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In this study, the response surface methodology was used to optimize the cryoprotective agent (skimmed milk powder, lactose and sucrose) formulation for enhancing the viability of Lactobacillus curvatus N19 during freeze-drying and storage stability of cells freeze-dried by using optimum formulation was evaluated. Our results showed that the most significant cryoprotective agent influencing the viability of L. curvatus N19 to freezing and freeze-drying was sucrose and skim milk, respectively. The optimal formulation of cryoprotective agents was 20 g/100 mL skim milk, 3.57 g/100 mL lactose and 10 g/100 mL sucrose. Using the optimum formulation during freeze-drying, the cell survival was found more than 98%. Under the optimal conditions, although only storage of the cells at 4 °C for 6 month retained the maximum stability (8.85 log cfu/g), the employed protectant matrix showed promising results at 25 °C (7.89 log cfu/g). The storage stability of cells under optimized conditions was predicted by accelerated storage test, which was demonstrated that the inactivation rate constant of the freeze-dried L. curvatus N19 powder was 9.74 × 10−6 1/d for 4 °C and 2.08 × 10−3 1/d for 25 °C. The loss of specific acidification activity after the storage at 4 and 25 °C was determined. • Composite cryoprotectants of freeze-dried L. curvatus N19 were optimized by RSM. • The optimized cryoprotectant formulation yields higher numbers of viable bacteria. • Optimized formulation provides a strong protection during long-term storage of dried cell. • Accelerated storage test appropriate for the shelf life prediction of dried N19 cell. • Storage period and temperature affects the acidification activity of dried cell. [ABSTRACT FROM AUTHOR]

In this study, various kinds of cryoprotectant (skim milk, lactose, and sucrose) formulations were tested to enhance the survival of Lactobacillus brevis ED25 after freezing and freeze‐drying. A Box–Behnken experimental design was used to optimize cryoprotective medium and the highest cell survival was observed with the 17.28% skim milk, 2.12% lactose, and 10% sucrose cryoprotectant as the optimum condition. The structural and physicochemical characteristics of freeze‐dried powder were acceptable for application with regards to particle surface morphology, moisture and water activity (Aw), glass transition temperature (Tg), Fourier transform infrared spectra, X‐ray structure, and also storage stability under the refrigeration and room temperature conditions. Accelerated storage test based on Arrhenius equation could be used to predict the freeze‐dried bacterial shelf life but only with a certain degree of predictability for long‐term storage. The acidification kinetics of fresh and stored culture in sourdough fermentation was also described on the basis of the Gompertz equation. Practical applications: Freezing and storage are crucial factors for the viability and acidification power of starter culture. Therefore, various types and concentrations of cryoprotectants have been used to preserve the microorganisms. L. brevis ED25 has been a good potential for the manufacture of industrial sourdoughs and this research has aimed to investigate long‐term protective effects of optimum cryoprotectant formulations on the viability of bacteria and also determine the acidification power in sourdough. The results showed the potential value of freeze‐dried L. brevis ED25 culture for commercialization. [ABSTRACT FROM AUTHOR]