# Non-Thermal Methods for Ensuring the Microbiological Quality and Safety of Seafood
## 2 High Hydrostatic Pressure--HHP  
### General Description of HHP Technology  
High pressure processing (HPP), which is also known as ultra-high pressure (UHP) or high hydrostatic pressure (HHP), is a non-thermal technique that has emerged in food production within recent decades, and since the 1990s, the use of this technique has been extended to various types of foodstuff, including fish and seafood [16]. The critical advantage of HHP is the homogenous and instantaneous transmission of pressure in the treated solid or liquid product from all sides, leading to pathogens inactivation and shelf-life extension, while the overall sensory quality of the processed product is hardly affected [17]. During HHP treatment, the product is exposed to a pressure ranging from 100 to 1000 MPa for a short period (up to some minutes) and processing temperatures at \(-20\) to 121 \({}^{\circ}\)C [13; 18]. During HP treatment, the already sealed food product in its final package with flexible plastic material is placed in a cylindrical pressure vessel capable of sustaining the required pressure. The industrial food processing equipment is around 500 L capacity, capable of operating at a pressure higher than 1000 MPa [19]. Subsequently, the product is fully submerged in the pressure-transmitting liquid medium. Different pressure-transmitting media can be used, either pure substances or mixtures containing castor oil, silicone oil, sodium benzoate, ethanol, or glycerol. Industrial HP treatment is currently a batch or semi-continuous process [13]. The selection of equipment depends on the kind of food product to be processed. The primary mechanism described for the bacterial inactivation under pressurizing time is the alteration of the cell membranes due to their structural changes in protein and membrane phospholipids leading to increased permeability, making the microorganism more vulnerable under the presence of antimicrobial agents [20; 21]. It is well established that HHP processing depends on [PERSON]'s principle, where pressure applied and pressure within the food should be equal [16]. Additionally, during pressurization, the food undergoes isostatic compression, and when pressure is released, food returns to its initial shape [22; 23]. These consecutive changes affect the cells' morphology and disorganize the large molecules without affecting the small molecules such as vitamins and flavor components [24]. In recent years, HP's treatment efficacy on bacterial inactivation has been extensively studied [25; 26; 27; 28]. The degree of inactivation depends on the type of microorganism and their growth phase, the pressure level, the process temperature, and time (PTM), including the food or lab medium's pH and composition. As a general practice, increasing treatment pressure, holding time, or temperature will usually increase the number of microorganisms inactivated [29]. Even though, Gram-negative bacteria are more sensitive to high pressure than Gram-positive bacteria, many differences in barotolerance can be found among various strains of the same species [30; 31; 32]. Bacterial spores are highly resistant, yet there are limitations in killing them with HHP, but [PERSON] and [PERSON] (2008) [33] showed that when pressurization is carried out at temperatures close to 100 \({}^{\circ}\)C, spore-forming pathogens can be inactivated. Pressure inactivation of common spoilage and vegetative microorganisms is markedly enhanced at temperatures of 50 to 70 \({}^{\circ}\)C and subzero temperatures [34; 35]. It has been generally observed that HHP and subzero temperatures (where the water exist in liquid phase, while pressure is increased, exert synergistic effects [36; 37]. At ultra-low temperatures, and more specifically at temperatures below \(-22\)\({}^{\circ}\)C, when the pressure increases above 200 MPa, an ice phase transition from type I to type III and further to type V, occurs [38]. This rapid phase shift of ice crystal structure enhances bacteria inactivation by causing membrane damage [39; 40].