Changes process in the cellular structures and constituents of Lactobacillus bulgaricus sp1.1 during spray drying

Spray drying is an efficient method for preparing powdered products; however, its application for sensitive biological material, such as live lactic acid bacterial agent, is limited. This study examines the relationship between survival rates of Lactobacillus bulgaricus sp1.1 and water content/activities and identifies the injury sequence in cellular structures and constituents during spray drying. The results revealed that 90% bacterial cells were killed when the water content was < 0.31 g/g (a(w) < 0.82). Fourier-transform infrared measurement indicated that the conformation of a-helices in the overall protein changed at the beginning stage of drying and membranes underwent phase transition after drying. Changes in sensitivity to lysozymes, penicillin G, NaCl, and bile salt showed that the cell wall was the first site to be injured when the water content reached < 1.71 g/g. Repair of the cell wall was inhibited only after spray drying and permeability of the cell membrane was damaged at < 0.31 g/g; this was lethal for most bacteria. Collapsed surfaces were observed at 0.07 g/g on scanning electron microscope. These results complement the knowledge of inactivation mechanisms in spray-dried lactic acid bacteria.