Barry Swanson, Ph.D.

Barry Swanson, Ph.D.

Emerging Non-Thermal Food Processing Technologies

Dr. Barry Swanson, who received his Ph.D. in Food Sciences from the University of Wisconsin, has been a professor in the Food Science and Human Nutrition Department of Washington State University for 27 years. Swanson’s research specialty is the validation of non-thermal food processing technologies—in particular, systems using pulse electric fields and hydrostatic pressure. It is a field of food processing technology that promises to revolutionize the industry as we enter the new Millennium.

FoodTechSource: Why is there suddenly so much interest in non-thermal technologies?

Barry Swanson, Ph.D.: It’s the equipment. We’ve been researching the technologies for a number of years. But the recent advances are due to the changes that have taken place in the defense industry technologies that have greatly improved the equipment. For example, there have major advances in switching systems—the ability to retain really high pressures—which wasn’t available years ago. The advances have really made a difference. And of course there are a lot of companies now that are interested in adapting the equipment to the food industry.

FTS: Why non-thermal? What is the advantage?

Swanson: Unlike thermal processing, which has a tendency to deteriorate the quality—the flavor, color, and texture of the product—with non-thermal methods we don’t alter the nutrient content or the color or the texture. We’re doing a lot less damage, so we have something that’s much more natural or fresh-like. But of course some people see that as a disadvantage.

FTS: Why?

Swanson: When we treat milk, for example, we end up with a milk product that doesn’t have the cooked flavor the American consumer is used to. We have a product that is much closer to raw milk. When we treat juices we have a product which is much closer to fresh-squeezed. People who like those qualities or flavors see it as an advantage; others do not.

FTS: What is the predominant non-thermal process?

Swanson: Probably pulse electric fields. Here we’re switching the flow of current in very high intensity electric fields. We’re talking between 10 and 100 kilovolts per centimeter. Initially most of those work was done with batch systems—as is generally the case—now the potential is there for continuous flow applications. There’s no potential for doing particulates and we have a bit of a problem deactivating some enzymes and some spores, which cause post-process problems and spoilage; so right now I’d say the pulsed electric field niche is with acid-based fluids, meaning fruit juices.

FTS: Isn’t it also being used to kill microorganisms in egg products?

Swanson: There is some interest in the treatment of eggs; in fact pulse electric fields technology was used by one company in New Jersey, and the FDA issued a letter of no objection to that company to allow them to use pulse electric fields as long as they followed up with additional processes to prohibit spores from germinating. I don’t know how successful they’ve been. But there also has been some interest in using it to pasteurize milk, although there is some question regarding consumer acceptance because of the taste.

FTS: Is there that much difference?

Swanson: I personally don’t think it makes a great deal of difference, but some people don’t like it.

FTS: How does pulse electric field processing work?

Swanson: Well, it’s a very high intensity varying electric field, through which the food passes. The temperature increase is minimal, about three degrees celsius per ten kilovolts. The microorganisms are deactivated via electro-permealization of the membrane—membrane destruction.

FTS: It explodes?

Swanson: It basically just breaks open the membranes of the cell. And we have been able to demonstrate with most microorganisms anywhere from a five- to nine-log reduction. So it is an adequate pasteurization technique. Of course what is adequate and what is safe is always a question, depending on initial load and things of that sort.

“Unlike thermal
processing, which
can deteriorate
the quality of
the product,
non-thermal methods
don’t alter the
nutrient content
or the color
or the texture.”

FTS: Is it as effective as heat?

Swanson: I think so. Pulse electric fields has proven to be as uniform as heat with specific products. But a lot of the work needs to be done to validate the process and petition the FDA.

FTS: Isn’t it more cost effective than heat?

Swanson: It depends on the product. For fruit juices, pulse electric fields use about 20% of the energy that you would find in a thermal system, so it is cost effective there. But not with milk. Ordinarily the pasteurization of milk using a thermal unit is 80 kilojoules per liter. Unfortunately right now, for pulse electric fields, we haven’t designed a regeneration system—just a straight forward system—and for that you’re talking 200 kilojoules per liter.

FTS: What is the difference between pulse electric field and pulse light processing?

Swanson: Pulse light is an irradiation system rather than a penetration system. It is primarily for surface kill—inactivation of microorganisms on surfaces, or if you are killing microorganisms in a liquid, you would have to run it through a very thin film. It’s your UV wavelengths that are giving you the kill. I don’t believe it’s as effective as pulse electric field, but like any other technology it has its place. I think it does a nice job, for example, reducing mold and fungi count on bread surfaces, that sorts of thing, reducing vegetative cells on packaging materials, etc.

FTS: Fruits?

Swanson: It probably would serve to do a pretty good job on the outsides of fruits, although I’m not sure it would do a whole lot better than water and scrubbing. There are a lot of microorganisms hanging onto the epidermal layer of fruits and vegetables and they’re removed a lot better if you scrub a little bit.

FTS: You might have problems with shadow areas as well.

Swanson: Right. and having a complete cycle of lights surrounding a product would not be very easy. As long as you’re dealing with a two-dimensional system you’re in pretty good shape. As soon as go to three dimensions you run into problems.

FTS: What is oscillating magnetic field technology?

Swanson: Actually it’s very similar to pulse electric fields but instead of using an electric field, which is a charged based system in which you are actually creating a change in potential across the membrane, oscillating magnetic fields changes the magnetic poles which are as necessary for many enzymes in metabolic systems for microorganisms as they are with people. It’s like the issue of high voltage power lines, where people say animals that are kept near them get confused and don’t reproduce and have all kinds of problems. What we do is generate high intensity magnetic fields and then we place microorganisms in those fields. There have been a number of research publications over the past ten years that have indicated that oscillating magnetic fields with a high enough intensity do indeed inactivate microorganisms and that they do reduce the growth of tumor cells. The theory behind it is that if it disrupts the metabolism enough it will halt the growth of the microorganisms. Does it work? We have no evidence to show that it is a plausible means of pasteurization or food preservative.

FTS: What about hydrostatic pressure? How does it work?

Swanson: Here you’re talking about subjecting foodstuffs to hydrostatic pressures around of 100,000 psi—which is substantial considering that atmospheric pressure is 15 psi. The technology was primarily introduced recently because we have equipment now that can withstand those pressures consistently and repeatedly. The system works by primarily inactivating microorganisms by imploding them—altering their membrane structure. We have shown pretty clearly that those membranes internal to the cell are destroyed more rapidly than the cell membranes themselves. But if you add enough pressure even the cell membranes are destroyed. Again, as we reduce the Ph of the food system, the organisms become more susceptible to high pressure. And if high pressure is used in conjunction with a little bit of heat it makes the organism much more susceptible, as does the addition of certain chemicals that sensitize the membranes.

FTS: This is for foods that are liquid in nature?

“Pulse light
is an irradiation
system rather
than a
penetration system.
It is primarily
for surface kill via
UV wavelengths.”

Swanson: It’s primarily fluid foods at this point in time, although high pressure is basically considered to be a mass and time independent process, which means you can use any size particulate as long as you have it uniformly pressurized. The biggest misconception people have is that when you think of high pressure you think of something being smashed because you only think in terms of two dimensions. Actually the pressure applied through a fluid is applied in a three-dimensional fashion, which means you don’t ordinarily have a change in structure unless you have a lot of air spaces. And then you have a reduction. For example, if you take a styrofoam cup and place it in a pressure unit you come out with a styrofoam thimble: it basically crunches it down to a smaller size.

FTS: You couldn’t use it for a peach, for instance, could you?

Swanson: Well, we use it for peaches and and for apples. Of course, we get a lot of browning after we take the product out of the high pressure unit. What it really does is just reduce the volume of the item. The company that is doing the most in this field is Flow International. They are designing not only a batch high pressure unit but also a sequential or continuous high pressure system. And there is a lot of interest in using that for acid products—primarily fruit juices. But also for eggs and low-acid foods, again in conjunction with some other method to protect against any spores, because high pressure systems don’t do a very good job of inactivating spores, and some enzymes may not be inhibited.

FTS: How large an item could you process?

Swanson: Again, anything that would withstand the pressure. Meat products would be no problem at all. In a batch system the problem is the capital expense. It takes it about five to ten minutes to bring it up to pressure and back down. So you have about a ten minute process with a batch operation. That can be costly.

FTS: Are there any products being processed this way right now?

Swanson: There is a guacamole or avocado puree that is high pressure processed and on the market in this country. Japan does a lot of processing of jams and jellies and acid products—dessert pastes and those sorts of things. In our research we have whey proteins to make a product very similar to cheese. So there are some innovative techniques that may be used to do some product development work.

FTS: What about the cost?

Swanson: That depends. There’s a lot of interest in doing some of this with seafoods like oysters, crawfish and shrimp, where you wouldn’t change the quality of the product but you would inactivate some of the microorganisms. Seafood is not very porous yet it would withstand the pressure without harming the texture of the flavor or appearance of the product.

FTS: And you couldn’t do that with electric pulse because it’s a particulate?

Swanson: Right. Too variable in composition. With electric pulse any time you get a variation in composition you get a variation in how the charge is going to penetrate the food product. And then you get sparking and you get heat produced and then you run into problems. But with high pressure that doesn’t happen. You can do particulates, you can do almost anything in a pressure unit--anything that would pass through the valve that would be required to keep it operating. And when you’re talking high pressure, the valve system is the basis for that. And that’s why I was speaking in terms of a sequential continuous unit, where they would actually have a number of batch operations sitting side by side just like pistons in a car where you would be filling and pressurizing and emptying all of them sequentially.

FTS: But with a product like an oyster, does the pressure go all the way through? One tends to think in terms of surface pressure...

“We are using
high pressure
systems to
perform changes
in whey and soy
protein molecules
in order to improve
their functional
properties as
flavor binders
and texturizers.”

Swanson: But it’s not. It’s volumetric pressure. There are no rays, no electrons that have to penetrate. It’s pressure that’s applied throughout, uniformly. It’s like a pressure cooker; once it gets up to temperature everything is at that temperature, not just the surface. We have to think in a little bit different terms. Unfortunately everything we’ve done in processing over the past 50 years has been thermal so we think of everything as heating from the outside. We had the same problem when microwave heating was introduced. Now everyone talks about it heating from the inside, which isn’t correct either. It’s uniform heating.

FTS: Which of these technologies do you find the most exciting?

Swanson: High pressure. We’re doing some fundamental chemistry with high pressure which is exciting. We’re trying to perform changes in whey protein molecules and soy protein molecules in order to improve them as ingredients for the food flavor binders, flavor releasers, texturizers...trying to improve their functional properties.

FTS: How so?

Swanson: We enhance the abilities of the whey proteins to bind flavors. Ordinarily when we alter the structure of proteins we change them into an intermediate form which is more hydrophobic. Many of the flavor compounds we would like to put into low fat foods are hydrophobic in nature, and we can’t put them into low-fat foods because we’ve taken the fat out. So we’re trying to take these flavor compounds which are fat soluble, and bind them to proteins which are now more hydrophobic. And then we can put those into reduced fat foods and improve the taste. And God knows those light foods could use some flavor. So it’s a way of getting flavor components into food systems that don’t have much fat in them.

FTS: So, you’re looking at hydrostatic pressure as more of a processing system than a pasteurization process?

Swanson: That’s right. We are altering the functionality of the proteins.

FTS: What do you see for the future?

Swanson: I think high pressure processing of food is going to have a niche. It’s a matter of the food industry and the equipment companies coming to terms. I suspect high pressure will be here with specific products, such as the jellies and the guacamole, and I think there will be other products coming about in the near future. I suspect it will be the first of these processes to be validated by the FDA. I think pulse electric fields will probably follow along behind—again in the egg industry perhaps. Or with fruit juices. Because both industries have an interest, and because both have been having some problems lately. They can develop new technologies and enhance their products, which works out good for all concerned.

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