Effects of ultrafiltration followed by heat or high-pressure treatment on camel and bovine yogurts

The aim of this study was to assess the effects of ultrafiltration (UF) combined with high-pressure processing (HPP) or heat treatment on the quality of yogurts produced from camel milk (CM) or bovine milk (BM). Milk was concentrated by UF (0, 1, and 2-fold) before applying heat (75 °C or 85 °C for 30 min) or HPP (350 MPa or 550 MPa for 5 min). Yogurts were produced using starter cultures (Streptococcus thermophilus and Lactobacillus bulgaricus subsp. delbrückii) and pH, viscoelastic and thixotropic rheological properties, and protein profiles were determined. Compared to HPP, heat-treated yogurts, especially at 85 °C, exhibited the highest rheological storage and loss moduli signifying stronger gels. Lower storage modulus values in HPP treated CM yogurts were explained by the lack of long-range β-lactoglobulin bridges at micelle surfaces and the reassociation of pressure-dissociated caseins by hydrophobic interactions. Percent structural regeneration, which increased with increasing milk concentration, revealed higher thixotropic behavior in the case of heat- than in HPP-treated samples. SDS-PAGE electrophoresis revealed extensive proteolysis in CM compared to BM yogurts suggesting the involvement of some enzyme activities in the low gel strength.