Effects of milk protein ingredients on physico-chemical properties of rice starch : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University Palmerston North, New Zealand
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Date
2009
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Massey University
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Abstract
The overall aim of this thesis is to determine if the interactions between normal and
waxy rice starch and milk proteins from four milk protein ingredients, namely skim
milk powder (SMP), milk protein concentrate (MPC), sodium caseinate (NaCAS) and
whey protein isolate (WPI) do occur, and to identify the mechanisms underlying these
interactions.
Different milk protein ingredients at various concentrations (0 to 10%, w/w) affected
markedly and differently the pasting behaviour of 10% (w/w) rice starches. SMP
delayed the pasting of both rice starches by increasing the onset temperature (Tonset) and
the peak viscosity temperature (Tpeak) of pasting. This was mainly due to the presence
of lactose and ions, which was further supported by the investigation of the effects of
UFSMP (a solution of salts and lactose present in SMP at their proper concentration)
and lactose. The addition of NaCAS also delayed the pasting of rice starch; Tpeak in the
case of both starches was increased. For normal rice starch paste, MPC and WPI
decreased the Tpeak. MPC had no affect on Tpeak of waxy rice starch paste.
The qualitative viscoelastic behaviour of rice starch/milk protein ingredient gels
obtained from the above pastes was dominated by the continuous phase made of the
starch molecules. There was evidence, as indicated by confocal microscopy, of phase
separation between the milk proteins of SMP and MPC and the two starches. The phase
separation was not observed in the addition of either NaCAS or WPI.
Studies on the thermal behaviour of rice starch/milk protein ingredient mixtures by
differential scanning calorimetry (DSC) showed that SMP, similarly to UFSMP,
delayed the gelatinization of both starches. NaCAS also delayed the gelatinisation of
both starches but had a greater effect on waxy than normal rice starch. The addition of
NaCAS did not affect Tonset but increased Tpeak for normal rice starch, whereas the
gelatinisation temperature of waxy rice starch was highly affected by the addition of
NaCAS with both Tonset and Tpeak shifted to higher temperatures. MPC had no affect on
the gelatinization temperature of normal rice starch, whereas the gelatinization
temperature of waxy rice starch was increased by the addition of MPC. The addition of
WPI to both rice starches showed two thermal transitions. The first of these was due to
the gelatinisation of the starches and the second to the denaturation of ß-lactoglobulin
(ß-lg). The addition of WPI to normal rice starch showed that the thermal behaviour of
normal starch and protein were independent from each other. In contrast, the thermal
behaviour of waxy rice starch was modified by the addition of WPI; both Tonset and
Tpeak were increased.
SMP decreased the Tonset of swelling, swelling ratio and the amount of starch leaching
from both starches. These observed changes were due to the presence of lactose and
ions in SMP. NaCAS slightly increased Tonset of swelling but the amount of starch
leaching was reduced for both rice starches. The rigidity of both starches tended to
increase in the presence of NaCAS. MPC and WPI affected the swelling behaviour of
normal and waxy rice starch differently. A dramatic increase in the swelling of normal
rice starch/MPC or WPI mixtures was observed, whereas this trend was not evident for
waxy rice starch/ MPC or WPI mixtures. The difference in the water holding ability
and gelatinization peak temperatures of the two starches over the temperature range at
which whey proteins denature and form gels are believed to be responsible for the
observed differences.
The results from confocal microscopy showed that milk proteins, such as a-casein, ß-
casein, ß-lg and a-lactalbumin (a-la), were adsorbed onto the granule surface of both
normal and waxy rice starch. The mechanism for this adsorption is the hydrophilic
interactions; hydrogen bonds between hydroxyl group from terminated glucan molecule
that protrude around starch granule surface-hydroxyl; amino, or other electron-donation
or electron-accepting groups of the added proteins. Using sodium dodecyl sulfatepolyacrylamide
gel electrophoresis (SDS-PAGE) it was found that for SMP and MPC
the adsorbed as- to ß-casein ratio on both starches was similar to the as-casein to ß-
casein ratio in the casein micelle at low SMP and MPC concentrations. But at high
concentrations of SMP or MPC, this ratio decreased indicating that more ß-casein was
adsorbed preferentially to as-casein. In the case of NaCAS, as-casein was adsorbed
preferentially to ß-casein. Moreover, there was evidence of multilayer adsorption of ascasein
into the surface of rice starch granules. Compared to the other milk protein
ingredients, very small amounts of the ß-lg and a-la from WPI were adsorbed onto
starch granules. However, the adsorbed amounts of ß-lg and a-la from WPI
continuously increased with increasing WPI concentration, suggesting that these two
proteins, particularly ß-lg, adsorbed in multilayers too.
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Keywords
Waxy rice starch, Lactoglobulin, Gelatinisation, Adsorption