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Milk Processors Work on Making Pasteurization Cool

May 11, 2011
Nonthermal pulsed electric field technology gets a boost from Arla Foods, European 'Smartmilk' researchers.

Milk processors for decades have relied on heat pasteurization to kill pathogens and prevent foodborne illness. But increasing numbers of people seem to be saying they don't want to settle for the product degradation of high-temperature conventional pasteurization.

"Probably for its superior taste, raw milk has continued to be consumed by significant numbers of people in rural areas across Europe and Eastern Europe," notes literature from a Spanish advanced engineering and research provider, Iris (www.iris.cat). The company is coordinating a "Smartmilk" (www.smartmilk.eu) consortium backed by the European Commission and joined by engineering, consulting and dairy firms. In addition to small and mid-sized companies, multinational Arla Foods (www.arla.com) has joined the effort.

The two-year Smartmilk project, officially launched in October 2010, seeks to better preserve the taste and nutrition of milk while retaining the safety and shelf-life of pasteurized milk.

The effort follows in the footsteps of prior, successful research into nonthermal pulsed electric field (PEF) technology. The technology has already proven it can process milk at a significantly lower temperature while delivering a safe product that preserves the integrity of milk fat and proteins, enzymatic activity and other desirable attributes. In addition, the Smartmilk project seeks to deliver those results along with evidence that shelf life can match that of conventionally pasteurized milk.

Diversified Technologies' units start with a 25 kW, $250,000 lab-scale system with 300 l/hr. capacity (shown), but are said to be scalable to thousands of gallons of milk per hour.

PEF has been researched and applied on both sides of the Atlantic. "It's a proven technology," says Iris' Edurne Gaston, Smartmilk project coordinator. "It has already been used for fruit juices in processing, also for waste management and water purification." Since 2005 the method has had clearance from the U.S. FDA, and research continues to move forward.

Heat-pasteurization alternative

A PEF processing system has three basic components: a high-voltage power supply, a pulse modulator to switch the voltage on and off very – very, very – rapidly, and a treatment chamber where pulses are applied to the product flowing through pipes and the electrodes.

In a PEF milk pasteurization processing system, milk flows through narrowly spaced electrodes and is subjected to quick pulses to break open the cell walls of vegetative bacteria, mold and yeast. Process parameters such as electrode spacing, voltage and pulse modulation vary with the application.

"To size a system, you start with desired flow rate, size your treatment chamber accordingly and then develop the electrical specifications," says Mike Kempkes, an electrical engineer turned vice president of marketing for Diversified Technologies Inc. (www.divtecs.com), Bedford. Mass., a manufacturer of PEF systems.

A large, commercial fluid milk application, for instance, might require a treatment chamber diameter (and electrode spacing) of a centimeter and a half. The larger the span, the higher the voltage required to maintain the desired electric field, measured in volts per meter. Most food applications, Kempkes says, are in the 30-35 kV/cm range.

And those quick pulses? They're measured and controlled down to a few microseconds, and applied at high frequency to ensure treatment of the entire volume of product flowing through the treatment chamber. This can translate to anywhere from hundreds of pulses to several thousand per second.

Other Nonthermal Technologies

Pulsed Electric Field or PEF technology is one of a handful of promising non-thermal technologies to minimize or replace high-heat processes in dairy processing. It may better retain the taste and nutrition while maintaining a practical shelf life for the end product.

High-pressure or hydrostatic pressure processing has found broad success in processing vegetables, juices, meats and shellfish. Pulsed light treatment uses brief but intense pulses of broad-spectrum light (from UV to near IR to control microorganisms on the surface of ready-to-eat meats, produce, cheese, baked goods, poultry and fish. Oscillating magnetic fields also have been studied, as has ultrasound, which is under investigation as part of the Smartmilk project.

Carmen Moraru, associate professor at Cornell University, says even if such nonthermal technologies don't fully replace thermal processing, "their unique benefits should be seriously considered…. the US dairy industry could benefit from these technologies, particularly in the 'healthy,' high-value product category."

PEF can be used alone or in conjunction with other technologies. For example, one application has treated milk for three seconds at roughly 50 degrees Celsius (122 degrees Fahrenheit) to control quality while avoiding the alleged heat degradation of conventional pasteurization. Citing PEF's track record and potential in dairy foods, Carmen Moraru, associate professor at Cornell University, noted that in milk processing, thermally assisted PEF can improve safety, freshness, nutritional value and retention of proteins such as the lactoferrin and lactoglobulin.

Similarly, the Smartmilk project is taking a cue from prior research at University College Dublin, where researchers combined PEF and manothermosonication (MTS), which combines ultrasound, pressure and heat, but less of the latter than conventional pasteurization requires. That research reduced inactivation of L. innocua on a par with conventional pasteurization.

Gaston says the MTS "assist" will provide "synergistic effects, so we expect to have a situation where one plus one equals three."

strong>Pulsed-milk challenges

Milk processing presents a special challenge owing, for example, to its higher initial microbial load compared to high-acid juice. "While PEF kills yeasts and molds in their vegetative state, it does not kill spores because they do not have an active cell wall PEF can work on," says Kempkes.

While "cranking-up the power" can affect enzymes, it's the high energy and not the PEF specifically that can break them down.

That said, combination of PEF and MTS may be an effective treatment for spores, and something the Smartmilk project will test, according to Gaston.

Milk is, however, well suited to PEF in other ways. It is a continuously pump-able fluid with low electrical conductivity and low or no solids, which affect power levels and electrode spacing. PEF, like conventional pasteurization (barring ultrapasteurization), does not kill spores or enzymes. With refrigeration, growth of spores can be inhibited for a conventional shelf life, and the presence of enzymes in milk can be seen as a benefit.

The Smartmilk project follows a long line of PEF developments in the food industry that extends as far back as a 1920 "Electro-Pure" process.

The heart of the pulsed electric field system at the Ohio State University pilot plant.

Kraft, General Mills and other processors and technology companies joined forces in 2000 with The Ohio State University and the U.S. Army's Natick Laboratory to create a PEF pilot system that continues to inform ongoing research. DTI provided the project with a solid-state PEF pulse modulator. OSU developed the PEF treatment chamber and pulse generator, which DTI has since licensed for sale to its customers. The system successfully demonstrated pasteurization of juices, milk, yogurt, soups and liquid eggs.

The project was over by mid-decade; the equipment now resides at the Eastern Regional Research Center of the USDA Agricultural Research Service in Wyndmoor, Pa.

The Smartmilk project started with a laboratory-scale system. A larger scale prototype will be ready in spring, 2012, Gaston says. This pre-industrial-scale processing system is as far as the two-year project expects to take the technology before, researchers hope, a suitor comes calling for a full-scale system.

Earlier in the decade, OSU reported that across food applications, PEF would add "only $0.03-$0.07/L to final food costs," and that the technology is scalable and capable of processing between 1,000 and 5,000 liters of liquid foods per hour.

And, says Kempkes: "The largest system we have is sized at 10,000 liters per hour for activated sludge in a wastewater application." He says that system could be applied to "about 6,000 liters an hour of orange juice… with or without the pulp."

The cost of that system would be roughly $800,000, not including fluid handling and packaging before and after the process. Larger systems are possible, but the economics of operating larger systems versus multiple smaller systems remains to be determined.

Followers of nonthermal processing already know the process works. What remains is to prove its mainstream commercial viability, especially following the short-lived PEF success of Genesis Juice Cooperative.

Genesis sold unpasteurized juices, and was shuttered in 2004 following FDA's warnings of "serious deviations" in Juice Hazard Analysis Critical Control Point regulations. The company was missing HACCP plans and had no sanitation control records. Genesis successfully introduced a PEF treated line of FDA-compliant juices in 2005, but was unable to overcome the financial impact of the shutdown, and was subsequently sold to another company, without the PEF process.

On the upside, researchers continue to show PEF is a promising method for juice, milk and other applications across the food industry. There remains significant potential for this and other nonthermal alternatives for producing safe, practical alternatives that improve upon the current compromises of high-heat processing.

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