Evaluation of exopolysaccharide producing Weissella cibaria MG1 strain for the production of sourdough from various flours

Abstract

This study determined exopolysaccharide (EPS) production by Weissella cibaria MG1 in sourdoughs prepared from gluten-free flours (buckwheat, oat, quinoa and teff), as well as wheat flour. Sourdoughs (SD) were fermented without sucrose, or by replacing 10% flour with sucrose to support EPS production. The amount of EPS depended on the substrate: high amounts of EPS corresponding to low amounts of oligosaccharides were found in buckwheat (4.2 g EPS/kg SD) and quinoa sourdoughs (3.2 g EPS/kg SD); in contrast, no EPS but panose-series oligosaccharides (PSO) were detected in wheat sourdoughs. Organic acid production, carbohydrates and rheological changes during fermentation were compared to the EPS negative control without added sucrose. Corresponding to the higher mineral content of the flours, sourdoughs from quinoa, teff and buckwheat had higher buffering capacity than wheat. Fermentable carbohydrates in buckwheat, teff and quinoa flours promoted W. cibaria growth; indicating why W. cibaria failed to grow in oat sourdoughs. Endogenous proteolytic activity was highest in quinoa flour; α-amylase activity was highest in wheat and teff flours. Protein degradation during fermentation was most extensive in quinoa and teff SD reducing protein peaks 18-29, 30-41 and 43–55 kDa extensively. Rheological analyses revealed decreased dough strength (A_F) after fermentation, especially in sucrose-supplemented buckwheat sourdoughs correlating with amounts of EPS. High EPS production correlated with high protein, fermentable sugars (glucose, maltose, fructose), and mineral contents in quinoa flour. In conclusion, W. cibaria MG1 is a suitable starter culture for sourdough fermentation of buckwheat, quinoa and teff flour.

Introduction

A gluten-free diet is currently the only effective treatment for coeliac and gluten intolerant patients. Gluten-free breads often have a low nutritional value and are characterized by a low bread volume and a poor texture owing to the use of refined ingredients (pure starches and proteins), and the lack of the network forming gluten proteins (Gallagher et al., 2003). The use of nutrient-dense flours, for example, quinoa, buckwheat or teff, may improve the nutritional value (Hager et al., 2012) but does not provide a network forming protein. Hydrocolloids, such as xanthan, carrageen and agar, are used in bakery products as a replacement for gluten and to bind water in dough. Hydrocolloids also retard starch retrogradation, which is intimately linked to bread staling and shelf-life (Belitz et al., 2008). Microbial exopolysaccharides (EPS) are high molecular weight carbohydrate polymers found in some bacteria and microalgae (Monchois et al., 1999; van Hijum et al., 2006). Depending on their composition, they can be divided into homopolysaccharides (HoPS), consisting of one type of monosaccharide being either glucose (glucans) or fructose (fructans), and into heteropolysaccharides (made of 3–8 multiple, repeated moieties) (De Vuyst et al., 2001; van Hijum et al., 2006). In contrast to heteropolysaccharides which are synthesized in smaller amounts from sugar nucleotide precursors (De Vuyst et al., 2001), HoPS are synthesized in larger amounts from sucrose (Monsan et al., 2001).

HoPS-producing lactic acid bacteria are already used in conventional bread making (Decock and Cappelle, 2005, Lacaze et al., 2007), but their use is particularly promising in gluten-free baking (Galle et al., 2012, Ruehmkorf et al., 2012a, Ruehmkorf et al., 2012b, Schwab et al., 2008) since EPS can potentially act as hydrocolloids (Schwab et al., 2008). Microbial production of exopolysaccharides during sourdough fermentation was reported to be more effective than the addition of comparable amount of EPS to the bread formulation (Brandt et al., 2003).

Alternatively to EPS synthesis, sucrase-type enzymes catalyse the reactions sucrose hydrolysis and oligosaccharide formation (Tieking et al., 2003, van Hijum et al., 2006).

The production of oligosaccharides and polysaccharides by glucansucrases is dependent on the concentration and type of suitable acceptor carbohydrates (Galle et al., 2010, Kaditzky and Vogel, 2008, Tieking and Gänzle, 2005). Maltose is an efficient acceptor carbohydrate for dextransucrase present in cereals and diverts sucrose conversion from exopolysaccharide synthesis to oligosaccharide production (Galle et al., 2010, Kaditzky and Vogel, 2008). The concentration of maltose and other acceptor carbohydrates in sourdough depends on the carbohydrate composition as well as the enzyme activity of the cereal substrate thus influencing the yield of EPS and oligosaccharides in sourdough fermentations with EPS-producing starter cultures (Galle et al., 2010, Galle et al., 2012). The carbohydrate composition of wheat and rye flours as well as the evolution of carbohydrate levels in wheat and rye sourdoughs is well described (Roecken and Vosey, 1995). However, little data is available for other cereals or pseudocereals that are used in gluten-free baking.

During sucrose hydrolysis, fructose is released which can be used as an electron acceptor by most heterofermentative LAB and results in acetate formation (Gänzle et al., 2007). An excess of acetate compromises the quality of bread (Galle et al., 2010, Kaditzky and Vogel, 2008, Tieking and Gänzle, 2005). The obligate heterofermentative strain Weissella cibaria MG1 produced high amounts of the HoPS dextran (a α-1,6-linked glucan) from sucrose (Galle et al., 2010, Galle et al., 2012). Yet, comparable to other Weissella species, acetate formation was low, since W. cibaria MG1 lacks mannitol dehydrogenase activity and does not convert fructose to mannitol with concomitant acetate formation (Galle et al., 2010).

The strain-specific ability to produce exopolysaccharides during sourdough fermentation depends on the metabolic activity of the fermentation microbiota (Gänzle et al., 2007), and contributes to the sourdough's ability to influence bread quality (Galle et al., 2012, Katina et al., 2009).

It was the aim of this study to assess the production of EPS, oligosaccharides and organic acids by Weissella cibaria in sourdoughs. Protein degradation in gluten-free sourdoughs, as well as the effect of fermentation on dough rheology was also determined.