Sourdough is the oldest form of leavened bread used as early as 2000 BC by the ancient Egyptians. It may have been discovered by accident when wild yeast drifted into dough that had been left out resulting in fermentation of good microorganisms, which made bread with better flavour and texture. The discovery was continued where sourdough was produced as a means of reducing wastage with little known (at that point of time) beneficial effects to health. With the progress and advent of science and technology in nutrition, sourdough fermentation is now known to possess many desirable attributes in terms of health benefits. It has become the focus of attention and practice in modern healthy eating lifestyles when linked to the secret of good health. The sourdough starter is an excellent habitat where natural and wild yeast plus beneficial bacteria grow by ingesting only water and flour. As each sourdough starter is unique, with different activities, populations and interactions of yeast and bacteria due to different ingredients, environment, fermentation time and its carbohydrate fermentation pattern, there is no exact elucidation on the complete make-up of the sourdough microbiome. Some lactic acid bacteria (LAB) strains that are part of the sourdough starter are considered as probiotics which have great potential for improving gastrointestinal health. Hence, from a wide literature surveyed, this paper gives an overview of microbial communities found in different sourdough starters. This review also provides a systematic analysis that identifies, categorises and compares these microbes in the effort of linking them to specific functions, particularly to unlock their health benefits.

Keywords: sourdough, fermentation, microbiota, benefits, health

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1. Introduction

The use of sourdough as a means of leavening is one of the ancient methods of grain fermentation [1]. Grinding of cereals, pseudo-cereals or legumes followed by addition of water brings about the formation of dough, which subsequently turns into sourdough after a period of time [2]. The presence of a symbiotic colony of lactic acid bacteria (LAB) and yeasts inhabiting our diverse ecosystem induce lactic acid fermentation of sourdough which eventually becomes a stable culture after many hours [3]. The culture can be sourced from three established methods, resulted in classification of sourdough into types I, II and III. Type I sourdough refers to the traditional sourdough that requires uninterrupted propagation (backslopping) by refreshing using fresh flour and water at regular intervals [4]; type II sourdough inoculates adapted cultures industrially as dough acidifiers; whereas type III sourdough is usually dried for easy storage and utilisation [5,6,7]. Type I sourdough can be categorised as type Ia, containing pure culture sourdough starters with different origin; type Ib, being refreshed everyday and fermented spontaneously; and type Ic, which its origin is tropical countries with higher fermentation temperature [8,9]. At the same time, some researchers also considered another type of sourdough as type 0, referring to pre-doughs or sponge doughs, with addition of baker’s yeast (Saccharomyces cerevisiae) [5]. To ensure the manufacture of products being optimised and consistent, manufacturers are always keen to identify and develop sourdough types II and III [10]. Despite being relatively unstable, costly and time consuming, the type I sourdough is more commonly utilised for microbiome study due to its natural diversity [11].

Scientists have started to discover that the variability in number and type of microbiota in dough depends not just on the native microbial flora of the baker’s environment and hands, but also other factors like choice of flour, when and how often the starter is fed, dough hydration level and type of cereal used, leavening temperature, fermentation time and sourdough maintenance temperature [12,13,14]. Different metabolic pathways in sourdough add flavourful metabolic by-products to the mixture, which result in subtler but more complex flavor [15]. Each microbial community has the ability to produce unique flavour profile as some yield yoghurt-like flavour from lactic acid while others produce sharper, more vinegary note with acetic acid (https://truesourdough.com/18-ways-to-make-sourdough-bread-more-or-less-sour/, accessed 21 March 2021) [16]. Lactic acid, together with vinegary acetic acid, contributes to sourdough’s tangy characteristic. Hence, with the aid of sourdough, dissatisfying sensory qualities especially the flavour and mouth-feel of some gluten-free bakery products can be enhanced [17,18], thus boosting their palatability and market values.

As microbial processes show significant potential in improving organoleptic characteristics and design of nutritional quality and health effects of foods and ingredients, sourdough uses have spread widely to other food products i.e., crackers, waffles, pancakes, tortillas, muffin and noodles beyond breads [19]. Bread, being one of the basic staple foods in many countries and designated the ‘staff of life’ is believed to be the prior option of sourdough addition from earlier days [20]. The sourdough bread trend was not a major highlight until recently when it re-emerged, and people have become enthused about home-baked sourdough breads. In 2020, there was a boom in the sourdough bread trend as millions are confined to their homes during the Covid-19 pandemic. The sourdough bread revival has gone into overdrive and was on third Google recipe search globally (https://trends.google.com/trends/yis/2020/GLOBAL/, accessed 21 March 2021) [21]. The underlining reasons that have been spurring sourdough bread boom include various health benefits which makes the discussion in this paper timely. The different factors affecting sourdough fermentation and their relationships to the benefits of sourdough intake due to the presence of beneficial microorganisms are also included.

Figure 1 shows a mindmap guide to sourdough. Sourdough is the result of fermentation from two basic ingredients, mainly flour and water. Microbiome in sourdough is not only affected by various components in different types of flours but also dough hydration level, backslopping time, fermentation time and temperature. Understanding the factors affecting a microbiome enables a greater accrued understanding of microbial ecology, which allows a better selection of starter cultures and fermentation conditions. Microbial succession is also mentioned, which involves the inheritance from ingredients, adaption of acidic conditions and domination by acid-tolerant sourdough bacteria. Three types of carbohydrate fermentation patterns of bacteria are included in sourdough which are obligately homofermentative, obligately heterofermentative and facultatively heterofermentative. With the presence of beneficial microorganisms, sourdough stands out to bring various benefits to properties of bread and health which are also listed in Table 1.

Sourdough starter, which is also known as natural yeast, has conducive habitat for microbes that can support growths of more than 50 species of lactic acid bacteria (LAB) and more than 20 species of yeasts [8]. As technology advances, many new species of LAB are discovered. For instance, in March 2020, there are already 262 species being reported under the genus Lactobacillus [54]. Most recently, in study of Zheng et al. [54], the genus Lactobacillus was proposed to be reclassified into 25 genera after reviewing the taxonomy of Lactobacillaceae and Leuconostocaceae. Consequently, some names of LAB were updated. They included Fructilactobacillus sanfranciscensis (formerly known as Lactobacillus sanfranciscensis), Lactiplantibacillus plantarum subsp. plantarum (formerly known as Lactobacillus plantarum subsp. plantarum) and Levilactobacillus brevis (formerly known as Lactobacillus brevis) [54]. For this review, the old names are referred to for reasons of familiarity amongst researchers due to this very new discovery.

The different species require different conditions for their optimal growth, hence the proportion of each species found in each sourdough starter also varies. Towards the final stage of fermentation, sourdough starters are asserted to be mature when the cell densities and abundances for all LAB and yeasts reach a plateau [55,56]. The species that are more competitive and adaptive in mature sourdough are usually those with larger population [57]. The divergence of microbial profile from one sourdough starter to another can also be observed when the types of ingredients (mainly flour and water), number of sourdough propagation steps, fermentation time, fermentation temperature, dough hydration, leavening temperature and baking environments are different [1,11,14,58,59,60,61].

2.1. Ingredients as Sources of Microorganisms

Without any prior sterilisation, flour is milled from raw cereals which are naturally inhabited by certain types of microorganisms. Corsetti et al. [62] isolated LAB from wheat and some non-conventional flours originated in Italy and revealed the harbouring of mainly lactobacilli and enterococci in the cereals. De Angelis et al. [63]’s list of microbes included Acinetobacter, Pantoea, Pseudomonas, Comamonas, Erwinia, Sphingomonas and Enterobacter from natural wheat (soft or durum) flour from Southern Italy. These bacterial populations are believed to be pioneers of sourdough microbiota since the lowest dissimilarity was found between microbial communities in flour and sourdough starters, as compared to those from water source, bakers’ hands and dust [58]. In the study of Rizzello et al. [64], all sourdough starters were harboured with Leuconostoc citreum, Lactobacillus plantarum and Lactococcus lactis, which are almost similar to the microbial profiles of dough prior to fermentation. These facts hence supported the assertion that the types of flour chosen for making sourdough affect the microbial communities remarkably. Some other factors affecting sourdough microbiota, including fermentation temperature and backslopping time, are also closely related to the types of flour used since microbes present in flour naturally prefers different fermentation conditions [65]. The types of microbes which grow in sourdough starters from different batches or types of flour are not necessarily the same due to the different quantities and qualities of carbohydrates, proteins, minerals, lipids and enzyme activities [11], different amount of microbial growth factors, and also due to the presence of microbial growth inhibitors [57].

The distinction of microbial profiles found in sourdough starters can be possibly seen by comparing the farming conditions (climatic conditions, farming systems, parasitic or fungal attack, etc.) and the degree of milling of flours [62,63]. Studies that directly relate farming systems and sourdough microbiome are still limited. Rizzello et al. [64] claimed that if cereals used for making sourdough are grown conventionally, the sourdough starter usually has lower biodiversity than those using flour from organically grown cereals. The study of Gobbetti et al. [66] then complemented the claim by limiting it within the Firmicutes phylum. For example, sourdough starter using conventionally grown flour only consisted of Leuconostoc, Lactococcus, and Lactobacillus, whereas in organically grown flour, other LAB such as Pediococcus were also found [64]. Adding on, during the milling process of grounding cereals into flour, some essential nutrients for sourdough LAB, particularly protein and ash, are being stripped together with the germs [67]. The degree of milling (amount of bran left on milled grains) is represented by the flour extraction rate, whereby a higher extraction rate is equivalent to a lower degree of milling [63]. By using flour with higher extraction rate, lactic acid fermentation can carry on for longer time with improved microbial growth and acidification power despite the low pH condition in sourdough starters [1,68]. This is due to the raised buffering capacity and contents of minerals and micronutrients in flour with higher extraction rate [69,70]. Hence, the sourdough environment for microbial activities is claimed to be more sustainable, leading to the release of a greater amount of lactic acid without affecting the final pH [68]. The sustainable sourdough environment also allows higher microbial diversities in sourdough starters using flour with higher extraction rate [63]. Generally, rye flour has higher extraction rate than wheat flour [71], thus the greater amount of protein and ash brings about a much wider variety of microbes in rye sourdough as compared to wheat sourdough.

Table 2 and Table 3, respectively, summarise some of the LAB and yeasts found in cereal dough before and after fermentation following the types of cereals, including wheat, rye, spelt, chickpea, etc. Among the flours, wheat and rye are most studied and mentioned. Naturally, both the flours possess different percentages of life-supporting components for LAB and yeasts, such as fermentable sugars, pentosans and amylase enzyme. Rye has a higher amount of amylase than wheat, thus boosting amount of readily fermentable sugars for LAB in a shorter period of time (http://www.baystatemilling.com/ingredients/why-not-rye/, accessed 21 March 2021) [72]. Fujimoto et al. [73] realised a greater rise in Gram-negative bacterial count together with reduction of sugar content on the first day of fermentation for rye sourdough due to higher enzymatic activity as compared to wheat sourdough. The subsequent content of organic acids was affected too. Fraberger et al. [74] who studied sourdough in Austria isolated only Leuconostoc spp. and streptococci from wheat sourdough, whereas some exclusive LAB such as Lb. diolivorans, Lb. gallinarum, Lb. kimchii, Lb. otakiensis, Lb. parabrevis, Lb. paralimentarius, and Lb. xiangfangensis were found in rye sourdough. Despite the different composition of every flour, Lb. plantarum is one of the frequently identified LAB [10] as it is found in wheat [75], spelt [76], rye [77] as well as sourdough utilising composite flour of wheat and legumes [78]. The other common sourdough LAB is Lb. sanfranciscensis and it has a high fitness in conditions of natural sourdoughs [13,64,78,79,80].

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