# Non-Thermal Methods for Ensuring the Microbiological Quality and Safety of Seafood
## 2 High Hydrostatic Pressure--HHP
### Effects of HHP on the Quality of Fish and Seafood  
The most common treatments use pressure levels between 100 and 600 MPa for a period ranging from a few seconds to 10-15 min to avoid the product's sensory quality degradation. In general, it seems that the physicochemical parameters are not significantly affected by the applied conditions of HHP immediately. [PERSON] et al. (2015) [49] studied the effects of HHP (200, 400, and 600 MPa) on the overall sensory appearance of mild smoked rainbow trout fillets (_O. mykiss_) and fresh European catfish fillets (_S. glanis_). They reported that the sensory appearance was dependent on the intensity of HHP in the catfish fillets, while the smoked trout was almost unaffected. On the aim of improving the shelf life of six edible seaweed species, using HHP at 400 or 600 MPa for 5 min, [PERSON] et al. (2020) [75] documented that HHP affected the physicochemical properties, color, and texture characteristics of the seaweeds at an acceptable level, while [PERSON] et al. (2020) [76] reported high odor characteristics and acceptance scores of HHP-treated kombu seaueed (_L. ochroleuca_) stored at 5 \({}^{\circ}\)C for 180 days. Several authors investigated the effect of HHP on sensory quality parameters of fresh fish [28; 47; 48; 57; 68; 77; 78; 79], mollusks [80; 81; 82; 83], crustaceans [57; 84; 85; 86], and RTE seafood products [28; 87; 88; 89; 90; 91].  
HP processing can retain or even improve the texture of seafood products and can be used to develop new products inducing desirable changes [92]. Furthermore, when HP treatment at 200 MPa applied to frozen raw fish to assist thawing, the water-holding capacity of HP-thawed fish in some cases increased [93]. The same phenomenon has been reported by several authors and may be due to an increase in hydration capacity of proteins during pressurization treatment [94; 95; 96]. However, according to [PERSON] et al. 2003 [93] the texture of HP-thawed raw salmon, cod, haddock, and trout fillets changed, showing a favorable increase in hardness in agreement with the results observed by [PERSON] et al. (2009) [97] on Atlantic salmon. In addition, to increase the consumer acceptance of HP-treated seafood, it is well known that the retention of color is imperative. Therefore, many authors investigated the effects of HP treatment on fish color and found that many factors affect the final color of the product such as muscle hydration capacity and denaturation of proteins [98; 99]. Among them, changes in the major pigments of astaxanthin and canhaxanthin (found on salmon, shrimps, etc.) [51; 100; 101], as well as myoglobin and porphyrin found in red meat fish [99] affect the color and are highly depended on the PTM conditions of HHP. Investigation of HP processing for a short time of some seconds showed no significant effect on cold-smoked salmon's redness [51]. Additionally, HHP at subzero temperatures retained the color of frozen cooked pink shrimps (_P. jordani_), exhibiting the positive effect of HHP on quality retention [56].  
Consequently, HHP represents a highly important innovative technology for the seafood industry. HP treatment is an environmentally friendly application that can achieve high log reduction comparable with heat pasteurization, while maintaining the food's desirable organoleptic characteristics. Even though HP treatment is considered effective and commercially used, there are still numerous opportunities and challenges for food scientists to overcome, because HP application depends on the food matrix and the PTM parameters used.