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Shocked tomatoes lose their skins

Researchers at Ohio State University found a new way to peel fruits and vegetables by placing them in an electric current. The discovery could make the food processing industry cleaner and more efficient.

Strictly speaking, you can't electrocute fruit. But that's roughly what engineers at Ohio State University are doing, to make tomatoes part with their peels.

The food processing industry currently uses two methods to peel tomatoes. Neither is ideal.

Soaking the tomatoes in a strong lye solution removes the skins cleanly and leaves a pretty tomato. But the lye-soaked skins are unusable. Neutralizing and disposing of the caustic waste is also problematic. Bill Hirzel of Hirzel Canning Company in Toledo says that, with the lye method, up to 20% of the tomato volume is lost as waste.

"In a plant like ours if we run 1000 tons of tomatoes it's not unusual to generate 20,000 gallons of waste, liquid waste," Hirzel said. "This material that comes off the tomato looks like ketchup, it's real vivid red because it's got all the lycopene in it, it's got skins mixed with it, but it looks like a heavy thick ketchup."

The caustic tomato sludge is neutralized and spread on farm fields.

The alternative to lye is steam. Steamed peels are usable, and there's no industrial waste to process. But Rob Neenan of the California League of Food Processors says the steamed tomatoes just don't look good.

"If you're going to use a whole tomato, it's less appealing looking," he said. "If you were to see a lye peeled tomato right next to a steam peeled tomato you would see the real difference in terms of color and appearance."

Sudhir Sastry, a professor of food process engineering at OSU, knew about these problems, but he was going after a completely different issue when he started to work with ohmic heating in his lab.

"Ohmic heating is when you pass an electric current through the food directly," Sastry said. "It's similar to passing an electric current through an electric range and the heater element gets red hot. Well the same thing happens in your food."

He was interested in the antibacterial effects of ohmic heating. The antibacterial research led to discovery of the peeling phenomenon.

"We had found that if you applied an electric field you ended up rupturing cells, rupturing different types of materials," he said. "So we said well, let's take this to the next step and see if it can peel a fruit or a vegetable, and that's what we did, and lo and behold it peels!"

The reason the tomatoes peel instead of just heating up is because the skin is waxy. It insulates the rest of the fruit from the current. When the cells in the skin finally give way, the entire peel splits off of the fruit.

The lab-scale prototype Sastry now operates can just peel a couple of tomatoes at a time. It's a plastic box with an electrode at each end. Ram Pandit, a researcher in Sastry's lab, is setting it up for a demonstration. He puts the tomatoes inside and adds hot salt solution

He flips on the current, sending 800 watts of electric energy through the solution. Although the water itself is not boiling, the tomatoes gradually start to bubble.

"You'll notice that there's a front if you will, a boiling type of front that passes from one end to the other of the fruit," Sastry said. "The two fronts eventually meet and pop the skin right off."

It takes about three minutes. Pandit turns off the electricity and drains the tomatoes under cold water. The skin slides right off, but the flesh is still fresh and uncooked.

So far every fruit and vegetable they've tried has given up its peel to the electric current: apples, potatoes, nectarines, pears, taro root and water chestnuts are among those peeled to date.

The method gives an attractive cleanly peeled product, like lye peeling, and a usable peel, like steam peeling. Sastry says the cost should be comparable to the other methods, but large scale commercial applications are still uncertain.

The California League of Food Processors says the technology is promising, but is concerned that equipment overhauls would be impractical, Neenan says.

"The question is would any equipment company make, design, test, make, and sell a whole new system?" he said. "And although we're open to those possibilities we're also interested in finding just better ways to manage lye peeling."

New technologies may have a better chance in the Midwest than in the massive California processing plants. Smaller companies like Hirzel have more leeway to be unconventional.

"It certainly has a lot of sex appeal in the Midwest because of the fact that we're much smaller and that our markets that we're aiming our products at aren't the general market, or the masses, but a very specialized market," Hirzel said. "We can do all the oddball stuff."

Hirzel says that ohmic peeling could be especially valuable in processing organic tomatoes, but that the process needs to be scaled up before canneries can begin to use it.