sealPurdue News

May 9, 1997

Food science prof wins agricultural research award

WEST LAFAYETTE, Ind. -- Suzanne Nielsen works in a Purdue laboratory that seems far removed from grocery stores and farms, but her work may put tastier cheese on grocery shelves and more pest-resistant corn plants in the field. It also has won her the 1997 Research Award from the Purdue University School of Agriculture.

Nielsen, a professor of food science, received the award and presented a seminar May 8.

She began doing research on the digestibility of food proteins shortly after earning her bachelor's degree in food science at the University of Nebraska in 1976. Later, as a graduate student at the University of Minnesota and as a professor at Purdue, she continued working on protein digestion, which led to her current work with enzymes.

"Enzymes are the tools we as humans use to digest proteins, so to study digestibility of proteins, we study enzymes," Nielsen says.

One enzyme Nielsen studies is plasmin, which adds flavor and texture to ripening cheese. That's the good side of the enzyme, she says. Plasmin also, however, shortens the shelf-life of boxed milk that has been processed at an ultra-high temperature by forming distasteful lumps of gel in the bottom of the milk boxes.

The plasmin enzyme naturally enters milk before it leaves the cow, Nielsen says. It sticks to milk proteins called casein, a chief constituent of milk and the basis of cheese.

"Casein molecules are like sponges with large holes. Plasmin molecules get stuck in the holes until other enzymes chew away the casein and release them," Nielsen says.

Once released, plasmin makes its way into the whey, which is the watery part of milk that separates from the thicker part (curds) after coagulation, as in cheese making. Processors also add whey as a dry ingredient to bakery products, meat and nutritional beverages. The plasmin may be able to degrade proteins in these foods, making them less useful.

Also, the casein part of milk, destined to become cheese, ripens more slowly when it lacks plasmin lost to the whey.

As Nielsen looked for ways to keep plasmin in cheese and out of whey, she found that she could influence levels of the active enzyme. Other researchers had hypothesized that molecules that they dubbed plasminogen activators changed plasminogen, the inactive form of the enzyme, into plasmin. Nielsen proved the activators existed. Now that she's found them, she's looking for ways to shut them down.

Nielsen's also looking for ways to shut down a different enzyme system, one that helps insects eat up corn roots and bean seeds.

"We started investigating the digestive system of bean weevils, because we wondered why they could digest bean proteins quite well and people couldn't," Nielsen says. She found that weevils' guts and people's guts contain different enzymes, and that the insect enzyme, cysteine proteinase, worked better.

Since Nielsen wanted to stop the activity of cysteine proteinase, she focused on finding enzyme inhibitors. She identified an inhibitor in soybeans, worked with others to clone the genes that produced it, and found that the inhibitors produced by the cloned genes were just as effective as the originals that had come from soybeans. By adding the cloned genes to weevil-susceptible plants, she hopes to save those plants from weevil damage.

"Then we decided that we needed to focus on an insect of greater economic significance," Nielsen says. She found one in the corn rootworm, which also uses cysteine proteinase to decimate root systems of corn plants. Farmers spend more than a billion dollars each year to control corn rootworm.

Nielsen used cloned genes to produce cysteine proteinase inhibitors, then fed the inhibitors to rootworms. The insects on the inhibitor diet grew more slowly. The plan now is to move the inhibitor into corn and test its effectiveness against rootworms in the field. If it works, rootworms attacking the inhibitor-enhanced corn would be slowed down or die before they could attack and decimate plant roots. Crop yields from enhanced corn would be higher, and farmers likely would use less pesticide on corn fields.

Source: Suzanne Nielsen, (765) 494-8328; e-mail,
Writer: Rebecca J. Goetz, (765) 494-0461; e-mail,
Purdue News Service: (765) 494-2096; e-mail,

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