Sourdough Fermented By Exopolysaccharide Forming Strain:Application And Mechanism Studies In Frozen Dough

Frozen dough technology has been widely used in bakery goods to facilitate centralized manufacturing and distribution processes as well as the standardization of product quality.Nevertheless,the final quality of frozen dough is often negatively affected by disruption of the gluten network and loss of yeast viability owing to ice formation and temperature fluctuation-driven ice recrystallization.To retard frozen dough quality deterioration,addition of novel biologicalfunctional agents to the matrix aimed at improving the properties of frozen dough has been an immediate areas of research focus and challenges.Sourdough fermentation technology is one of the frontier areas of baking and food science in recent years.In the modern baking industry,sourdough is often used as a natural new bio-improver to improve the texture,flavor and nutrition of the cereal food quality.However,the study of combination of sourdough and fozen dough technology has been rarely reported.In the present study,a high yield exopolysaccharide(EPS)-producing strain was isolated from Chinese traditional sourdoughs,and was used to ferment sourdough.The effect of sourdough fermented by EPS-forming strain on the quality of frozen dough and steamed bread as well as the related mode of action were also investigated.1.In this study,120 lactic acid bacteria(LAB)strains were isolated from 30 Chinese traditional sourdoughs.A combination method of phenotype screening on EPS-producing solid media,viscocity determination in liquid media and determination of EPS production were used to screen EPS-producing LAB.Of all screened isolates,Weissella confusa QS813(W.confusa QS813)was identified and found to produce large amounts of EPS.The optimum growth temperature of Weissella confusa QS813 is 35°C,while its optimum EPS-producing temperature is 20°C.The high yileds of EPS and high sucrose conversion rate ability make this strain has a great industrial application potential.2.The monosaccharide composition,molecular weight,molecular structure,rheological properties,thermal stability and microstructure of EPS produced by W.confusa QS813 were studied.The results showed that EPS produced by Weissella confusa QS813 was a high molecular mass,low branched dextran.EPS has good solubility in water and presents typical shear thinning characteristics.Thermal analysis shows that EPS has good thermal stability.At low concentration,EPS presented intertwined fibrous structure,while at higher concentration,it showed regular sheet structure.3.Sourdoughs were produced using an EPS-producing strain W.confusa QS813.The cell counts,pH,TTA,microbial metabolism and proteolytic activity were measured during sourdough fermentation.Results showed that fermentation at a lower temperature using EPS-producing strain W.confusa QS813 delayed the acidification of sourdoughs and promoted the EPS accumulation.The highest EPS content and lowest titratable acidity value were obtained in sucrose-enriched sourdough fermented at 20 °C.The free amino acid contents were also markedly affected by fermentation temperature and metabolism of exopolysaccharide.The extent of the decrease of protein degradation in sourdoughs was decreased when fermented at 20 °C with sucrose addition.4.The effects of EPS-producing sourdough on the rheofermentation,hydration,rheological and microstructure of dough and specific volume of steamed bread were studied.The results showed that the EPS-producing sourdough fermented at lower temperature(20°C EPS+)enhanced the gas production and holding power of the frozen dough,weakened the influence of freezing on the moisture mobility of the dough,significantly improved the stability of the frozen dough,and effectively delayed the moisture migration of the dough caused by ice crystals.The 20°C EPS+ sourdough promotes gluten crosslinking and delays gluten network damage and starch granule dissolution caused by freezing.20°C EPS+ sourdough raises the specific volume of frozen dough steamed bread.Sourdoughs fermented at higher temperature negatively effect the quality of frozen dough and steamed bread.5.The effect of EPS produced by Weissella confusa QS813 on the hydration,water distribution,rheology and microstructure of wheat gluten during freeze-thaw cycles(FTC)was investigated.Addition of EPS increased the water content of fresh gluten and delayed the dehydration of gluten during FTC.Low field-nuclear magnetic resonance showed that the presence of EPS reduced the mobility of both confined and bulk water in the fresh gluten matrix.The presence of EPS effectively reduced the release of bulk water and retarded the redistribution of confined water induced by FTC.Dynamic oscillatory studies indicated that the presence of EPS induced a softening effect on the fresh gluten.FTC significantly decreased the rheological parameters,(storage modulus(G′)and loss modulus(G″),of all gluten samples.Gluten with higher concentrations of EPS(above 0.5%)exhibited a delayed decrease in G′ and G″ and maintained a constant tanδ value after FTC.Laser scanning confocal microscope and scanning electron microscope revealed that EPS maintained the structural integrity of gluten during FTC,suggesting an inhibitory effect on the recrystallization of ice crystals.6.The effect of EPS produced by Weissella confusa QS813 on the syneresis rate,water distribution and microstructure of a wheat starch gel during FTC was investigated.When added in wheat starch(WS)at different concentrations,the EPS at 1% significantly(p<0.05)reduced the syneresis rate among all samples,thus the highest stability during FTC.The LF-NMR results showed that the addition of EPS at high concentration to WS appeared to alter the distribution and mobility of water in the starch gel.The microstructure of the wheat starch gel with 1% EPS was denser and more uniform compared to that of the wheat starch without EPS.In conclusion,the EPS produced by Weissella confusa QS813 significantly improved the freeze-thaw stability of wheat starch gel.To sum up,a high EPS-producing Weissella confusa QS813 which was screened from Chinese traditional sourdough could successfully produce pure fermented sourdough with high yield of EPS and low yield of acid.EPS-producing sourdough can effectively improve the quality of frozen dough.This research has broken through the barrier of the combination of biological fermentation technology and frozen dough technology,and provided a new and effective technical path for the industrialization of steamed bread frozen dough in China.