Rising stars in the bakery: novel yeasts for modern bread

Keywords: aroma, bread, fermentation, FODMAP, gluten, leaven, non-conventional yeast, sourdough.

The microbial fermentation of bread dough leads to the production of CO₂ and other metabolites that give bread its characteristic texture and aroma. Today, the chief microbe used by humans in this endeavour is Saccharomyces cerevisiae, also known as brewer’s or baker’s yeast. Its domestication to food environments occurred long ago,¹ and it has been the dominant breadmaking organism since the advent of purified S. cerevisiae monocultures in the 19th century.² Despite its predominance, S. cerevisiae has several major drawbacks as a fermenting yeast, namely its limited use of only a few carbon substrates and an inability to withstand certain stresses associated with baking, such as osmotic, oxidative, temperature, and ethanol stresses.³ The range of nitrogen sources the yeast is able to assimilate is also relatively limited, due to a whole-genome duplication (WGD) event affecting several genera within the Saccharomycetaceae.⁴ This may be due to WGD-driven gene partitioning: copies of nitrogen assimilation genes that each perform only a subset of an ancestral gene’s function.⁵ It is also likely that most S. cerevisiae strains used for breadmaking arose from interbreeding of an ale and wine strain, and while this has advantages for the consistency and speed of the fermentation process, it limits the taste and aroma complexity of the final products.⁶ Therefore, there is a desire to seek out diverse breadmaking yeasts that can be used to make baked goods with improved technological and organoleptic properties, as well as those that can cater to the gluten-free and low-FODMAP (fermentable oligo-, di-, monosaccharides and polyols) demands of modern consumers.

There are many yeast species from the family Saccharomycetaceae (to which S. cerevisiae belongs) that are found in food environments. Non-S. cerevisiae yeasts are involved in the production of cocoa, kefir, fermented vegetables, wine, and beer.⁷ Bread was historically leavened using traditional sourdough starters: flour and water mixtures that are left at room temperature for several days to ferment ‘spontaneously’, i.e. without a starter culture.² This practice continues today, and sourdough microbial composition – specifically unique functional properties of constituent microbes – continues to be an active field of study. A large proportion of non-S. cerevisiae yeasts are commonly found in sourdough starters, and recent research suggests that the fungal diversity of sourdoughs may be greater than previously thought.⁸

Mature sourdough starters tend to contain only one or two yeast species. The most common non-S. cerevisiae yeasts found in sourdoughs are Kazachstania exigua, K. humilis, Candida glabrata, Torulaspora delbrueckii, Pichia kudriavzevii, and Wickerhamomyces anomalus.^9,10 Novel non-conventional yeasts continue to be isolated from sourdoughs, such as K. saulgeensis which was first described in 2016.¹¹ It is important to note that while S. cerevisiae is also found in many sourdoughs, it remains unclear whether this is due to contamination from purified baker’s yeast often used in the same bakeries¹² and industrial contexts or due to its autochthonous presence there. Sourdough yeasts occupy an environment described as ‘specific and stressful’ due to cereal dough forming an environment with low pH, low oxygen tension, and carbohydrates (mainly maltose) needing to be shared with fermenting lactic acid bacteria.¹⁰ Sourdough starters are therefore a significant source of novel yeasts with interesting applications.

In addition to using sourdoughs themselves as reservoirs of novel breadmaking yeasts, the original sources of sourdough microbes (such as soils, plants, and insects) have yielded non-conventional yeasts of interest. Madden et al.¹³ found that from a pool of yeast strains isolated from sugar-seeking insects, thirteen from the Candida, Lachancea and Pichia genera were able to produce bread loaves of comparable quality to those made with baker’s yeast. Furthermore, it was found that some of the Lachancea strains isolated could grow in osmotically challenging conditions, meaning that isolates of this genus may be suitable for growth as purified monocultures on an industrial scale. Potentially, ‘bioprospecting’ in non-food environments could yield novel food fermentation organisms.

It is frequently observed that the taste, texture, and aroma profiles of sourdough bread are different to those of ‘straight’ dough breads produced with purified baker’s yeast. While some of this difference must be attributed to the different process parameters involved in making sourdough bread and the presence of lactic acid bacteria in the sourdough ecosystem, attention has now turned to sourdough yeasts as a source of valuable aroma volatiles in bread. In fact, yeast metabolism has been reported to be the main source of aromatic diversity in fermented foods such as alcoholic beverages and bread.¹⁴ When used as the sole fermenting yeasts in bread dough, Wickerhamomyces supelliculosus and Kazachstania gamospora were found to produce unique aromatic compounds.¹⁵ These compounds may include volatile esters, associated with a fruity aroma, which have been reported in increased amounts when novel yeasts are used in bread fermentation.⁶ A co-culture of S. cerevisiae and T. delbrueckii was found to improve production of succinic acid and acetic acid (Fig. 1) in steamed bread compared to dough fermented with mono-cultures.¹⁶ Interestingly, when a strain of S. cerevisiae that had been isolated from an Australian sourdough was used to ferment bread dough, the resulting bread had a distinct, different chemical aroma profile compared to that made with commercial baker’s yeast.¹⁷ This suggests that even for S. cerevisiae, the derivation of yeasts from a sourdough environment may be related to important aroma- and flavour-generating properties.

Fig. 1.  Metabolic pathways of fermenting bread yeasts. These are well understood in S. cerevisiae, but may differ in non-conventional yeasts (e.g. Kluyveromyces spp.), in which they are poorly characterised. Green labels represent substrates, red labels represent secreted metabolites and pink bubbles represent extracellular enzymes. Dotted arrows indicate omitted metabolic steps. Based on De Vuyst et al. 2016.¹⁰ TCA, tricarboxylic acid cycle. Created using BioRender.

Gluten and FODMAP contents in bread are, for those with coeliac disease, irritable bowel syndrome,¹⁸ and other gastrointestinal disorders, significant obstacles to the consumption and enjoyment of bread. Recent research has shown that non-conventional yeasts may play a key supporting role in the predominantly LAB-mediated degradation of gluten in sourdough (Table 1). The presence of T. delbrueckii in co-culture with bacterium Pediococcus acidilactici was shown to enhance the latter’s protein metabolism and accelerate its ability to degrade proteins.¹⁹ Additionally, although not considered bakery yeasts, fungal proteases derived from Aspergillus oryzae and A. niger have been used in conjunction with sourdough lactobacilli to initiate primary hydrolysis of wheat proteins. Ultimately, these enzymes could detoxify wheat flour.²⁰ Interestingly, growth on a synthetic gluten-limited medium showed that strains of W. anomalus could be classified as ‘gluten-degrading’ in their own right, and that the extent of gluten degradation varied slightly between individual strains.²¹ FODMAPs are a class of small, osmotically active carbohydrate molecules. These properties mean that they are not well absorbed in the small intestine and pass into the large intestine, where they undergo rapid bacterial fermentation, which in turn causes abdominal swelling and luminal distention.²² Novel yeasts derived from sourdoughs have been shown to have the enzymatic capabilities to break these carbohydrates down, notably fructans. As an example, Kluyveromyces marxianus was found to be able to degrade >90% of the fructan component of whole wheat bran, due to its ability to produce inulinase.²³ Struyf et al. have also demonstrated the successful use of co-cultures of K. marxianus and S. cerevisiae to ensure sufficient CO₂ production, producing bread with adequate loaf volume and ≤0.2% fructan content.²⁴ Non-conventional yeasts, both as mono-cultures and co-cultures, might therefore present an attractive opportunity to create high-quality bread that caters to the dietary needs of modern consumers.

Table 1.  Non-conventional yeasts and their attributes and applications in breadmaking.

An additional notable aspect of novel bakery yeasts is their capacity to form associations with bacteria. This is important in fermented food products such as kefir and kombucha that rely on a consortium of both yeasts and bacteria to produce their characteristic properties. These co-cultures have been shown to have interesting and valuable effects on final food products that their constituent monocultures are not capable of producing. When inoculated in bread dough as co-cultures alongside sourdough-derived lactic acid bacteria, novel sourdough-derived yeasts are capable of producing distinctive aroma profiles (with predominant sour aromas) and crumb structures preferred by sensory panels.¹⁷ It is likely that interactions between yeasts and lactic acid bacteria affect the manner in which the bacteria use carbohydrates to produce metabolites.²⁵ It has been suggested that the oft-documented association between Kazachstania humilis and the sourdough heterofermentative LAB Fructilactobacillus sanfranciscensis may be driven by cross-feeding, as maltose metabolised by F. sanfranciscensis into glucose may provide a source of nutrition for the maltose-negative yeast.²⁶ Although the molecular mechanisms underpinning this interaction are yet to be fully elucidated, the frequent detection of established yeast-bacteria pairs in food environments suggests that there is a strong natural tendency for such partnerships to form.

Ongoing research is required to render novel non-conventional yeasts suitable for baking, especially in an industrial context. For instance, Kluyveromyces marxianus cannot ferment maltose,²³ so it requires added sucrose or an enzyme (i.e. amyloglucosidase) to release glucose from amylose to produce sufficient CO₂ to fulfil its leavening requirements. This emphasises the need to consider the metabolic demands of novel yeasts, and to consider whether they might function best as co-cultures, or in conjunction with certain substrates or enzymes. Co-cultures of yeasts or yeasts in combination with bacteria present an attractive area for future research into the applications of novel yeasts in breadmaking. Particularly for yeasts derived from non-food environments, safe usage status (generally-regarded-as-safe or qualified presumption of safety status) must also be established¹⁵ before they can be approved for use on an industrial scale.

Data sharing is not applicable as no new data were generated or analysed during this study.

The authors declare no conflicts of interest.

This work has been supported by an Australian Government Research Training Program Scholarship provided by the Australian Commonwealth Government and The University of Melbourne to AW.