## 1 Introduction  
Different fish species play an important role in human nutrition as an excellent source of proteins and unsaturated fatty acids, especially long-chain omega-3 fatty acids, and vitamin D. However, raw fish and many mildly processed fish products are highly perishable due to neutral pH and high water activity, and the oxidative process that starts almost immediately when fish are caught [1]. Smoking and salting enhance the taste of many fish products, and at the same time, they are traditional ways to increase their safety. Food preservatives provide longer shelf life, but consumers tend to prefer clean label ingredients and are driving the food industry to search for \"green\" food preservation technologies [2]. This has pushed both food researchers and food industry to seek alternative processing methods, such as high pressure processing (HPP), for various food items, including hot and cold smoked rainbow trout (_Oncorhynchus mykiss_). HPP research started sometime in the 1990s and since then, experiments with different kinds of food have been investigated, and later also utilized, in industrial applications. The essential advantage of HPP is based on the isostatic principle that pressure is uniformly distributed throughout the product independently of its size or geometry. Moreover, HPP preserves nutritional properties better than processes involving heating. Compared with non-treated samples, HPP extends the shelf-life and tackles challenges regarding microbial quality. As with any food processing method, HPP also has its challenges: it requires pre-packaging and product-specific optimal conditions. One factor that needs to be considered is the high initial investment of the equipment [3; 4; 5]. [PERSON] et al. [1] reviewed the effects of high pressure on the quality of pressurised fish meat. High pressure changes the quaternary, tertiary and secondary structures of proteins, leading to dissociation, unfolding, denaturation, aggregation, precipitation, and gelatinization in varying intensities. Already pressures 150-200 MPa may be able to dissociate oligomeric proteins, potentially leading to protein aggregation or precipitation, and beyond those pressures, unfolding of proteins starts to take place [6]. Depending on compression rate and rearrangements in the secondary structures of proteins, pressures above 300-700 MPa can cause non-reversible denaturation [6]. Denaturation of fish proteins by HPP opens the structure of the protein, enabling better action of hydrolytic enzymes and thus, potentially contributes to the digestibility and bioavailability of proteins [7]. However, along with denaturation, the hydrophobic components of proteins are revealed which, together with other interactions, can lead to protein aggregation. Excessive protein aggregation limits the activity of peptidase enzymes and thus, reduces protein digestibility [7]. Therefore, HPP may have both positive and negative impacts on protein digestibility and bioavailability. [PERSON] et al. [8] found that the digestibility of cod (_Gadus morhua_) was improved when the samples were processed at 200 MPa for 20 min. HPP treatment can also cause protein oxidation in fish [1]. Oxidation of proteins due to pressure can produce free radicals that cause various oxidizing reactions. As a result, the digestibility of food proteins may be reduced. Data on the effects of high-pressure treatment on the digestibility of fish proteins, especially hot or cold smoked rainbow trout, is limited. The compaction of muscle fibres leads to changes in hardness by decreasing sarcomere length, and the liquid release. The fluid loss retained by capillary forces alters the water holding capacity (WHC). Protein denaturation alters protein solubility and may lead to exposure of acidic or alkaline amino acid residues and hydrogen bonds, which can alter the pH, and furthermore, cause changes in WHC [1]. HPP can also induce changes in the sensory quality of fish; for example, protein denaturation affects fish muscle texture and colour [9].