Effects of ice-nucleation active bacteria on the freezing of some model food systems

Cells of ice-nucleation active (INA) bacteria, Pseudomonas syringae and Erwinia herbicola, were cultured at 18 degrees C with media of nutrient broth and/or yeast extract and harvested at late log phase for maximum ice nucleation activity. These cells were able to nucleate water to freeze at temperatures as high as -2 degrees C. They were incorporated into model food systems, including sugar, protein solutions and oil/water suspensions, representing all major components of foods, to investigate their effects on freezing. The nucleation temperatures of all the treated models were significantly raised by between 3.0 and 5.9 degrees C compared with controls when the freezer temperature was set at -6 to -7 degrees C. The application of the INA cells also caused freezing of certain model solutions at -6 degrees C, such as sucrose solution (10%), which did not freeze at the same conditions without INA bacterial cells. Additions of INA cells also shortened the total freezing time of the model systems by between 20 and 38%. These results suggest that with the application of bacterial ice nucleation, some current food freezing processes may be modified to operate at higher subzero temperatures to provide guaranteed freezing, energy savings and improvement of efficiency and product quality.