Freezing of water in the presence of the ice nucleation active bacterium, Erwinia ananas, and its application for efficient freeze-drying of foods.

Abstract

Factors affecting the development of the ice nucleation (IN) activity of Erwinia ananas IN-10 were examined, using the degree of supercooling (DS) as a parameter.An aqueous suspension of the bacterial cells or the outer membranefraction showedtwo particular DSvalues, i.e., near 0°C and 3°C.The development of the DS of 0°C required two conditions: a high cell (or membrane) concentration and the presence of a water/glass interface in the system.Sugars inhibited the development of this type of supercooling.The IN activity was stable in the pH range of 5 to 10.On the basis of these data, a possible mechanism for the IN activity development was proposed.The application of the IN activity of the cells to the freeze-drying of high salt containing foods was attempted.It led to a shortening of their freezing times and efficient formation of powdered products.A variety of ice nucleation active (INA) bacteria exist ubiquitously.1}In particular, Erwinia ananas IN-10 has been reported to make water freeze more efficiently.2)While extensive studies are being conducted in many countries from the standpoint of the prevention offrost damage,3~9) the demandfor a meanof freezing water without supercooling is growing in the field offreeze-drying in the food industry.This work was undertaken with the idea that an INA bacterium would inhibit supercooling of the bulk water existing in food items and that this must facilitate their freezedrying. MATERIALS AND METHODSMaterials.Corn starch (Nihon Shokuhin Kako), soybean protein isolate (SPI, Fiji Oil), soy sauce (Kikkoman) and soybean paste (Takeya) were obtained from commercial sources.Ovalbumin was obtained from Seikagaku Kogyo.Sodium alginate (Kanto Chemicals) was used for entrapping cells and the outer membranefraction.Glass beads (5/mi in diameter) were obtained from Oshinriko Co.Other chemicals used were of reagent grade.INA bacterium.Erwinia ananas IN-10 was kindly donated by Mr. T. Makino, The Shizuoka Agricultural Experiment Station.It wascultured on a slant of Pseudomonas Agar F (Difco) at 20°C for 16hr.The resulting