Mechanistic study of the differences in lactic acid bacteria resistance to freeze- or spray-drying and storage.

Lactobacillus delbrueckii subsp. bulgaricus and Lactiplantibacillus plantarum are two lactic acid bacteria (LAB) widely used in the food industry. The objective of this work was to assess the resistance of these bacteria to freeze- and spray-drying and study the mechanisms involved in their loss of activity. The culturability and acidifying activity were measured to determine the specific acidifying activity, while membrane integrity was studied by flow cytometry. The glass transitions temperature and the water activity of the dried bacterial suspensions were also determined. Fourier transform infrared (FTIR) micro-spectroscopy was used to study the biochemical composition of cells in an aqueous environment. All experiments were performed after freezing, drying and storage at 4, 23 and 37 °C. The results showed that Lb. bulgaricus CFL1 was sensitive to osmotic, mechanical, and thermal stresses, while Lpb. plantarum WCFS1 tolerated better the first two types of stress but was more sensitive to thermal stress. Moreover, FTIR results suggested that the sensitivity of Lb. bulgaricus CFL1 to freeze-drying could be attributed to membrane and cell wall degradation, whereas changes in nucleic acids and proteins would be responsible of heat inactivation of both strains associated with spray-drying. According to the activation energy values (47-85 kJ/mol), the functionality loss during storage is a chemically limited reaction. Still, the physical properties of the glassy matrix played a fundamental role in the rates of loss of activity and showed that a glass transition temperature 40 °C above the storage temperature is needed to reach good preservation during storage. KEY POINTS: • Specific FTIR bands are proposed as markers of osmotic, mechanic and thermal stress • Lb. bulgaricus CFL1 was sensitive to all three stresses, Lpb. plantarum WCFS1 to thermal stress only • Activation energy revealed chemically limited reactions ruled the activity loss in storage.

Colorant and antioxidant properties of freeze-dried extracts from wild berries: use of ultrasound-assisted extraction method and drivers of liking of colored yogurts.

This work aimed at developing powders rich in antioxidants and pigments from two wild berries: maqui (Aristotelia chilensis) and murra (Rubus ulmifolius). Fruits were subjected to successive ultrasound-assisted extractions (UAE) and then freeze-dried. Physical properties, anthocyanin stability of powders, and their performance as natural colorants in yogurts were evaluated. The optimum extraction methods were: UAE for 10 min in murra, and without UAE (control) in maqui, with juice extraction yields ranging between 80 and 82%. Maqui powder exhibited ≈ 2.8 times more polyphenol and anthocyanin content than murra. However, murra powder showed better stability characteristics as powder colorant since it exhibited greater protection of anthocyanins by means of copigmentation phenomena. Regarding consumer's perception of colored yogurt, samples with 4% and 8% maqui powder could be considered as future prototypes to be launched into the market. The obtained powders may be used in different industrial food applications.