Chemistry Reference
In-Depth Information
unsalted curd to 50% (i.e. removing 88% of the initial water) will maintain a
w
very close to 1.
Characterizing water molecular mobility by NMR seems to be the
method of choice to elucidate the retention mechanisms and to reveal the
possible influence of product formulation and processing parameters. NMR
relaxation times of protons (T
1
, called longitudinal or spin-lattice relaxation
time, and T
2
, transversal or spin-spin relaxation time) can be used to monitor
the rotational motion of water molecules. From them (particularly T
2
, which
depends more univocally on mobility), a rotational correlation time,
c
(the
average time taken for a molecule to rotate through one radian), can be
deduced. Transverse relaxation in food systems is often found to be multi-
exponential and always faster than relaxation in bulk water (i.e. water mole-
cules are less mobile). The common interpretation has been for long that
water exists in a number of populations that are ''bound'' in various ways to
the solutes.
Water molecules in close contact with a solute molecule have a reduced
mobility, which induces an increase in relaxation rate (their T
2
is decreased).
Via rapid exchange of energy between spins, the observed T
2
of the aqueous
bulk phase is a weighted average of those of the unmodified water and of the
small motionally modified fraction. The mono-multi-exponential behaviour
of relaxation is a consequence of the spatial heterogeneity of the sample
(Lillford et al., 1980). If a water molecule can reach a macromolecule surface
within a diffusion time shorter than its intrinsic relaxation time, its relaxation
will be hastened. The diversity of distances will result in a diversity of relaxa-
tion times. The occurrence of multi-exponential relaxation was shown to be
predictable on the basis of the diffusion coefficient of water molecules, the
characteristic dimension in the sample heterogeneity and the difference in
relaxation rates in the two sites allowing chemical exchange of protons (Hills
et al., 1990; Belton, 1990). Fat-free milk powder dispersed in water showed a
single exponential relaxation, as predicted for a casein micelle of diameter
0.1 mm, which is small enough for the fast exchange limit to apply. On the
contrary, with a skim milk powder paste with particles of aggregated casein of
the order of 400-500 mm in radius, multi-exponential relaxation was
observed, as expected on the basis of the diffusive exchange model. As
stressed by the authors, ''the transition from single to multiexponential
behaviour is explained on the basis of morphology without recourse to any
speculation as to changes in the nature or state of water present''.
The dramatic enhancement of the relaxation rate of protons in solutions
must also be discussed further.
17
O relaxation, which does not suffer from
complications such as hydrogen exchange between water and protein, allows
a reasonable picture of water dynamics in protein solutions to be described,
together with other methods, e.g. molecular dynamics simulation. Only water
