Role of protein interactions on microstructure and rheological properties of Greek-style Yogurt
Disposal of acid whey is a major concern for the manufactures of Greek-style yogurt (GSY) because of its potential environmental impact. An alternate way of preparing GSY is to eliminate de-wheying step by using functional milk proteins and manipulating process-induced protein interactions. The objective of this study was to investigate the influence of micellar and non-micellar casein to globular proteins ratios on the properties of GSY prepared using this alternate process.
GSY (7.5% w/w proteins, 15% w/w total solids) were prepared with either milk protein concentrates (MPC) as a source of micellar casein or carbon-dioxide treated milk protein concentrate (T-MPC) as a source of non-micellar caseins. Whey protein concentrates (WPC) and de-proteinized whey was used to adjust the globular protein and total solids level respectively. The casein to whey ratio was adjusted to 4:1, 2.3:1 and 1.5:1. All samples were pre-adjusted to pH 6.5 before heating at 90°C/10 min. Acid gels were prepared using Glucono-δ-lactone to obtain final pH 4.4 after 4h of incubation@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 characterization of acid gels were performed using confocal laser scanning microscopy (CLSM) in the fluorescence mode and small amplitude oscillatory rheology (1% strain, 0.1Hz frequency), respectively.ANOVA was used to test the results and statistical significance at P<0.05 was determined, using the statistical software SAS.
SDS-PAGE results indicated significantly higher proportion (P<0.05) of soluble disulfide-linked aggregates in serum phase of heated milks prepared from T-MPC. Gels prepared using T-MPC as a source of non-micellar casein had significantly higher (p<0.05) elastic modulus (G') (e.g.1.12 x 103 Pa) compared to gels prepared using MPC (e.g.6.59 x 102 Pa).Acid gels containing T-MPC in different ratios with globular proteins had a significantly higher (p<0.05) G' than acid gels prepared with MPC and WPC. CLSM images revealed that T-MPC gels had smaller, well-connected aggregates with uniform, homogenous pore sizes, which strongly supported the results of rheological characterization.
It can be concluded that T-MPC as a source of non-micellar casein is an ideal ingredient to alter the ratio of non-micellar to globular protein ratio and thereby increase the gel strength of Greek-style yogurts. This invention is patent pending and can be used to produce protein structures having different gels strengths suitable for commercial applications.
Keywords: Greek-style yogurt, Carbon dioxide, Rheology