Document Type : Research Article
Authors
Department of Food Science and Technology, Gorgan University of Agricultural Sciences and Natural Resources, Iran.
Abstract
Introduction: Sourdough is a very complex biological system and an important modern fermentation method of cereal flours and water. Sourdough fermentation is based on lactic acid and alcoholic fermentation depending on the composition of micro flora and fermentation conditions. Commercial sourdough processes do not rely on fortuitous flora but on the use of specific starter cultures. There has also been much progress in the development of tools that allow for the selection of key sourdough microorganisms for particular activities such as those concerned with enzymatic, antimicrobial, nutritional and additive replacement aspects. Most of the beneficial properties attributed to sourdough are determined by the acidification activity of dominant Lactobacillus starters. Sourdough fermentation can improve texture and palatability of whole grain fiber-rich, stabilize or increase levels of various bioactive compounds, retard starch retro-gradation and improve mineral bioavailability. The acidification of the sourdough and the partial acidification of the bread dough will impact on structure-forming components like gluten, starch and arabinoxylans. The swelling of gluten in acid is a well-known effect and mild acid hydrolysis of starch in sourdough systems has also been hypothesized for delay bread staling. The objectives of this research were to apply the dominant Lactobacillus starter isolated from traditional sourdough for cup bread production with whole wheat flour and delay it's staling. Materials and methods: In this study, following isolation of dominant Lactobacillus starter from traditional sourdough produced with whole wheat flour, the starter was identified by specific PCR. The single colonies obtained from streak plate of the sourdough culture, were subjected to species specific PCR. Afterwards, the mentioned starter was used for sourdough preparation. For this purpose, the effect of flour components (extraction rate, moisture, protein, ash and falling number), fermentation times (8, 16, 24 h) and sugar contents (0.5, 1, 1.5%) on starter activity were evaluated. pH and total titratable acidity (TTA) of sourdough treatments were measured. After processing of cup breads with sourdough treatments, the staling of these breads were also examined 2, 48 and 96 h after baking, based on crumb firmness (texture analysis) and amount of porosity (Image j method). Finally for statistical analysis a completely randomized design with factorial arrangement and 3 replications was used. To study the relationship between bread hardness and porosity with fermentation conditions, multiple linear regression was used and regression models were exhibited. Results and Discussion: By sequencing of the PCR products (obtained from sourdough culture), dominant Lactobacillus starter was identified as Lactobacillus plantarum. The TTA profile for the sourdoughs was also quite similar (starters interestingly continue to produce acid) and by increasing of TTA, the pH values were decreased. The acid production depends on factors such as fermentation temperature, time and dough yield. In general, a higher temperature, a higher water content of sourdough and the utilization of whole meal flour enhances the production of acids in wheat sourdoughs. The effect of sourdough on softness improvement was partly due to a higher porosity. Among the bread samples, 96 h after baking, lowest crumb firmness was observed in sample produced with sourdough with 24 h fermentation and 0.5%. sugar content. The maximum amount of crumb firmness was observed in sample produced with sourdough after 8 h fermentation and 1.5% sugar content. Furthermore, the maximum amount of porosity was obtained after 24 h sourdough fermentation and 1.5% sugar content, while the lowest amount was obtained after 8 h sourdough fermentation and 0.5% sugar content. After evaluation the results of texture analysis and porosity tests, significant correlation coefficients were established between porosity and softness, and it is reported that volume improvement is the main reason for a better shelf life in sourdough breads. The relationship between factors affecting on sourdough fermentation including fermentation time, sugar content and flour components, were also exhibited as regression models for examination texture characteristics of sourdough breads based on those viscoelastic behavior. By increasing the fermentation time in all of the sourdoughs, crumb hardness was decreased. Acids strongly influence the mixing behavior of doughs. Doughs with lower pH values require a slightly shorter mixing time and have less stability than normal doughs. Fundamental rheological evaluation of acid effect on gluten systems model indicated that both softness and elasticity of gluten were increased. Further to the direct impact of low pH on dough characteristics, secondary effects of acidification and fermentation time including changes in the activity of cereal or bacterial enzymes associated. Wheat flour proteases have optimal activity around pH=4. In addition, proteolytic enzymes with acidic pH optima in vital wheat gluten have been detected. Process requirements for optimum quality were strain-specific and different for textural improvement which should be taken in to account in designing future sourdough baking processes. According to results of this research, the influence of sourdough on bread softness during storage was depended on fermentation conditions and significant effect of sourdough process conditions on bread staling was clarified in comparison to control sample.
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