Food-canning technique uses enzyme to keep cooked vegetables crispy.

Touch base with scientists at Cornell University's New York State Agricultural Experiment Station if you're looking to make canned vegetables nearly as firm and crunchy as fresh vegetables. Researchers have received four U.S. patents for a process that incorporates lower-than-normal blanching temperatures, a brief holding period that allows a naturally occurring enzyme to fix the vegetables' crispness and the use of calcium and citric acid.

The scientists are willing to discuss their technology, which is available for licensing. They would like to see this technology put into commercial practice because it can improve the texture of processed vegetables extensively. The technology works for most succulent vegetables that are thermally processed (canned), frozen, dehydrated and frozen for later canning.

Some background: investigators wondered why vegetables lose their firmness during the blanching stage of canning and thought that using less heat during that process would preserve crispness. They found that the cooking heat breaks down the large pectin molecules responsible for firmness in vegetables in a process called depolymerization. Investigators discovered that one enzyme found naturally in almost all plant tissue, pectinmethylesterase (PME), has potential for reversing the softening effects of cooking. PME builds salt bridges from spare calcium atoms in vegetables. If conditions favor enzymatic action, you get more linkages, a larger pectin polymer and crisper foods.

As part of their effort, researchers prepared thousands of cans of beans, carrots, and other vegetables in different blanching temperatures and holding times before sterilization. The holding time gives the PME a chance to work before vegetables are reheated during sterilization. Investigators estimated that commercial canners could probably afford up to about a 30 min holding period.

They found a blanching temperature of 140 F to 150 F was best for the desired enzymatic activity. After vegetables were sterilized and canned, chantenay carrots were nearly twice as firm as those from conventional canning processes, an increase from 219 newtons, the force needed to crush vegetable pieces in a testing machine, to 437 newtons, similar to fresh carrots lightly cooked in the kitchen. The test was performed in a strength-of-materials testing machine.

Cauliflower increased in firmness from 65 newtons to 127 newtons, similar to lightly cooked fresh cauliflower. The Cornell experts also found that adding citric acid to lower the pH allows for a gentler heat treatment during sterilization and creates a firmer-textured product. Also, adding calcium gives a firmer texture. After these additions, researchers further increased crispness in Danvers carrots, BBL47 green beans and cauliflower. Any increase needed by food processors in holding times can probably be achieved by placing the blanched vegetables on a slow-moving conveyor belt on the way to the can-filling machine and sterilizer.

Patents. 5,843,511. Firmness in processed vegetables and fruits. Issued: Dec. 1, 1998. Inventor: Malcolm Bourne. 5,645,879. Firmness in processed vegetables and fruits. Issued: July 8, 1997. Inventor: Malcolm Bourne. 5,607,712. Processing of vegetables that are frozen before canning and sterilizing. Issued: March 4, 1997. Inventor: Malcolm Bourne. 5,599,572. Firmness in processed vegetables. Issued Feb. 4, 1997. Inventor: Malcolm Bourne. These patents generally cover improving the firmness of processed canned foods, including fruits and vegetables, by subjecting the fruit or vegetable to a low temperature blanching step at a temperature in the range of 125 F to 160 F. Preferably this can be done from about 140 F to 155 F before conventional sterilization is undertaken.

Further information. Malcolm Bourne, Cornell University New York State Agricultural Experiment Station, Geneva, NY 14456; phone: [phone]; fax: [phone]; email: [e-mail].