**2. Materials and Methods**

### *2.1. Hydrolysis of Whey Protein Concentrate*

The substrate was a 5% (*w*/*w*) aqueous dilution of whey protein concentrate powder with approximately 80% (*w*/*w*) protein content (WP80) derived from sheep/goat Feta cheese whey (Epirus Protein SA, Ioannina, Greece). The composition of WP80 (Table 1) was determined as described by Zoidou et al. [7]. The aqueous dilution of WP80 remained overnight at 4 ◦C–6 ◦C for proper hydration of the powder. Hydrolysis was carried out by means of trypsin and Protamex. The ratio enzyme to protein was 0.25%, and 0.1% for trypsin and protamex, respectively. Incubations were carried out at 50 ◦C without pH adjustment at pH 6.2–6.3, which was the pH of the WP80 dilution. After 60 min, the pH was 5.6–6.1 and hydrolysis was stopped by heat treatment at 68 ◦C for 10 min, which are the conditions for batch pasteurization of milk. Hydrolysates were powdered by lyophilization. Trypsin and protamex hydrolysates were symbolized as WPHt and WPHp. Hydrolysis conditions resulted from previous experiments (data not shown).

**Table 1.** Composition of whey protein concentrate (WP80) g/100 g used as substrate for the production of whey protein hydrolysates (WPHs).


CMP, caseinomacropeptide; α-la, α-lactalbumin, SA, serum albumin; β-lg, β-lactoglobulin; TNP, total native protein.

### *2.2. Assessment of Heat Stability of Various Sheep Milk Bases*

The aim of this experimental section was the selection of heat stable milk bases that could be used for the manufacture of yoghurt-type gels. Raw sheep milk was defatted by centrifugation at 3000× *g* for 20 min at 40 ◦C. Skim sheep milk (0.1% fat) was mixed with variable quantities (0.5%–2% *w*/*w*) of WP80, WPHt and WPHp powders with or without trisodium citrate at 0.2% (*w*/*w*), i.e., approximately 7 mM. Experiments were performed in duplicate and three replicates of each mixture were prepared in each experiment. Mixtures were kept overnight at 5 ◦C–6 ◦C for hydration. Then, two of the replicates were heated at 70 ◦C, 75 ◦C, 80 ◦C, 85 ◦C, 90 ◦C and 95 ◦C for 5 min. At the end of the heat treatment, the milk mixtures were cooled down immediately at room temperature using an ice-bath. One of the triplicates was not heated (control). Heat-treated (HT) and control mixtures were centrifuged at 10,000× *g* for 10 min. Appearance of sediment indicated heat instability.
