The effect of interactions of denatured whey proteins with casein
micelles
on the rheological properties of acid milk gels was investigated. Gels
were made by
acidification of skim milk with glucono-δ-lactone at 30°C using
reconstituted skim
milk powders (SMP; both low- and ultra-low-heat) and fresh skim milk (FSM).
The
final pH of the gels was ∼4·6. Milks containing associated or
‘bound’ denatured
whey proteins (BDWP) with casein micelles were made by resuspending the
ultracentrifugal pellet of heated milk in ultrafiltration permeate. Milks
containing
‘soluble’ denatured whey protein (SDWP) aggregates were formed
by heat treatment
of an ultracentrifugal supernatant which was then resuspended with the
pellet. Acid
gels made from unheated milks had low storage moduli, G′,
of <20 Pa. Heating
milks at 80°C for 30 min resulted in acid gels with G′
in the range 390–430 Pa. The
loss tangent (tan δ) of gels made from heated milk increased after
gelation to attain
a maximum at pH ∼5·1, but no maximum was observed in gels made
from unheated
milk. Acid gels made from milks containing BDWP that were made from low-heat
SMP, ultra-low-heat SMP and FSM had G′ of about 250, 270
and 310 Pa respectively.
Acid gels made from milks containing SDWP that were made from ultra-low-heat
SMP or FSM had G′ values in the range 17–30 Pa, but
gels made from low-heat SMP
had G′ of ∼140 Pa. It was concluded that BDWP were important
for the increased
G′ of acid gels made from heated milk. Addition of N-ethylmaleimide
(NEM) to low-heat
reconstituted milk, to block the —SH groups, resulted in a reduction
of the G′
of gels formed from heated milk but did not reduce G′ to
the value of unheated milk.
Addition of 20 mm-NEM to FSM, prior to heat treatment, resulted
in gels with a
lower G′ value than gels made from reconstituted low-heat
SMP. It was suggested
that small amounts of denatured whey proteins associated with casein micelles
during low-heat SMP manufacture were probably responsible for the higher
G′ of gels
made from milk containing SDWP and from milk heated in the presence of
20 mm-NEM,
compared with gels made from FSM.
The interfacial structure and Young's modulus of peptide films having switchable mechanical properties