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Newly-discovered 'glassy' state dynamics may enhance quality of frozen foods.

Newly-Discovered `Glassy' State Dynamics May Enhance Quality of Frozen Foods

A lot of research still needs to be done, but Dr. David Reid of the University of California-Davis thinks he's onto something hot . . . well, cold: a "glassy" state of frozen food that may be a breakthrough in preservation of quality.

"Glass Dynamics," as Reid calls it, applies the principles of polymer science to frozen food. He made a preliminary report to the Refrigeration Research Foundation in April, and was scheduled to give updates to the American Frozen Food Institute (AFFI) Sept. 12, and to the National Frozen Food Convention Oct. 14.

Glassy State Threshold

"At some temperature, if crystalization of a second component does not occur, the material between the ice crystals [in a frozen food product] becomes so viscous that it enters the glassy state," Reid explained. "In the glassy state, motion is restricted, and reactive species may no longer diffuse."

Reid coined the symbol [T.sup.g]' to symbolize the temperature at which frozen food enters the glassy state. Since it is therefore also the highest temperature at which the glassy state can be maintained, it "would be expected to be an important threshold temperature for frozen product stability," he told the research group.

Because "the kinetics of change above the glass temperature would depend on the temperature difference between the glass temperature and the storage temperature," Reid continued, "Expectations of the temperature dependence of optimum storage life might have to be revised." But there are still a lot of unanswered questions awaiting further research.

For example, he said, "It is necessary to determine whether one, homogeneous, glassy phase is formed at the appropriate low temperature in many frozen foods. In many cases, it may be that several discrete glassy phases, with different compositions, and different properties, are formed. This would have important effects on product stability."

It's Still a Theory

So far, he stressed, it's largely theoretical. "The validity of correlations between [T.sup.g]' and product stabilities at different temperatures has to be established. An extensive data base is lacking for real products. This data base will be necessary to establish the value of `Glass Dynamics' in describing and explaining frozen product stabilities."

Still, Reid said, the Glass Dynamics approach "holds great promise for use in the prediction of frozen product stability, and for raw material screening prior to processing." At U.C.-Davis, research is being carried on by the Physical Properties Group of the Department of Food Science and Technology. Glass transitions are being studied with a variety of techniques. Calorimetry is used to locate the temperature of transformation, and the role of system composition is being explored. Relaxation techniques are being used to probe the properties of the glassy phase, and the role of kinetics in determining the composition of the glass is also being looked into.

Reid's work challenges the validity of the standard time-temperature tolerance (T-TT) theory of frozen food stability, whch holds that each degree drop in temperature extends the shelf life of a product by an equal amount, without any break points or plateaus in the time-temperature tolerance curve.

Hugh W. Symons, senior vice president of research and technical services at AFFI, goes farther than Reid himself has (on record, at least) as to the "exciting possibilities [that] unfold for the industry." Symons says the industry can look forward to reformulation of products to give a warmer glass transition temperature, the breeding of cultivars of fruits and vegetables with warmer glass transition temperatures, the rotation of inventory during storage with regard to these temperatures, and the re-evaluation of optimal storage temperatures to take acount of glass-transition temperatures of stored products.

New Research Group

Under the chairmanship of Dennis G. Spink of Agripac, Inc., AFFI has formed its own Coalition for Research into Food Freezing, which will "enable those interested in the work to participate in prioritizing research tasks, introducing products into the program, and learning the results first hand." U.C.-Davis' work, meanwhile, will "enable individual product development laboratories to determine [glass transition] temperatures in formulated products."
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Publication:Quick Frozen Foods International
Date:Oct 1, 1990
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