COLUMBUS, Ohio -- Sudhir Sastry has studied ohmic heating for decades, but the process continually sparks his curiosity.
"We've been finding some things that are really interesting," Sastry said.
Apparently, others agree. Sastry's team recently won a $492,000 U.S. Department of Agriculture grant on using the process to control enzymatic activity in tomatoes.
Ohmic heating uses electricity to heat and process food in an unconventional way: The food is subjected directly to electric fields to be heated, much like an electric range's coiled element.
"You know how the element gets red hot?” said Sastry, professor of food, agricultural and environmental engineering, a researcher with the Ohio Agricultural Research and Development Center, and an internationally recognized expert in the fields of food processing and packaging. “Well, food that's ohmically heated does something similar."
The process provides a rapid, uniform heat source that, according to Sastry's research, kills or otherwise inactivates microorganisms, including bacterial spores, which are much more resistant than other pathogens. And, it keeps the products tasting fresh.
The process can be widely used, but is most effective for processing products that combine chunks of food with a liquid base, such as soups, stews, or fruit slices in syrup. It is not effective for oils or pure fats, which have zero conductivity, but it's ideal for heat-sensitive liquids. For example, in just a fraction of a second, liquid egg can be treated ohmically without coagulation.
Furthermore, the process works on multiple levels: The jolt of electricity appears to have an added impact over and above the effect of the heat produced.
"If you compare a sample heated conventionally and a sample heated ohmically at the exact same temperatures and time, your ohmically heated sample will have more inactivation of bacteria and bacterial spores," Sastry said. "There's something else going on. The electricity itself is doing something more."
In the current USDA project, Sastry's team is examining another aspect of ohmic heating -- its effect on enzymes.
Any kind of heat can inactivate enzymes. That's one reason why vegetables, for example, are blanched before being frozen -- it stops enzymes that would cause browning and other types of deterioration. But sometimes, it would be beneficial to activate a food's enzymes, Sastry said.
"One of my post-doctoral researchers was doing a study with enzymes, and he found that under certain frequencies, enzymes were actually more active," Sastry said. "At other frequencies, they were a little less active.
“And at really high frequencies, there was no effect at all. So there are a lot of interesting things going on with the chemistry of a food system based on the frequency that you apply."
The ability to control enzymatic activity, or being able to turn some enzymes on and others off at will, would allow companies to enhance the color, texture, flavor and nutritional value of the fruits and vegetables they process.
"If you process tomatoes for paste, for example, you're crushing the tomatoes and heating them at the same time," Sastry said. "The moment you crush tomatoes, enzymes become active.
“You want to inactivate some of those enzymes as quickly as possible because they may affect how thick the paste will be. Now, if you're making tomato juice, there's another enzyme at play. Ideally you want to boost the activity of that enzyme a little bit more before you finally inactivate it."
For the current project, which runs through August 2014, the team will examine the effect different frequencies have on different enzymes important in tomato processing.
"With ohmic heating, we're getting better at it and we're understanding more and more what it can do," Sastry said. "And the applications are pretty broad, so we may have a pretty broad-based industrial impact."