Protective effects of sorbitol during the vacuum drying of Lactobacillus helveticus: an FT-IR study

Although the addition of sugars to drying media is a general practice for the viability improvement of lactic acid starter cultures during drying, the mechanism behind the protective effects is not clear. To gain insight into the protective mechanisms of sugars, Lactobacillus helveticus was dried by vacuum drying in the presence of sorbitol, whose protective effects were shown in our previous study (Santivarangkna et al. Lett Appl Microbiol 42:271-276, 2009). In this study, membrane phase transition temperatures (T(m)) of fresh cells and cells dried with and without sorbitol were measured by Fourier transform infrared spectroscopy (FT-IR). Generally, an increase in membrane phase transition temperature (T(m)) is believed to cause membrane damage during drying. Lactobacillus helveticus cells exhibited the transitions at the temperatures of 15.5 and 37.5 degrees C. Drying of cells without sorbitol increased the transition temperatures to 16.5 and 41.5 degrees C, while it was depressed by 6 and 10.5 degrees C when dried with sorbitol. The interaction between membrane phospholipids and sorbitol was observed from hydrogen-bonding sensitive C = O and P = O stretching bands. The position of the P = O band was shifted to the lower frequency in cells dried with sorbitol, which reflects the hydrogen bonding interaction. We suggest that sorbitol protects cells during drying by depressing T(m) via the interaction with phosphate groups of membranes.