Abstract
Proteolysis in fermented milk, a complex and dynamic process, depends on the starter cultures used. This study aimed to evaluate the influence of Lactobacillus plantarum or Bifidobacterium animalis subsp. lactis, or both, co-fermented with Streptococcus thermophilus, on the changes in the proteolysis profile of fermented milk during 21-day storage at 4 °C, including the pH value, proteolytic degree, protease activity, aminopeptidase activity, free amino acid content, and electrophoresis performance. The results showed that the treatments with co-cultures exhibited a higher amount of free amino groups and neutral protease activity at an extracellular level, whereas lower pH values and aminopeptidase activities towards the six substrates at an intracellular level than the ones with a single-strain of S. thermophilus over the refrigerated storage were observed. In co-fermentation with S. thermophilus, B. animalis subsp. lactis did not significantly affect the concentrations of most free amino acids, while contributions of L. plantarum were found. Electrophoresis indicated that the mixed starters, especially the co-cultures containing L. plantarum, showed a stronger degradation for caseins than the pure S. thermophilus culture. These findings suggest that culture combinations may influence the proteolysis characteristics of the fermented products, and probiotic cultures must be carefully chosen for fermented production.
Highlights
Fermented milk is defined as the products of milk acidification by lactic acid bacteria (LAB)during metabolic fermentation
All of the batches of fermented milks showed a rapid decline in pH values with the increase of storage time, which may reflect the growth of the bacteria and the accumulation of lactic acid [21]
This may be due to the continuous fermentation of LAB during storage [23], which may lead to the hydrolysis of milk proteins
Summary
Fermented milk is defined as the products of milk acidification by lactic acid bacteria (LAB)during metabolic fermentation. Fermented milk is defined as the products of milk acidification by lactic acid bacteria (LAB). LAB depend on their complex proteolytic system to hydrolyze milk proteins to release free amino groups for proliferation. It is reported that proteolytic action in milk is correlated with the processes for forming polypeptides and oligopeptides through the action of bacterial proteases [4,5], and further forming free amino acids and smaller peptides by bacterial peptidases [6]. Aminopeptidases are thought to be of important peptidase in fermented milk, because of their capabilities to release single amino acid residues from the oligopeptides formed by extracellular protease activity [3,10]
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