Chapter 1. General Introduction
1.5 Sourdough and lactic acid bacteria
endo-dextranase and only α-(1→6) glucan synthesizing dextransucrase using sucrose as sole substrate. Leuconostoc mesenteroides NRRL B-1299 dextransucrase synthesizes specifically α-(1→2) linear and branched glucooligosaccharides which are highly resistant to glycolytic digestive enzymes and can be fermented by beneficial species of the intestinal microflora (Ramaud-Simeon et al., 1994; Dols et al., 1998). Branched oligosaccharides produced by GTFs coming from Leuconostoc mesenteroides M2860 were readily catabolized by lactobacilli but not by Escherichia coli and Listeria innocua strains, pointing toward their application in intestinal microflora modification. Thermo acid-stable oligosaccharides (TASO) were produced from Leuconostoc mesenteroides B-512 FMCM (Seo et al., 2007). These resistant oligosaccharides inhibited the growth of Streptococcus sobrinus. However, it stimulated the growth of probiotic organisms such as Bifidobaterium sp (Seo et al., 2007). Gluco-oligosaccharides are known to selectively stimulate the growth of bifidobacteria. In particular, long-chain glucooligosaccharides with a degree of polymerization of 3 or higher are preferred to short-chain oligosaccharides because of the longer persistence in the colon (Chung and Day, 2002). These probiotic strains get advantage over other harmful bacteria in the sense that they can ferment these oligosaccharides. Short chain fatty acids like propionic acids are formed as the results of the fermentation of these oligosaccharides which play decisive role in the prevention of colon cancer (Cummings, 1981).
together and the fermentation takes place by microbes from preceding sourdough, commercial starter culture, bakery equipment or from flour. The most important quality characteristics for wheat breads are high volume, soft and elastic crumb structure, good shelf-life and microbiological safety of the product (Cauvain, 2003).
Sourdough fermentation is based on lactic acid and alcoholic fermentation depending on the composition of microflora and fermentation conditions. The dominating microbes in spontaneously fermented dough are homofermentative lactobacilli and Pediococci. Typical homofermentative lactic acid bacteria in spontaneous sourdoughs are Lactobacillus casei, Lb. delbrueckii, Lb.farciminis, Lb. plantar and Pediococcus.
pentosaceus. Among heterefermentative LAB are Lactobacillus brevis, Lb. buchneri, and Lb. fermentum (Stolz, 2003). Various yeast strains have also been isolated from spontaneous fermentations of sourdough such as Saccharomyces cerevisiae and Pichia satoi (Beech and Davenport, 1971). Homofermentative lactic acid bacteria ferment hexose sugars to produce mainly lactic acid. Many heterofermentative strains can also ferment pentosans to produce lactic acid, acetic acid and ethanol. The formation of end product is dependent on the processing conditions of sourdough and type of heterofermentative strain used (Röcken et al., 1992). The most common lactic acid bacteria identified in sourdoughs are capable of fermenting pentoses, hexoses, sucrose and maltose, although some species such a Lb. sanfransiscensis are specific to maltose.
In wheat products, textural characteristics are mainly based on the formation of gluten network during bread baking, which has the ability to extend and keep the gas from yeast fermentation and makes a direct contribution to the formation of a cellular crumb structure. In order to improve particularly texture and shelf-life of the breads on the market, several food additives such as emulsifiers and enzymes have
been introduced in bread baking. However, most of these additives are either E- numbered additives or produced with genetically modified organisms (GMOs).
Nowadays these additives do not fulfill the current consumer trends which prefer GMO-free, high-quality products baked without chemical additives. Therefore, Sourdough baking is an alternative to the use of additives. Sourdough fermentation is known to improve the nutritional value of cereal products (Liukkonen et al. 2003, Kariluoto et al. 2004).
The majority of gluten-free breads available are of poor sensory and textural quality. Bran supplementation usually weakens the structure and baking quality of wheat dough and decreases bread volume and the elasticity of the crumb. Recently, it has been reported that certain lactic acid bacteria are able to produce exopolysaccharides, which might have a positive effect on bread volume and shelf- life (Korakli et al., 2001, Tieking et al., 2003). In gluten-free bread, dextran from Weissella cibaria improved nutritional and organoleptic properties of gluten-free bread (Schwab et al., 2008). Dextran enriched Weissella cibaria MG1 sourdough increased loaf volume and improved crumb softness and provided mildly acidic bread with an improved shelf life of gluten-free sorghum sourdough bread (Galley et al., 2012). In addition to dextran, Weissella ciberia MG1 also produced oligosaccharides during sorghum sourdough fermentation contributing to the nutritional benefits of gluten-free sorghum bread (Galley et al., 2012). Bread prepared by adding 50%
sourdough produced with the starter cultures of Leuconostoc citreum HO12 and Weissella koreensis HO20, exhibited consistent ability to retard the growth of bread spoilage fungi (Penicillium roqueforti and Aspergillus niger) and rope-forming bacterium (Bacillus subtilis) (Choi et al.,2012).
Hydrocolloids have been reported to improve bread quality (Rosell et al., 2001). The exopolysaccharides influence the product texture, mainly due to their ability to influence viscosity. Weissella sp. has emerged as potential in-situ dextran producer in sourdough. Katina et al., 2009 established the potential of Weissella confusa VTT E-90392 in sourdough fermentation. Weissella confusa VTT E-90392 produced significant amounts of polymeric dextran and isomaltooligosaccharides in wheat sourdough without strong acidification (Katina et al.,2009).
The endogenous cereal proteases of flours have been shown to degrade cereal prolamins under acidic conditions (Kawamura and Yonezawa, 1982, Brijs et al., 1999). Increased proteolysis during sourdough fermentation leads to the liberation of amino acids in wheat and rye dough (Spicher and Nierle, 1988, Collar et al., 1991, Gobbetti et al., 1994). Furthermore, sourdough fermentation results in a solubilization and depolymerization of the gluten macropolymer (Thiele et al., 2004). Lactic acid bacteria also produce volatile compounds which is strain-specific. Homofermentative Lactobacilli are characterized by the high production of diacetyl, acetaldehyde, hexanal and heterofermentative strains are characterized by the production of ethyl acetate, alcohols and aldehydes. Isoalcohols (2-methyl-1-propanol, 2,3-methyl-1- butanol), with their respective aldehydes and ethylacetate are characteristic volatile compounds of yeast fermentation (Damiani et al., 1996).