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Improving properties of acid skim milk gels by adjusting non-micellar to micellar protein ratio and controlling protein interactions
In addition to the protein content, the type and status of milk protein in yogurt milk may affect the final rheological properties of yogurts. In the present study, we investigated the effects of altering non-micellar to micellar casein ratio on the rheological properties and microstructure of acid gels.
Model acid gel formulations containing 0, 10, 30 and 60% protein substitution from carbon-dioxide-treated milk proteins (T-MPC) as a source of non-micellar casein and non-fat dry milks (NFDM) as a source of micellar casein were developed. All the samples were standardized to 4 % w/w protein and 12% w/w total solids. The pH was adjusted to 6.5 before their pre-heating to 90°C/10 min. Acid milk gels were prepared using Glucono-δ-lactone to obtain final pH4.4 ± 0.05 after 4 h incubation at 30°C. The soluble (serum) phases obtained by centrifugation of heated and unheated milk samples at 25000g/1h were characterized using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Microstructure and rheological properties of acid gels were characterized using confocal laser scanning microscopy (CLSM) in the fluorescence mode and small amplitude oscillatory rheology (SAOR) (1% strain, 0.1Hz frequency), respectively. Photon correlation spectroscopy was used to study the particle size of the heated milks. ANOVA was used to test the results and statistical significance at P<0.05 was determined, using the statistical software SAS.
The SDS-PAGE pattern of model formulations containing T-MPC showed significantly higher (P<0.05) proportion of soluble caseins and disulfide-linked casein-whey protein complexes in the serum phase of unheated and heated milk, respectively. Particle size of formulations containing T-MPC was significantly lower than control samples containing untreated proteins. This can be attributed to the preferential interaction of whey proteins with κ-casein in the soluble phase. SAOR showed a significant increase (P<0.05) in the elastic modulus (G') of acid gels formulated with T-MPC to an optimum level. CLSM images revealed that gels containing treated proteins had smaller, well-connected aggregates with uniform, homogenous pore-sizes, which explained the results of rheological characterization.
It can be concluded that the soluble casein-whey protein complexes and optimum non-micellar to micellar casein ratio in the yogurt formulation yielded acid gels with significantly improved rheological properties. Overall, the results suggested that yogurt with varying texture can be made by altering the ratio of non-micellar to micellar casein and by manipulating interactions of milk proteins at soluble and micellar phase. This invention is patent pending.
Keywords:
Yogurt, Carbon dioxide, Rheology, Microstructure