## 1 Introduction  
Raw ewe's milk _Serra da Estrela_ cheese, with Protected Denomination of Origin (PDO) certification in the European Union, owes its unique characteristics mainly to the milk's origin and the traditional manufacturing process. _Serra da Estrela_ cheese is made exclusively with raw milk from Bordaleira and/or Churra Mondegueira sheep breeds, salt and a crude plant rennet extract (dried flowers of _Cynara cardunculus_ L.), and it is commercialized only after 30 days of mandatory ripening [1,2]. These conditions result in a cheese with distinctive organoleptic characteristics, with a closed, moderately buttery, deformable when cutting, well connected, creamy and uncuous texture, with few or no eyes and a smooth,clean and slightly acidic _bouquet_[3]. These organoleptic features are largely associated with proteolysis, the most important biochemical and complex process in cheese ripening.  
Proteolysis in cheese can be evaluated via proteolytic indexes, in particular by the ripening extension index (ratio of water-soluble nitrogen to total nitrogen--WSN/TN ratio). For _Serna da Estrela_ cheese, this index was found to increase from 9.5-11%, just after manufacturing to 23-59% after 35-180 days of ripening [4; 5; 6; 7]. These high values were associated with the role that the vegetable rennet plays in cheese proteolysis [8], causing more extensive proteolysis, which is associated with a more homogeneous cheese structure and increased creaminess and softness [9]. It is known that cardoon flowers influence the texture of cheese [10].  
Since _Serna da Estrela_ cheese is a non-pasteurized product manufactured from raw milk, microbial safety is a pertinent issue. From a safety point of view, previous results have demonstrated the usefulness of HPP (400-600 MPa, 3-10 min) to render _Serra da Estrela_ cheese microbiologically safer during a more extended shelf-life of up to 100 days under refrigeration [11]. The observed effect of HPP on cheese properties is dependent on the pressure intensity, the holding time under pressure and the cheese ripening stage at which HPP is applied.  
The effect of HPP on proteolysis in whole cheeses manufactured with raw ewe's milk has been studied during ripening of La Serena cheese [12] and during storage of Casar [13] and Torta del Casar [9] cheeses and for small pieces (~20 g) of _Serra da Estrela_ cheese [11]. La Serena cheese that was pressure-treated at 300 and 400 MPa (10 min, 10 \({}^{\circ}\)C) at 50 days of ripening showed no changes in proteolysis level after 10 days under conventional ripening conditions [12]. During storage of Casar cheese, casein degradation was significantly retarded in HPP-treated cheeses (600 MPa/5 min) [13]. Also, [PERSON] et al. (2015) [9] treated Torta del Casar cheese at 600 MPa/5 for 20 min at 60 days of ripening and demonstrated a reduction in proteolysis of casein fractions (WSN/TN ratio decreased) after 240 days of storage. In a previous study with small portions of _Serra da Estrela_ cheese, pressure treatments caused no significant changes in WSN content up to 100 days of refrigerated storage [11].  
According to the above-mentioned studies, HPP treatments applied at the end of the ripening period slowed down or maintained the casein hydrolysis state and nitrogen ratio during storage. Since proteolysis has a marked effect on texture and, thus, on _Serra da Estrela's_ sensorial characteristics, this type of HPP treatment opens an interesting possibility to maintain the degree of cheese proteolysis during extended storage and, thus, to avoid undesired changes in texture. HPP can be used to process Serra da Estrela cheese in conditions that assure microbial safety without influencing cheese lipid profiles [14].  
Thus, in this work, the effects of HPP (450 MPa for 6 or 9 min or 600 MPa for 6 min) applied to whole _Serra da Estrela_ cheeses after 45 days of ripening on the evolution of cheese proteolysis, texture and sensorial properties over 15 months of refrigerated storage were studied.