High hydrostatic pressure (HHP) refers to the exposure of foods within vessels to high pressures (300 to 700 MPa) for a short period, typically ranging from a few seconds to several minutes. Food is pressurized by direct and indirect methods utilizing water as a pressure-transmitting medium. HHP is a nonthermal process, as it only involves minor increases in temperature during pressurization. For a working pressure of 600 MPa, the temperature increment of pure water is only approximately 15ºC

Principle

High hydrostatic pressure technology is based on the use of pressure to compress food located inside a pressure vessel. The pressure vessel is the most important component of HHP equipment, consisting of a forged monolithic cylindrical piece built of alloy steel with high tensile strength. Multilayer or wire-wound prestressed vessels are used for pressures higher than 600 MPa. Prestressed vessels are purposely designed with residual compressive stress in order to lower the maximum stress level in the vessel wall during pressurization, hence reducing the cost of producing this important piece of equipment. In HHP equipment utilized in food applications, pressure is transmitted by two methods: direct or indirect. In the direct method, a piston is pushed at its larger diameter end by a low-pressure pump, directly pressurizing the pressure medium at its smaller diameter end. This method allows very fast compression but requires a pressure-resistant dynamic seal between the piston and the internal vessel surface to avoid leaks and contamination of the food. In the indirect method, high-pressure intensifiers are used to pump the pressure medium from the reservoir into the closed vessel until the desired pressure is achieved.

The applied pressure is isostatically transmitted by a fluid. In this way, uniform pressure from all directions compresses the food, which then returns to its original shape when the pressure is released.

Effects of High Hydrostatic Pressure on Microbial Inactivation

Pressures between 300 and 600 MPa can inactivate pathogenic microorganisms. Pressure induces a number of changes in the microbial cell membrane, cell morphology, and biochemical reactions that ultimately can cause microbial inactivation. Cell membranes are the primary site of pressure damage done to microbial cells. The microbial membranes play an important role in the transport and respiration functions; thus, a great change in membrane permeability can cause the death of cells.