Exploring the integrity of cellular membrane and resistance to digestive juices of dehydrated lactic acid bacteria as influenced by drying kinetics.

The survival of probiotics after spray drying is largely related to drying kinetics experienced by cells in atomized droplets. In this study, the effect of drying kinetics on the viability, integrity of cellular membrane, and digestive stability of Lactobacillus rhamnosus GG and Lactococcus lactis subsp. cremoris was quantitatively examined, using the single droplet drying technique with reconstituted skimmed milk as protectant. Distinct inactivation histories of the strains were observed at 70 °C, with droplet sizes of 1 and 2 μL and air flow velocities of 0.45 and 1 m/s. The two strains showed similar inactivation profiles under the same condition. The order of inactivation rate and the degree of damages on the cellular membrane of dehydrated cells followed the order of drying rate, which was the highest for 1 µL at 1 m/s, followed by 1 µL at 0.45 m/s and 2 µL at 1 m/s, while 2 µL at 0.45 m/s showed the slowest inactivation. For individual drying process, the percentage of live cells with intact membrane was constantly higher than cell survival ratio. Dehydrated cells showed increased viability after in situ gastric digestion for 30 min, which could be due to the resistance of milk proteins to simulated gastric fluid and the rich nutrients of the protectant. Bacterial viability declined again after subsequent in situ intestinal digestion. The findings demonstrated the sensitivity of lactic acid bacteria toward minor changes in drying kinetics. Optimizing drying kinetics may effectively increase the survival and lower the cellular injury of spray-dried probiotics.