Improving the drying of Propionibacterium freudenreichii starter cultures.

Propionibacterium freudenreichii is a beneficial food-grade actinobacterium, widely implemented, and thus consumed, in various food products. As the main application, P. freudenreichii is used as a cheese-ripening starter, mostly in hard type cheeses. Indeed, during manufacture of "Swiss-type" cheeses (or opened-body cheeses), the technological process favors propionibacteria growth, as well as the corresponding propionic fermentation. This leads to the characteristic flavor of these cheeses, through the release of short chain fatty acids and through lipolysis, as well as to their specific texture. To fulfil this ripening, massive amounts of propionibacteria are industrially produced, dried and stored, prior to cheese making. Furthermore, P. freudenreichii is commercialized in various probiotic food supplements aiming at preserving intestinal health and comfort, in line with its ability to produce beneficial metabolites (short chain fatty acids, vitamins), as well as immunomodulatory compounds. Other industrial applications of P. freudenreichii include the production of food-grade vitamins of the B group, of trehalose, of conjugated linoleic acid, and of biopreservatives. For these different applications, maintaining survival and activity of propionibacteria during production, drying, storage and finally implementation, is crucial. More widely, maintaining live and active probiotic bacteria represents a challenge as the market for probiotic products increases. Probiotic bacteria are, for a bulk majority, freeze-dried, but spray drying is also more and more considered. Indeed, this process is both continuous and more cost-efficient, as it utilizes less energy compared to freeze-drying; on the other hand, it exposes bacteria to higher heat and oxidative stresses. Apart from process optimization and strain selection, it is possible to enhance the resistance of bacteria by taking advantage of their adaptation capacity. Indeed, P. freudenreichii stress tolerance can be boosted by different pretreatments applied before the drying step, thus considerably increasing its final survival. In particular, adaptation to hyperosmotic conditions improves stress tolerance, while the presence of osmoprotectants may mitigate this improvement. Thermal adaptation also modulates tolerance towards these technological challenges. The composition of the growth medium, including the ratio between the carbohydrates provided and the non-protein nitrogen, plays a key role in driving the accumulation of osmoprotectants. This, in turn, determines P. freudenreichii tolerance towards different stresses, and overall towards both freeze-drying and spray-drying. As an example, the accumulation of trehalose enhances its spray-drying survival, while the accumulation of glycine betaine enhances its freeze-drying survival. Growth of propionibacteria in hyperconcentrated whey was used to trigger multiple stress tolerance acquisition, underpinned by overexpression of key stress protein, accumulation of cytoplasmic storage compounds, and leading to enhanced spray-drying survival. A simplified process, from cultivation to atomization, was developed by using whey as a 2-in-1 medium in which propionibacteria were grown, protected and dried with minimal cell death. This innovative process was then subjected to scaling up at the industrial level. In this aim, a gentle multi-stage drying process offering mild drying conditions by coupling spray drying with belt drying, led to final probiotic survival close to 100% when stress tolerance acquisition was previously implemented. Such innovation opens new avenues for the efficient, cost-effective and sustainable development of new probiotic production technologies, as well as probiotic application in the context of food and feed. KEY POINTS: • Propionibacteria acquire multi-stress tolerance when grown in hyper-concentrated whey. • Spray drying of osmo-adapted probiotic bacteria is possible with limited cell death. • A two-in-one drying method is developed to grow and dry probiotic bacteria in the same matrix.