| The mixed fermentation of various microorganisms such as lactic acid bacteria and yeast in sourdough can give Chinese steamed bread a unique flavor.However,traditional sourdough is not easy to preserve,and the production process of Chinese steamed bread is cumbersome.Therefore,this paper intends to use freeze-drying,ripening process(ohmic heating combined with hot air drying)and spray-drying three different drying methods to make sourdough into easy-to-preserve sourdough powder,with the main purpose of improving the sensory quality of steamed bread,combined with the analysis of volatile flavor substances,comprehensively evaluate the applicability and feasibility of ripening process and spray-drying in the production of sourdough powder,provide technical guidance for its application in sourdough drying,and promote the industrial production of sourdough powder.The specific research results were as follows:(1)The effects of different drying methods on the physicochemical properties of sourdough powder were studied.The results showed that the ripening process promoted the aggregation of sourdough powder particles,resulting in an increase in particle size and bulk density,saving packaging and transportation costs.At the same time,the ripening process not only destroys the secondary structure of protein in sourdough(α-helix into β-sheet,β-turn and random curl),but also destroys the structure of starch in sourdough,resulting in an increase in the content of damaged starch,a decrease in the content of amylose and resistant starch,a change in the crystal form of starch(A-type → V-type),and a decrease in crystallinity.The protein conformation and starch molecular structure of the sourdough powder prepared by spray-drying and freeze-drying were similar to those of the control flour.The secondary structure of the protein was composed of 18.39 ～ 19.06% α-helix,16 ～ 17%β-turn,about 46% β-sheet and 18.37 ～ 19.96% random groups.The crystal structure of starch was A-type structure.Therefore,the results obtained in this study suggest that different drying methods had different effects on the physicochemical properties of sourdough powder,and freeze-drying had the least effect on the metabolites and molecular structure of sourdough powder.(2)The effects of sourdough powder prepared by different drying methods on the quality characteristics of flour and Chinese steamed bread were studied.The results showed that the spray-drying and freeze-drying sourdough powder reduced the overall viscosity of the mixed powder,while the sourdough powder prepared by ripening process increased the overall viscosity of the mixed powder,and the starch was easier to absorb water and expand,making the gluten network structure more compact and orderly.The sourdough powder prepared by freeze-drying and spray-drying will reduce the quality of gluten in the dough,the gluten strength is weak,and the stirring resistance is poor.The sourdough powder prepared by the ripening process will increase the content of hydrophobic bonds and disulfide bonds in the dough with sourdough power,the gluten quality is better,the water absorption of the dough is improved,and the ability to resist mechanical damage is strong,which is conducive to the formation of loose and soft texture of Chinese steamed bread.The addition of sourdough powder prepared by freeze-drying and spray-drying will lead to the deterioration of the viscoelasticity and creep recovery properties of the dough,and the dough is more likely to collapse and the deformation resistance is weak,while the sourdough powder prepared by the ripening process can improve the viscoelasticity of the dough.The addition of sourdough powder did not affect the color of Chinese steamed bread,and the addition of sourdough powder prepared by ripening process had the least effect on the hardness,chewiness and gumminess of Chinese steamed bread.Therefore,the results obtained in this study suggest that compared with freeze-drying and spray-drying,the sourdough powder prepared by ripening process had less negative effects on flour and Chinese steamed bread,and could be used as an alternative to freeze-drying for the industrial production of sourdough powder.(3)The effects of different fermentation conditions and drying methods on the composition of volatile flavor substances in sourdough,and the effect of sourdough powder addition on the flavor of Chinese steamed bread were compared.The results showed that dough yield and fermentation temperature could affect the accumulation of volatile flavor substances in sourdough.Refrigerated solid-state fermentationwas beneficial to the accumulation of aldehydes,esters and ketones,while liquid fermentation could give more alcohols to sourdough.The content of ethyl esters and ketones in the sourdough powder prepared by the ripening process decreased,and the content of aldehydes and acids increased,resulting in new substances such as benzyl alcohol,3-octene-1-ol,trans-2-nonenal,2-tridecenal and phenol.The total content of volatile flavor substances was 1.08～1.15 times that of freeze-drying.The content of alcohols and small molecular esters in the sourdough powder prepared by spray-drying decreased,but new substances such as peach aldehyde,trans-2-decenal,2-methyl-2-butenal,5-methylfuran aldehyde,coconut aldehyde and pyrazine were produced,and the content of volatile flavor substances was the highest.Sourdough powder can endow Chinese steamed bread with unique flavor substances such as heptanol,1-octen-3-ol,heptanal,methyl valerate,hexyl acetate,2-heptanone,and orange-based acetone.Compared with freeze-drying sourdough powder,Chinese steamed bread added with sourdough powder prepared by ripening process(ohmic heating combined with 50℃ hot air drying)and spray-drying had more types and contents of volatile flavor substances,and had better sensory quality(sensory scores were 89.85 and 89.20,respectively).Therefore,the results obtained in this study suggest that the flavor substances in the sourdough were lost and new substances were generated after the ripening process and spray-drying.The sourdough powder prepared by the ripening process(ohmic heating combined with 50℃ hot air drying)had the best effect on the sensory quality of Chinese steamed bread,and had the advantages of short drying time and low energy consumption,which was the best drying method for the preparation of sourdough powder. |
