Glutamine, glutamate, and arginine-based acid resistance in Lactobacillus reuteri.

This study aimed to determine whether glutamine deamidation improves acid resistance of Lactobacillus reuteri, and to assess whether arginine, glutamine, and glutamate-mediated acid resistance are redundant or complementary mechanisms of acid resistance. Three putative glutaminase genes, gls1, gls2, and gls3, were identified in L. reuteri 100-23. All three genes were expressed during growth in mMRS and wheat sourdough. L. reuteri consistently over-expressed gls3 and the glutamate decarboxylase gadB. L. reuteri 100-23ΔgadB over-expressed gls3 and the arginine deiminase gene adi. Analysis of the survival of L. reuteri in acidic conditions revealed that arginine conversion is effective at pH of 3.5 while glutamine or glutamate conversion were effective at pH of 2.5. Arginine conversion increased the pHin but not ΔΨ; glutamate decarboxylation had only a minor effect on the pHin but increased the ΔΨ. This study demonstrates that glutamine deamidation increases the acid resistance of L. reuteri independent of glutamate decarboxylase activity. Arginine and glutamine/glutamate conversions confer resistance to lactate at pH of 3.5 and phosphate at pH of 2.5, respectively. Knowledge of L. reuteri's acid resistance improves the understanding of the adaptation of L. reuteri to intestinal ecosystems, and facilitates the selection of probiotic and starter cultures.

Propionic acid production by cofermentation of Lactobacillus buchneri and Lactobacillus diolivorans in sourdough.

Cooperative metabolism of lactobacilli in silage fermentation converts lactate to propionate. This study aimed to determine whether propionate production by Lactobacillus buchneri and Lactobacillus diolivorans can be applied for bread preservation. Propionate formation was observed in cofermentation with L. buchneri and L. diolivorans in modified MRS broth as well as sourdough with low, medium and high ash contents. 48 mM of propionate was formed in sourdough with medium ash content, but only 9 and 28 mM propionate were formed in sourdoughs prepared from white wheat flour or whole wheat flour, respectively. Acetate levels were comparable in all three sourdoughs and ranged from 160 to 175 mM. Sourdough fermented with L. buchneri and L. diolivorans was used in breadmaking and its effect on fungal spoilage was compared to traditional sourdough or propionate addition to straight doughs. Bread slices were inoculated with Aspergillus clavatus, Cladosporium spp., Mortierella spp. or Penicillium roquefortii. The use of 20% experimental sourdough inhibited growth of three of the four moulds for more than 12 days. The use of 10% experimental sourdough deferred growth of two moulds by one day. Bread from traditional sourdough with added acetate had less effect in inhibiting mould growth. In conclusion, cofermentation with L. buchneri and L. diolivorans represents a process to increase antifungal capacities of bread.

Novel metabolites from cereal-associated lactobacilli - novel functionalities for cereal products?

Predictions from genome sequence data of sourdough lactobacilli, novel applications of known metabolic traits such as glycansucrases, as well as the exploitation of biodiversity of lactobacilli from traditional fermentations remain an important resource for identification of novel metabolic traits of lactobacilli for use in bread production and the production of value-added food ingredients. Cornerstones of heterofermentative lactic metabolism in cereal fermentations are the rapid utilization of maltose as preferred carbon source, and the production of lactate, CO(2), and the alternative products ethanol and acetate. This review will highlight selected novel aspects of carbohydrate metabolism that are related to the production of maltose and the utilisation of lactate by lactobacilli in cereal fermentations. Several species of lactobacilli convert glycerol and lactate to 1,3 and 1,2 propanediol, respectively. Both metabolic pathways are relevant for food preservation as reuterin is an intermediate of 1,3 propanediol formation, and 1,2 propanediol is further converted to propionate. Glycansucrases, disaccharide hydrolases and disaccharide phosphorylases catalyse oligosaccharide formation from sucrose, maltose, or lactose. Lactobacilli in sourdough generally harbour several enzymes capable of oligosaccharide formation from disaccharides. Oligosaccharide formation by sourdough lactobacilli can be exploited for fermentative production of novel oligosaccharides in bread and a wide spectrum of other food applications.

Prolamin hydrolysis and pentosan solubilization in germinated-rye sourdoughs determined by chromatographic and immunological methods.

The assortment and quality of bakery products designed for celiac patients may be improved by designing whole grain ingredients with low residual prolamin contents. The main objective of this study was to evaluate the extent of prolamin hydrolysis and pentosan solubilization in germinated-rye sourdoughs (GRSDs). Size-exclusion chromatography analyses, the fate of fluorescent prolamin, and immunological analyses determined the extent of prolamin hydrolysis and pentosan solubilization. Hydrolysis of rye prolamins was extensive in GRSDs, and according to enzyme-linked immunosorbent assay analyses, more than 99.5% of the prolamins were hydrolyzed. Pentosan solubilization occurred in native-rye sourdoughs, whereas in GRSDs, pentosans were partially hydrolyzed to monosaccharides. Test baking showed that the use of GRSD improved the overall quality of oat bread and that an estimated daily gluten intake from 100 g of bread would be less than 10 mg. However, the clinical safety must be assured before making any recommendations for celiac patients to use such products.