Probiotics, as functional supplements, are good for both the immune system and for intestinal health. But how do they get into the yogurt jar? So far, probiotic bacteria are mostly freeze-dried, before they are used in high concentrations in foods. However, the freeze-drying process is problematic: for some probiotics it means certain death, and it is also quite energy consuming. The probiotics must first be frozen, and in a second step, heat is inserted in the sample to transform the ice directly into steam. Thus, water is removed from the bacterial culture. The TUM researchers from the Chair of Food Process Engineering and Dairy Technology decided to save "detour" and tried to find a drying process that is both gentler and more environmentally friendly.
That is how the TUM researchers came across low temperature vacuum drying (LTVD) a process that runs under mild conditions. The product remains in a liquid state, since in a vacuum the evaporation takes place at low temperatures: For instance, water boils at 8 degree Celsius in an atmosphere of 10 mbar air pressure. Compared to freeze-drying, this method requires 40% less energy. Dr. Petra Först's team at the Chair of Food Process Engineering and Dairy Technology conducted experiments on this process using three probiotic bacterial strains. The TUM researchers first determined the optimal LTVD conditions and then, in a second step, compared the results with conventional freeze-drying.
The results were unexpected: Low Temperature Vacuum Drying resulted partially in a higher survival rate than conventional drying. For instance, the yoghurt strain Lactobacillus bulgaricus, which barely survives freeze-drying, showed a 10 times higher yield following LTVD. The new process will allow health-fostering probiotic "candidates" that are too fragile for conventional manufacturing processes to be used in the food industry. Conversely, it turned out that probiotics that handle freeze-drying very well, performed poorly in low temperature vacuum drying. To cut a long story short: The optimal drying process depends on the respective bacterial strain.
Dr. Jörgen Behr and his research team at the TUM Chair of Technical Microbiology focused on the molecular background of this phenomenon. They looked into possible differences between bacterial strains that might explain the disparate behavior during drying. The secret could be traced back to the bacterial cell membranes, which protect the bacteria from environmental influences. The researchers demonstrated that in probiotics this adaptable "shield" has a different fatty acid composition for every bacterial strain. The researchers can now even control this composition by adjusting the cultivation conditions before the drying process. In an experiment, they successfully increased the survival rate of a bacterial strain by about 50% -- merely by optimizing growth conditions.
The low temperature vacuum drying process is not only the most energy efficient one; it also has a positive influence on storage stability. Compared to probiotics from conventional freeze-drying, probiotics in powder form produced using LTVD keep significantly longer in mueslis or infant formula once the packaging is opened. Thus, more active bacteria remain in the product until it is consumed, even when stored under unfavorable conditions.
From the July 11, 2011, Prepared Foods' Daily News.
