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

Abstract

Pseudomonas fluorescens MACK-4, isolated from mackerel surface, has an ice-nucle- ating activity (INA). In addition to bacterial cells, this strain could also produce an extracellular ice- nucleating 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 ∞ C. However, glycerol dramatically lowered the INA in both bacterial cells and the EINS. The addi- tion 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.