Machine Design

Fresher with Pressure

...there's another method for preserving foods that better retains their natural taste and nutritional value: ultrahigh pressure (UHP).

In batch mode, prepackaged products are placed into a vessel and pressurized.

Automated loading in a batch system speeds handling.

The isolator system for in-line production pumps products into and out of the pressure vessel.

Isolator-based in-line pressure production system.

Chemical preservatives, irradiation, and heating are common ways to prevent raw or minimally processed foods from spoiling. But heating, for instance, can also destroy nutrients and vitamins. The good news is, there's another method for preserving foods that better retains their natural taste and nutritional value: ultrahigh pressure (UHP). Exposing foods to hydrostatic pressures of 60,000 psi and higher, destroys pathogenic microorganisms such as salmonella and E-coli, while leaving beneficial nutrients intact.

For example, fresh juice brought to 80,000 psi for 30 sec, shows a 5-log reduction in pathogens. It also retains more vitamins and has a longer shelf life than heat-processed juice. Similar pressure levels kill vibrio bacteria in raw oysters without impacting the product's texture or taste.

There are two types of equipment used for hydrostatic food treatment; batch and in-line. In batch mode, prepackaged, solid, or liquid products are placed in a vessel and pressurized. Containers must have one elastic surface, however. The process can be automated to speed handling.

The in-line arrangement is for pumpable products only. Here, a pump transports product into and out of a pressure vessel (isolator) through valves in the vessel top. The food is separated from the pressure source by a floating piston. During pressurization, the product's temperature increases slightly from compression (adiabatic) heating, but that rise is immediately reversed upon depressurization. And, because product isn't discharged under pressure, exit and entrance temperatures are the same. Packaging choices are basically unlimited because filling is done after UHP exposure.

Best of all, the amount of energy needed to process foods with UHP is comparable to conventional methods. The low compressibility of water-based foods means little energy is stored and wasted. Other products preserved by UHP include fresh jams and jellies, sliced ham, and guacamole. These items are already marketed in Europe, Japan, and North America, and more in development. UHP can also firm the texture of foods high in starch and protein. For example, yogurt exposed to UHP forms better gels and separates less, and cheese products retain more moisture and have improved yields.

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