*2.3. Heat- and High Pressure- Treatment of Selected Skim Sheep Milk Bases*

Raw ovine milk was defatted by means of a lab-scale milk fat separator. The composition and treatments of milk bases used in the subsequent yoghurt-type gel experiments are shown in Table 2. The criterion for their selection was the heat stability (Section 2.2). Heat treatments (HT) were carried out under batch conditions. The same mixtures without trisodium citrate were processed by high pressure (HP) as follows. Treatments were performed using a laboratory-scale HP system with a maximum operating pressure of 1000 MPa (Food Pressure Unit FPU 1.01, Resato International BV, Roden, Netherlands), consisting of an HP unit with a pressure intensifier, an HP vessel of 1.5 L and a multi-vessel system consisting of six vessels of 42 mL capacity each. All HP vessels are surrounded by a water circulating jacket connected to a temperature control system. The pressure-transmitting fluid used was the polyglycol ISO viscosity class VC 15 (Resato International BV, Netherlands). Milk bases were put into a multilayer (PP, foil, PE) packaging and placed in the 1.5 L chamber for processing. The desired value of pressure was set and, after pressure build-up (ca. 20 MPa·s<sup>−</sup>1), the pressure vessel was isolated; this point defined the zero time of the process. The pressure of the vessel was released after a preset time interval (10 min pressurization time) by opening the pressure valve (release time <3 s). The initial temperature increase during the pressure build-up (ca. 3 ◦C per 100 MPa) was taken into consideration in order to achieve the desired operating temperature. The pressure and temperature were constantly monitored (intervals of 1 s) and recorded during the process. All samples were pressurized at 619 ± 7 MPa and 55.2 ± 1.3 ◦C for a process time of 10 min. After processing, samples were kept overnight at 4 ◦C. The heat and high-pressure experiments were performed in duplicate.


**Table 2.** Composition and treatments of skim sheep milk bases. Means of two experiments ± standard deviation.

<sup>1</sup> defatted sheep milk (control); <sup>2</sup> whey protein concentrate powder with approximately 80% (w/w) protein content (WP80) derived from sheep/goat Feta cheese whey; <sup>3</sup> trypsin hydrolysate of WP80; <sup>4</sup> protamex hydrolysate of WP80.
