The effects of MRS, whey protein and blueberry alone, and mixed fermentation on the survival rate of lactic acid bacteria under various freeze-drying conditions were investigated. The surface structure of the freeze-dried powders was also investigated to explore the anti-freezing protection mechanism of mixed whey protein and blueberry fermentation on the bacteria. It was found that the mixed fermentation medium of blueberry and whey protein has a protective effect on the freeze-drying bacteria and is better than the traditional MRS and whey protein medium. The optimal concentration of blueberry juice addition was 9%. The survival rate of the pre-freezing temperature at −80 °C was higher than at −20 °C after the pre-freezing and freeze-drying processes. The freeze-drying thickness of 0.3 cm could improve the survival rate of the bacteria. The Fourier transform infrared spectroscopy results indicated the interaction between the whey protein, anthocyanins, and the surface composition of the lactic acid bacteria.

The properties of blueberry juice and whey protein gels formed by the mixed fermentation of L. plantarum 67 and L. paracasei W125 were investigated. The state of the gels, including the colour and surface morphology of the microspheres, showed significant changes with different fermentation times. The polyphenolic, flavonoid, and protein release of whey protein or combined blueberry juice fermented gels under in vitro digestion were investigated. The whey protein and blueberry juice fermented gels had more small pores, with a honeycomb structure, compared to whey protein fermented gels. The hardness of the gels was increased after fermentation for 7 h for the whey protein gels and whey protein mixture blueberry juice gels. The storage modulus and water-holding capacity of the gels were increased between fermentation times of 6 h and 8 h. The swelling rates of the whey protein gels fermented for 7 h and whey protein mixed blueberry juice gels fermented for 8 h and kept in pepsin-free simulated gastric fluid for 1 h had higher values. The release of polyphenols, flavonoids, and protein for the fermented gels was higher at fermentation of 7 h in the in vitro digestion experiment. We found that the chewiness of the whey protein gels, or whey protein mixed fermentation gels, was higher at a fermentation time of 7.5 h and 8 h. However, the cohesiveness values were not significantly different. Therefore, whey protein fermented gels and whey protein mixed blueberry juice fermented gels should be fermented for more than 7 h. This facilitates the release of polyphenols, flavonoids, and protein in the gastric juices.

The interaction of blueberry and whey protein has strong antioxidant properties and potential antibacterial and anti-aging functions during the fermentation process. In this study, the properties of fermented gels derived from whey protein mixed with blueberry juice were investigated for the production of probiotic-rich products such as jelly and pudding. The microstructure, water-holding capacity, texture changes, rheological properties, and digestive characteristics of fermented gels were evaluated in vitro. The fermented gels with a mixture of whey protein and blueberry exhibited a honeycomb structure, observed by SEM. The adhesiveness of the gel with a mixture of blueberry and whey protein was the highest at 7.5 h and 8.0 h, respectively. The storage modulus (G′) and loss modulus (G″) of the mixed gels were higher than those of whey protein gels before 6 h of fermentation. When the fermentation time was 8 h, the release of polyphenols, flavonoids, and proteins was fastest and greatest during the digestion of gastric and intestinal fluid ether for the whey protein fermented gel and the mixed fermented gel. The water-holding capacity of the mixed gels was lower than that of the whey protein fermented gels during the fermentation period of 8 h. The viable counts of the mixed fermented gels could reach 10⁷ CFU/mL, which was higher than those of whey protein gels after 6 days of storage.

In this study, different molecular weight cut-off ultrafiltration membranes (100 and 50 kDa) were used to concentrate and separate whey protein and blueberry juice fermented system by lactic acid bacteria (LAB) to demonstrate that the mixed fermentation had a protective effect on LAB and antioxidant activity during in vitro gastrointestinal digestion. The antioxidant activity after gastrointestinal digestion, the survival rate of LAB after membrane filtration, and their ability to reflect ultraviolet light of the membrane concentrated fermented components were investigated. It was found that the ABTS cationic free radical scavenging ability of L. plantarum 67 and L. paracasei W125 fermented group increased by 7.85%, especially after digestion, which increased by 3.17% compared with the unfermented blueberry and whey protein mixed group. The survival rates of L. plantarum 67 and L. paracasei W125 in the blueberry-whey protein mixture were greater than 82.45% after digestion. The ultrafiltration membrane concentration process had little effect on the viability of LAB during 0–50 min. The mixture of L. bulgaricus 134 and S. thermophiles Grx02 fermented blueberry-whey protein showed strong UV reflection at 300 nm, reaching 27.39%. The mixture of L. plantarum 67 and L. paracasei W125 fermented blueberry-whey protein showed strong UV reflection at 335 nm, reaching 34.29%.