**Lambros Sakkas 1, Maria Tzevdou 2, Evangelia Zoidou 1, Evangelia Gkotzia 1, Anastasis Karvounis 1, Antonia Samara 1, Petros Taoukis <sup>2</sup> and Golfo Moatsou 1,\***


Received: 30 June 2019; Accepted: 9 August 2019; Published: 12 August 2019

**Abstract:** An objective of the present study was the enrichment of skim sheep yoghurt milk base with hydrolysates (WPHs) of whey protein concentrate (WP80) derived from Feta cheesemaking. Moreover, the use of high hydrostatic pressure (HP) treatment at 600 MPa/55 ◦C/10 min as an alternative for heat treatment of milk bases, was studied. In brief, lyophilized trypsin and protamex hydrolysates of WP80 produced under laboratory conditions were added in skim sheep milk. The composition and heat treatment conditions were set after the assessment of the heat stability of various mixtures; trisodium citrate was used as a chelating agent, when needed. According to the results, the conditions of heat treatment were more important for the physical properties of the gel than the type of enrichment. High pressure treatment resulted in inferior gel properties, irrespective of the type of enrichment. Supplementation of skim sheep milk with whey protein hydrolysates at >0.5% had a detrimental effect on gel properties. Finally, skim sheep milk base inoculated with fresh traditional yoghurt, resulted in yoghurt-type gels with high counts of *Lb. delbrueckii* subsp. *bulgaricus* and *Str. thermophilus* -close to the ideal 1:1- and with a high ACE inhibitory activity >65% that were not essentially affected by the experimental factors.

**Keywords:** high hydrostatic pressure; whey protein hydrolysates; sheep milk; yoghurt; ACE inhibitory activity; gel properties; heat stability; traditional yoghurt starter; biofunctionality
