Reference Number: 130
The drivers for the establishment and composition of the sourdough microbiota, with particular emphasis on lactic acid bacteria, are reviewed and discussed. More than 60 different species of lactobacilli were identified from sourdoughs, showing the main overlapping between sourdough and human intestine ecosystems. The microbial kinetics during sourdough preparation was described by several studies using various methodological approaches, including culture-dependent and -independent (e.g., high throughput sequencing), and metabolite and meta-transcriptome analyses. Although the abundant microbial diversity harbored by flours, a succession of dominating and sub-dominating populations of lactic acid bacteria suddenly occurred during sourdough propagation, leading to the progressive assembly of the bacterial community. The contribution of all the potential sources (house microbiota, flour, types of flours and additional ingredients) for contaminating lactic acid bacteria was compared with the aim to find overlapping or specific routes that affect the sourdough microbiota. Once established and mature, pros and cons regarding the stability of the sourdough lactic acid bacteria biota were also reviewed, showing contradictory results, which were mainly dependent on the species/strains. Probably, the future research efforts should be dedicated to decrease the sources/drivers of noticeable variation rather than to full standardization of the process for sourdough preparation and use.
SIGNIFICANCE OF THIS STUDY
The current paper aims at describing and discussing the recent data on the diversity and factors (house microbiota, flour, flour varieties and ingredients) that drive the establishment and composition of the sourdough lactic acid bacteria.
Diversity of lactobacilli:
The authors analysed sourdough starters made from a variety of flours and of different geographical location and found that more than 60 different species of lactobacilli were associated to sourdough which represents the very large diversity of LAB species in sourdough. When they carried out an analysis to check if the species found in sourdough were also present in other ecosystems such as dairy products, fermented meats, human saliva, human intestine and faeces, they found the biggest overlap was found between sourdough and the human intestine and faeces. For example, Lactobacillus reuteri inhabits gastro-intestinal tract of humans and some sourdoughs. The main role that L reuteri plays in both the human GI micrbiome and the sourdough microbiota is energy conversion and carbohydrate metabolism. Scientist suggest that the presence of this human GI bacteria in sourdough could mainly be due a genetic evolution that could have made it adaptable to grow in sourdough starters allowing it to perform its main function of energy conversion and carbohydrate metabolism. The paper provides an interesting lists of other bacterial species that over lap with other ecosystems similar to L . reuteri.
Sources of microbial contamination in sourdough:
The current paper was one of the first papers looking at the influence of in-house bacteria on the final microbial composition of sourdough starters. The bacterial isolates were collected from sourdoughs, flour, hands of the baker, air and equipment of several artisanal bakeries. They found that specific strains of lactic acid bacteria persisted in artisan sourdoughs and circulated in the bakery environment, which acted as a potential carrier. For example, a consistent presence of L. sanfranciscensis in storage box and dough mixer, as well as in the sourdough, was shown. For one bakery, the abundance of this species from equipment was markedly higher than that found in the flour. On the contrary, L. plantarum showed higher adaptability in sourdough than in bakery equipment, suggesting that its main origin was flour. The lactic acid bacteria biota of sourdoughs also differed depending on the type of additional ingredients used. For example, the addition of apple, grape or yoghurt (typical Spanish protocols) as ingredients. Heterofermentative P. pentosaceus was the only dominant species identified in the sourdough added with yoghurt. On the contrary, the sourdoughs added with either apple or grape harbored L. plantarum, L. brevis, L. sakei, and W. cibaria as the most dominant species.