Soybean Protein Hydrolysate Enhances Growth and Freeze-Drying Survival of Bifidobacterium breve and Bifidobacterium longum Strains

The study proposed a strategy using soybean protein hydrolysate (SPH) as the sole nitrogen source for promoting the proliferation and freeze-drying survival of <i>Bifidobacterium breve</i> and <i>Bifidobacterium longum</i> strains. High proportions of SPH replacing traditional nitrogen sources in MRSL significantly enhanced bacterial viable cell counts and OD₆₀₀ values. The small peptides (<3 kDa) and hydrophilic amino acid residues in SPH are considered to be the key factors for promoting bacterial growth. The exclusive use of SPH (100%-SPH) as the nitrogen source induced a morphological change in <i>B. breve</i> 1206 with the Y-shape transformation into smaller rod-shaped cells, while <i>B. longum</i> 070103 and 050101 became shorter rods. Cells with these morphological changes could more effectively maintain cell membrane integrity in an acidic condition and during the freeze-drying process. Consequently, MSPH improved cell viability and freeze-drying survival for <i>B. breve</i> 1206 in PBS and 10% skim milk compared to MRSL. It also significantly increased viable cell counts and the ability to survive freeze-drying for functional <i>B. longum</i> strains 070103 and 050101 in 10% skim milk, with survival rates increasing by 16.2% and 43.1%, respectively. These results showed the applicability of SPH in the industrial-scale cultivation of functional <i>Bifidobacterium</i> strains. It also provides new insights into soybean-derived nitrogen sources that can affect amino acid composition and bacterial morphology to enhance probiotic stability. This study supported the application of soybean peptides as the sole nitrogen source for producing high-viability probiotics and the potential prebiotic with health benefits.