Lifestyles of sourdough lactobacilli – Do they matter for microbial ecology and bread quality?

Functional and Healthy Features of Conventional and Non-Conventional Sourdoughs

Sourdough is a composite ecosystem largely characterized by yeasts and lactic acid bacteria which are the main players in the fermentation process. The specific strains involved are influenced by several factors including the chemical and enzyme composition of the flour and the sourdough production technology. For many decades the scientific community has explored the microbiological, biochemical, technological and nutritional potential of sourdoughs. Traditionally, sourdoughs have been used to improve the organoleptic properties, texture, digestibility, palatability, and safety of bread and other kinds of baked products. Recently, novel sourdough-based biotechnological applications have been proposed to meet the demand of consumers for healthier and more natural food and offer new inputs for the food industry. Many researchers have focused on the beneficial effects of specific enzymatic activities or compounds, such as exopolysaccharides, with both technological and functional roles. Additionally, many studies have explored the ability of sourdough lactic acid bacteria to produce antifungal compounds for use as bio-preservatives. This review provides an overview of the fundamental features of sourdoughs and their exploitation to develop high value-added products with beneficial microorganisms and/or their metabolites, which can positively impact human health.

Comparison of the effects of acetic acid bacteria and lactic acid bacteria on the microbial diversity of and the functional pathways in dough as revealed by high-throughput metagenomics sequencing

Knowledge of the effects of various strains of acetic acid bacteria (AAB) on sourdough remains limited. In this study, the diversity of microbial taxa in sourdoughs fermented by different starters was assessed and their functional capacity was evaluated via high-throughput metagenomics sequencing. Results showed that Erwinia (29.43%), Pantoea (45.89%), and Enterobacter (9.16%) were predominant in the blank CK treatment. Lactobacillus (91.40%), Saccharomyces (6.13%), as well as the AAB genus Acetobacter (0.61%) were the dominant microbial genera in the sourdoughs started by yeast and a strain of lactic acid bacteria (YL treatment). By contrast, the dominant genera in the sourdoughs started by yeasts and various LAB and AAB strains (YLA treatment) were Komagataeibacter (0.39%) except for the inoculated Lactobacillus (68.37%), Acetobacter (20.17%), and Saccharomyces (8.31%) species. Functional prediction of these changes in microbial community and diversity revealed that various metabolism-related pathways, including alanine, aspartate, and glutamate metabolism (21.95%), as well as amino acid biosynthesis (19.14%), were predominant in the sourdoughs started by yeast and an AAB strain (YA treatment). Moreover, arginine biosynthesis (11.65%) were the dominant pathways in the YL treatment. The fermented dough added with sourdoughs started with yeast + AAB and yeast + AAB + LAB strains had substantially higher contents (more than 48.58% in total) of essential amino acids than the dough added with sourdoughs started with yeast + LAB strain. These results demonstrated that amino acid biosynthesis has a beneficial effect on sourdoughs inoculated with an AAB strain.

Innovations in Sourdough Bread Making

The application of sourdough is considered to be a key tool for the production of high-quality bread. Several advantages have been presented through the application of sourdough in bread making, such as increased shelf life, improved aromatic profiles and sensory characteristics, increased nutritional value, and health benefits. Technological benefits have also been recorded, such as the successful application of sourdough in gluten-free breads. Likewise, an upsurge of interest in sourdough applications in bread making as well as in other foodstuffs (pasta) has been witnessed in recent years. Many factors are considered important for sourdough preparations; however, the proper selection of the starter cultures is considered the most central. This Special Issue of Fermentation aims to disseminate recent innovative research regarding sourdough bread making, as well as authoritative reviews that compile information from previously published material.

Degradation of Wheat Germ Agglutinin during Sourdough Fermentation

Non Celiac Wheat Sensitivity (NCWS) is an intolerance to wheat products and individuals with NCWS often adhere to a gluten free diet. However, gluten free diets are often associated with a reduced sensory and nutritional quality. Wheat Germ Agglutinin (WGA) is one of the wheat components linked to NCWS. This study explored the fate of WGA during sourdough fermentation. To assess the role of thiol-exchange reactions and proteolysis, sourdoughs were fermented with Fructilactobacillus sanfranciscensis DSM20451, F. sanfranciscensis DSM20451ΔgshR, which lacks glutathione reductase activity, or Latilactobacillus sakei TMW1.22, with or without addition of fungal protease. The conversion of WGA was determined by size exclusion chromatography of fluorescence-labeled WGA, and by enzyme-linked immunosorbent assay (ELISA). Commercial whole wheat flour contained 6.6 ± 0.7 μg WGA/g. After fermentation with L. sakei TMW1.22 and F. sanfranciscensis DSM20451, the WGA content was reduced (p < 0.05) to 2.7 ± 0.4 and 4.3 ± 0.3 μg WGA/g, respectively, while the WGA content remained unchanged in chemically acidified controls or in doughs fermented with F. sanfranciscensis DSM20451ΔgshR. Protease addition did not affect the WGA content. In conclusion, the fate of WGA during sourdough fermentation relates to thiol-exchange reactions but not to proteolytic degradation.

Limosilactobacillus balticus sp. nov., Limosilactobacillus agrestis sp. nov., Limosilactobacillus albertensis sp. nov., Limosilactobacillus rudii sp. nov. and Limosilactobacillus fastidiosus sp. nov., five novel Limosilactobacillus species isolated from the vertebrate gastrointestinal tract, and proposal of six subspecies of Limosilactobacillus reuteri adapted to the gastrointestinal tract of specific vertebrate hosts

Ten strains, BG-AF3-AT, pH52_RY, WF-MT5-AT, BG-MG3-A, Lr3000T, RRLNB_1_1, STM3_1T, STM2_1, WF-MO7-1T and WF-MA3-C, were isolated from intestinal or faecal samples of rodents, pheasant and primate. 16S rRNA gene analysis identified them as Limosilactobacillus reuteri. However, average nucleotide identity and digital DNA-DNA hybridization values based on whole genomes were below 95 and 70 %, respectively, and thus below the threshold levels for bacterial species delineation. Based on genomic, chemotaxonomic and morphological analyses, we propose five novel species with the names Limosilactobacillus balticus sp. nov. (type strain BG-AF3-AT=DSM 110574T=LMG 31633T), Limosilactobacillus agrestis sp. nov. (type strain WF-MT5-AT=DSM 110569T=LMG 31629T), Limosilactobacillus albertensis sp. nov. (type strain Lr3000T=DSM 110573T=LMG 31632T), Limosilactobacillus rudii sp. nov. (type strain STM3_1T=DSM 110572T=LMG 31631T) and Limosilactobacillus fastidiosus sp. nov. (type strain WF-MO7-1T=DSM 110576T=LMG 31630T). Core genome phylogeny and experimental evidence of host adaptation of strains of L. reuteri further provide a strong rationale to consider a number of distinct lineages within this species as subspecies. Here we propose six subspecies of L. reuteri: L. reuteri subsp. kinnaridis subsp. nov. (type strain AP3T=DSM 110703T=LMG 31724T), L. reuteri subsp. porcinus subsp. nov. (type strain 3c6T=DSM 110571T=LMG 31635T), L. reuteri subsp. murium subsp. nov. (type strain lpuph1T=DSM 110570T=LMG 31634T), L. reuteri subsp. reuteri subsp. nov. (type strain F 275T=DSM 20016T=ATCC 23272T), L. reuteri subsp. suis subsp. nov. (type strain 1063T=ATCC 53608T=LMG 31752T) and L. reuteri subsp. rodentium subsp. nov. (type strain 100-23T=DSM 17509T=CIP 109821T).

Metabolic and Gut Microbiota Responses to Sourdough Pasta Consumption in Overweight and Obese Adults

Increasing consumer interest in fermented products has driven the emergence of a number of novel foods including shelf-stable sourdough pasta. This study comprehensively examined the impact of fermentation on the microbial composition of the culture, pasta, its subsequent effects on glycemic responses and gut microbiota in overweight men and women (>25 kg/m²) compared to a conventional, non-fermented pasta. Two, randomized crossover trials were performed. Study A examined acute feeding responses to each product wherein fasted participants completed a meal tolerance test comprised of 75 g of conventional or sourdough pasta to examine glycemic responses. Results showed enhanced gastric emptying with sourdough, but no difference in overall blood glucose, insulin or satiety hormone responses between the treatments. Study B consisted of three standard oral glucose tolerance tests as well as fecal collection for sequencing at baseline and following each pasta intervention (150 g or 2 serving/d for 5 days) followed by a 2-week washout period. Results showed no differential impact of either pasta treatment on glucose tolerance. Analysis of fecal bacterial and fungal (mycobiome) microbiota showed no change at the individual species or genus levels. However, fungi were adaptive following chronic pasta consumption with decreases in alpha diversity of fungi following sourdough, but not conventional pasta. This was accompanied by reductions in total fecal short chain fatty acid concentrations. In conclusion, sourdough fermentation did not change the overall glycemic properties of the pasta, incretin responses or bacterial gut microbiota, but appears to impact microbiome fungal community structure with chronic consumption.

Functional Properties and Sustainability Improvement of Sourdough Bread by Lactic Acid Bacteria

Preventing food spoilage without the addition of chemical food additives, while increasing functional properties of wheat-based bakery products, is an increasing demand by the consumers and a challenge for the food industry. Within this study, lactic acid bacteria (LAB) isolated from sourdough were screened in vitro for the ability to utilize the typical wheat carbohydrates, for their antimicrobial and functional properties. The dual culture overlay assay revealed varying levels of inhibition against the examined fungi, with Lactiplantibacillus plantarum S4.2 and Lentilactobacillusparabuchneri S2.9 exhibiting the highest suppression against the indicator strains Fusarium graminearum MUCL43764, Aspergillus fumigatus, A. flavus MUCL11945, A. brasiliensis DSM1988, and Penicillium roqueforti DSM1079. Furthermore, the antifungal activity was shown to be attributed mainly to the activity of acids produced by LAB. The antibacillus activity was evaluated by the spot-on-the-lawn method revealing a high inhibition potential of the majority of LAB isolated from sourdough against Bacillus cereus DSM31, B. licheniformis DSM13, B. subtilis LMG7135, and B. subtilis S15.20. Furthermore, evaluating the presence of the glutamate decarboxylase gen in LAB isolates by means of PCR showed a strain dependency of a potential GABA production. Finally, due to improved functional activities, LAB isolated from sourdoughs exhibit promising characteristics for the application as natural preservatives in wheat-based bakery products.

Microbial Ecology of Greek Wheat Sourdoughs, Identified by a Culture-Dependent and a Culture-Independent Approach

The aim of the present study was to assess the microecosystem of 13 homemade spontaneously fermented wheat sourdoughs from different regions of Greece, through the combined use of culture-dependent (classical approach; clustering by Random Amplified Polymorphic DNA-Polymerase Chain Reaction (RAPD-PCR) and identification by PCR species-specific for Lactiplantibacillus plantarum, and sequencing of the 16S-rRNA and 26S-rRNA gene, for Lactic Acid Bacteria (LAB) and yeasts, respectively) and independent approaches [DNA- and RNA-based PCR-Denaturing Gradient Gel Electrophoresis (DGGE)]. The pH and Total Titratable Acidity (TTA) values ranged from 3.64–5.05 and from 0.50–1.59% lactic acid, respectively. Yeast and lactic acid bacteria populations ranged within 4.60–6.32 and 6.28–9.20 log CFU/g, respectively. The yeast: LAB ratio varied from 1:23–1:10,000. A total of 207 bacterial and 195 yeast isolates were obtained and a culture-dependent assessment of their taxonomic affiliation revealed dominance of Lb. plantarum in three sourdoughs, Levilactobacillus brevis in four sourdoughs and co-dominance of these species in two sourdoughs. In addition, Companilactobacillusparalimentarius dominated in two sourdoughs and Fructilactobacillussanfranciscensis and Latilactobacillus sakei in one sourdough each. Lactococcus lactis, Lb. curvatus, Leuconostoc citreum, Ln. mesenteroides and Lb. zymae were also recovered from some samples. Regarding the yeast microbiota, it was dominated by Saccharomyces cerevisiae in 11 sourdoughs and Pichia membranifaciens and P. fermentans in one sourdough each. Wickerhamomyces anomalus and Kazachstania humilis were also recovered from one sample. RNA-based PCR-DGGE provided with nearly identical results with DNA-based one; in only one sample the latter provided an additional band. In general, the limitations of this approach, namely co-migration of amplicons from different species to the same electrophoretic position and multiband profile of specific isolates, greatly reduced resolution capacity, which resulted in only partial verification of the microbial ecology detected by culture-dependent approach in the majority of sourdough samples. Our knowledge regarding the microecosystem of spontaneously fermented Greek wheat-based sourdoughs was expanded, through the study of sourdoughs originating from regions of Greece that were not previously assessed.

Insights into the Potential of Sourdough-Related Lactic Acid Bacteria to Degrade Proteins in Wheat

Sourdough processing contributes to better digestible wheat-based bakery products, especially due to the proteolytic activity of lactic acid bacteria (LAB). Therefore, sourdough-related LAB were screened for their capacity to degrade immunogenic proteins like gluten and alpha-amylase-trypsin inhibitors (ATIs). Firstly, the growth of 87 isolates was evaluated on a gluten-based medium. Further, the breakdown capacity of selected isolates was determined for gluten with a focus on gliadins by measuring acidification parameters and MALDI-TOF MS protein profiles. ATI degradation after 72 h of incubation within an ATI-based medium was investigated by means of acidification, HPLC, and competitive ELISA. All isolates exhibited the potential to degrade ATIs to a high degree, whereas the gliadin degradation capacity varied more greatly among tested LAB, with Lacticaseibacillus paracasei Lpa4 exhibiting the strongest alterations of the gliadin pattern, followed by Lactiplantibacillus plantarum Lpl5. ATI degradation capacities ranged from 52.3% to 85.0% by HPLC and 22.2% to 70.2% by ELISA, with Lacticaseibacillus paracasei Lpa4 showing superior breakdown properties. Hence, a selection of specific starter cultures can be used in sourdough processing for wheat-based bakery products with reduced gluten and ATI content and, further, better tolerated products for patients suffering from non-celiac wheat sensitivity (NCWS).

Microbial Diversity and Nutritional Properties of Persian "Yellow Curd" (Kashk Zard), a Promising Functional Fermented Food

“Yellow curd” (YC) is one of the most popular homemade Persian fermented foods and is consumed by many people. Notwithstanding, no studies are available to date on its nutritional and microbiological composition. In this study, we examined YC samples obtained from different local markets of Sistan and Baluchestan province, Iran. The results of the chemical analyses revealed a homogenous content of protein (13.71% ± 1.07), lipids (4.09% ± 0.73), and carbohydrates (61% ± 2.13) among the samples. By comparing the average mineral content of YC with yogurt, many relevant differences were detected. Apart from the calcium content, which was similar on average to that of YC, all other minerals tested are present in higher amounts in YC than in yogurt. The analysis of the main sugars present (i.e., lactose, galactose and glucose) highlighted relevant differences among samples, indicating that different YC samples contain natural strains with different capabilities to metabolize sugars. The concentration of galactose in YC samples should be taken into consideration by galactose intolerant people. From the microbiological perspective, the metagenomics analysis revealed that lactic acid bacteria, and particularly the genera Lactobacillus, Pediococcus, and Streptococcus, were dominant in YC. The information provided shows that YC is an interesting base for the preparation of novel functional foods with a good content of beneficial bacteria.

Intraspecies diversity and genome-phenotype-associations in Fructilactobacillus sanfranciscensis

In this study the intraspecies diversity of Fructilactobacillus (F.) sanfranciscensis (formerly Lactobacillus sanfranciscensis) was characterized by comparative genomics supported by physiological data. Twenty-four strains of F. sanfranciscensis were analyzed and sorted into six different genomic clusters. The core genome comprised only 43,14 % of the pan genome, i.e. 0.87 Mbp of 2.04 Mbp. The main annotated genomic differences reside in maltose, fructose and sucrose as well as nucleotide metabolism, use of electron acceptors, and exopolysacchride formation. Furthermore, all strains are well equipped to cope with oxidative stress via NADH oxidase and a distinct thiol metabolism. Only ten of 24 genomes contain two maltose phosphorylase genes (mapA and mapB). In F. sanfranciscensis TMW 1.897 only mapA was found. All strains except those from genomic cluster 2 contained the mannitol dehydrogenase and should therefore be able to use fructose as external electron acceptor. Moreover, six strains were able to grow on fructose as sole carbon source, as they contained a functional fructokinase gene. No growth was observed on pentoses, i.e. xylose, arabinose or ribose, as sole carbon source. This can be referred to the absence of ribose pyranase rbsD in all genomes, and absence of or mutations in numerous other genes, which are essential for arabinose and xylose metabolism. Seven strains were able to produce exopolysaccharides (EPS) from sucrose. In addition, the strains containing levS were able to grow on sucrose as sole carbon source. Strains of one cluster exhibit auxotrophies for purine nucleotides. The physiological and genomic analyses suggest that the biodiversity of F. sanfranciscensis is larger than anticipated. Consequently, "original" habitats and lifestyles of F. sanfranciscensis may vary but can generally be referred to an adaptation to sugary (maltose/sucrose/fructose-rich) and aerobic environments as found in plants and insects. It can dominate sourdoughs as a result of reductive evolution and cooperation with fructose-delivering, acetate-tolerant yeasts.

Comparison of the Functionality of Exopolysaccharides Produced by Sourdough Lactic Acid Bacteria in Bread and Steamed Bread

Exopolysaccharides (EPSs) produced by lactic acid bacteria improve the quality of bread; however, their functionality in steamed bread is unknown. This study aimed to compare the impact of EPS produced during sourdough fermentation on the quality of bread and steamed bread. Sourdoughs were fermented with EPS-producing Fructilactobacillus sanfranciscensis, Weissella cibaria, and Leuconostoc mesenteroides; Latilactobacillus sakei LS8 and chemically acidified sourdough were prepared as controls. EPS production generally enhanced the specific volume, improved the texture, and reduced the staling rate of bread. The effect of EPS on steamed bread quality was more pronounced when compared to its effect on bread quality. Remarkably, the beneficial effects of F. sanfranciscensis bread quality were largely independent of EPS formation and may relate to gluten modifications rather than EPS production. In conclusion, the direct comparison of sourdough and EPS functionality in steaming and baking provides novel insights for the optimization of commercial (steamed) bread production.

Commercial Saccharomyces cerevisiae as a starter culture in "Wanergao": A traditional fermented food in China

To explore the effect of commercial Saccharomyces cerevisiae on the quality of traditional "Wanergao" for reasonable consuming guidance, the dominant microbes, physicochemical property, free amino acid content, texture, and sensory properties during fermentation of "Wanergao" were illustrated in this paper. Compared with the samples in the control group, "Wanergao" samples in the two groups that were subjected to S. cerevisiae inoculation had weaker acidity (the pH values dropped from 4.39 ± 0.08 to 4.36 ± 0.07 and 4.36 ± 0.07 within 2 h during fermentation), higher fermenting rate (volume increased from 100 ± 1.31 to 305 ± 4.61 and 316 ± 4.93 mL separately within 3 h), and the dominant lactic acid bacteria and yeast being leukonid and S. cerevisiae. More amylose, ethanol, and free amino acid were detected in "Wanergao" produced with S. cerevisiae inoculation compared with "Wanergao" produced by sourdough. The two kinds of "Wanergao" presented various hardness (2318 ± 112, 2279 ± 103), springiness (0.76 ± 0.03, 0.71 ± 0.03), chewiness (1.43 ± 0.05, 1.41 ± 0.06), and cohesiveness (0.68 ± 0.03, 0.62 ± 0.03) after fermentation. The result of sensory analysis revealed that "Wanergao" in the S. cerevisiae group had higher elasticity, aroma, and restoring force. The experiment demonstrated that "Wanergao" produced by using S. cerevisiae is a kind of fermented rice product with rich fragrance, high amount of nutrients, and strong elasticity.

Diverse Microbial Composition of Sourdoughs From Different Origins

Hundreds of sourdoughs have been investigated in the last decades. However, many studies used a culture-dependent and/or culture-independent microbiological approach [mainly based on denaturing gradient gel electrophoresis (DGGE) of PCR amplicons], seldomly combined with a metabolite target analysis, to characterize the microbial species communities of the sourdoughs examined. Moreover, attention was mainly paid on lactic acid bacteria (LAB) and yeast species. In the present study, distinct household-scale (including an artisan lambic brewery) and artisan bakery-scale backslopped sourdoughs (17 in total), obtained from different regions (Belgium, France, United Kingdom, and USA), were examined through a multiphasic approach, encompassing a culture-dependent analysis [targeting LAB, acetic acid bacteria (AAB), and yeasts], different culture-independent techniques [rRNA-PCR-DGGE, metagenetics, and metagenomics (four bakery sourdoughs)], and metabolite target analysis. It turned out that the microbial species diversity of the sourdoughs was influenced by the house microbiota of the producer. Further, when the producer made use of different flours, the sourdoughs harbored similar microbial communities, independent of the flour used. AAB were only present in the Belgian sourdoughs, which might again be related to the processing environment. Fructilactobacillus sanfranciscensis (formerly known as Lactobacillus sanfranciscensis) was the prevalent LAB species of the eight sourdoughs produced by two of the three bakeries of different countries analyzed. These sourdoughs were characterized by the presence of either Saccharomyces cerevisiae or Kazachstania humilis. Moreover, the presence of Fl. sanfranciscensis was positively correlated with the production of mannitol and negatively correlated with the presence of other LAB or AAB species. Sourdoughs produced in an artisan lambic brewery were characterized by the presence of the yeast species Dekkera anomala and Pichia membranifaciens. One household sourdough was characterized by the presence of uncommon species, such as Pediococcus parvulus and Pichia fermentans. Metagenomic sequencing allowed the detection of many more LAB and AAB species than the other methods applied, which opened new frontiers for the understanding of the microbial communities involved during sourdough production processes.

A taxonomic note on the genus Lactobacillus: Description of 23 novel genera, emended description of the genus Lactobacillus Beijerinck 1901, and union of Lactobacillaceae and Leuconostocaceae

The genus Lactobacillus comprises 261 species (at March 2020) that are extremely diverse at phenotypic, ecological and genotypic levels. This study evaluated the taxonomy of Lactobacillaceae and Leuconostocaceae on the basis of whole genome sequences. Parameters that were evaluated included core genome phylogeny, (conserved) pairwise average amino acid identity, clade-specific signature genes, physiological criteria and the ecology of the organisms. Based on this polyphasic approach, we propose reclassification of the genus Lactobacillus into 25 genera including the emended genus Lactobacillus, which includes host-adapted organisms that have been referred to as the Lactobacillus delbrueckii group, Paralactobacillus and 23 novel genera for which the names Holzapfelia, Amylolactobacillus, Bombilactobacillus, Companilactobacillus, Lapidilactobacillus, Agrilactobacillus, Schleiferilactobacillus, Loigolactobacilus, Lacticaseibacillus, Latilactobacillus, Dellaglioa, Liquorilactobacillus, Ligilactobacillus, Lactiplantibacillus, Furfurilactobacillus, Paucilactobacillus, Limosilactobacillus, Fructilactobacillus, Acetilactobacillus, Apilactobacillus, Levilactobacillus, Secundilactobacillus and Lentilactobacillus are proposed. We also propose to emend the description of the family Lactobacillaceae to include all genera that were previously included in families Lactobacillaceae and Leuconostocaceae. The generic term 'lactobacilli' will remain useful to designate all organisms that were classified as Lactobacillaceae until 2020. This reclassification reflects the phylogenetic position of the micro-organisms, and groups lactobacilli into robust clades with shared ecological and metabolic properties, as exemplified for the emended genus Lactobacillus encompassing species adapted to vertebrates (such as Lactobacillus delbrueckii, Lactobacillus iners, Lactobacillus crispatus, Lactobacillus jensensii, Lactobacillus johnsonii and Lactobacillus acidophilus) or invertebrates (such as Lactobacillus apis and Lactobacillus bombicola).

Synergistic Effect of Multiple Saccharifying Enzymes on Alcoholic Fermentation for Chinese Baijiu Production

Chinese Jiuqu (fermentation starter) provides saccharifying enzymes for baijiu (Chinese liquor) fermentation, which undergoes a simultaneous saccharification and fermentation process. However, the key saccharifying enzymes associated with alcoholic fermentation from Jiuqu and their effects on ethanol production remain poorly understood. In this study, we identified 51 carbohydrate hydrolases in baijiu fermentation by metaproteomics analysis. Through source-tracking analysis, approximately 80% of carbohydrate hydrolases in the baijiu fermentation were provided by Jiuqu Among these enzymes, alpha-amylase (EC 3.2.1.1) and glucoamylase (EC 3.2.1.3), from Aspergillus, Rhizomucor, and Rhizopus, were positively related to starch hydrolysis and ethanol production, indicating that they were the key saccharifying enzymes associated with alcoholic fermentation in the baijiu fermentation. Moreover, a combined mixture of alpha-amylase and glucoamylase (in a ratio of 1:6, wt/wt) enhanced ethanol production in a simulative baijiu fermentation under laboratory conditions. This result revealed a synergistic effect of multiple saccharifying enzymes on ethanol production in baijiu fermentation. Our study provides a potential approach to improve the efficiency of saccharification and alcoholic fermentation by optimizing the profile of saccharifying enzymes for fermentation of baijiu and other beverages.IMPORTANCE Jiuqu starter provides enzymes to the simultaneous saccharification and fermentation process of baijiu (Chinese liquor) production; however, the key saccharifying enzymes associated with alcoholic fermentation from Jiuqu and their effects on ethanol production remain unclear. We confirmed that Jiuqu was the main source of carbohydrate hydrolases for baijiu fermentation and identified two types of saccharifying enzymes from multiple microbes as the key enzymes associated with alcoholic fermentation. Moreover, a proper combination of multiple saccharifying enzymes could enhance ethanol production in baijiu fermentation. This combination provides an approach to optimize the profile of saccharifying enzymes for enhancing ethanol production in baijiu and other food fermentations.

Selection of non-Lactobacillus strains to be used as starters for sourdough fermentation

The suitability of forty-one non-Lactobacillus strains to be used as selected starters for sourdough fermentation was evaluated. According to the data collected, Pediococcus pentosaceus OA1 and S3N3 and Leuconostoc citreum PRO17 were selected based on the optimal acidification and growth performances and the intense proteolytic activity (increase of TFFA up to 80%) on whole wheat flour doughs. A relevant degradation of phytic acid (up to 58%) and the increase of phenols content and scavenging activity (4- and 2-folds, respectively) were also observed. The technological performances were compared to two representative Lactobacillus strains (Lactobacillus plantarum and Lactobacillus sanfranciscensis). The investigation of the robustness of the selected strains during the propagation (back-slopping procedure) showed their long-term dominance only when singly-inoculated; while Leuc. citreum PRO17 dominated the fermentation when the strains were co-inoculated. The sourdoughs obtained by the non-Lactobacillus selected strains (singly or pooled) were used for breadmaking. Selected sourdoughs allowed the production of breads characterized by in-vitro protein digestibility (IVPD) higher than that of breads obtained with Lactobacillus strains or baker's yeast. The aroma profile, estimated by GC/MS, was complex and characterized by high concentration of the typical compounds (hexanol, 3-methylbutanol and 2-pentylfuran) of sourdough bread.

Interactions between Kazachstania humilis Yeast Species and Lactic Acid Bacteria in Sourdough

Sourdoughs harbor simple microbial communities usually composed of a few prevailing lactic acid bacteria species (LAB) and yeast species. However, yeast and LAB found in sourdough have been described as highly diverse. Even if LAB and yeast associations have been widely documented, the nature of the interactions between them has been poorly described. These interactions define the composition and structure of sourdough communities, and therefore, the characteristics of the final bread product. In this study, the nature of the interactions between strains of two commonly found sourdough yeast species, Kazachstania humilis and Saccharomyces cerevisiae, and lactic acid bacteria isolated from sourdoughs has been analyzed. Population density analysis showed no evidence of positive interactions, but instead revealed neutral or negative asymmetric interaction outcomes. When in coculture, the yeasts´ population size decreased in the presence of LAB regardless of the strain, while the LAB´s population size was rarely influenced by the presence of yeasts. However, a higher maltose depletion was shown in maltose-negative K. humilis and maltose-positive obligately heterofermentative LAB cocultures compared to monocultures. In addition, tested pairs of obligately heterofermentative LAB and K. humilis strains leavened dough as much as couples of LAB and S. cerevisiae strains, while K. humilis strains never leavened dough as much as S. cerevisiae when in monoculture. Taken together, our results demonstrate that even if higher fermentation levels with increased maltose depletion were detected for K. humilis and obligately heterofermentative LAB pairs, these interactions cannot be ecologically classified as positive, leading us to rethink the established hypothesis of coexistence by facilitation in sourdoughs.

Bacterial community dynamics in spontaneous sourdoughs made from wheat, spelt, and rye wholemeal flour

Sourdough fermentation is a traditional process that is used to improve bread quality. A spontaneous sourdough ecosystem consists of a mixture of flour and water that is fermented by endogenous lactic acid bacteria (LAB) and yeasts. The aim of this study was to identify bacterial diversity during backslopping of spontaneous sourdoughs prepared from wheat, spelt, or rye wholemeal flour. Culture-dependent analyses showed that the number of LAB (10⁹  CFU/ml) was higher by three orders of magnitude than the number of yeasts (10⁶  CFU/ml), irrespective of the flour type. These results were complemented by next-generation sequencing of the 16S rDNA V3 and V4 variable regions. The dominant phylum in all sourdough samples was Firmicutes, which was represented exclusively by the Lactobacillales order. The two remaining and less abundant phyla were Proteobacteria and Bacteroidetes. The culture-independent approach allowed us to detect changes in microbial ecology during the 72-hr fermentation period. Weissella sp. was the most abundant genus after 24 hr of fermentation of the rye sourdough, but as the process progressed, its abundance decreased in favor of the Lactobacillus genus similarly as in wheat and spelt sourdoughs. The Lactobacillus genus was dominant in all sourdoughs after 72 hr, which was consistent with our results obtained using culture-dependent analyses. This work was carried out to determine the microbial biodiversity of sourdoughs that are made from wheat, spelt, and rye wholemeal flour and can be used as a source of strains for specific starter cultures to produce functional bread.

Evaluation of Pediococcus pentosaceus SP2 as Starter Culture on Sourdough Bread Making

In the present study, a novel Pediococcus pentosaceus SP2 strain, recently isolated from kefir grains, was evaluated as a starter culture in sourdough bread making. The novel starter was applied in fresh, freeze-dried, and freeze-dried immobilized (on wheat bran) form. The type of culture (fresh, freeze-dried, immobilized cells) influenced the bread characteristics. Specifically, the application of freeze-dried immobilized cells led to higher total titratable acidity (TTA) values (9.81 mL NaOH N/10), and the produced bread presented higher resistance to mold and rope spoilage. Moreover, the produced sourdough breads were significantly better in terms of pH, TTA, organic acids content, and resistance to mold and rope spoilage, compared to breads made with a commercial, wild microbiota, sourdough. The organic acids content was also significantly higher than the commercial sourdough sample (2.93 g/kg lactic acid; 1.01 g/kg acetic acid). Determination of volatile compounds through solid-phase microextraction (SPME) gas chromatography/mass spectrometry (GC/MS) analysis and sensorial assessments indicated no significant differences between the tested sourdough breads.

Effect of Mixed Cultures of Yeast and Lactobacilli on the Quality of Wheat Sourdough Bread

In this study, mixed starter cultures of yeast and lactobacilli were used for type I sourdough bread making to evaluate their ability to improve bread quality and increase the amount of flavor volatiles. Kazachstania humilis, Saccharomyces cerevisiae, Wickerhamomyces anomalus, and Lactobacillus sanfranciscensis DSM20451T and Lactobacillus sakei LS8 were used in different combinations to ferment wheat sourdough. S. cerevisiae produced the highest amount of CO2 among all strains and thus enhanced bread volume and crumb texture. S. cerevisiae also increased the free thiol level in bread dough, and this study confirms that thiol accumulation was not strongly related to the content of the glutenin macropolymer (GMP) or bread volume. The role of thiol exchange reactions on bread quality differs between long fermentation sourdough and straight dough with baker's yeast only. The influence of different starter cultures on wheat sourdough bread volatiles was established by using head space solid-phase microextraction and gas chromatography/mass spectrometry analysis (SPME-GC/MS). The sourdough breads fermented with a combination of lactobacilli and yeast had a more complex profile of volatiles, particularly with respect to esters.

Composition and Origin of the Fermentation Microbiota of Mahewu, a Zimbabwean Fermented Cereal Beverage

Mahewu is a fermented cereal beverage produced in Zimbabwe. This study determined the composition and origin of mahewu microbiota. The microbiota of mahewu samples consisted of 3 to 7 dominant strains of lactobacilli and two strains of yeasts. Enterobacteriaceae were not detected. Candida glabrata was present in high cell counts from samples collected in summer but not from samples collected in winter. Millet malt is the only raw ingredient used in the production of mahewu and is a likely source of fermentation microbiota; therefore, malt microbiota was also analyzed by culture-dependent and high-throughput 16S rRNA gene sequencing methodologies. Millet malt contained 8 to 19 strains of Enterobacteriaceae, lactobacilli, bacilli, and very few yeasts. Strain-specific quantitative PCR assays were established on the basis of the genome sequences of Lactobacillus fermentum FUA3588 and FUA3589 and Lactobacillus plantarum FUA3590 to obtain a direct assessment of the identity of strains from malt and mahewu. L. fermentum FUA3588 and FUA3589 were detected in millet malt, demonstrating that millet malt is a main source of mahewu microbiota. Strains which were detected in summer were not detected in samples produced at the same site in winter. Model mahewu fermentations conducted with a 5-strain inoculum consisting of lactobacilli, Klebsiella pneumoniae, and Cronobacter sakazakii demonstrated that lactobacilli outcompete Enterobacteriaceae, which sharply decreased in the first 24 h. In conclusion, mahewu microbiota is mainly derived from millet malt microbiota, but minor components of malt microbiota rapidly outcompete Enterobacteriaceae and Bacillus species during fermentation.IMPORTANCE This study provides insight into the composition and origin of the microbiota of mahewu and the composition of millet malt microbiota. Fermentation microbiota are often hypothesized to be derived from the environment, but the evidence remains inconclusive. Our findings confirm that millet malt is the major source of mahewu microbiota. By complementing culture methods with high-throughput sequencing of 16S rRNA amplicons and strain-specific quantitative PCR, this study provides evidence about the source of mahewu microbiota, which can inform the development of starter cultures for mahewu production. The study also documents the fate of Enterobacteriaceae during the fermentation of mahewu. There are concerns regarding the safety of traditionally prepared mahewu, and this requires in-depth knowledge of the fermentation process. Therefore, this study elucidated millet malt microbiota and identified cultures that are able to control the high numbers of Enterobacteriaceae that are initially present in mahewu fermentations.

Members of the Lactobacillus Genus Complex (LGC) as Opportunistic Pathogens: A Review

Microorganisms belonging to the Lactobacillus genus complex (LGC) are naturally associated or deliberately added to fermented food products and are widely used as probiotic food supplements. Moreover, these bacteria normally colonize the mouth, gastrointestinal (GI) tract, and female genitourinary tract of humans. They exert multiple beneficial effects and are regarded as safe microorganisms. However, infections caused by lactobacilli, mainly endocarditis, bacteremia, and pleuropneumonia, occasionally occur. The relevance of Lactobacillus spp. and other members of the LGC as opportunistic pathogens in humans and related risk factors and predisposing conditions are illustrated in this review article with more emphasis on the species L. rhamnosus that has been more often involved in infection cases. The methods used to identify this species in clinical samples, to distinguish strains and to evaluate traits that can be associated to pathogenicity, as well as future perspectives for improving the identification of potentially pathogenic strains, are outlined.

Toward rational selection criteria for selection of probiotics in pigs

An accurate understanding of properties of probiotics is a prerequisite for selecting probiotic organisms for use in swine production. This review aims to review selection criteria for probiotic organism in swine. The systematically investigated ecological history rather than the source of isolates should be regarded as the natural origin of probiotic strains, which helps to correct the inconsistencies arising from incorrect identification of the source. Moreover, in vivo studies are suggested as follow-up assessment to validate the characteristics of probiotic predicted by in vitro experiments. In addition, the intended probiotic effect depends on the age of the animal and disease prevention in young animals may require different probiotic strains when compared to growth promotion in older animals. With adequate selection criteria, the inclusion of probiotic in feed supplementation is a promising way to exert positive effects on sows, newborns, weanling animals and grower-finisher pigs. Both host-adapted probiotics and nomadic probiotics can be applied for pathogen inhibition but host adapted organisms appear to have a different mode of action. Host-adapted probiotic strains are likely to be associated with exclusive colonization while the nomadic or environmental strain exert better immune stimulating functions. Strains with potent enzymatic activity are fitter for grower pigs favoring feed digestion and enhancing growth performance.

γ-Glutamyl Cysteine Ligase of Lactobacillus reuteri Synthesizes γ-Glutamyl Dipeptides in Sourdough

Kokumi-active γ-glutamyl dipeptides (γ-GPs) accumulate in fermented food. γ-Glutamyl transferase, glutaminase, glutathione synthetase, and γ-glutamyl cysteine ligase (GCL) may synthesize γ-GPs. The genome of Lactobacillus reuteri encodes GCL but not glutathione synthetase or glutamyl transferase; therefore, this study investigated the role of GCL in γ-GP synthesis by L. reuteri LTH5448. Phylogenomic analysis of gcl in lactobacilli demonstrated that three genes coding for GCL are present in L. reuteri; two of these are present in L. reuteri LTH5448. Two deletion mutants of L. reuteri LTH5448, L. reuteri LTH5448Δ gcl1 and LTH5448Δ gcl1Δ gcl2, were constructed by double crossover mutagenesis. Growth and oxygen resistance of the mutants were comparable to the wild type. γ-Glu-Glu, γ-Glu-Leu, γ-Glu-Ile, γ-Glu-Val, and γ-Glu-Cys were quantified in buffer and sourdough fermentations by liquid chromatography-mass spectrometry. The wild type and L. reuteri Δ gcl1 but not Δ gcl1Δ gcl2 converted amino acids to γ-Glu-Cys. γ-Glu-Ile accumulation was reduced in both mutants; however, the disruption of gcl did not alter the biosynthesis of the other γ-GPs. In conclusion, gcl1 in L. reuteri mediates γ-Glu-Ile synthesis, gcl2 mediates γ-Glu-Cys synthesis, but neither gene affected synthesis of other γ-GPs. This study facilitates selection of starter cultures that synthesize γ-Glu peptides with kokumi activity and, thus, improve the taste of fermented foods.