Biomedical Engineering Reference
In-Depth Information
When water is bound (i.e. unavailable to take part in reactions) to the solid matrix
or non-solvent, then no deterioration reactions could be expected. The glass transi-
tion concept is based on the molecular mobility of the reacting components at
micro-level in a matrix, thus diffusion of the reactants through the system is very
slow and stability is achieved. Thus a successful combination of water activity and
glass transition could open more precise and unified determination of stability crite-
ria (Rahman
2010
).
2.4.4
Critical Temperature Concept and Molecular Mobility
It is expected that there should be a break in the plot of
k
(i.e. reaction rate) versus
T
/
T
g
(i.e. change in slope between above and below the critical ratio) at
T
/
T
g
equal
to 1, if glass transition concept is valid or
X
w
/
X
b
equal to 1, if water activity concept
is valid. Buitink et al. (
2000
) measured molecular mobility by ST-EPR and
1
H- NMR,
and observed two distinct changes: first one minor shift just close to
T
g
and second
abrupt decrease due to solid-like to liquid-like defined as
T
c
. In the case of sugars,
T
c
was observed at 17-35 °C higher than
T
g
and for biological materials it was more
than 50 °C. This variation was also explained by the density of hydrogen bond and
molecular packing measured by FTIR. This higher
T
c
was also correlated with the
observed collapse or softening of sugars at 10-17 °C above glass transition (Levine
and Slade
1988
; Roos
1995
; Sun et al.
1996
) and crystallization above 30 °C. It is
generally believed that crystallization over practical time scale occurred above glass
transition, although some report showed it was below 30 °C than glass temperature
(Le Meste et al.
2002
). ʱ-amylase was more stable in rubbery matrices of lactose or
trehalose than in a glassy PVP matrix and the protective efficiency of saccharides,
maltodextrins and PVPs did not increase with their respective glass transition tem-
perature (Rossi et al.
1997
; Terebiznik et al.
1998
). In addition dielectric and other
spectroscopy determine ʱ, ʲ and ʳ relaxations below glass transition (Adrjanowicz
et al.
2009
). However it is not clear how these could be related or linked to deter-
mine the stability of foods. Considering the fact that glass transition is not the criti-
cal limit, Rahman (
2010
) tested the hypothesis that there is a critical temperature as
a ratio of
T
c
/
T
g
(
T
c
is the critical temperature) which could vary with moisture con-
tent. Above the critical temperature, an increase in the water content or temperature
significantly increased the reaction rate while below the critical temperature the rate
was relatively less affected by water content and temperature. He observed values
of
T
c
/
T
g
varied from 0.78 to 1.5 depending on the types of reaction and the matrices.
In some instances, the values of
T
c
/
T
g
were close to 1.0 indicating only glass tran-
sition could explain the process. Moreover, the deviations of
T
c
/
T
g
from 1 explain
why in many instances in the literature both stability and un-stability were
observed above and below glass transition. The glass transition by thermal or
mechanical relaxations measure mobility in a 20-300 nm range, while other relax-
ation techniques, such as Nuclear magnetic Nuclear Magnetic Resonance (NMR)
measures the molecular relaxation in a 1-2 nm range (McBrierty and Packer
1993
).
