HPP Overview

Today, many processed foods we buy from a supermarket have been heated to kill bacteria. For example, juice, milk, and canned soups are heat processed. While these foods are safe, one problem is heat can destroy quality. On the other hand, high pressure processing preserves quality without compromising safety. The technology has its roots in the material and process-engineering industry where it has been commercially used in sheet metal forming and isostatic pressing of advanced materials such as turbine components and ceramics.

High Pressure Processing is a food processing method wherein the food is subjected to very high pressures (up to 120,000 pounds per square inch) to kill bacteria present in the raw food. The technology can also be used to enhance desired food attributes in some foods. High pressure processing can improve food safety by destroying the bacteria that cause food borne illness and spoilage, and parasites that cause diseases. High pressure works like heat to kill bacteria, but the food remains fresh and rich. In a typical process, pre-packaged raw product is loaded inside a pressure chamber and subjected to very high pressures for specific time. This whole process may take 10 minutes or less.

Pressures used in are almost ten times greater than in the deepest oceans on earth. High-pressure processing causes little change in the 'fresh' characteristics of foods. In fact, it is possible to keep many foods longer and in better condition. Small molecules that are responsible for flavor and nutrition are typically not changed by pressure. Pressure processed foods are also reported to have better texture, nutrient retention, and color compared to heat processed foods.

In principle, any food material with sufficient moisture content can be subjected to high pressure processing. HPP can be used to process both liquid and solid foods. Food material containing large quantity of air pockets (such as marshmallows) are not suitable candidates for high pressure processing because the high pressure processing remove the air from these foods and thus destroying their appearance.

At present, HPP is used mainly for processing high-value or novel products of superior quality. High pressure processed foods are commercially available in the US market, from mid 1990 onwards Food products that have been brought to market that currently employ high pressure processing in their manufacture include guacamole, oysters, ham, fruit jellies and jams, fruit juices, pourable salad dressings, salsa, poultry and rice products. Other potential applications include processing shelf-stable products, blanching, and pressure assisted freezing and thawing. Equipment and processing costs are typically estimated to be less than $0.10 per kg of the food processed.

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	Historical Timeline
1895	H. Royer used high pressure to kill bacteria.
1899	Bert H. Hite at the West Virginia Agricultural Experimental Station examined pressure effects on milk, meat, fruits and vegetables.
1914	P. W. Bridgman coagulated egg albumen under high pressure.
1990	First commercial products like fruit juices, jams, fruit toppings and tenderized meats introduced in Japan.
1995	Orange juice commercialized in France.
1997	Market introduction of guacamole in the US and sliced cooked ham in Spain.
1999	Oysters introduced in the US.
2000	Range of salsas launched in the US market.

In order to understand the uniqueness and potential of a high hydrostatic pressure process, consider the following:

Principle of Le Chatelier

Chemical reactions which result in a decrease in total volume (negative activation volume) are enhanced by pressure. Conversely, reactions resulting in an increased total volume (positive activation volume) are slowed down by pressure.

Isostatic rule

The hydrostatic pressure process is volume independent. Therefore pressure is instantaneous and uniform throughout the pressure vessel. Pressure gradients do not exist.

Adiabatic heating

Pressurization is accompanied by a uniform temperature increase. The adiabatic heating rate is specific for each chemical compound (water ~3ºC/100 MPa, fats and oils ~6 ºC-8 ºC-/100 MPa). The adiabatic heating is reversible upon pressure release.

Electrostriction

Pressure leads to increased ionization, because water molecules arrange more compact around electric charges. This results in more or less pronounced negative and reversible pH shifts dependant on the chemical nature of the buffer.

Compression energy

Energy input during the pressure process is very small compared to thermal processes. Therefore no chemical reactions involving covalent bonds are observed.

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