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The influence of Protein content of Milk Protein Concentrates on the rheological properties of Greek style acid skim milk gels
Greek-style yoghurt (GSY) derives its popularity from combining nutritionally desired high protein content with a rich smooth texture. The straining process traditionally applied in GSY manufacture leads to large amounts of acid whey, which is an industrial concern in terms of further processing or disposal. An alternative process, preventing the creation of acid whey, is producing GSY from suitable milk protein ingredients for attaining the desired protein concentration. The aim of this study was to evaluate the rheological properties of Greek- style acid milk gels prepared from milk protein concentrates (MPCs) of varying protein content.
MPC powders containing 50% (MPC50) to 85% (MPC85) protein were prepared by ultrafiltration and spray-drying from the same lot of milk. Solution (7.5% protein and 15% total solids) were prepared with these MPCs and lactose and pH was adjusted to pH 6.7 before preheating at 90°C for 10 minutes. Acid gels were prepared using glucono-δ-lactone to obtain final pH 4.6 after 4h of incubation at 30°C. Small amplitude oscillatory rheology (SAOR) measurements at 1% strain and a frequency of 1 Hz were performed for rheological characterization. pH was also monitored continuously during acidification. The pH of gelation (pHG) and time of gelation were taken as the point where elastic modulus (G') was >1 Pa. Statistical significance (P<0.05) of effects observed was tested by ANOVA.
The SAOR measurements showed a significant differences (P<0.05) in G' of acid milk gels prepared from MPC60 – MPC85 compared to gels prepared with MPC50. This was also reflected in a significant increase (P<0.005) in the gelation pH and decrease in gelation time of acid gels. Such differences in the rheological properties, gelation time and gelation pH could be attributed to increased diafiltration during the preparation of MPC powders with higher protein contents, thereby reducing serum calcium and phosphate, increasing calcium ion activity and increasing the amount of denatured whey proteins associated with the casein micelles after heating.
From these studies, it can be concluded that the use of MPC60-MPC85 in the manufacturing of GSY has a positive influence on G’, gelation pH and gelation time. There was no significant differences in the rheological properties of gels manufactured with MPCs with >60% protein. Hence, opportunities exist to produce GSY without acid whey as a by-product using MPC like MPC60 or MPC70, which are not prone to excessive solubility loss during storage.
Keywords: Greek style yogurt, Milk protein concentrate. Rheology