What is high pressure processing? 

Extremely high pressures are used in this type of processing. Pressures of up to 600 MPa are often used in commercial processing. This corresponds to a pressure of 6000 bar (1 MPa = 10 bar).  It can be a little difficult to imagine how extreme these pressures are.  

The Mariana Trench is the world’s deepest oceanic trench. At its deepest point, it is 11,034 m deep, and here the pressure is approx. 1100 bar (110 MPa). Up to 600 MPa is generally used in high pressure processing, which corresponds to a water column of about 60,000 metres — to put things into perspective!  

How is food subjected to high pressure processing?  

The packaged food product is placed in a large steel or plastic container. The container is introduced into the high pressure machine, and water is added as a pressure transmitting medium. When the container is full, more water is pressed into the container by powerful pumps, forming the high pressure.   

The pressure is transferred immediately and produces a homogeneous effect throughout the product regardless of geometry and size. In standard commercial use of high pressure, with the desired effect being extending shelf life or inactivation of microorganisms, processing typically occurs at 400-600 MPa for 1-5 minutes.   

Building up a pressure of several hundred MPa often takes a little time. For more recent machines, this can mean around two minutes to build up a pressure of 600 MPa. In turn, release of pressure takes 1-2 seconds, see the figure below for a schematic presentation of a high pressure process.  

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High pressure also generates increased temperature as a result of adiabatic heating. In general, an increase of approx. 3°C/100 MPa is achieved. If the product contains large amounts of fat, the increase may be 7-9°C/100 MPa.  

Definition and equipment 

The chambers in which high pressure processing takes place are made of solid steel that can withstand extreme changes in pressure. After the food products have been inserted into the high pressure machine, it is filled with water or other liquid. When the chamber is full, powerful pumps are used to pump in more water. This results in the water molecules being packed more tightly, thus forming the extreme pressure.   

Another possibility of creating high pressure is by using a so-called piston model.  Here, pressure is applied directly to a piston, which further compresses the water molecules and creates the high pressure.   

High pressure processing is primarily a batch process, but, a few years ago, continuous processing also became possible for liquid products. Most often, the pressure is pushed up to the desired level for a certain holding time, but there are also options for running pressure processing in multiple stages.   

Processing parameters    

The most relevant parameters that can be adjusted in connection with high pressure processing are:   

• pressure   
• processing time   
• temperature  

High pressure processing usually takes place at room temperature or under refrigerated conditions. The advantage is that the use of heat is avoided, which means that the products preserve a more natural flavour and there is very little effect on colour and texture. The same applies to nutrients.   

Read also: 

High pressure processing in the food industry

How high pressure affects food safety and shelf life