Effect of drying methods on microencapsulated bacteria on secondary protein structure and glass transition temperature as studied by FTIR and DSC

Abstract

The objective of this study was to examine the effect of drying methods on microencapsulated L. acidophilus and L. cremoris on secondary protein structure and glass transition temperature. Protective mechanism of casein-based microcapsule containing mannitol on L. acidophilus and L. lactis ssp. cremoris and glass transition of the microcapsules were studied after spray- or freeze-drying and after 10 week of storage in aluminum foil pouch containing different desiccants (NaOH, LiCl or silica gel) at 25°C. An in situ FTIR analysis was carried out to recognize any changes in fatty acids of bacterial cell envelopes; interaction between polar site of cell envelopes and microcapsules, as well as alteration of their secondary protein structures, whereas DSC was used to determine glass transition (Tg) of microcapsules. Hierarchical cluster analysis based on functional groups of phospholipid bilayers of cell envelopes and secondary protein structures was also carried out to classify the microencapsulated bacteria. The results showed that drying process did not affect fatty acids and secondary protein structures of bacteria, however, those structures were affected during storage depending upon the type of desiccant. Interaction between bacterial cell envelopes and microencapsulant occurred after drying as shown by alteration of wavelength of P = O symmetric of cell envelopes from 1075 cm−1 to 1047 and 1048 cm−1 (fresh L. acidophilus, freeze-dried and spray dried cells, respectively). Similar phenomenon was demonstrated by L. cremoris. However, these structures were maintained after storage in foil pouch containing NaOH. Method of drying and type of desiccants influenced the level of similarities of microencapsulated bacteria. Desiccants and method of drying affected glass transition; yet no Tg ≤ 25°C was detected. This study demonstrated that the changes in fatty acids and secondary structures of the microencapsulated bacteria still occurred during storage at Tg above the room temperature indicating that glass state did not completely prevent chemical activities.