High Pressure Processing Draws High Interest

High hydrostatic pressure processing (HPP) of foods has gone from a relative curiosity to a fairly widespread process in just a few years, according to Carole Tonello of NC Hyperbaric, Spain, who spoke at the IFT Nonthermal Processing Division Workshop held in October 2010, in Montreal. She said there were 156 HPP installations in the world, up from 11 in 1999. The applications include vegetables, especially avocados, ready-to-eat meats, and seafood, especially shucking oysters and extracting meat from lobsters and crabs.

Typical conditions are 600 MPa (about 87,000 psi) for up to 5 min. HPP does not impact spores or enzymes unless heat is added to supplement the adiabatic heat of compression, which can raise temperature about 20°C.

Other speakers discussed temperature distribution in HPP and two approaches to using high pressure in combination with heating—pressureassisted thermal sterilization (PATS) and temperature-assisted pressure sterilization (TAPS). Pasteurization, in contrast to sterilization, provides 5 log reduction in vegetative cells in about 1 min at 500–550 MPa for foods such as orange juice, according to Swamy Ramaswamy of McGill University.

Under high pressure, decimal reduction times are greatly reduced, but the usual principles of thermal processing still apply. However, heating due to compression can be quite fast (3°–5°C for every 100 MPa). Upon release of pressure, cooling is also fast, but cooling must be completed conventionally, which is then limited by internal heat transfer rates.

Phase Changes
High pressure can affect phase behavior of water and other materials, according to Alain Le Bail of ENITIAA, France. If solid specific volume of a material is less than the liquid specific volume, then increasing pressure increases the melting point. The reverse is also true: increasing pressure lowers the melting point if solid volume is greater than liquid volume. This happens to be true for water, which we all learned is why ice skating works and why ice floats on water. (The focused high pressure under a skate blade causes ice to melt and provide a thin film of water on which the skater glides.)

Applying this phenomenon to food, foods are pressurized to about 200 MPa, cooled, and when pressure is released, about 30% of the water turns instantly to ice, forming very small crystals.

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One challenge is that ice formation releases the heat of fusion, which raises the temperature of the food because the heat is slow to diffuse by conduction from inside the food to the cold surroundings. Denys et al., writing in Hendrickx and Knorr (2001), present several numerical models for the heat transfer processes in high pressure freezing and thawing. A distinction is made between pressure-shift freezing and thawing and pressure-assisted freezing and thawing. In addition, some foods benefit from storage under high pressure at low temperatures without freezing.

In normal freezing at ambient pressure, water is first super cooled, and then nuclei form, initiating ice crystal formation. The rate of freezing depends on the heat removal rate, which is by conduction within the food, and progresses from the outside in. External heat transfer is maximized by high velocities, in the case of air blast freezers, and by using the lowest available external temperatures, as in cryogenic freezing, with liquid nitrogen or carbon dioxide. Ultimately, the rate of freezing depends on heat conduction through the frozen layer, which increases with time.

In pressure-shift freezing, the food is placed under high pressure and then cooled to well below its normal freezing point. (Most foods have freezing points below 0°C because dissolved solids lower the freezing point of pure water.) Up to 20°C super cooling may be obtained, in contrast to just a few degrees under atmospheric pressure. When the pressure is released, ice spontaneously forms throughout the food as very fine crystals. In normal freezing, ice crystals are typically large, formed as water diffuses out from cells. Large crystals degrade food texture and lead to drip loss on thawing.

Practical applications of pressure-shift freezing typically result in improved texture compared with normal freezing. However, the benefits can be lost if subsequent storage conditions are not optimized. An approach to optimizing equipment is to remove products from the high pressure chamber once the pressure is released and complete freezing elsewhere.

The high pressure equipment must be carefully designed for low temperature operation. For example, the pressure transmitting fluid, typically water in other applications, cannot be a fluid that freezes, therefore not pure water. Glycol/water, alcohol/water, or edible oils are candidates. Packaging materials must be chosen to be compatible with such fluids.

Seals and metal used to construct the pressure vessel must be selected for the low temperatures, which can approach -40°C. Normal steels become brittle at such temperatures, as do many of the flexible materials used for seals. Pressure vessels used for HPP are subject to fatigue from cycling between high and low temperatures. Such fatigue typically limits the useful life of such vessels to a few years, contributing significantly to the cost of HPP. At low temperatures, the costs increase due to the special requirements, so the benefits need to be significant.

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Thawing Issues
Some textural damage occurs during normal thawing of frozen foods, but most damage occurs during freezing and storage. On the other hand, thawing takes time, and microbial growth can occur as parts of a frozen food thaw while others remain frozen. Applying high pressure can quickly convert ice to liquid water. There are competing thermal effects. Compression can raise the temperature, as previously mentioned. However, thawing requires supply of latent heat, so the temperature of the food may actually drop until enough heat can be supplied.

Provided heat can be supplied, pressure thawing can significantly reduce the time to uniformly thaw a food. Upon release of pressure, recrystallization may occur. There can be negative effects, such as whitening of meats, caused by denaturation of proteins under high pressure. Starch viscosity is changed under pressure, and lipids may crystallize.

Pressure thawing produces higher quality fish than thawing under running water, and reduces drip loss in strawberries. Equipment for thawing is similar to that used in HPP pasteurization and sterilization because of the higher temperatures.

Pressure-assisted freezing and thawing refer to phase transitions under constant pressure rather than by changing pressure. There do not appear to be significant benefits from such approaches, so they are of more theoretical than practical interest.

High Pressure, Low Temperature Storage
Foods can be stored at low temperatures under high pressure without phase transition. This avoids the negative effects of freezing and requires removal of much less energy because there is no heat of fusion to be removed. On the other hand, enzymes that are typically inactivated by freezing are still active even at low temperatures and may limit storage times for foods where enzymatic activity can be detrimental.

The fact that some microorganisms and enzymes are not inactivated by low temperatures can be beneficial when these need to be preserved, while freezing might damage them. Drip loss in pork was reduced by such storage, compared with freezing. Some harmful microbes are reduced by unfrozen storage at low temperature under pressure more than by normal freezing. Strawberries and tomatoes maintained fresh taste, texture, and color after such storage.

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The practicality of such storage may be questionable at present because most high pressure vessels are relatively small. The largest equipment from NC Hyperbaric, for instance, holds 600 L in two vessels of 300 mm diameter. The company claims throughput for 3-min cycles at 600 MPa of 2,000 kg/hr, assuming 50% fill factor. It would be difficult to justify using such equipment for storage, so obviously some other approach is required to take advantage of the observed benefits.

NC Hyperbaric is one of several suppliers of HPP equipment. Others include Avure of Kent, Wash; Elmhurst Research, Albany, New York; and BaoTou KeFa High Pressure Technology Co. Ltd. of Baotou, China.

Issues in equipment design include vertical or horizontal orientation, pressure transmitting fluid, metallurgy, and degree of automation. Horizontal orientation makes automated loading and unloading easier but occupies more floor space. Heating and cooling the vessel and contents is another challenge, as is reliable instrumentation, especially for research. In commercial practice, HPP for low acid foods must demonstrate that the food reaches temperatures that are known to kill spores (121°C) for sufficient time. Because the technology is still relatively new, there is some uncertainty about obtaining approval for low acid, shelf-stable foods. Consequently, most current applications are for refrigerated foods, such as guacamole and ready-to-eat meats or for specialized uses, such as shucking oysters.

J. Peter Clark,
Contributing Editor,
Consultant to the Process Industries, Oak Park, Ill. 
[email protected]

References