CONICET | Buscador de Institutos y Recursos Humanos

During spray-drying of bacterial cultures, thermal and oxidative stresses are important factors acting against cell viability. In previous studies, L. rhamnosus 64 showed immunostimulating activity in mice, capacity to growth in dairy by-products (cheese and ricotta whey and whey permeate) in test tubes and moderate resistance to spray-drying and storage in cheese whey. The aim of this work was to enhance biomass production in whey permeate, in a biofermentor, and to evaluate the resistance to oxidative and thermal challenges after the exposure to thermal, acid and oxygen mild stresses. A factorial experimental design was used to optimize the growth of L. rhamnosus 64 in whey permeate, using yeast extract, tryptone, Tween 80 and Mg-Mn solution as additives. After growth in the optimized culture medium (37°C, anaerobiosis, pH 6), the strain was exposed for 1 h to 45°C (thermal mild stress), pH 5 (acid mild stress) or air influx (oxygen mild stress). Then, oxidative (0.4 mM H2O2 37°C) or thermal (55°C) challenges were applied, separately, for 15 min. In the optimized culture medium and conditions, cell counts reached ca. 1010 CFU/ml. Resistance to oxidative or thermal challenge was different depending on the previous application (or not) of a mild stress (p = 0.002). In particular, for oxidative challenge, the exposure to the factors under study had no significant effects (p = 0.8974) on survival (cell death ca. 2 log CFU/ml). On the contrary, mild stresses significantly enhanced survival to thermal challenge (p < 0.0001). The lowest cell death (0.62 log CFU/ml) was obtained for thermal stress, compared to untreated cells (1.41 log CFU/ml). We conclude that mild stresses, particularly heat stress, were efficient for enhancing the strain resistance to one of the stresses commonly found during spray drying. This work was partly funded by MIUR, Italy (FIRB n. RBFR107VML).

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