Underwater processing inhibits deterioration of fruit quality.

According to the International Fresh-Cut Produce Association, fresh-cut fruits and vegetables make up one of the fastest growing food categories in U.S. supermarkets. U.S. sales of fresh-cut produce sprang from $3.3 billion in 1994 to $11 billion in 2000 and are projected to reach $15 billion in 2005. Fresh-cut vegetables, including ready-to-eat salads, account for most of these sales.

But what is holding fresh-cut fruits back from even greater consumption are the physiological and biochemical changes that occur when they're processed and stored. Compared to vegetables, fruits commonly destined for the fresh-cut market generally have a higher pH and water content, making them more vulnerable to microorganisms and enzymatic changes. When fruit is sliced, there's a good chance that its sensory qualities--taste and texture--will be altered, nutrients lost, and bacteria and molds introduced into the product. When fruit is cut, electrical, chemical and hormonal signals move through the plant's tissues. These signals, which can be transmitted within seconds, initiate defense responses that promote wound healing, guard against bacterial attack and generally protect plant cells from further stress.

USDA-ARS researchers have devised a way to use plant biology to their advantage. Plant cells will communicate stress signals if something changes the turgor (fluid) pressure within their tissues. Slicing through a piece of fruit will cause a change in pressure. So to keep the signals from initiating defense responses, scientists have tried cutting fruit while it's submerged in water.

By slicing a melon under water, investigators can control turgor pressure because water forms a barrier that prevents the movement of fluids while the melon's being cut. The watery environment also helps to flush potentially damaging enzymes away from plant tissues. When fruit is cut, enzymes, which are normally isolated from other plant cell components, are forced to interact with plant substrates. This mingling can lead to destructive chemical reactions, as happens when a freshly cut apple turns brown.

With underwater processing, researchers can add compounds, such as calcium, that can further suppress wound-signaling mechanisms. Antimicrobial agents can also be mixed into the water. This approach also works well for vegetables, since they don't require as much peeling and separating--tasks more difficult to do underwater. In the meantime, the scientists are working to improve the engineering designs surrounding their submerged-processing technology.

Further information. Olusola Lamikanra, USDA-ARS Food Processing and Sensory Quality Research Unit, Southern Regional Research Center, 1100 Robert E. Lee Blvd., New Orleans, LA 70124; phone [phone]; email: [e-mail].