Juming Tang, professor of food engineering, Washington State University, Pullman, WA. Source: Washington State University

Scientific proof of the consistency and reliability of new processing technologies has stopped cold more than one novel process with the potential to improve the taste and nutritional quality of food. When the FDA recently approved a microwave sterilization process for prepackaged mashed potatoes that will remain safe at ambient temperatures, researchers at Washington State University (WSU) achieved what several novel processes had failed to do: they validated to regulators that they could deliver a safe product with an unconventional process.

The WSU system was patented in 2006, and a 76-by-12 ft. pilot unit went into service soon after in the pilot plant at WSU’s Pullman, WA campus, processing salmon fillets and other low-acid foods. But developing a computer-vision method based on chemical-marker formation to determine the cold spot in trays being conveyed through the system required a combination of perseverance and good fortune. With October’s FDA approval, the project entered a new phase, as investment groups and equipment builders began exploring commercialization opportunities.

In the meantime, the WSU team led by Professor Juming Tang continues to optimize the design and prepare FDA filings for additional products. Tang has taught food engineering at WSU since 1995, following similar academic duties at South Dakota State University and Canada’s Acadia University. Since 2001, he has served as director of the Microwave Sterilization Research and Development Consortium, which includes Kraft, Hormel, Rexam Containers and the US Department of Defense. Tang earned an undergraduate degree in mechanical engineering at Central-South University of Technology in Hunan, China, before studying in Canada, receiving a master’s degree at University of Guelph and a PhD in agricultural engineering at the University of Saskatchewan.


FE: Describe the microwave system’s design.

Tang: We use single-mode microwave energy and concepts found in a conventional retort: recirculated water spray in a pressurized vessel and an environment where you raise the temperature close to 250˚F to get an effective kill of Clostridium botulinum.  In our design, trays move in single file through a tunnel, with windows in pairs on either side of interconnected cavities. A shallow bed of water with over-pressure of 25-35 psi circulates through the tunnel.

Our lab unit has four interconnected cavities, but a commercial tunnel could have any number needed to meet desired throughput. Unlike the multi-mode microwave units for home use, only one pattern of microwave is supported in each cavity. We use wavelengths of 915 MHz, much larger than the 2450 MHz waves in home-use ovens, and the tighter cavity results in a trajectory that is much less random.

This design helps overcome the unpredictable microwave field patterns that occur in multi-mode systems, which is a killer for FDA approval.


FE: Does the water help produce even heating?

Tang: The dielectric property of water is such that peak conversion of electric energy to heat energy occurs by rotation of polar water molecules at frequencies between 10-20,000 MHz. Our waves are 915 MHz, capturing the tail end of that range. However, as you increase the temperature of water, viscosity is reduced and the absorption peak shifts toward the high end of the range. The water that circulates through the tunnel is preheated to 250˚. As a result, the frequency response of the water molecule to the magnetic molecule is negligible.

Augmenting microwave energy with hot water also results in much more stable performance and the removal of edge heating. Think of the optical properties of light: as the wave moves from low density to high density, you have a lot of refraction and focusing of energy along the edges. That’s manifested in the burned edges of food in a home microwave. By passing the waves through water, we reduce edge heating.


FE: Years of research preceded FDA approval of your process. What was the biggest challenge?

Tang: There are very reliable models available to predict sterility with conventional heating, but when you add wave equations, you introduce significant variation. Development of a simulation model and taking it from a concept to a system and all the software modifications and methodologies to support it is a major challenge. Several years ago, a supercomputer would have been required for the calculations. Today, an upper-end desktop computer can do the job.

Outside our lab, scientists at the US Army Natick Soldier Center identified the chemical marker M-2 as a tool to evaluate heating patterns in a microwave. We used M-2 to monitor and predict the location of cold spots in a microwave-treated product. We start with solidified mass, then look for color change from the markers, slicing the mass into sections and transferring the blue and red areas to a 3D image. That lets us visualize the cold spots, which in turn allows us to measure temperature at the cold spot in a continuous system. After many, many runs under different conditions, we were able to predict where the cold spot shifts and ultimately determine the heating pattern in the microwave system, which is key.


FE: The predictive model was developed three years ago. What’s been the focus since then?

Tang: Designing and putting together a functional system and calibrating the instrumentation was a major engineering challenge. Many meetings involving the WSU team, consortium members and technical consultants from the Sea Food Association in Seattle were held to discuss the steps in preparing documentation for review by FDA. The FDA filing was in October 2008, after which we responded to questions raised and went through a second round of reviews. This is a novel process, so naturally there were many questions on the testing protocols and the system’s ability to produce a repeatable and predictable process. There also was the matter of identifying an acceptable surrogate that mimics the pathogenic Clostridium botulinum spore. PA 3679 was used to inoculate samples and prove the process delivered measurable power to ensure no survival.


FE: In the FDA evaluation, what type of container was used with the product?

Tang: Rexam supplied rigid trays with barrier film. It’s not much different than the containers used for commercially produced shelf-stable retorted products like Hormel’s Compleats.

Now that we’ve proved the process is stable, we simply have to confirm to the FDA that the cold spot is known as approval is sought for other products. That will allow us to focus more on the packaging side for both shelf-stable and pasteurized products. This will open a spectrum of products, including meats and pastas, which are sensitive to high-temperature processing and suffer quality degradation in a retort.


FE: Will the quality improvement be sufficient to justify the higher price that will have to be charged at retail?

Tang: Some sensory panels have been done at Natick, with very good results. But there has not been any systematic analysis of retention of vitamins and other nutrients. I’m not in a position to say if consumers will pay a high enough premium to justify the capital investment, but I trust the judgment of companies that have visited the WSU facility and are excited about the products they saw and tasted. Serious consideration is being made of the multi-million investment needed to design and build a commercial unit.

Safety is a given, and the technology is proven to deliver a safe product. Quality is the other dimension, and that is why this has such great commercial potential. If you use 200-year-old technology to retort salmon in a can for an hour, you cannot present a wholesome-looking product. With microwave, you can process salmon in eight minutes and end up with a visually appealing product.