local availability of water and its displacement from one to another region of the system.
In systems where water is practically absent or strictly confined within micelles (hydrophobic systems) the phase transitions are governed by the rates of nucleation and growth of crystal phases in specific regions, e.g. around the liquid droplets and air bubbles.
Thermal analysis is of great help in the follow up of these events. DSC and TGA are the most used techniques in these studies, but require skilful operators and adequate mathematical treatments of the data.
Some examples will be given of phase transitions in either kind of systems together with the general lines of the relevant kinetic parameterization. The latter includes definition of TTT (Temperature, Time, Transformation) diagrams and kinetic models for the polymorph transitions. All these examples concern investigations performed with DSC and TGA equipments
Thermal analyses and combined techniques in food physical chemistry
No full textThis chapter reviews thermal analyses and relevant combined techniques, that can be of help in the study of food and related systems. Instruments which show promising arrangements, but still requiring further arrangements to be of practical use, are also presented. These thermal analyses techniques include, differential scanning calorimetry, isothermal calorimetry and thermogravimetry
KINETICS OF POLYMERIZATION OF EPOXY ADHESIVES AND COMPOSITES
No full textThe polymerization kinetics of epoxy materials are examined with particular emphasis in assessing (i) the proper use of non-isothermal DTA or DSC data, (ii) the correlation between glass transition temperature and cure degree, (iii) the effect of the filling agent on the phenomenological kinetics of the cure process
Microbial Growth in Planktonic Conditions
Full text linkA two-parameter model describes the microbial growth trend of planktonic cultures. Based on the assumption
that cell duplication underlies the growth, the model defines an average generation time that depends on time and
complies with the phenomenological evidence that the growth rate is naught at the start and at the end of the process.
This is tantamount as to replace the real growth process with a virtual one, where all the generation lines stemming
from the inoculum are synchronous and imply a duplication tree with no truncated branches. A simple function that
complies with these constraints is τ=(a/t+bt), where a and b are parameters defined through a best fit treatment of
the experimental plate count data. Surprisingly simple relationships come out for specific items of the growth trend,
like maximum specific growth rate, eventual cell number, Nmax, duration of lag phase, etc., as well as some intriguing
correlations between them. Published plate count data allowed testing the reliability of the model. The agreement is
satisfactory being in line with the accuracy of the data (R2 ≥ 0.98)
Growth and Decay of a Planktonic Microbial Culture
Full text linkThe paper shows that the phenomenological trends of both growth and decay of a microbial population in a given medium are easily reproducible with simple equations that allow gathering the experimental data (plate counts) related to different microbial species, in different mediums and even at different temperatures, in a single master plot. The guideline of the proposed approach is that microbes and surrounding medium form a system where they affect each other and that the so-called “growth curve” is just the phenomenological appearance of such interaction. The whole system (cells and medium) changes following a definite pathway described as the evolution of a “virtual” microbial population in planktonic conditions. The proposed equations come from the assumption of a duplication mechanism with a variable generation time for the growth and of an exponential-like decline with a linear increase of the rate for the decay. The intermediate phase between growth and decay is a time span during which growth and death counterbalance each other and age differences within the virtual cell population tend to level off. The proposed approach does not provide an a priori description of this phase but allows the fit of the whole evolution trend of a microbial culture whenever the experimental data are available. Deviations of such a trend concern microbes able to form spores, modify their metabolism, or express phenotypic heterogeneity, to counterbalance adverse medium conditions
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