702 research outputs found
Shear work induced changes in the rheology of model Mozzarella cheeses : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Manawatu, New Zealand
Mozzarella cheese is a pasta filata type of cheese. Its manufacture includes a kneading – stretching step that creates a fibrous protein network and distributes fat-serum channels to attain desirable melt functionality on a pizza. During processing and manufacturing of pasta-filata cheese, large deformations take place. For appropriate characterization of a food material, rheological evaluation should be conducted in similar operating conditions, length scales and time scales to those taking place in the actual process. Development of the typical fibrous pasta-filata structure of mozzarella cheese depends on composition and process variables. Critical process variables in the development of cheese structure are time, temperature and shear. In this study we studied the effect of shear work on rheology, structure and melt functionality of model Mozzarella cheese.
Three types of model cheeses (full-fat, non-fat and full-fat with added tri-sodium citrate) were prepared by working cheese components together at 70 oC in a twin screw Blentech cooker. Varied amounts of shear work input (2.8-185 kJ/kg) were given to the cheese samples using 50, 150 and 250 rpm screw speeds. Samples were subjected to a range of rheological tests, confocal laser scanning microscopy, fat particle size measurements (DLS) and melt functionality evaluation.
While measuring steady shear viscosity of Mozzarella-type cheeses in a rotational rheometer at 70oC, three main difficulties were encountered; wall slip, structural failure during measurement and viscoelastic time dependent effects. A flow curve method was successfully devised to measure steady shear rheology by using serrated plates as surface modification to avoid wall slip, giving enough measurement duration at low shear rate to avoid viscoelastic effects and selecting limited shear steps to cause minimum structural changes. These techniques enabled successful measurement of steady shear viscosity of molten Mozzarella-type cheeses at 70oC at shear rates up to 250 s-1.
Strong work thickening was observed for full fat Mozzarella cheese from steady shear rheology, oscillatory rheology, creep, elongational viscosity and tensile testing data. Steady shear rheology and melt functionality were found to be strongly dependent on total shear work input. An exponential increase in consistency coefficient (K from power law model) was observed with increasing amounts of accumulated shear work, indicating work thickening behaviour. An exponential work thickening equation is proposed to describe this behaviour. Excessively worked cheese samples exhibited liquid exudation, poor melting and poor stretch. Nonfat cheese exhibited similar but smaller changes after excessive shear work input.
At lower shear work inputs (70 kJ/kg) it behaved like a viscoelastic solid with low frequency dependence. A definite critical point for structural and viscoelastic transition was identified at a medium shear work level (~ 58 kJ/kg at 150 rpm). Similar viscoelastic property changes occurred in non-fat cheese suggesting that major changes were taking place in the protein matrix during working.
Confocal microstructures plus macroscopic observations showed systematic changes in structure with increased shear work inputs with unmixed buttery liquid observed at 58 kJ/kg. At very high shear work inputs, > 75 kJ/kg, striations or anisotropy in the microstructures had disappeared and small micro-cracks were evident. Volume-weighted mean fat particle size decreased with shear work input and particle size distributions also changed. To account for the short and long term relaxation response behaviour, a 4-element Burger‘s model was found adequate for fitting the creep data of model cheese at 70 oC but a 6-element model was required at 20 oC. As shear work input increased, retarded compliance decreased and zero shear viscosity increased indicating the more elastic behavior of the cheeses with higher shear work input.
Fracture stress and strain for longitudinal samples from elongated full fat cheese did not vary significantly with shear work input up to 26.3 kJ/kg then decreased dramatically at 58.2 kJ/kg. Longitudinal samples with shear work input <30 kJ/kg, demonstrated significant strain hardening. At shear work inputs <30 kJ/kg strong anisotropy was observed in both fracture stress and strain. After a shear work input of 58.2 kJ/kg anisotropy and strain hardening were absent. Perpendicular samples did not show strain hardening at any level of shear work input.
A good correlation was found between the steady shear, oscillatory shear and transient rheological properties and the melting properties of the cheeses. The order for the rheological properties in terms of their sensitivity towards both shear work input and melt functionality is ηapp > G‘ > elongational viscosity > consistency coefficient, K. It was concluded that the dominant contributor to the changes in rheology, structure and melt properties with increased shear work was shear induced structural changes to the protein matrix. An increase in calcium induced protein-protein interactions after high shear work at 70 oC.
In summary, this thesis provides useful insights to shear work induced changes in material properties. It proposes useful linkages between the manufacturing process and the application of model Mozzarella cheese using appropriate rheological methods. Since the linkages were validated for only one composition and in only one processing environment, it is proposed that they should be tested in other conditions. In order to build a more complete picture, a molecular level study is proposed for future work to elucidate chemical changes during working and find appropriate linkages with physical and functional characteristics
3D global simulations of RIAFs: convergence, effects of azimuthal extent and dynamo
We study the long-term evolution of non-radiative geometrically thick
() accretion flows using 3D global ideal MHD simulations and a
pseudo-Newtonian gravity. We find that resolving the scale height with 42 grid
points is adequate to obtain convergence with the product of quality factors
and magnetic tilt angle . Like previous global isothermal thin disk simulations,
we find stronger mean magnetic fields for the restricted azimuthal domains.
Imposing periodic boundary conditions with the azimuthal extent smaller than
make the turbulent field at low appear as a mean field in the runs
with smaller azimuthal extent. But unlike previous works, we do not find a
monotonic trend in turbulence with the azimuthal extent. We conclude that the
minimum azimuthal extent should be to capture the flow structure,
but a full extent is necessary to study the dynamo. We find an
intermittent dynamo cycle, with -quenching playing an important role in
the nonlinear saturated state. Unlike previous local studies, we find almost
similar values of kinetic and magnetic -s, giving rise to an irregular
distribution of dynamo-. The effects of dynamical quenching are shown
explicitly for the first time in global simulations of accretion flows.Comment: 24 pages, 27 figures, 4 tables. Accepted in MNRAS. We welcome
comments and suggestion
Turbulence in the intracluster medium: simulations, observables & thermodynamics
We conduct two kinds of homogeneous isotropic turbulence simulations relevant
for the intracluster medium (ICM): (i) pure turbulence runs without radiative
cooling; (ii) turbulent heatingradiative cooling runs with global thermal
balance. For pure turbulence runs in the subsonic regime, the rms density and
surface brightness (SB) fluctuations vary as the square of the rms Mach number
(). However, with thermal balance, the density and SB
fluctuations are much larger. These scalings have implications
for translating SB fluctuations into a turbulent velocity, particularly for
cool cores. For thermal balance runs with large (cluster core) scale driving,
both the hot and cold phases of the gas are supersonic. For small scale (one
order of magnitude smaller than the cluster core) driving, multiphase gas forms
on a much longer timescale but is smaller. Both
small and large scale driving runs have velocities larger than the Hitomi
results from the Perseus cluster. Thus turbulent heating as the dominant
heating source in cool cluster cores is ruled out if multiphase gas is assumed
to condense out from the ICM. Next we perform thermal balance runs in which we
partition the input energy into thermal and turbulent parts and tune their
relative magnitudes. The contribution of turbulent heating has to be in order for turbulence velocities to match Hitomi observations. If the
dominant source of multiphase gas is not cooling from the ICM (but say uplift
from the central galaxy), the importance of turbulent heating cannot be
excluded.Comment: MNRAS accepted version; for movies see:
http://www.mso.anu.edu.au/~rajsekha/BT_movies.htm
Cold gas in cluster cores: Global stability analysis and non-linear simulations of thermal instability
We perform global linear stability analysis and idealized numerical
simulations in global thermal balance to understand the condensation of cold
gas from hot/virial atmospheres (coronae), in particular the intracluster
medium (ICM). We pay particular attention to geometry (e.g., spherical versus
plane-parallel) and the nature of the gravitational potential. Global linear
analysis gives a similar value for the fastest growing thermal instability
modes in spherical and Cartesian geometries. Simulations and observations
suggest that cooling in halos critically depends on the ratio of the cooling
time to the free-fall time (). Extended cold gas condenses out
of the ICM only if this ratio is smaller than a threshold value close to 10.
Previous works highlighted the difference between the nature of cold gas
condensation in spherical and plane-parallel atmospheres; namely, cold gas
condensation appeared easier in spherical atmospheres. This apparent difference
due to geometry arises because the previous plane-parallel simulations focussed
on {\em in situ} condensation of multiphase gas but spherical simulations
studied condensation {\em anywhere} in the box. Unlike previous claims, our
nonlinear simulations show that there are only minor differences in cold gas
condensation, either in situ or anywhere, for different geometries. The amount
of cold gas condensing depends on the shape of the gravitational potential
well; gas has more time to condense if gravitational acceleration decreases
toward the center. In our idealized simulations with heating balancing cooling
in each layer, there can be significant mass/energy/momentum transfer across
layers that can trigger condensation and drive far beyond the
critical value close to 10. Triggered condensation is very prominent in
plane-parallel simulations, in which a large amount of cold gas condenses out.Comment: 17 pages, 16 figures, 2 tables, version accepted in MNRAS. Links to
python codes for global stability analysis:
https://drive.google.com/folderview?id=0B2HaDXI2USsZWUdESVVsN2RGeVU&usp=sharin
Thermal Conduction and Multiphase Gas in Cluster Cores
We examine the role of thermal conduction and magnetic fields in cores of
galaxy clusters through global simulations of the intracluster medium (ICM). In
particular, we study the influence of thermal conduction, both isotropic and
anisotropic, on the condensation of multiphase gas in cluster cores. Previous
hydrodynamic simulations have shown that cold gas condenses out of the hot ICM
in thermal balance only when the ratio of the cooling time () and
the free-fall time () is less than . Since thermal
conduction is significant in the ICM and it suppresses local cooling at small
scales, it is imperative to include thermal conduction in such studies. We find
that anisotropic (along local magnetic field lines) thermal conduction does not
influence the condensation criterion for a general magnetic geometry, even if
thermal conductivity is large. However, with isotropic thermal conduction cold
gas condenses only if conduction is suppressed (by a factor )
with respect to the Spitzer value.Comment: 7 pages, 4 figures; replaced by the MNRAS-accepted versio
Detection of polarized quasi-periodic microstructure emission in millisecond pulsars
Microstructure emission, involving short time scale, often quasi-periodic,
intensity fluctuations in subpulse emission, is well known in normal period
pulsars. In this letter, we present the first detections of quasi-periodic
microstructure emission from millisecond pulsars (MSPs), from Giant Metrewave
Radio Telescope (GMRT) observations of two MSPs at 325 and 610 MHz. Similar to
the characteristics of microstructure observed in normal period pulsars, we
find that these features are often highly polarized, and exhibit quasi-periodic
behavior on top of broader subpulse emission, with periods of the order of a
few s. By measuring their widths and periodicities from single pulse
intensity profiles and their autocorrelation functions, we extend the
microstructure timescale - rotation period relationship by more than an order
of magnitude down to rotation periods 5 ms, and find it to be consistent
with the relationship derived earlier for normal pulsars. The similarity of
behavior is remarkable, given the significantly different physical properties
of MSPs and normal period pulsars, and rules out several previous speculations
about the possible different characteristics of microstructure in MSP radio
emission. We discuss the possible reasons for the non-detection of these
features in previous high time resolution MSP studies along with the physical
implications of our results, both in terms of a geometric beam sweeping model
and temporal modulation model for micropulse production.Comment: 6 pages, 4 figures, 1 table. Accepted for publication in ApJ Letter
- …