Properties of extracellular ice-nucleating substances from Pseudomonas fluorescens MACK-4 and its effect on the freezing of some food materials

Pseudomonas fluorescens MACK-4, isolated from mackerel surface, has an ice-nucleating activity (INA). In addition to bacterial cells, this strain could also produce an extracellular icenucleating substance (INS) with a maximal INA at pH 6.0. The extracellular INS (EINS) was stable at pH 6-9 during 1-h incubation with 3-5% of saccharides including maltose, trehalose and sucrose at 15 deg C. However, glycerol dramatically lowered the INA in both bacterial cells and the EINS. The addition of either the EINS or bacterial cells significantly elevated the ice-nucleating temperatures of pure water, full-cream milk, and 10% starch solution, but not orange juice and mackerel mince. The EINS produced from this strain could serve as crystal nuclei and accelerate ice crystallization during freezing.