Novel Nonthermal Technologies

Two intriguing technologies described at the October 2010 workshop of the IFT Nonthermal Processing Division in Montreal are cold plasma and pulsed electric field (PEF) cooking. As it happens, OMVE Netherlands B.V., Schalkwijk, the Netherlands (www.omve.com), offers small-scale equipment for both technologies, as well as laboratory and pilot plant equipment for other purposes, such as beverage carbonation, homogenization, and more. Some other nonthermal technologies deserving mention include chlorine dioxide, pulsed light, ultraviolet (UV) pasteurization, and high pressure carbon dioxide.

Cold Plasma
Hennie Mastwijk ([email protected]), Wageningen University, the Netherlands, described cold plasma as the same physical phenomenon as the aurora borealis or northern lights—ionized molecules of inert gas, such as nitrogen, argon, or helium. The sensible temperature is about 40°C, but the electron temperature can be tens of thousands of degrees. The ionized molecules are very reactive and can kill microbes on surfaces without damaging most surfaces. Thus, a cold plasma could conceivably reduce the biological burden on fresh produce. In other applications, a cold plasma can modify polymer surfaces, making an otherwise hydrophobic surface wettable. In disinfection applications, 2–3 log reductions at the surface are seen. There is no need for water, no residue, and the process is performed at a low temperature. Application of cold plasma to meat dries the surface.

Application times are currently minutes, with research directed at reducing these to seconds. The mechanism is not fully understood, although it is not thought to involve ultraviolet radiation, and there is a need to scale up to make the process practical. OMVE’s equipment operates at about 300 L/hr of gas with a copper electrode. Power consumption is about 1 watt. The plasma “flame” can be three meters long, and gases can be nitrogen, helium, or air. Instruments are provided to regulate gas flow, measure electron temperature, and control the time the generator is on.

The hope is that scale-up will reduce gas consumption and reduce decimal reduction times. Conditions for various applications need to be standardized, and, while there is no residue, toxicological studies still need to be performed on foods disinfected with cold plasma before wide application of the technology is permitted.

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PEF Cooking
Huub Lelieveld ([email protected]), representing the European Federation of Food Science and Technology (EFFoST), described what he considers a revolution in food cooking and preparation using pulsed electric fields. PEF has been used to pasteurize fruit juices and to treat contaminated water. PEF involves applying short bursts of electric power between narrowly spaced electrodes. The electric field is typically 30,000 volts/cm and pulses are thousands per second. Treatment times are seconds. To prevent erosion of the electrodes, the field direction is alternated. The pulses are typically square waves, though sinusoidal waves have also been studied. (Square waves require more expensive signal generators, similar to those used in military radar, as compared with radio-like generators for sinusoidal pulses. Square waves deliver peak power longer than sinusoidal waves.)

PEF kills microbes by perforating their cell membranes, permitting cellular contents to leak out. The same phenomenon improves extraction of juice from fruit and of sugar from sugar beets. Applied to sewage sludge, PEF improves clarification and increases the solids content by killing the microorganisms that bind water.

Developments in PEF have involved electrode design, in which clearances are typically small to reduce power requirements, but which then restrict flow rates and optimization of pulse rate and shape. Ohio State University has patented a co-current electrode design and built several small units, one of which is used in research at the Eastern Regional Research Center of the U.S. Dept. of Agriculture.

Lelieveld observed that cooking was a great breakthrough in the progress of civilization, as early man probably spent many hours chewing raw food to make it digestible. He observed that PEF had analogous effects on food as does cooking—the softening of tissues and the release of nutrients. This led to the development of Nutri-Pulse, a novel food preparation device offered by IXL Nederland BV, Schalkwijk, the Netherlands, a partner company of OMVE.

The full potential of this technology remains to be developed. The electric field is about 20,000 V/cm, somewhat lower than that used in flowing systems for fruit juice, and 3,000 to 6,000 pulses are applied. Temperature change is quite low: 46–69°C. Flavors and textures are unique. Both solids and liquids can be treated. Solids need to be immersed in a liquid. The commercially available unit has a working volume of 60 ml, but other sizes may be possible. Meats, fish, and vegetables have been prepared.

This application of PEF radically changes the concept of cooking. Raw foods are made edible, but acceptance may require some adjustment to familiar expectations.

Chlorine Dioxide
Chris Doona ([email protected]), U. S. Army Natick Soldier Research Development and Engineering Center, Natick, Mass., described a portable chemical sterilizer using a novel generator of chlorine dioxide that can fit into a small suitcase. The need arose from the fact that solutions of sodium hypochlorite (bleach) are hazardous and difficult to transport. The Army uses bleach or chlorine dioxide to disinfect possibly contaminated fruits and vegetables, as well as other items. Because of pending patent applications, Doona could not describe the chemistry in detail, but it may offer advantages over current methods of generating chlorine dioxide.

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Pulsed Light
Carmen Moraru ([email protected]), Cornell University, Ithaca, N.Y., described research on pulsed light for disinfection of surfaces. The technology has been approved by the U.S. Food and Drug Administration since 1996 for cumulative doses less than 12 joule/cm². The typical approach is a xenon lamp discharge of 1 microsecond to 0.1 sec in the wavelength range of 200–1,100 nm. This is mostly ultraviolet radiation. So far, no acquired resistance has been observed. Kinetics of microbial destruction fit a Weibull distribution better than first order. There has been a wide range of reported microbial reductions. One challenge is identifying suitable surrogate microorganisms. Factors that influence effectiveness include shading, distance of the surface from the source, and penetration of the radiation. Liquids need to be in a thin and turbulent layer to insure adequate treatment. Porous surfaces are hard to treat, as are those that reflect light, such as aluminum.

Pulsed light might be used synergistically with biocides. There is only slight heating, if any, and little effect on quality. A xenon pulsed light has no mercury, as do conventional UV lamps, and is on for shorter periods than a continuous UV source. Moraru pointed out that even a 1 log reduction in microbial burden can extend shelf life of some materials.

Ultraviolet Pasteurization
Tatiana Koutchma ([email protected]), Alberta Agriculture Food Center, described conventional UV pasteurization. This technology has been used since 1930 for liquid foods and beverages, including water. The typical sources are low pressure mercury lamps, medium pressure mercury lamps, and low pressure amalgam lamps. There are also excimers and pulsed lamps emitting in the range of 100–400 nm wavelengths. The intensity varies from 0.001–30,000 W/cm², depending on electrical efficiency and temperature. UV radiation reduces microbes, viruses, parasites, and toxins.

Additional uses include synthesizing vitamin D in milk (the discovery that originally funded the Wisconsin Alumni Research Foundation) and synthesizing vitamin D-2 in mushrooms. UV also reduces patulin. Koutchma said that existing UV reactors are not very effective and can deliver an overdose, impacting quality. Dosing of UV is affected by temperature, pH suspended particles, and viscosity. A Taylor-Couette concentric cylinders reactor has been developed and is used to test various UV sources.

High Pressure Carbon Dioxide
Liao Xiaojun ([email protected]), China Agricultural University, discussed the effect of high pressure carbon dioxide (CO₂) on various enzymes. Pressures are typically under 50 MPa (about 500 atm). The enzymes considered include pectin methyl esterase, polyphenyl oxidase, lipoxidase, and peroxidase. After exposure times of 15–300 min, and temperatures of 30–65°C, residual activity was less than 10%. Xiaojun claimed that enzyme particles aggregated and that secondary structures were changed. Various mechanisms were offered, including lower pH, complexes with CO₂, decomposition of subunits, hydrophobic interactions resulting in removal of water from the molecule, and the effect of depressurization at gas/liquid interfaces and from gas expansion. High pressure carbon dioxide has been studied in the United States, primarily at the University of Florida. One drawback that has been noted is the loss of volatile flavors upon release of pressure and the escape of dissolved CO₂.

There are many possible nonthermal processing technologies, none of which, so far, have proven ideal. The goal of preserving foods safely while retaining the highest quality remains elusive. A particular challenge is sharing information among competing researchers while helping regulatory authorities understand the emerging technologies sufficiently to approve some of them with confidence. Conferences like the Nonthermal Processing Division workshop contribute in this effort by building a community of experts who can share their learning and insights. The next workshop is scheduled for October 12–14, 2011, in Osnabrueck, Germany ([email protected]).

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