Food-grade galactooligosaccharide (GOS) has a low water activity (aw). We investigated the survival of Escherichia coli, Salmonella spp., Pseudomonas aeruginosa, Listeria monocytogenes, Staphylococcus aureus, Aspergillus flavus and Saccharomyces cerevisiae in GOS as affected by inoculum levels and storage temperatures. The five bacteria (inoculated at 3 or 6 log cfu/g), and the fungus A. flavus (3 log cfu/g) and S. cerevisiae (3 log cfu/g), were inoculated into GOS and the material stored at 4, 22 or 50C for 28 days. When ca. 6 log cfu/g of microorganisms were inoculated into GOS, the final numbers of L. monocytogenes and S. aureus (thus after 28 days of storage) were higher than those of E. coli, Salmonella spp. and P. aeruginosa, at all storage temperatures tested. In GOS inoculated with ca. 3 log cfu/g of microorganisms, A. flavus numbers were the highest after 28 days. S. cerevisiae levels were lower than those of gram-positive but higher than those of gram-negative bacteria. The overall level of survival by temperature was 4C > 22C > 50C, regardless of inoculum level. Thus, microbial survival in low-aw GOS was affected by temperature and inoculum level but the microorganisms did not grow during the storage.
This study showed that survival of microorganisms in GOS was affected by storage temperature and inoculum level. Microorganisms did not grow in GOS at all temperatures tested, but they still survived in GOS for up to 28 days upon storage temperature. This information will be useful in development of effective microbiological safety strategies to be incorporated in preparation or storage of low-aw food and food ingredients.