**1. Introduction**

The manufacture of low-fat, high-protein fermented milks is one of the current trends in food technology. High-protein, high-mineral set-style yoghurt manufactured from partially defatted or skim sheep milk can be stable throughout four weeks of storage. However, the fat removal reduces the water holding capacity and the firmness of the skim yoghurt in comparison to the reduced-fat counterpart [1]. An objective of the present study was the enrichment of the skim sheep milk base with hydrolysates (WPHs) of whey protein concentrate (WP80) derived from Feta cheesemaking. The hypothesis was that this intervention could modify the texture and increase the biofunctionality, in terms of angiotensin converting enzyme inhibitory activity (ACE-IA), in accordance to our recent findings for reduced-fat cow milk yoghurt [2]. Fortification or enrichment of yoghurt cow milk base with WPC has been widely studied [3,4], but similar interventions in small ruminants yoghurt milk are scarce and have been applied in goat milk [5–7].

Hydrolysis of whey or individual whey proteins or whey protein concentrates by various enzymes results in mixtures of proteins and peptides known as whey protein hydrolysates (WPHs). WPHs can exhibit improved biofunctionality and modified physical properties compared to the substrate [4,8,9]. The use of whey protein hydrolysates (WPHs) in the cow milk yoghurt base has been studied in regard to the growth of probiotics [10,11] or biofunctional and textural properties [2,12].

An important and essential step of the manufacture of fermented milks is the heat treatment of the milk base. The heat stability of sheep milk is lower than cow milk due to differences in the casein micelle structure and mineral content [13,14]. Therefore, another objective of the present study was the use of high hydrostatic pressure (HP) as an alternative for the heat treatment of yoghurt milk bases. HP processing has been associated with variable effects on yoghurt properties that depend on the composition of cow milk base [15–17]. However, information for the effect of HP-treatment on sheep milk that could affect yoghurt manufacture and properties is limited [18–23].
