Because the concentrations of free amino acids and peptides are very low in milk, the starter cultures depend for growth in milk heavily on their proteolytic systems. The degradation of milk proteins (caseins) leads to peptides and free amino acids, which can subsequently be taken up by the cells (99,100). Proteolysis is initiated by a single cell-wall-bound extracellular proteinase (Prt), which can be either chromosomally or plasmid-encoded. Most dairy lactic acid bacteria strains contain such an extracellular proteinase, but several do not and these are mainly dependent on other strains in the starter culture for the production of peptides and amino acids. Such dependency of strains is rather common in starter cultures and indicates the relevance of knowledge on the population dynamics between strains in order to be able to develop stable starter cultures.
Peptide and amino acid transport systems have been studied extensively in lactococci, but far less is known for other lactic acid bacteria such as mesophilic and thermophilic lactobacilli [see for a review Kunji et al. (99)]. Peptide and amino acid uptake occurs via oligopeptide transport systems, and di-/tripeptide transporters as well as various amino acid
transport systems (101). Following uptake, the peptides are degraded intracellularly by a variety of peptidases (99,100). These peptidases of lactic acid bacteria can be divided into endopeptidases, aminopeptidases, di-/tripeptidases, and proline-specific peptidases. The specialized peptidases in lactic acid bacteria for hydrolysis of Pro-containing peptides have been postulated to be important for the degradation of casein-derived peptides, because these are known to have a high proline content.
The balance between the formation of peptides and their subsequent degradation into free amino acids is very important. Because accumulation of peptides might lead to a bitter off-flavor in cheese (38,39,102,103). Various bitter-tasting peptides have been identified and especially these peptides should be degraded rapidly in order to prevent off-flavors (102104). Specific cultures have been selected with high bitter-tasting peptide degrading abilities (38,39) and such cultures are nowadays frequently used in the preparation of various types of cheese (Fig. 3).
The ability of cultures to degrade bitter-tasting peptides was not only found to be dependent on the strain used, but also on the growth conditions. For example, it was found that pH-controlled growth conditions resulted in a higher debittering activity (40). The mechanism behind these differences was found to be strongly correlated with the sensitivity of the cells to lysis. Meijer et al. (105) showed that the introduction of a transposon in L. lactis SK110 not only increased the stability of the cells, but also the bitterness in cheese made with this culture. These results indicate that the cell membrane can be a barrier between the enzymes, located intracellularly, and the peptide substrates, present in the cheese matrix. Apparently, there is not enough active transport anymore with the starter cultures once they are present in the cheese matrix for a certain ripening time.
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