Heat stability of Lactobacillus rhamnosus GG and its cellular membrane during droplet drying and heat treatment

Dehydration and thermal stresses are generally considered as two mains factors deactivating probiotic cells during droplet drying, as typically in industrial spray drying for producing active dry probiotics. However, little is known about how cells respond to these interplaying stresses in the short period of drying. This study showed that dehydration process could alleviate the detrimental effect of thermal stress to a certain extent, evidenced by that probiotic cells could withstand higher temperature in a single droplet drying (SDD) process compared to sole heat treatment. During SDD at 90 °C, droplet temperature increased with time, and the inactivation of Lactobacillus rhamnosus GG (LGG) was initially observed at droplet temperature of 61–65 °C. By contrast, the transition from the maintenance of LGG viability to rapid deactivation occurred at around 54 °C in heat treatment without dynamic dehydration. Possible mechanisms for the enhanced thermotolerance were investigated from drying kinetics level and cellular level. The favorable temperature profile and the decrease in droplet water activity during drying may benefit cell survival. The cytoplasmic membrane of LGG was more stable at elevated temperatures of 60–65 °C during drying, which might be related to the high viscosity of semi-dried particles mitigating the leakage of intracellular substances. Trehalose demonstrated a strong thermoprotective effect over lactose in heat treatment, but the protection was less effective at the later stage of drying. These results dissected the influence of the interplaying stresses on probiotic cells for the first time during droplet drying and also suggested possible approaches for improving cell survival in dried particles. Components capable of protecting cellular membrane are recommended for developing protectant formulation in spray drying of probiotics.