Survival during long-term storage, membrane integrity, and ultrastructural aspects of Lactobacillus acidophilus 05 and Lacticaseibacillus casei 01 freeze-dried with freshwater microalgae biomasses

[Display omitted] •Microalgae biomasses enhanced the probiotic survival during freeze-drying.•Microalgae protection exerted during storage varied with probiotic strain tested.•Microalgae biomasses protected probiotic cultures during in vitro digestion.•Microalgae biomasses reduced probiotic membrane damage during freeze-drying.•Ultrastructural aspects of probiotic cells varied with the microalgae tested. This study aimed to assess Spirulina platensis, Chlorella vulgaris, Scenedesmus quadricauda, and Lagerheimia longiseta microalgae potential as protective agents for probiotic cultures [(Lactobacillus acidophilus (La-05) and Lacticaseibacillus casei (Lc-01)] during freeze-drying, refrigeration storage (4 °C, 120 days), and in vitro simulated gastrointestinal conditions (SGIC). The occurrence of membrane damage and ultrastructural aspects of the cells were also verified. Fructooligosaccharides (FOS) were used as a positive control and saline solution as a negative control. The effects of the cryoprotectants on probiotic survival depended on the tested probiotic culture and microalgae biomass. For La-05, all tested cryoprotectants caused a lower reduction in probiotic counts during the freeze-drying and up to 90 days of storage. S. platensis kept higher probiotic counts during storage, while C. vulgaris protected the probiotic against the SGIC. L. longiseta decreased the probiotic membrane damage, mainly due to the production of exopolysaccharides, which was observed in the scanning electron microscopy (SEM). For Lc-01, all tested cryoprotectants promoted a lower reduction in probiotic counts up to 120 days of storage. FOS and S. quadricauda protected the probiotics during freeze-drying and refrigeration storage, while C. vulgaris protected the probiotic against the SGIC and caused lower membrane damage, mainly due to physical protection observed in SEM. In conclusion, microalgae biomasses exerted similar or better cryoprotectant effects on probiotics than FOS, a recognized cryoprotective agent.