Frequently Asked Questions

Why high pressure processing?

High Pressure Processing (HPP) is a method of food processing wherein the food is subjected to elevated pressures (pressures up to 87,000 pounds per square inch or approximately 6000 atmospheres) with or without the addition of heat to achieve microbial inactivation or to alter the food attributes in order to achieve consumer-desired qualities. Pressure is effective in inactivating most of the vegetative bacteria, yeast, mold, virus. Pressure treatment retains food quality, maintains natural freshness, and extends microbiological shelf life. HPP can be used to process both liquid and solid (water-containing) foods. The process is also called as high hydrostatic pressure processing (HHP) and ultra high-pressure processing (UHP) in the literature.

How does this technology benefit consumers?

High pressure processing causes minimal changes in the ‘fresh’ characteristics of foods by eliminating thermal degradation. Compared to thermal processing, HPP results in foods with fresher taste, better appearance, texture and nutrition. High pressure processing can be conducted at ambient or refrigerated temperatures, thereby eliminating thermally induced cooked off-flavors. The technology is especially beneficial for heat sensitive products.

How does HPP work?

Most processed foods today are heat processed to kill bacteria. Heat oftentimes diminishes the quality of a product. High pressure processing provides an alternative means of killing bacteria which can cause spoilage or food-borne disease without a loss of sensory quality or nutrients. In a typical HPP process, the product is packaged in a flexible container (pouch, plastic bottle or semi-rigid container) and is loaded into a high pressure chamber filled with a pressure transmitting (hydraulic) fluid. The hydraulic fluid in the chamber is pressurized with a pump and this pressure is transmitted through the package into the food itself. Pressure is applied for a specific time, usually 3-5 minutes. The processed product is then removed and stored/distributed in the conventional manner. Because of the uniform manner in which the pressure is transmitted (in all directions simultaneously), food retains its shape, even at extreme pressures. And because no heat is needed, the sensory characteristics of the food are retained without compromising microbial safety.

Can HPP be used for processing of all foods?

Like any other processing method, HPP cannot be universally applied for processing all types of high moisture content foods. The treatment can be used for both liquid and solid products. At the moment, HPP is being used in the United States, Europe, South America, and Asia for variety of high value foods either to extend shelf-life or to improve food safety. Some products that are being commercially produced using HPP are cooked ready-to-eat meats, salads, avocado products (guacamole), tomato salsa, applesauce, variety of juices and seafoods.

What type of products high pressure processing cannot be used?

High pressure treatment requires that food must contain water and not have internal air pockets. Food materials containing entrapped air such as strawberries, marshmallows, leafy vegetables would be crushed under high pressure treatment.  Similarly, the pressure treatment may not work very well for dry solids or powders. Pressure treatment may compact ( form cake) the products that do not have sufficient moisture. Further due to reduced water activity, pressure treatment may be ineffective on such products for microbial destruction.

Will the process damage the food product?

During HPP processing, pressure is uniformly applied around and throughout the food product. For example, a grape placed between fingers can be easily squeezed and broken; this is because the pressure is not applied evenly from all sides simultaneously. On the other hand, if the same grape is squeezed from all sides simultaneously, it will not be crushed. This can be demonstrated by placing a grape inside soda bottle filled with water. By squeezing the bottle, you pressurize the water inside as well as the grape. Yet the grape is not damaged, no matter how hard you squeeze. In the same way, foods processed by high pressure will not be damaged by the applied pressure.

What is the shelf life of HPP processed product?

Pressure pasteurization kills vegetative bacteria and, unless the product is acidic, it requires refrigerated storage. For foods where thermal pasteurization is not an option (due to flavor, texture or color changes) HPP can extend the shelf-life by 2-3 fold over a non-pasteurized counterpart, and improve food safety. As commercial products are developed, shelf life can be established based on microbiological and sensory testing.

What are the pressure pasteurized products I can find retail stores?

High pressure processed products are commercially available in 30 different countries around the world including United States, European, Australia and Asian retail markets. Examples of high-pressure processed products commercially available in the US include fruit smoothies, guacamole, ready meals with meat & vegetables, oysters, salad dressing, hummus, ham, chicken strips, fruit juices, and salsa.

Can pressure treatment be used for preserving shelf stable products?

Pressure treatment can be combined with heat to inactivate harmful bacterial spores present low-acid foods and make them shelf-stable. The process is called pressure-assisted thermal processing (PATP). During 2009 and 2015, FDA issued no objection to two industrial petitions for preserving low-acid foods (mashed potato and seafood) by PATP.  At the moment PATP treated low acid shelf-stable products are not commercially available yet.

What functional effect does the HPP bring up in the food product?

It has been generally known that high pressure has very little effect on low molecular weight compounds such as flavor compounds, vitamins and pigments compared to thermal processes. Accordingly, the quality of HPP pasteurized food is very similar to that of fresh food products and the quality degradation is influenced more by subsequent storage and distribution rather than the pressure treatment. Pressure also provides unique opportunity to create and control novel food textures in protein based foods. In some cases, pressure can be used to form protein gels and increase viscosity without using heat.

How are pressure pasteurized foods stored?

HPP products currently marketed worldwide are primarily distributed refrigerated. In some cases, this is necessary for safety (to prevent the growth of spores in low acid foods). For acid foods, refrigeration is not a necessity for microbial stability, but is employed to preserve flavor quality for extended periods of time.

Is commercial scale equipment available?

Yes. Avure Technologies, Middle Town, Ohio, Hiperbaric, Burgos, Spain, Multivac, Wolfertschwenden, Germany are some the major equipment vendors sell commercial size (up to 525-liter capacity) batch HPP equipment.

Is HPP equipment safe to operate?

High pressure equipment design is a mature technology and has its origin in chemical process industry. In the USA, high-pressure vessels are manufactured under guidelines established by the American Society of Mechanical Engineers (ASME) boiler and pressure vessel codes (ASME Section VIII, Div. 3 pressure vessel codes). Similar regulations (e.g., EU Directives and CE Mark in Europe) are available in different countries to ensure safety of the pressure equipment.

Processors should also ensure that the vessels are manufactured, installed, tested and operated according to relevant state regulations. It is worthwhile to ensure the hygenic design of the pressure equipment. With little training, food plant personnel can learn to safely operate the equipment.

How economical is HPP processing?

A commercial scale high pressure vessel costs approximately between $600,000 to $4 million dollars depending upon the equipment capacity, and the extent of automation. As a new processing technology, pressure processed products may cost 3-10 cents per pound more to produce than thermally processed products. With a 525-liter HPP unit operating under typical food processing conditions, a throughput of approximately 63 million pounds per year is achievable. High throughput may be also accomplished with smaller size machines by using multiple pressure vessels.  As demand for HPP equipment grows, capital cost and operating cost will continue to decrease. Consumers benefit from the increased shelf life, quality, and availability of value-added and new types of foods (which are otherwise not possible to make using thermal processing methods).

What regulatory approval is required for commercializing HPP processed product?

HPP does not present any unique issues for food processors as regards to regulatory matters. The requirements are similar to traditional thermal pasteurization in the United States, where the Food and Drug Administration (FDA) and Department of Agriculture (USDA) are responsible for evaluating and monitoring the safety of HPP processed foods.

Can I work with OSU to evaluate high pressure processing, before commercial operation?

Yes, OSU researchers can assist food processors in conducting confidential product evaluations for food safety, quality and shelf-life, and to obtain guidance on product development. The resources at OSU can be accessed for a nominal fee. Contact Dr. Bala if there is an interest.