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Engineering R&D: Currents of change

April 10, 2003
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David Reznik conducts research on the physics of wave phenomena to reduce water and biological oxidation.

David Reznik, founder and CEO of Raztek Corp.
The time lag between new process development and commercial adoption usually is measured in decades, so it’s not surprising that only a handful of firms have deployed David Reznik’s electroheating technology since its introduction in 1991. Similar to resistive heating and the ohmic heating process developed by the UK’s Electricity Research and Development Centre and licensed to APV, electroheating applies an electrical current to food in a conduit, with the product itself providing the electrical resistance. Electroheating offers processors another short time, high temperature treatment to effectively pasteurize fluids without side effects such as coagulation or product scorching.

A chemical engineer with an advanced degree in food science and engineering, Reznik began his career in metal packaging and food processing in 1967, later forming his own R&D company in Israel before joining Tri Valley Growers. In 1982 he launched Tri-Valley’s Technical Research Center, which was spun off as Raztek Corp. five years later. Reznik serves as president and R&D director of the Sunnyvale, Calif.-based firm, which develops technology for food processing and can manufacturing. Electroheaters pass high-voltage electricity via graphite electrodes along the flow line of a fluid. Voltages of opposite polarity are applied to the first and second electrode assemblies to heat the product as it flows through the apparatus. The conduit is made of nonconducting, food grade materials.

Electroheating has been used commercially by Papetti’s Egg Products in New Jersey to extend the shelf life of liquid eggs without coagulation and, more recently, by Grupo Jumex SA de CV to create shelf-stable fresh orange juice.

With rapid vacuum cooling, incoming product serves as a condenser and is preheated to the median temperature between the hot and cold stream before entering the electroheater or heat exchanger. Treated hot product enters submerged coils in boiling water under a vacuum.
.FE: What advantage does electroheating provide?

Reznik: Heat damage is minimized because temperature control is more accurate, heating is more uniform, and no fouling or scorching of product occurs on the unit’s walls, which means lower maintenance and sanitation requirements. Fresher tasting UHT milk can be produced. Taste tests with electroheated milk show that U.S. consumers prefer it to commercial UHT milk by a margin of two to one.

FE: What products are suited to this process?

Reznik: It’s particularly useful in the treatment of proteinaceous fluids, such as liquid egg and blood, which tend to denature and coagulate when heated. When combined with rapid vacuum cooling, liquid egg can be heated to 75°C in a fraction of a second, then cooled to 65°C in one second, before coagulation can occur at 68°C. Nobody thought that was possible before.

Electroheating can be used to pasteurize beer to have a taste like draught beer. One company plans to use it to pasteurize wine as an alternative to sulfite and without adversely affecting the flavor. Grape juice and other delicate juices can be treated to inactivate enzymes without affecting the flavor, and milk sterilization without the burned-on flavor is possible

FE: What are the most significant technical refinements since you first began working with electroheating?

Reznik: Carbon electrodes had to be developed. If you use a metallic electrode, it dissolves by electrolysis. We solved that issue by using nonmetallic electrodes of pure carbon.

Arcing was another major problem. The introduction of a lot of power in an extremely short time results in arcing that can burn the product and the equipment. The solution was to develop equipment that can handle 10,000 volts at just a few amperes. A critical consideration is to have equipment designed for low current density but very high power. Very high-velocity flow produces uniform heating of large volumes of fluid in a very short time, and that allows us to sterilize food before damage occurs.

FE: Is anyone using it for particulates?

Reznik: We developed the system for liquid food with particles up to about one-quarter inch, and we’ve been experimenting with larger particles in the more difficult field of pumpable foods. Very uniform heating occurs with milk, but with particles, resistance is not uniform, and the electricity tends to go through the more conductive parts, such as the lean portion of meat as opposed to the fat. Each particle has to be studied individually to determine if the desired temperature was reached and if cooling occurred within desired parameters. Sudden expansion and other problems can occur with fast heating of large particles.

FE: How does performance differ from a conventional heat exchanger?

Reznik: With conventional technology, fouling occurs when processors attempt to pasteurize at high temperatures for a short time because the walls of the processing unit have to be at a higher temperature than the product. The temperature difference is magnified with rapid heating, which results in even more product damage. With our technology, the wall is heated by the product and not the other way around. The customer dictates the parameters, but we recommend extremely high temperatures and short holding times.

FE: What advantages are there to using this in conjunction with rapid vacuum cooling?

Reznik: With electroheating, we had to first understand the problems before we could develop practical solutions. With rapid cooling, we were able to take advantage of a significant existing body of knowledge about vacuum cooling. Vacuum cooling exploits the high rate of heat exchange during boiling, evaporation and condensation of a fluid in the absence of non-condensable gas. We get cooling rates close to flash cooling without exposing the product to the vacuum and therefore without the loss of the product’s water and volatile compounds.

Incoming product is cold and serves as a coolant in the condensing coil. The outgoing hot-product coil is submerged in water that is boiling in the vacuum chamber. The heat transfer coefficients are maximized during evaporation and condensation under a vacuum. There is no contact between the two coils, so there is no possibility of contamination by leakage from the cold line. If either tube leaked, product would go into the vacuum chamber.

Using one vacuum cooler, we can recover 50 percent of the energy used to cool products. With two vacuum coolers, we recover two-thirds of the energy. With three, 75 percent recovery is realized, and this preheating reduces the load on the electroheater. Rapid cooling could be applied independently, without electroheating. Instead of coils, a modified scraped-surface heat exchanger would be put inside the vacuum chamber to cool puddings, ice cream mixes and other viscous products. The vacuum cooler could be used as a vacuum heater because the final temperature of the product is virtually the same as the heating vapors inside the vacuum chamber.

FE: Have advances in other technologies helped improve electroheating?

Reznik: We have been able to leverage advances in sensors, electronics and controls. We have a very sensitive, fast-acting thermocouple that responds in 0.15 seconds. Its input translates to voltage corrections that keep temperatures within 0.1°C of the set point.

FE: How big is the electroheating unit?

Reznik: A commercial system consists of three tubes, each about 3 feet high and 4 inches in diameter. Normally the tubes are located in a transparent enclosure. Unit size is the same for a wide range of flow rates, with larger internal tube diameters accommodating higher flow rates.

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