Look to antifreeze proteins to control ice recrystallization.

Researchers who must control the growth of ice crystals in foods should take a look at antifreeze proteins. In addition to lowering freezing temperature, the proteins retard recrystallization and the growth of large ice crystals when a frozen product is stored and then thawed. Antifreeze proteins have been a lab curiosity for years.

The scientific community has performed a number of studies on the proteins' structure, function and mechanism of action. Initially identified in the blood and tissues of Antarctic and some northern marine fish, they prevent the fish from freezing at low temperatures not tolerated by other fish. These materials may offer a new way to improve the quality of food if used properly, and if they can be incorporated economically into products.

Scientists at the University of California (Department of Food Science and Technology, One Shield Ave., Davis, CA 95616) are involved in antifreeze protein research. Most of their current studies concern the mechanism of action in lowering the freezing temperature without affecting the melting point. These are mainly done using atomic force microscopy, nuclear magnetic resonance and theoretical molecular projections.

Two main types of antifreeze proteins come from fish: glycoproteins and nonglycoproteins. Many researchers agree that the proteins function by adsorption on the ice crystal surface or possibly by some interaction at the ice-water interface.

Even though the proteins lower freezing temperatures, the melting temperature is usually about 0 C. This thermal hysteresis indicates that the mechanism is non-colligative. The non-colligative activity may be as great as 500 times that of colligative-acting substances. But there is an asymptotic condition at which the activity saturates.

Food uses for antifreeze proteins are limited for the most part to lab curiosity. Work has been carried out on ice cream products as well as on chilled and frozen meat. In research, small amounts of the proteins were added to thawed samples of commercial product. Researchers froze the samples at about -80 C and stored them at a temperature between -6 C and -8 C for different periods of time. A control sample of an ice cream pop, after 1 hour at -6 C to -8 C, had large crystals while the sample with the antifreeze proteins did not show any crystal growth microscopically.

Scientists have added antifreeze glycoproteins to lambs before slaughter, reducing drip and ice crystal size. The smallest crystals occurred when the proteins were injected into a final concentration of 0.01 mg/kg, particularly when the injection occurred 24 hours before slaughter.

It appears that the utilization of the proteins in foods will depend on economics. Both glycoproteins and winter flounder proteins are commercially available, but at about $500/g. Advancements in synthesis research and molecular biology may help lower costs. Studies also are needed to show that antifreeze proteins do not react with other additives or the food itself. Researchers tell us that liquid foods might be easier to protect than nonliquid systems, such as meats, in which it may be necessary to add large numbers of proteins.

Further information. Robert Feeney; phone: [phone]; fax: [phone]; email: [e-mail].