Ultrasound-assisted freezing of Lactobacillus plantarum subsp plantarum: The freezing process and cell viability

Lactic acid bacteria, which are preserved commercially in a frozen or freeze-dried state, play a crucial role in the food industry due to their preservative and health effects. Hence, maintaining highly viable cells in frozen products by reducing the damage caused by freezing is quite important. Power ultrasound has been considered recently as a novel technique for improving the freezing and crystallization processes. In this article, the effects of power ultrasound on the freezing process and the viability of Lactobacillus plantarum subsp. plantarum after freezing were assessed. Freezing was performed in an ultrasonic bath system (25 kHz) equipped with a refrigerated circulator. The bacteria were grown in de Man-Rogosa-Sharpe (MRS) broth and transferred into 1.2 ml tubing vials. The freezing medium was MRS broth with or without dimethyl sulfoxide (DMSO). Our results indicated that the non-irradiated sample exhibited a high degree of supercooling. Ultrasound irradiation (0.25 W cm(-2)) for 3 s at different supercooled temperatures (-2, -4, -6, -8, -10 and -12 degrees C) caused nucleation to occur close to the irradiation temperature. Lower nucleation temperatures resulted in a shorter phase change stage and reduced the cell viability. A quicker phase change is known to diminish the size of the ice crystals and cause the formation of intracellular crystals, which adversely affect the viability of frozen suspended cells. Ultrasound-assisted irradiation at higher temperatures (-2 and -4 degrees C), on the other hand, increased the viability of the cells significantly. Ultrasound irradiation during the phase change stage of the freezing process (4 min) led to a further increase in the viability of the cells while reducing the freezing time. The higher mass transfer rate of water molecules achieved by ultrasound irradiation might have caused the improved viability and faster freezing process observed for the irradiated samples. Our results revealed that ultrasound irradiation during the nucleation stage or phase change step of the freezing process holds promise as a tool to ensure the higher viability of frozen suspended cells. Industrial relevance: Our results revealed that ultrasound irradiation during the nucleation stage or phase change step of the freezing process holds promise as a tool to ensure the higher viability of frozen suspended cells for the industry. (C) 2013 Elsevier Ltd. All rights reserved.