Dough Properties and Bread Texture

1. Wheat Dough and Bread

Incorporation of sourdoughs in wheat bread making influences the gluten proteins and the viscoelastic behaviour of doughs due to the drop in pH value caused by the organic acids produced. Several investigations have shown that the addition of acid to wheat dough decreased the dough stability during mixing, and the acidified doughs became considerably softer than a nonacidified control dough (82-84).

Dough stability was also decreased when it was prepared with the addition of sourdough (85,86). The dough consistency was unchanged when the sourdough was fermented by a heterofermentative culture and softer if a homofermentative culture was used (86). Proteolytic breakdown of proteins was enhanced at low pH during fermentation of wheat dough, and major effects were attributed to changes in pH rather than to microbial proteolytic activity from the sourdough (87).

In spite of the decreased stability in doughs with added sourdough, increased bread volume is reported for bread containing up to 20% sourdough (31,86). The crumb structure of bread containing up to 20% sourdough has been comparable to standard bread without sourdough, whereas inferior crumb structure was observed in bread containing 40% sourdough (88).

2. Rye Dough and Bread

The main component of rye and wheat is starch, and its content has a crucial influence on the bread texture. It becomes sticky and pasty if the starch is degraded during the bread making due to too high activity of amylases. This problem is greater in rye bread making than for wheat bread, as the activity of the sprout-induced enzyme a-amylase is highest in rye (89). This is caused by rainy summers in the rye-growing area. Furthermore, the period from harvest to possible sprouting is extremely short for rye; it can even sprout in the fields (89). One of the main functions of sourdough in rye bread making is inactivation of the a-amylase activity, and a general rule in bakeries is to add a larger amount of sourdough when the activity of enzymes in the flour is high. Bread with a rye content of more than 20% normally requires the addition of sourdough to prevent degradation of starch (88,90). Rye starch begins to swell as low as 52 °C and subsequently the a-amylase can degrade the starch until it will be heat-inactivated at 80 °C (91). Rye a-amylase has a pH optimum at pH 5.5 (92), and the activity is totally inactivated in sourdough at pH below 4. Wassermann and Dorfner (93) found that the viscosity of rye doughs (rye flour and water) was lowest at pH 5.

The activity of a-amylase is not only reduced considerably in the sourdough, but also in the rye dough with added sourdough. The activity of a-amylase was totally inactivated in an imitated sourdough acidified to pH 3.5 (TTA 32) by lactic and acetic acid (94). The activity of a-amylase in the bread dough after resting (pH 4.5), with 20% sourdough added, was about half the activity in the flour.

Pentosans (arabinoxylans) play a key role in the viscosity of rye doughs due to high water-binding capacity. The viscosity of sourdoughs decreases during the sourdough fermentation due to the activity of the pentosan-degrading enzymes at the beginning of the fermentation. However, those enzymes are inactivated in the fermented sourdough (94). Rye proteins are different from wheat proteins, as they do not form gluten structure.

Kratochvil and Holas (95) found that proteolytic activity in rye sourdough was caused by enzymes from the flour.

B. Flavor and Taste

The flavor of bread crumb depends mainly on the flour type and the enzymatic reactions taking place due to yeast and sourdough fermentations, whereas the flavor of bread crust is more influenced by the thermal reactions during the baking process. Including sourdough in the bread recipe is recommended for a more aromatic bread flavor (31,96) and sourdough bread has a higher content of volatile compounds (31,39,97-99) and higher scores in sensory tests (31,100,101). The content of volatile compounds produced during sourdough fermentation depends on the flour type (wheat, rye, maize), the extraction rate of the flour, the fermentation temperature, the water content in the sourdough, and the microorganisms in the sourdough. Generally, the LAB in the sourdough are mostly responsible for the acidification of the dough, and the sourdough yeasts for the production of flavor compounds. Factors that favor the propagation of yeasts will also result in higher content of yeast fermentation products.

The extraction rate of the flour and the water content in the sourdough mostly influences the acidification of the sourdough. Higher extraction rate of the flour results in higher production of lactic and acetic acid (38,102), however, sourdoughs fermented with heterofermentative cultures have much higher content of ethyl acetate (38). The production of acids calculated per gram dry matter is higher in fluid sourdoughs than in firm sourdoughs. Higher water content in the sourdough and increased fermentation temperature result in higher propagation of yeasts and in higher content of iso-alcohols (36,81,102).

Sourdoughs fermented with heterofermentative LAB have, aside from much higher content of acetic acid and ethanol, a higher content of ethylacetate and ethyl-hexanoate compared to sourdoughs fermented with homofermentative LAB, which have higher contents of diacetyl and some other carbonyls (36,81,102,103). The production of acetic acid in sourdoughs can be increased in heterofermentative cultures with the addition of fructose as a hydrogen acceptor (102,104). When sourdough yeasts are added in the preparation of the sourdough, the production of ethanol, iso-alcohols, esters, and diacetyl increase considerably (37,103).

In sourdough bread, the content of esters is very low compared to the corresponding sourdoughs (31,39). Sensory evaluation of rye bread crumb shows that the most intense and breadlike flavor is associated with 2-propanone, 3-methyl-butanal, benzylalcohol, and 2-phenylethanol (39). However, vanillin, 2,3-butandione, 3-hydroxy-4,5-dimethyl-fura-none, and methylbutanoic acids also contribute to the overall crumb flavor (98). The perceived taste of salt is enhanced in sourdough rye bread compared to wheat bread, so less salt can be added in sourdough rye bread (105).

Sensory evaluation of wheat bread crumb showed that bread made with sourdough fermented with the heterofermentative L. sanfranciscensis had a pleasantly mild, sour odor and taste. Bread fermented with L. plantarum had an unpleasant metallic sour taste, but when the sourdough was also supplemented with the sourdough yeast S. cerevisiae, the bread acquired a more aromatic bread flavor. That bread had a higher content of methyl-butanol, methylpropanoic acids, and 2-phenylethanol, which may, in part, cause the more aromatic flavor (31). Mixed cultures with both LAB and yeast are recommended for an aromatic and pleasant sourdough bread flavor (31,103,106). A lexicon for description of the flavor of wheat sourdough bread has been developed (107).

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