Submitted to: Letters in Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/15/2009
Publication Date: 8/13/2009
Citation: Paul, M., Somkuti, G.A. 2009. DEGRADATION OF MILK-BASED BIOACTIVE PEPTIDES BY YOGURT FERMENTATION BACTERIA. Letters in Applied Microbiology. 49:345-350. Interpretive Summary: There is significant interest in developing ways to improve the nutritional and health-promoting properties of commercially available foods. Compounds that have been discovered to have beneficial health-promoting effects include peptides that are derived from proteins found in milk. This research explores the possibility of incorporating two biologically active small peptides, one that helps reduce blood pressure and one with antibacterial activity, into fermented milk products such as yogurt. Since yogurt fermentation relies on the metabolic activities of starter cultures, the challenge was to evaluate enzyme activities in these bacteria that may result in the breakdown of the bioactive peptides. We demonstrated that addition of the peptides at the start of yogurt fermentation results in extensive degradation due to peptidase activity of the starter bacteria. However, the production of lactic acid by the bacteria increases acidity in the product which creates an unfavorable environment for peptidase activity. Therefore, the integrity and efficacy of the bioactive peptides can be preserved if they are added near the end of the fermentation process when pH levels drop to 4.5 or lower. This may be difficult to carry out in traditional yogurts where the coagulum must remain intact. However, supplementation with these beneficial peptides may be possible in drinkable yogurt-type products that have significantly increased in consumer acceptance.
Technical Abstract: The effect of cell associated peptidases in yogurt starter culture strains Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus on milk-protein based antimicrobial and hypotensive peptides was evaluated to determine their survival in yogurt-type dairy foods. The 11mer antimicrobial and 12mer hypotensive milk protein-derived peptides were incubated with mid-log cells of Lactobacillus delbrueckii subsp. bulgaricus (LB) and Streptococcus thermophilus (ST), which are required for yogurt production. Incubations were performed at pH 4.5 and 7.0, and samples removed at various time points were analyzed by reverse phase-high performance liquid chromatography (RP-HPLC). LB strains caused rapid degradation of both peptides at pH 4.5 and 7.0, while the peptides remain intact at pH 4.5 in the presence of ST strains. The 11mer and 12mer bioactive peptides may be added at the end of the yogurt-making process when the pH level has dropped to 4.5, limiting the overall extent of proteolysis. The scope and limitations of utilizing bioactive peptides as supplements in yogurt-fermentation based foods is examined. The results show the feasibility of using milk-protein based antimicrobial and hypotensive peptides as food supplements to improve the health promoting qualities of liquid and semi-solid dairy foods prepared by the yogurt fermentation process.