Bread spoilage is mainly due to the growth of filamentous fungi, and metabolites produced during sourdough fermentation by lactobacilli can inhibit fungal growth. One of these metabolites is phenyllactic acid (PLA), which is a catabolite from phenylalanine. In this work, the influence of peptide supply and cosubstrates was determined on PLA formation from phenylalanine by Lactobacillus plantarum TMW1.468 and Lactobacillus sanfranciscensis DSM20451(T). Transport of single amino acids is not efficient in lactobacilli, and only 1% of the offered phenylalanine was converted to PLA. PLA yields were increased 2-4-fold when peptides instead of single amino acids were used as a substrate. The accumulation of phenylalanine after peptide addition indicated that, after transport, transamination was the second limiting factor. In L. plantarum TMW1.468, PLA yields were increased from 5 to >30% upon the addition of alpha-ketoglutarate. In L. sanfranciscensis DSM20451, a combination of both citric acid and alpha-ketoglutarate increased PLA formation. The combined effect of citric acid and alpha-ketoglutarate can be attributed to changes in the NAD/NADH ratio.
Protein hydrolysis and amino acid metabolism contribute to the beneficial effects of sourdough fermentation on bread quality. In this work, genes of Lactobacillus sanfranciscensis strain DSM 20451 involved in peptide uptake and hydrolysis were identified and their expression during growth in sourdough was determined. Screening of the L. sanfranciscensis genome with degenerate primers targeting prt and analysis of proteolytic activity in vitro provided no indication for proteolytic activity. Proteolysis in aseptic doughs and sourdoughs fermented with L. sanfranciscensis was inhibited upon the addition of an aspartic protease inhibitor. These results indicate that proteolysis was not linked to the presence of L. sanfranciscensis DSM 20451 and that this strain does not harbor a proteinase. Genes encoding the peptide transport systems Opp and DtpT and the intracellular peptidases PepT, PepR, PepC, PepN, and PepX were identified. Both peptide uptake systems and the genes pepN, pepX, pepC, and pepT were expressed by L. sanfranciscensis growing exponentially in sourdough, whereas pepX was not transcribed. The regulation of the expression of Opp, DtpT, and PepT during growth of L. sanfranciscensis in sourdough was investigated. Expression of Opp and DtpT was reduced approximately 17-fold when the peptide supply in dough was increased. The expression of PepT was dependent on the peptide supply to a lesser extent. Thus, the accumulation of amino nitrogen by L. sanfranciscensis in dough is attributable to peptide hydrolysis rather than proteolysis and amino acid metabolism by L. sanfranciscensis during growth in sourdough is limited by the peptide availability.Wheat sourdough fermentation has a positive influence on the overall bread quality because it improves flavor (14,29,32) and texture (18) and prolongs shelf life due to the formation of antifungal compounds (20) and delayed staling (6). The fate of the protein fraction of the flour during sourdough fermentation is of crucial importance for bread quality. The protein network in wheat doughs determines dough rheology, gas retention, and thus bread volume and texture. Proteolytic events during fermentation provide the substrates for microbial growth and conversion of amino acids to flavor precursor compounds and antifungal metabolites (20,32). Furthermore, some peptides from wheat proteins are involved in human cereal intolerance and their levels in bread hydrolysates are reduced by selected sourdough lactic acid bacteria (9).The most abundant proteins in wheat flour are gluten proteins. After dough mixing, the proteins are linked to each other by disulfide and hydrogen bonds, resulting in an insoluble network with a molecular weight of up to several million (35). Proteolytic enzymes in the flour are associated with wheat gluten. The pH-optimum of activity of these aspartic proteinases is below 4.0 (2, 3). Accordingly, the proteolytic activity in wheat doughs is strongly increased in acidified doughs independent of the presence of lactic acid bacteria (32, 33) and prote...
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