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Dehydration redux

October 1, 2009
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Applying microwave energy in a vacuum to dry or dehydrate foods has a checkered past, but a Canadian scientist believes he has solved the challenges.

A view of the commercial dehydration machine at CAL-SAN in British Columbia. Source: EnWave Corp.


A 1988 article in  about MIVAC could have been written 20 years later. Shorthand for microwave and vacuum drying, MIVAC promised to remove moisture from fruit without destroying nutrients or adversely affecting flavor and color. Pacific Gas & Electric and California State University-Fresno built the dehydration unit, and a few systems were sold to food processors. In the end, MIVAC wound up in the ashbin of food processing innovations.

MIVAC may not have panned out, but scientists in Vancouver, BC believe they can deliver on its promise with radiant energy vacuum (REV). The first commercial REV machine went into full-scale production last fall at Cal-San Enterprises Ltd., a Richmond, BC blueberry grower and processor that is leveraging REV’s lower energy requirements to compete with freeze-drying. The equipment was developed by EnWave Corp., which also is working with Danisco to create powder from liquid streams containing live cultures, and with Aridis Pharmaceuticals to use the technology to produce shelf-stable vaccines.

The co-CEO of EnWave and developer of REV is Timothy D. Durance, a food scientist and professor at the University of British Columbia. EnWave expects the global market for industrial freeze-drying equipment to balloon 26% in the next seven years to $2.19 billion. Durance’s hope-and the expectations of his investors-is that REV will capture a share of that spending, thanks to its speed and energy-conserving advantages over freeze-drying.

After receiving a BA in anthropology, Durance switched gears and earned a second undergraduate degree in microbiology before getting a job at a dairy company, where he worked in QC before getting involved in R&D at the company’s pilot plant. He returned to school to pursue MS and PhD degrees in food science from the University of British Columbia, where he accepted a faculty position in 1987. His academic coursework includes principles of food engineering and food process science.

Timothy D. Durance, co-CEO, EnWave Corp., Vancouver, BC

FE: When did you begin investigating microwave drying under vacuum?

 I was involved in overseeing an engineering project at the university in the mid-1990s, in which a student was drying potatoes with radio frequency, convection heating, freeze-drying and microwave, though not in combination with vacuum. That inspired me to shift from thermal processing to a focus on dehydration. With microwave, you almost get a straight drying curve: As fast as energy is delivered, moisture is removed without collapsing the cell structure. The problem is that the boiling temperature is so high, which suggests a vacuum to lower the temperature.

I began researching the literature and found lots of applications involving vacuum to dry, but was discouraged by the lack of commercial success for any of them. The food-science groundwork for those applications hadn’t been done.

FE: How does your design differ from earlier efforts?

Getting uniform heating in a large chamber is the difficulty. We basically agitate or stir the product in the microwave field. It’s the same principle as a turntable in a home microwave, though we’re working in three dimensions instead of two.

I started experimenting with home microwave units, but you have to get beyond that very quickly, and that requires collaboration. I teamed up with mechanical and electrical engineers to build prototype equipment, and that requires more money than the university grant system can provide. Fortunately, we started the intellectual-property patent process at the very beginning. Local businessmen saw the commercial potential and were willing to invest. We probably spent $10 million in R&D building the prototype systems.

FE: Did that create any tension in academia?

Alliances with commercial interests are still frowned on by some, though school administrators see the value. But if I didn’t have tenure, I wouldn’t have tried it.

Food science is conducive to these types of collaborations because the food industry wants real problems solved. That brings you down to reality and encourages interdisciplinary work. Our goal was to duplicate freeze-drying quality and do it at convective air-drying prices. Freeze-drying is great technology, but the cost is prohibitive for many products-herbs cost $50 a lb. to freeze-dry. If you can improve a process, the commercial prospects are great.

FE: What makes REV more cost effective than freeze-drying?

Overcoming the latent heat of dehydration in freeze-drying requires tremendous energy because of the multiple phase changes that occur. The water changes from ice to liquid to gas, and you’re still not done because you have to take out the vapor stream to prevent burnout of your vacuum pump. The energy cost is probably 10 times greater than with convective drying.

REV uses a conventional ring pump to pull a vacuum, which is another cost advantage. And processing speed is 50-100 times faster.  Condenser costs also are much lower than with freeze-drying.

FE: How did you go about proving the REV concept?

The initial experiments were done in collaboration with someone on the university’s forestry faculty who was using microwave to dry wood laminates. Next, I hired a post-doc student with a background in microwave and did more experimenting. Enzyme activity is a big problem with shrimp and krill, so we used microwave to pretreat those foods before subjecting them to air drying. Next, we made zero-fat potato chips, using a rotating basket in a field to replicate the effect that hot oil has. The chips crumpled into balls, and we recognized the need for pretreatment with air drying.

It was a major challenge to find the engineering expertise to design the system and then to fabricate a prototype. We worked with a physicist to develop instrumentation to control and measure power, define the microwave field, get good vacuum and monitor activity with video.

FE: Even before installing the REV unit, Cal-San Enterprises had invested $4 million into blueberry drying. How is the additional investment panning out?

The owner has targeted the dried fruit market for years, and the business case convinced him to make the additional investment. Besides the machine, there has to be a building to put it in, and Cal-San had a 30,000-sq.-ft. processing plant. The owner had one of the original MIVAC machines, built in the 1980s and long since mothballed. He also had a convection air dryer. That unit now is being used as a predryer to take out half the water before product goes through the REV unit.

Installation began in April 2008, with trials starting around November. We spent months working through redesign issues, mostly involving the baskets that convey product. They are cylindrically shaped, and the first ones had to be handmade. They had to handle product well, be easy to clean and be easy for the operator to open and close.

Properly speaking, it’s a semi-continuous process. There is a vacuum lock on either end of the basket, so there is a cycle, but from the operator’s point of view, it’s continuous. There are 22 baskets. One goes in and another comes out every two minutes. The process is very predictable and lends itself to automation.

FE: How does REV dehydration for pharmaceuticals and nutraceuticals differ from food dehydration?

For biologicals like vaccines and enzymes, we’re working with 10ml serum vials instead of baskets, and the material has to be dried to a powder. We’ve also developed hybrid systems using microwave and freeze-dryers, operating at temperatures of -25

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