Driving the nitrogen/carbon ratio of the culture medium determines Propionibacterium freudenreichii survival during spray-drying.

IntroductionP. freudenreichii is used both as Swiss-type cheese starters and as probiotics. For these purposes, it should be converted into a powdered form to be stabilized, without significant loss of viability. Freeze-drying and spray drying can be implemented in order to obtain dry starters or probiotics. Compared to freeze-drying, spray-drying requires less energy for water removal; however, the use of spray-drying involves exposure to air at high temperatures during the process. It is thus a stressful process, with simultaneous thermal, oxidative and osmotic stresses. To resist to these latter, bacteria should adapt their physiology. Culture media constitute a means to adapt bacteria in the aim to increase their survival during spray drying.AimIn this study, we explored the influence of the growth medium composition on 1) osmoprotectants accumulation and 2) bacteria survival during spray drying.MethodsIn rich YEL medium, the salt concentration chosen was the highest still allowing growth of P. freudenreichii CIRM BIA 129. For all experiments, cells were collected at the beginning of the stationary phase. Accumulated osmoportectants were identified and quantified by C and H NMR. The different cultures were then submitted to oxidative, heat, and spray drying challenges.ResultsDuring growth, P. freudenreichii CIRM-BIA 129 accumulated trehalose, glutamate and glycine betaine. In hyper-osmotic conditions, the accumulation of these osmoprotectants increased.Lactose, which is naturally present in sweet whey, or lactose added to YEL growth medium, triggered acid adaptation, but also increased intracellular trehalose accumulation. The glycine betaine/trehalose ratio accumulated in intracellular content was determined by the culture medium carbon/azote ratio.With an optimized intracellular glycine betaine/trehalose ratio, P. freudenreichii better survived heat and oxidative lethal challenges, as well as spray drying. ConclusionDesigning the culture medium composition allows driving bacterial adaptation and leads to better survival after spray drying. This, in turn, should lead to high yield and high-quality production of starters and probiotics.