Stability of freeze-dried Lactobacillus delbrueckii subsp. bulgaricus in the presence of galacto-oligosaccharides and lactulose as determined by near infrared spectroscopy

Abstract

The ability of two types of galacto-oligosaccharides (GOS) and lactulose to protect Lactobacillus delbrueckii subsp. bulgaricus upon freeze-drying was evaluated on the basis of their capacity to form glassy structures, using near infrared (NIR) spectroscopy. After freeze-drying in the presence of protective agents, microorganisms were stored at 4 °C at different relative humidities (RH) (from 11 to 80%) up to 21 days. The loss of viability was determined by plate count and referred to viability before freeze-drying. For each storage condition, glass transition temperatures (Tg) were determined by differential scanning calorimetry (DSC). In parallel, near infrared spectra registered in the 900–1700 nm region provided information about viability and RHs that could be correlated with the results obtained by plate counts and DSC. The comparison of NIR spectra of freeze-dried samples stored at the same RH but differing in the time of storage allowed the identification of spectral features related with viability. The spectral differences observed on samples stored in the same conditions (same time of storage and same protective agent) but differing in RHs were ascribed to the formation of amorphous and rubbery states. Based on the spectral information, a multivariate based classification model (PLS-DA) was defined to determine whether samples are in a rubbery or an amorphous state. As the physical state of starters is related with their shelf-life, this model provided support for the use of NIR spectroscopy as a quick method to assess the proper storage of starters.