Use Of Cryoprotectants, Prebiotics And Microencapsulation of Bacterial Cells in Improving the Viability of Probiotic Organisms in Freeze-Dried Yoghurt

The main aim of this study was to improve the viability of probiotic organisms within freeze-dried yoghurt during processing and storage. Initially, probiotic organisms including Lactobacillus acidophilus, Lactobacillus casei, Lactobacillus rhamnosus and Bifidobacterium spp. were incorporated in reconstituted skim milk and freeze-dried in order to select the most robust strains for use in further experiments The viability of B. infantis 17930 and L. rhamnosus GG was reduced by 0.07 log, while that of L. casei 1520 and B. longum 1941 was reduced by 0.28 and 0.39 log, respectively. The probiotic organisms selected for further experiments were L. acidophilus 33200, L. casei 279, L. rhamnosus GG and B. longum 536. The effectiveness of adding cryoprotectants and prebiotics in improving the viability of the selected probiotic organisms was investigated. There was a 7% improvement in the viability of L. casei 1520 when cryoprotectant 'UnipectineTM RS 150' was added at 2.5% (w/v). The addition of the prebiotic 'Raftilose P95 at 1.5% (w/v) to yoghurt improved the viability of the combined selected probiotic organisms by 1.42 log during four weeks of storage at 4°C. Microencapsulation of probiotic organisms was assessed to determine whether further improvements in viability were possible. Microencapsulation with calciumalginate using the emulsion technique was found to improve the viability of the combined selected probiotic organisms by 0.31 log in freeze-dried yoghurt stored at 21C. Further studies were carried out to compare the extrusion technique with the emulsion technique. It was found that the viability of the probiotic organism was 1.0 log10 CFU/g higher using the emulsion technique after freeze-drying. Coating with poly-L-lysine or chitosan further improved the survival of the probiotic organisms by 0.03 and 0.07 log10 cfu/g in yoghurt during 4 weeks of storage at 4ºC. The survival of probiotic organisms in yoghurt stored at 4ºC was highest (8.69 log10 cfu/g) in beads coated with chitosan. In freeze-dried yoghurt, coating with poly-L-lysine or chitosan improved survival of probiotic organisms by 0.2 and 0.4 log10 cfu/g in freezedried yoghurt during 16 weeks of storage at 21C. The survival of probiotic organisms in freeze-dried yoghurt stored at 4ºC was highest (8.47 log10 cfu/g) in beads coated with chitosan. Although microencapsulation was effective in improving the viability of probiotic organisms in freeze-dried yoghurt, the calcium-alginate beads were too large and influenced the texture of the yoghurt. Consequently, further work was carried out to identify an effective technique for reducing bead-size during microencapsulation of probiotic organisms. The diameters of calcium-alginate beads were reduced by forming emulsions using homogenisation with Ultra-Turrax benchtop homogeniser, Avestin Inc. Piston homogeniser and Silverson mixer. The smallest beads were formed using the Ultra-Turrax benchtop homogeniser with a modal diameter of 56 μm. The counts of L. casei 279 after microencapsulation using the Ultra-Turrax benchtop homogeniser at 8000 rpm for 2 min were not adversely affected as the population was 8.8 log10 cfu/g. However, the count of microencapsulated B. longum 536 using the Silverson mixer was 6.3 log10 cfu/g. Overall, small beads containing viable probiotic organisms were created when an emulsion formed by homogenisation during microencapsulation. It is recommended that probiotic organisms present in freeze-dried yoghurt should be microencapsulated to improve their viability. The emulsion technique should be used and the emulsion. The sodium alginate/probiotic and oil mixture should be homogenised using the Ultra-Turrax Benchtop mixer at 8000 rpm for 2 minutes to effectively reduce bead size. Ideally, calcium-alginate beads should be coated in chitosan to provide maximum protection. Cryoprotectants should also be added to the yoghurt to provide probiotic organisms with additional protection during freeze-drying