The effects of ultra-high pressure homogenization (UHPH) on skim milk yogurt making properties were investigated. UHPH-treated milk was compared with conventionally homogenised (15 MPa) heat-treated skim milk (90°C for 90 s), and to skim milk treated under the same thermal conditions but fortified with 3% skim milk powder. Results of the present study showed that UHPH is capable of reducing skim milk particle size which leads to the formation of finer dispersions than those obtained by conventional homogenisation combined with heat treatment. In addition, results involving coagulation properties and yogurt characteristics reflected that, when increasing UHPH pressure conditions some parameters such as density of the gel, aggregation rate and water retention are improved.

In this study, high pressure (HP)-induced denaturation of α-lactalbumin (α-la) and β-lactoglobulin (β-lg) in dairy systems was examined. In both milk and whey, β-lg was less baroresistant than α-la; both proteins were considerably more resistant to HP-induced denaturation in whey than in milk. HP-induced denaturation of α-la and β-lg increased with increasing proportion of milk in mixtures of milk and whey. Addition of a sulphydryl-oxidising agent, KIO3, to milk or whey increased HP-induced denaturation of β-lg, but reduced the denaturation of α-la. Denaturation of both α-la and β-lg was prevented by adding a sulphydryl-blocking agent, N-ethylmaleimide, to milk or whey prior to HP treatment, highlighting the crucial role of sulphydryl-disulphide interchange reactions in HP-induced denaturation of α-la and β-lg. Removal of colloidal calcium phosphate from milk also reduced HP-induced denaturation of α-la and β-lg significantly. The higher level of HP-induced denaturation of α-la and β-lg in milk than in whey may be the result of the abscence of the casein micelles and colloidal calcium phosphate from whey, which facilitate HP-induced denaturation of α-la and β-lg in milk.