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Rheology of Concentrated Milk Protein Dispersions in the Presence of Lactose
I. Marti*, P. Fischer, E. J. Windhab
ETH Zürich, Institute of Food Science and Nutrition, Food Process Engineering
Building LFO E12.1, Schmelzbergstrasse 9, 8092 Zürich, Switzerland
*[email protected]
INTRODUCTION
Proteins are natural polymeric macromolecules. Their
structure, conformation and physico-chemical
properties are determined by the amino acid sequence
(Fig. 1).
O
R2
C
CH
H2N
CH
NH
R1
NH
O
Ri
C
CH
C
NH
Concentrated milk protein dispersions exhibits
shear dependent flow behaviour and pronounced
viscoelastic properties as shown for the occurrence of
rod climbing in Fig. 3.
COOH
O
Figure 1. Schematic representation of a protein
molecule; R1, R2 and Ri indicate the amino acid side
chains [1]
Depending on protein structure, environmental
conditions and chemical nature of the side chains
three-dimensional structures are built up and
stabilised by means of intra- and/or intermolecular
forces. Fig. 2 gives an overview of the governing
interaction mechanisms.
NH3
+
O
O
_
Œ
O
C
Ž
CH2OH

O
H O C

CH2OH
Figure 2. Non-covalent forces stabilising structure of
protein: 1: ionpair interaction; 2: hydrogen bond;
3: dipole-dipole interaction; 4: hydrophobic
interaction [1]
Milk protein can be considered as a complex
mixture of proteins. It typically contains 80 %
caseins and 20 % whey proteins. The caseins are
amphiphilic and mostly random coil polypeptides
with a molecular weight of 19’000 to 23’000. The
whey proteins are with a molecular weight of 14’000
to 18’300 smaller in size and so called globular
proteins. They have a more organised structure and
are sensitive to heat and pH [1].
Although all proteins are built from amino acids,
the flow properties of their dispersions differ greatly
due to factors such as protein type, concentration,
temperature, heat treatment, pH, and solvent
environment. Rheological data of milk protein
systems have been reviewed by Kinsella [2]. The
majority of the studies, however, refer either to
casein, caseinate or whey protein dispersions with
low concentrations and focus on the impact of
variation in pH, ionic environment and temperature.
Figure 3. Rod climbing of a concentrated milk
protein dispersion
Protein dispersions relevant for the design and
production of food products typically contain besides
proteins other solutes such as sugars. Several
researchers [3-5] have investigated the interactions
between non-ionic solutes and ionic polymers, such
as sugar and proteins. It was demonstrated that in the
presence of sugars the proteins are preferentially
hydrated. Consequently, sugars are preferentially
excluded from contact with the surface of the
proteins. The degree of preferential hydration was
shown to be dependent on sugar concentration, and
that physico-chemical properties such as partial
specific volume of the proteins, structure, stability,
and protein-protein interaction are altered in the
presence of non-ionic solutes. As the flow properties
are strongly related to these factors it is evident, that
also the flow properties of protein dispersions is
altered with the addition of sugars to the dispersion,
in this case milk protein dispersion with the addition
of the sugar lactose.
Large deformation processes like spinning are
sensitive to flow instabilities. Thus, the knowledge
of the rheological behaviour of the processed milk
protein dispersions is of practical significance for
process design and quality control of both the
manufacturing process and the final products.
EXPERIMENTAL METHODS
The experiments reported in this paper were
performed using a Rheometric Scientific ARES
rheometer equipped with cone-plate geometry of
50 mm diameter and a cone angle of 0.04 rad. A
RESULTS
Fig. 4 shows the shear stress and first normal stress
difference at various milk protein concentrations
measured at a temperature of 10 ˚C. The respective
temperature dependencies for the milk protein
dispersion with a concentration of 19.4 % by weight
are depicted in Fig. 5.
104
τ
N1
22.5 wt %
τ [Pa], N1 [Pa]
103
19.4 wt %
17.0 wt %
15.3 wt %
102
1
101
2
1
1
100
10-3
10-2
10-1
100
101
102
103
γ [s-1]
Figure 4. Shear stress and first normal stress
difference as function of shear rate at various
concentrations of milk protein (ϑ = 10 ˚C)
At low shear rates a zero shear viscosity can be
observed as shown by the slope of 1 in the shear
stress curves. At higher shear rates the milk protein
dispersions exhibit shear thinning behaviour. With
decreasing concentration and increasing temperature
the onset of shear thinning is shifted to higher shear
rates. At the onset of shear thinning the first normal
stress difference can be detected.
Zero shear viscosity and shear thinning can be
illustrated through microstructural aspects. At rest,
protein molecules are randomly oriented and
entangled. When this material is sheared, the
asymmetric dispersed molecules tend to align
themselves in flow direction so that frictional
resistance is reduced. The random structure changes
to a shear-oriented structure with progressively
decreasing resistance to flow reflected in decreasing
viscosity [2].
For all concentrations and temperatures the slope
of the first normal stress difference curve is slightly
steeper than the one of the stress curve leading to a
cross-over at the highest measured shear rates for all
concentrations and temperatures but the lowest.
104
τ
103
τ [Pa], N1 [Pa]
waterbath was used to maintain temperature at the
defined value. After loading, the samples were
covered with mineral oil to prevent evaporation
during measurements. The measurements were carried
out in triplicates.
Milk protein dispersions containing 15.3-22.4 %
by weight of dry matter were prepared in a stirring
apparatus at a temperature of 55 ˚C using
commercially available milk protein powder
AME100 from Emmi Milch AG, CH-Dagmersellen
and tap water. Stirring was performed during 4 hours
in a low-pressure environment (0.5 bar) to remove
dispersed air.
Lactose containing protein dispersions were all
made from the same milk protein dispersion with
20 % by weight of dry matter and lactose
monohydrate from DMV International, The
Netherlands. Dispersions with lactose concentrations
from 0-20 g anhydrous lactose per 100 g water were
used.
N1
10 °C
20 °C
30 °C
102
1
100
10-2
2
1
101
1
10-1
100
101
102
103
γ [s-1]
Figure 5. Shear stress and first normal stress
difference as function of shear rate at various
temperatures (concentration = 19.4 % by weight)
The flow behaviour of the examined milk protein
dispersions shows a strong dependency on
concentration and temperature. Elastic behaviour
predominates over viscous behaviour at shear rates
that are below processing conditions, which might
lead to flow instabilities during processing.
In the case of lactose containing milk protein
dispersions the curves of shear stress and the first
normal stress difference are shifted to higher values
with increasing lactose concentration in the solvent.
The increased stiffness and elasticity of the
dispersions, which were also be detected in
oscillatroy experiments, cannot solely be explained
by the impact of solvent viscosity increase. The
impact of changed protein-solvent interactions caused
by the addition of the sugar lactose may, as
discussed for other protein properties in [3-5], be the
critical factor governing the flow behaviour of the
dispersions.
REFERENCES
1.
Belitz H.-D., Grosch W., Lehrbuch der
Lebensmittelchemie (1987) 3rd Ed. Springer
2.
Kinsella J. E., CRC Critical Reviews in
Food Sci. and Nutrition (1984) 21 197-262
3.
Arakawa T., Timasheff S. N., Biochemistry
(1982) 21 6536-6544
4.
Mora-Gutierrez A., Farrell Jr. H. M., J.
Agric. Food Chem. (2000) 48 3245-3255
5.
Timasheff S. N., In Protein-Solvent
Interactions; Gregory R. B. Eds.; Marcel
Decker, Inc. (1995) 445-482