Lactiplantibacillus plantarum strains with proteolytic abilities showed diverse effects on casein gel formation during fermentation

Proteolysis is often used to improve gel-forming properties of protein, however, the proteolytic effect of lactic acid bacteria on protein gel properties has been rarely reported. The purpose of this study is to investigate the potential role of proteolysis by Lactiplantibacillus plantarum in casein gel formation during fermentation. A total of fifty-four L. plantarum strains were subjected to a screening of proteolytic ability and twenty-one strains were found possessing extracellular protease-producing abilities. The top nine strains with varying proteolytic abilities were investigated for their impacts on casein gel formation. Interestingly, the casein gels fermented by the strains with the highest and lowest proteolytic abilities both showed relatively low hardness and lower storage modulus (G ') and loss modulus (G '') in rheological analyses. However, two strains, namely L. Plantarum YM13-8.1 and L. Plantarum YN22-3.1, which possess a relatively moderate level of proteolytic ability among the nine strains, significantly improved the hardness and water-holding capacity of casein gels and presented a more dense and uniform gel structure, which was accompanied by an increased hydrophobicity and disulfide bond content. The results indicate that the strains YM13-8.1 and YN22-3.1 can facilitate the formation of casein gel by improving the aggregation of protein molecules in fermentation. Concludingly, we propose that the proteolysis by L. plantarum proteinases dramatically influences the casein gel properties, and the effects, to a certain degree, are strain-specific.